DNA and genes. How reliable is DNA analysis? DNA test for twins

Our world is inhabited by a variety of organisms: from microscopic, visible only through a powerful microscope, to huge ones, whose weight reaches several tons. Despite such species diversity, all organisms on Earth have a very similar structure. Each of them consists of cells, and this fact unites all living beings. At the same time, it is impossible to meet two identical organisms. The only exception is identical twins. What makes every organism living on our planet so unique?

Each cell has a central organ - this is the nucleus. It contains certain material units - genes located in chromosomes. From a chemical point of view, genes are deoxyribonucleic acid, or DNA. curled into a double helix, is responsible for the inheritance of many traits. Thus, the meaning of DNA is the transfer of genetic information from parents to offspring. To discover this truth, scientists around the world for two centuries have made incredible experiments, put forward bold hypotheses, suffered setbacks and experienced the triumph of great discoveries. It is thanks to the work of great researchers and scientists that we now know what DNA means.

By the end of the 19th century, Mendel established the basic laws of the transfer of traits in generations. The beginning of the 20th century and Thomas Hunt Morgan revealed to mankind the fact that hereditary traits are transmitted by genes that are located on chromosomes in a special sequence. Scientists guessed about their chemical structure in the forties of the twentieth century. By the mid-fifties, the double helix of the DNA molecule was revealed, and replication. In the 1940s, scientists Boris Ephrussi, Edward Tatum, and George Beadle made the bold hypothesis that genes produce proteins, that is, they store specific information about how to synthesize a specific enzyme for certain reactions in the cell. This hypothesis was confirmed in the works of Nirenberg, who introduced the concept of the genetic code and deduced a pattern between proteins and DNA.

DNA structure

In the nuclei of the cells of all living organisms there are nucleic acids whose molecular weight is greater than that of proteins. These molecules are polymeric, their monomers are nucleotides. Proteins are made up of 20 amino acids and 4 nucleotides.

There are two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Their structure is similar in that both substances contain a nucleotide: a nitrogenous base, a phosphoric acid residue and a carbohydrate. But the difference is that DNA has deoxyribose and RNA has ribose. Nitrogenous bases are purine and pyrimidine. DNA contains the purines adenine and guanine and the pyrimidines thymine and cytosine. RNA includes in its structure the same purines and pyrimidine cytosine and uracil. By combining the phosphoric acid residue of one nucleotide and the carbohydrate of another, a polynucleotide skeleton is formed, to which nitrogenous bases cling. Thus, quite a lot of different compounds can be obtained, which determine the species diversity.

The DNA molecule is a double helix of two large polynucleotide chains. They are linked by a purine of one chain and a pyrimidine of another. These connections are not accidental. They obey the law of complementarity: the bonds are able to form between themselves an adenyl nucleotide with a thymylic one, and a guanyl one with a cytosyl one, since they complement each other. This principle gives the DNA molecule the unique property of self-replication. Special proteins - enzymes - move and break the hydrogen bonds between the nitrogenous bases of both chains. As a result, two free nucleotide chains are formed, which are completed by the free nucleotides available in the cytoplasm and nucleus of the cell according to the principle of complementarity. This leads to the formation of two strands of DNA from one parent.

and his secrets

DNA research allows us to understand the individuality of each organism. This can be easily seen from the example of tissue incompatibility in organ transplantation from a donor to a recipient. A "foreign" organ, for example, donor skin, is perceived by the recipient's body as hostile. This starts a chain of immune reactions, antibodies are produced, and the organ does not take root. An exception in this situation may be the fact that the donor and recipient are identical twins. These two organisms originated from the same cell and have the same set of hereditary factors. During organ transplantation, in this case, antibodies are not formed, and almost always the organ takes root completely.

The definition of DNA as the main carrier of genetic information was established empirically. Bacteriologist F. Griffiths conducted an interesting experiment with pneumococci. He injected a dose of the pathogen into mice. The vaccines were of two types: form A with a capsule of polysaccharides and form B without a capsule, both heritable. The first species was thermally destroyed, while the second did not pose any danger to mice. What was the bacteriologist's surprise when all the mice died from form A pneumococci. Then a reasonable question arose in the scientist's head about how the genetic material was transferred - through protein, polysaccharide or DNA? Almost twenty years later, the American scientist Oswald Theodor Avery managed to answer this question. He set up a series of experiments of an exclusive nature and found that with the destruction of a protein and a polysaccharide, inheritance continues. The transfer of hereditary information was completed only after the destruction of the DNA structure. This led to the postulate: the molecule carrying hereditary information is responsible for the transmission of hereditary information.

The discovery of the structure of DNA and the genetic code has allowed humanity to take a tremendous step forward in the development of such areas as medicine, forensics, industry and agriculture.

DNA analysis in forensic science

At present, progressive record keeping of criminal and civil processes is not complete without the use of DNA expertise in forensic science for the study of biological material. With the help of this study, forensic scientists can detect traces of an intruder or victim on objects or bodies.

Genetic expertise is based on a comparative analysis of markers in biological samples of people, which gives us information about the presence or absence of relationship between them. Each person has a unique "genetic passport" - this is his DNA, which stores complete information.

In forensic examination, a high-precision method called fingerprinting is used. It was invented in the UK in 1984 and is a study of samples of biological samples: saliva, semen, hair, epithelium or body fluids in order to identify traces of a criminal in them. Thus, the DNA forensic examination is designed to investigate the guilt or innocence of a certain person in unlawful acts, to clarify cases of doubtful motherhood or paternity.

In the sixties of the last century, German specialists organized a society to promote genome research in the judicial and legal sphere. By the beginning of the nineties, a special commission was created, which publishes important works and discoveries in this area, being a legislator of standards in the work of a forensic medical examination. In 1991, this organization was given the name "International Society of Forensic Geneticists". Today, it consists of more than a thousand employees and 60 global companies that are engaged in research in the field of judicial proceedings: serology, molecular genetics, mathematics, biostatics. This has brought uniform high standards to the world forensic practice, which improve the detection of crimes. DNA forensic examination is carried out in specialized laboratories that are part of the complex of the judicial and legal system of the state.

Tasks of forensic genomic analysis

The main task of forensic experts is to examine the submitted samples and make a DNA conclusion, according to which it is possible to determine the biological “imprints” of a person or establish blood relationship.

DNA samples can be contained in the following biological materials:

• sweat marks;
• pieces of biological tissues (skin, nails, muscles, bones);
• body fluids (sweat, blood, semen, transcellular fluid, etc.);
• hair (must have hair follicles).

To conduct a forensic medical examination, a specialist is provided with material evidence from the crime scene containing genetic material and evidence.

A DNA database of criminals is currently being created in a number of progressive countries. This allows you to increase the detection of crimes even with expired statute of limitations. The DNA molecule can be stored for many centuries without change. Also, the information will be very useful for identifying a person in case of mass death of people.

Legislative framework and prospects for forensic DNA testing

In Russia, in 2009, the law “On Mandatory Genomic Fixation” was adopted. This procedure is carried out for prisoners, as well as for people whose identity has not been established. Citizens who are not included in this list take the test voluntarily. What can such a genetic base give:

• reduce the number of atrocities and lower the crime rate;
• may become the main evidentiary fact in solving a crime;
• solve the problem of inheritance in disputable cases;
• establish the truth in matters of fatherhood and motherhood.

The conclusion of DNA can also provide interesting information about a person's personality: a genetic predisposition to diseases and addictions, as well as a propensity to commit crimes. An amazing fact: scientists have discovered a certain gene that is responsible for a person's propensity to commit atrocities.

DNA expertise in forensics has already helped to solve more than 15 thousand crimes around the world. It is especially fascinating that it is possible to solve a criminal case only by a hair or a piece of the criminal's skin. The creation of such a base prophesies great prospects not only in the judicial sphere, but also in such industries as medicine and pharmaceuticals. DNA research helps to cope with intractable diseases that are inherited.

DNA analysis procedure. Establishing paternity (maternity)

Currently, there are many private and public accredited laboratories where DNA testing can be done. This examination is based on a comparison of DNA fragments (loci) in two samples: the intended parent and the child. Logically, a child receives 50% of its genes from its parents. This explains the similarity to mom and dad. If we compare a certain section of the child’s DNA with a similar section of the DNA of the intended parent, then they will be the same with a probability of 50%, that is, 6 out of 12 loci will coincide. %. If only one of the twelve loci matches, this probability is minimized. There are many accredited laboratories where DNA testing can be done privately.

The accuracy of the analysis is affected by the nature and number of loci taken for the study. DNA studies have shown that the genetic material of all people on the planet matches 99%. If we take these similar sections of DNA for analysis, it may turn out that, for example, an Australian aborigine and an Englishwoman will be absolutely identical personalities. Therefore, for an accurate study, areas unique to each individual are taken. The more such areas will be subjected to research, the higher the probability of accuracy of the analysis. For example, with the most thorough and high-quality study of 16 STRs, a DNA conclusion will be obtained with an accuracy of 99.9999% when confirming the probability of maternity / paternity and 100% when refuting the fact.

Establishment of close relationship (grandmother, grandfather, niece, nephew, aunt, uncle)

A DNA test for kinship is not fundamentally different from a test for paternity and motherhood. The difference is that the amount of total genetic information will be half that of a paternity test, and will be approximately 25% if 3 out of 12 loci match exactly. In addition, the condition must also be observed that the relatives between whom kinship is established belong to the same line (by mother or father). It is important that the decoding of the DNA analysis is as reliable as possible.

Establishing DNA similarities between siblings and half-siblings

Siblings receive one set of genes from their parents, so DNA testing reveals 75-99% of the same genes (in the case of identical twins, 100%). Half-siblings can only have a maximum of 50% of the same genes and only those that are passed down through the maternal line. A DNA test with an accuracy of 100% is able to show whether brothers or sisters are relatives or half-brothers.

DNA test for twins

Twins by nature of biological origin are identical (homozygous) or dizygotic (heterozygous). Homozygous twins develop from one fertilized cell, are of only one sex and are completely identical in genotype. Heterozygous, on the other hand, are formed from different fertilized eggs, are of different sexes and have slight differences in DNA. Genetic examination is able to determine with 100% accuracy whether twins are monozygous or heterozygous.

DNA testing on the Y chromosome

The transmission of the Y chromosome occurs from father to son. With the help of this type of analysis, it is possible to determine with high accuracy whether men are members of the same family and how close they are related. Determination of DNA by the Y chromosome is often used to create a genealogical family tree.

Mitochondrial DNA analysis

mtDNA is inherited through the maternal line. Therefore, this type of survey is very informative for tracing kinship through the mother's side. Scientists use mtDNA analysis to control evolutionary and migratory processes, as well as to identify people. The structure of mtDNA is such that two hypervariable regions HRV1 and HRV2 can be distinguished in it. By conducting research on the HRV1 locus and comparing it with the Standard Cambridge Sequence, you can get a DNA conclusion about whether the people under study are relatives, whether they belong to the same ethnic group, the same nationality, the same maternal line.

Deciphering genetic information

In total, a person has about a hundred thousand genes. They are encoded into a sequence that consists of three billion letters. As mentioned earlier, DNA has the structure of a double helix connected to each other through a chemical bond. The genetic code consists of numerous variations of five nucleotides, designated: A (adenine), C (cytosine), T (thymine), G (guanine) and U (uracil). The order of localization of nucleotides in DNA determines the sequence of amino acids in a protein molecule.

Scientists have discovered a curious fact that about 90% of the DNA chain is a kind of genetic slag that does not carry important information about the human genome. The remaining 10% are broken down into their own genes and regulatory regions.

There are times when doubling the chain fails. Such processes lead to the appearance of mutations. Even a minimal error of one nucleotide can cause the development of a hereditary disease that can be fatal to humans. In total, scientists know about 4,000 such disorders. The danger of the disease depends on which part of the DNA chain the mutation will affect. If this is an area of ​​genetic slag, then the error may go unnoticed. This will not affect normal operation. If a replication failure occurs at an important genetic segment, then such an error can cost a person his life. DNA research from this position will help geneticists find a way to prevent gene mutation and defeat hereditary diseases.

The DNA genetic code table helps genetic scientists to add up complete information about the human genome.

DNA genetic code table

Amino acid	mRNA codons
Arginic acid Isoleucine tryptophan Methionine Glutamine Aspartic acid Histidine Asparagine	TsGU, TsGTs, TsGG, TsGA CUG, UCA, AUU, AUA, UAC GCC, GCG, GCU, GCA UUG, CUU, UUA, CUU GUTs, GUG, GUU, GUA CCC, CCG, CCU, CCA UCC, UCG, UCU, UCA ACC, ACG, ACC, ACA

Genetic screening during planning and during pregnancy

Geneticists recommend that couples undergo genetic research at the stage of offspring planning. In this case, you can find out in advance about possible changes in the body, assess the risks of having children with pathologies and determine the presence of genetically inherited diseases. But practice shows that women's DNA tests are most often carried out when they are pregnant. Under such circumstances, information will also be obtained on the likelihood of malformations in the fetus.

Is voluntary. But there are a number of reasons why a woman must undergo such a study. These indications include:

• biological age over 35 years;
• hereditary diseases on the maternal line;
• a history of miscarriages and;
• presence during the conception period: radioactive and x-ray radiation, the presence of alcohol and drug addiction in parents;
• previously born children with developmental pathologies;
• viral diseases transferred by a pregnant woman (especially rubella, toxoplasmosis and influenza);
• indications detected by ultrasound.

A blood test for DNA will allow with a high degree of probability to determine the predisposition of the unborn baby to diseases of the heart and blood vessels, bones, lungs, gastrointestinal tract and endocrine system. This study also shows the risk of having a baby with Down and Edwards syndromes. The DNA report will give the doctor a complete picture of the condition of the woman and child and will allow them to prescribe the correct corrective treatment.

Methods of genetic research during pregnancy

Traditional research methods include ultrasound and a biochemical blood test, they do not pose any danger to the woman and the fetus. This is the so-called screening of pregnant women, carried out in two stages. The first is carried out at a gestational age of 12-14 weeks and allows you to identify serious fetal disorders. The second stage is carried out at 20-24 weeks and provides information about minor pathologies that may occur in the baby. If there is evidence or doubt, doctors may prescribe invasive methods of analysis:

• Amniocentesis or sampling of amniotic fluid for examination. A puncture is made in the uterus with a special needle, the necessary amount of amniotic fluid is collected for analysis. This manipulation is carried out under ultrasound guidance to avoid injury.
• Chorionic biopsy - sampling of placental cells.
• Pregnant women who have had an infection are prescribed placentogenesis. This is a rather serious operation and is carried out starting from the twentieth week of pregnancy, under general anesthesia;
• Collection and analysis or cordocentesis. It can only be done after the 18th week of pregnancy

Thus, it is possible to find out from genetic analysis what your child will be like, long before his birth.

Cost of DNA testing

For a simple layman who does not encounter this procedure, after reading this article, a reasonable question arises: “How much does a DNA examination cost?”. It is worth noting that the price of this procedure depends on the chosen profile of the study. Here is the approximate cost of a DNA test:

• paternity (motherhood) - 23,000 rubles;
• close relationship - 39,000 rubles;
• cousin relationship - 41,000 rubles;
• identification of a sibling / half-brother (sister) - 36,000 rubles;
• twin test - 21000 rubles;
• on the Y chromosome - 14,000 rubles;
• for mtDNA - 15,000 rubles;
• consultations on establishing kinship: oral - 700 rubles, written - 1400 rubles.

In recent years, scientists have made many great discoveries that are redefining the postulates of the scientific world. DNA research is ongoing. Scientists are driven by a great desire to discover the secret of the human genetic code. Much has already been discovered and explored, but how much of the unknown lies ahead! Progress does not stand still, and DNA technologies are firmly introduced into the life of every person. Further studies of this mysterious and unique structure, which is fraught with many secrets, will reveal to mankind a huge number of new facts.

Incredible Facts

DNA is the blueprint for our body, and without it, we wouldn't exist. It is a molecule that contains the genetic instructions for development and continues to function in every living organism.

DNA is in every cell of our body, telling it which proteins to produce. We inherit the DNA in our cells from our parents, so we have many similarities.

She has the shape double helix, similar to a huge spiral ladder, and each rung on this ladder consists of a pair of nucleotides. When DNA is copied, errors sometimes occur and these errors are known as mutations.

Here are some interesting facts about DNA that will help you understand yourself better.

DNA molecule

1. Bdelloid rotifers - These are microscopic animals that for 80 million years remained exclusively females. They reproduce by borrowing the DNA of other animals.

2. If you had to type one word per second for 8 hours a day, you would it took 50 years to print the human genome.

4. If you suddenly undergo a bone marrow transplant, in the DNA of your blood donor DNA will be present which has led to false arrests in the past.

5. With siblings 50% shared genes like parents with children.

6. DNA is damaged about 1 million times a day in every cell of our body. Fortunately, our body has a complex system for its recovery. If this were not the case, it would lead to cancer or cell death.

7. When it comes to invertebrates, then earthworms are our closest relatives. We have more DNA in common than cockroaches and even octopuses.

8. Four families in Iceland have DNA found only in Native Americans, according to scientists. Evidence indicates that the Vikings brought a Native American woman back to Europe about 1,000 years ago.

9. The International Space Station has a hard drive called " disc of immortality". It contains the DNA of people like Lance Armstrong and Stephen Hawking in case of a worldwide catastrophe.

10. Brooke Greenberg, the girl who looked like a child all her life, died at the age of 20. Scientists believe that it DNA could be the key to biological immortality.

Human DNA

11. Around 8 percent of our DNA is made up of ancient viruses that once infected humans.

12. According to DNA research, the Polynesians visited Chile in the 1300s and overtook Columbus by setting foot in the Americas almost 200 years earlier.

13. Around 2 grams of DNA could hold all of the world's digitally stored information.

14. Scientists recorded a song from the Disney cartoon("It's A Small World After All") in bacterial DNA, which is resistant to radioactivity, so that in the event of a nuclear catastrophe, people in the future or other life forms can find it.

15. Zambian doctor John Schneeberger was accused of sexual assault. He implanted himself with a tube with the blood of another person, and when they took blood from him for DNA, he was able to deceive the experts. In the end, he still managed to detain.

16. Human DNA is 99.9 percent the same. The differences are the last 0.1 percent.

17. The genetic content of an egg can be replaced with a male's DNA and then fertilized with a sperm. Thus, two men can become the parents of a child.

18. The DNA in all your cells can stretch over 16 billion kilometers if it is unrolled. This is approximately the distance from Earth to Pluto and back.

19. Although there are websites that offer genetic tests on saliva that confirm your lineage, scientists warn that this is a kind of "genetic astrology" and should not be taken seriously.

20. 50 percent of your DNA is similar to that of a banana.

21. Scientists have determined that the half-life of DNA is 521 years, and after 1.5 million years, even DNA preserved in its best form will not be readable.

22. Due to the destruction of DNA, it is unlikely that we will ever be able to clone dinosaurs or other prehistoric animals.

23. German police once took DNA samples during a jewelry robbery. The samples pointed to the twins Hassan and Abbas O. Both denied involvement in the crime, despite the fact that the police knew that one of them had committed the crime.

They were unable to determine which of them committed it, since the DNA was almost identical, and according to German law, the suspects could not be held indefinitely. Thus, the police had no other choice but to release the suspects.

24. All people of non-African descent have traces of Neanderthal DNA.

25. During the Hornslet Deep Burial Project, a Danish artist Christian von Hornslet in 2013 to the deepest part of the oceanthe time capsule was lowered. The capsule contained blood, hair and animal DNA samples. The aim of the project was preservation of DNA so that extinct species can be brought back to life in the future.

What does DNA mean

What does DNA mean

Deoxyribonucleic acid is a molecule that is literally the building block of all life. The rudimentary knowledge of DNA appeared only 50 years ago, and despite all the advances in the study of DNA, our knowledge of the molecule is still far from complete. Below are ten facts that prove that we had interesting ancestors, some of us may be twins of ourselves, and that we may soon learn the secret of immortality.

10. Outstanding personalities

Fact: There is an "Immortal Disc" aboard the International Space Station containing the DNA of prominent personalities.

On October 12, 2008, the Russian Soyuz spacecraft flew to the International Space Station in Earth's orbit. On board the ship was a small storage medium containing digitized DNA sequences from various (arguably) important people, including comedian Stephen Colbert, physicist Stephen Hawking, Playboy model Jo Garcia, and professional cyclist Lance. Armstrong (Lance Armstrong). The purpose of this disc is to provide the building blocks for the rebirth of humanity in the event that the planet is affected by an apocalyptic event.

9. Serial killer


Fact: a mistake in the analysis of DNA "spawned" a serial killer

Female serial killers are surprisingly rare, but in 2007 a woman nicknamed "The Phantom of Heilbronn" became known throughout Germany after the murder of a female police officer. The crimes of the Phantom could be listed endlessly: a lot of brutal murders and thefts that began back in 1993. The Phantom worked in France and Austria along with many accomplices of various nationalities. There was no discernible pattern of crimes, no security footage of the Phantom walking through walls. And then the police found male fingerprints that matched the desired DNA. It was only then that it was discovered that no Phantom of Heilbronn existed. Q-tips used throughout Europe to collect DNA were "contaminated", most likely by someone who packaged them at the factory. During the sterilization of the sticks, bacteria, fungi and viruses were destroyed, but the DNA remained on them. The consequences of this blunder were profound: in addition to thousands of wasted man-hours, there were dozens of murders, the perpetrators of which were virtually ignored by investigators chasing the ghost.

8. Immortality


Fact: There are creatures in the world whose DNA makes them virtually immortal.

The science of aging, or gerontology, is overly complex, but to put it simply, our DNA is subject to entropy. To put it even more simply, with each replication, DNA becomes a little weaker or less efficient. However, there are also species whose DNA does not appear to deteriorate over time (or does so so slowly that it is too hard to notice). Lobsters, some fish, and many types of turtles age very slowly and, under optimal circumstances, could live indefinitely. This is called negligible aging. The oldest tortoise in the world was Adwaita, a giant tortoise that lived to be 255 years old. It is important to note that such animals, although they do not suffer from aging, are still prone to illness, injury, etc., and the longer the animal lives, the greater the chance that it will become ill. Advaita died of liver failure after his shell broke.

7 Identical Cheetahs


Fact: Cheetahs are almost identical genetically

Laboratory mice and rats were specially crossed among themselves for several generations so that it was possible to achieve the same results in scientific experiments. However, DNA tests on cheetahs show that they are also almost similar to each other. It is believed that some event occurred around 10,000 years ago during the Pleistocene era that reduced the cheetah population to 7 individuals. Perhaps it is no coincidence that this period of time coincides with the appearance of modern humans and the extinction of many other large land mammals, like the saber-toothed tiger. Somehow, not without a successful mating, cheetahs were able to restore their species, but their genetic similarity makes them prone to disease.

6. Vikings in America


Fact: DNA proves that the Vikings brought their wives from America centuries before Columbus

There is evidence that the Vikings were far more sophisticated than their barbaric reputation would suggest, and traveled incredibly far. A DNA study of four different families from Iceland showed that their genetics are similar to Native Americans and Asians. A study by the University of Iceland proves that at least one Native American woman was brought to Iceland at least as early as 1700, and one of the mutations suggests that this could have happened even hundreds of years earlier. There is little historical evidence for this - although the Vikings did have contact with Native Americans (whom they called Screlings), most of the contacts were hostile.

5 Human Chimpanzee

Fact: it may be possible to derive a "human-chimpanzee"

Even though chimpanzees and humans evolved along two different paths (chimpanzees have 2 more chromosomes than humans), our DNA is incredibly similar. Although the exact number of similarities is controversial, it is believed that the DNA of chimpanzees and humans can match up to 99.4%. Moreover, some scientists believe that under the right conditions, these two species can produce offspring. The first attempts to create a cross between a human and a monkey date back at least a hundred years ago. Just as in the case of the crossing of donkeys and horses, resulting in the birth of a mule, if a creature combining chimpanzee and man were bred, it would be barren. Rumors have been circulating for years that a chimpanzee named Oliver is actually such a creature, however, despite his unusual appearance, genetic research has proven that he was an ordinary monkey.

4. Information carrier DNA


Fact: DNA is the best information carrier in the world

While the beginnings of digital storage in DNA date back to the 1980s, it wasn't until last year that researchers at Harvard actually cracked the code. They managed to store 700 terabytes of information in one gram of DNA. 700 terabytes is equivalent to about 150 kilograms of hard drives, and in DNA form, this is just a drop that fits on your fingertip. Using current technology, sequencing DNA to read information takes hours and is incredibly expensive, so the practicality of this form of information storage is limited, but the very idea that all human knowledge of the world could be stored in a space no larger than the volume of your closet is amazing.

3. Two sets of DNA


Fact: A person can have two sets of DNA

Many pregnancies begin as twins - often one of the twins absorbs the other before the fetus can be seen. In 99% of cases this is the end of the story. However, under certain conditions, if a person "absorbed" their twin, there may be two different sets of DNA in their body. The phenomenon of "chimerism", named after the chimera, a creature from Greek mythology with parts of a lion, a snake and a goat, is not uncommon and most people go through their lives without even knowing that they are an example of chimerism. This is most often noticed when a person is tested for compatibility with organs for transplantation. In particular, the case of Lydia Fairchild is interesting. In 2002, Fairchild applied for state benefits in the state of Washington, and her family had to undergo DNA testing in order to prove they were her relatives. To her surprise, during testing, it turned out that she was not the mother of her own children. Her case was taken to court for attempted fraud, and the possibility of taking her own children from her was considered. Fairchild won the trial as testing finally revealed that she was, in fact, her own twin.

2. Radioactive DNA


Fact: People born after 1955 have radiocarbon in their DNA.

In the 1950s, the Cold War began, and the US and the USSR began to show their power by detonating atomic warheads in their wastelands. Due to a large radioactive burst released into the atmosphere, there is a small amount of carbon-14 present in the DNA of all people born after 1955. Cells that did not divide before a person was born did not contain carbon-14. While it does not appear to affect the body in any way, the phenomenon has been useful in medical experiments such as tracking cell division in the human heart.

1. Crossbreeding between humans and Neanderthals


Fact: Neanderthals and humans interbred

And it was previously believed that Homo sapiens interbred with Neanderthals tens of thousands of years ago, but only recently DNA research has shown exactly how this happened. It is believed that Homo sapiens met the Neanderthals in the Middle East after they left Africa. This is an interesting fact showing how early humans migrated. Recently, a skeleton was discovered in Italy that was 30-40,000 years old, and whose jaw indicates that it was the product of a cross between a female Neanderthal (this was determined by inherited mitochondrial DNA) and a male Homo sapiens.

On the right is the largest human DNA helix built from people on the beach in Varna (Bulgaria), which was included in the Guinness Book of Records on April 23, 2016

Deoxyribonucleic acid. General information

DNA (deoxyribonucleic acid) is a kind of blueprint of life, a complex code that contains data on hereditary information. This complex macromolecule is capable of storing and transmitting hereditary genetic information from generation to generation. DNA determines such properties of any living organism as heredity and variability. The information encoded in it determines the entire development program of any living organism. Genetically embedded factors predetermine the entire course of life of both a person and any other organism. Artificial or natural influence of the external environment can only slightly affect the overall severity of individual genetic traits or affect the development of programmed processes.

Deoxyribonucleic acid(DNA) is a macromolecule (one of the three main ones, the other two are RNA and proteins), which provides storage, transmission from generation to generation and implementation of the genetic program for the development and functioning of living organisms. DNA contains information about the structure of various types of RNA and proteins.

In eukaryotic cells (animals, plants, and fungi), DNA is found in the cell nucleus as part of chromosomes, as well as in some cell organelles (mitochondria and plastids). In the cells of prokaryotic organisms (bacteria and archaea), a circular or linear DNA molecule, the so-called nucleoid, is attached from the inside to the cell membrane. They and lower eukaryotes (for example, yeast) also have small autonomous, mostly circular DNA molecules called plasmids.

From a chemical point of view, DNA is a long polymeric molecule consisting of repeating blocks - nucleotides. Each nucleotide is made up of a nitrogenous base, a sugar (deoxyribose), and a phosphate group. The bonds between nucleotides in a chain are formed by deoxyribose ( WITH) and phosphate ( F) groups (phosphodiester bonds).

Rice. 2. Nuclertide consists of a nitrogenous base, sugar (deoxyribose) and a phosphate group

In the overwhelming majority of cases (except for some viruses containing single-stranded DNA), the DNA macromolecule consists of two chains oriented by nitrogenous bases to each other. This double-stranded molecule is twisted in a helix.

There are four types of nitrogenous bases found in DNA (adenine, guanine, thymine, and cytosine). The nitrogenous bases of one of the chains are connected to the nitrogenous bases of the other chain by hydrogen bonds according to the principle of complementarity: adenine combines only with thymine ( A-T), guanine - only with cytosine ( G-C). It is these pairs that make up the "rungs" of the helical "ladder" of DNA (see: Fig. 2, 3 and 4).

Rice. 2. Nitrogenous bases

The sequence of nucleotides allows you to "encode" information about various types of RNA, the most important of which are information or template (mRNA), ribosomal (rRNA) and transport (tRNA). All these types of RNA are synthesized on the DNA template by copying the DNA sequence into the RNA sequence synthesized during transcription and take part in protein biosynthesis (translation process). In addition to coding sequences, cell DNA contains sequences that perform regulatory and structural functions.

Rice. 3. DNA replication

The location of the basic combinations of DNA chemical compounds and the quantitative ratios between these combinations provide encoding of hereditary information.

Education new DNA (replication)

1. The process of replication: the unwinding of the DNA double helix - the synthesis of complementary strands by DNA polymerase - the formation of two DNA molecules from one.
2. The double helix "unzips" into two branches when enzymes break the bond between the base pairs of chemical compounds.
3. Each branch is a new DNA element. New base pairs are connected in the same sequence as in the parent branch.

Upon completion of the duplication, two independent helices are formed, created from the chemical compounds of the parent DNA and having the same genetic code with it. In this way, DNA is able to rip through information from cell to cell.

More detailed information:

STRUCTURE OF NUCLEIC ACIDS

Rice. 4 . Nitrogenous bases: adenine, guanine, cytosine, thymine

Deoxyribonucleic acid(DNA) refers to nucleic acids. Nucleic acids is a class of irregular biopolymers whose monomers are nucleotides.

NUCLEOTIDES consist of nitrogenous base, connected to a five-carbon carbohydrate (pentose) - deoxyribose(in the case of DNA) or ribose(in the case of RNA), which combines with a phosphoric acid residue (H 2 PO 3 -).

Nitrogenous bases There are two types: pyrimidine bases - uracil (only in RNA), cytosine and thymine, purine bases - adenine and guanine.

Rice. Fig. 5. The structure of nucleotides (left), the location of the nucleotide in DNA (bottom) and the types of nitrogenous bases (right): pyrimidine and purine

The carbon atoms in a pentose molecule are numbered from 1 to 5. Phosphate combines with the third and fifth carbon atoms. This is how nucleic acids are linked together to form a chain of nucleic acids. Thus, we can isolate the 3' and 5' ends of the DNA strand:

Rice. 6. Isolation of the 3' and 5' ends of the DNA strand

Two strands of DNA form double helix. These chains in a spiral are oriented in opposite directions. In different strands of DNA, nitrogenous bases are connected to each other by means of hydrogen bonds. Adenine always combines with thymine, and cytosine always combines with guanine. It is called complementarity rule(cm. principle of complementarity).

Complementarity rule:

A-T G-C

For example, if we are given a DNA strand that has the sequence

3'-ATGTCCTAGCTGCTCG - 5',

then the second chain will be complementary to it and directed in the opposite direction - from the 5'-end to the 3'-end:

5'- TACAGGATCGACGAGC- 3'.

Rice. 7. The direction of the chains of the DNA molecule and the connection of nitrogenous bases using hydrogen bonds

DNA REPLICATION

DNA replication is the process of doubling a DNA molecule by template synthesis. In most cases of natural DNA replicationprimerfor DNA synthesis is short snippet (created again). Such a ribonucleotide primer is created by the enzyme primase (DNA primase in prokaryotes, DNA polymerase in eukaryotes), and is subsequently replaced by deoxyribonucleotide polymerase, which normally performs repair functions (correcting chemical damage and breaks in the DNA molecule).

Replication occurs in a semi-conservative manner. This means that the double helix of DNA unwinds and a new chain is completed on each of its chains according to the principle of complementarity. The daughter DNA molecule thus contains one strand from the parent molecule and one newly synthesized. Replication occurs in the 3' to 5' direction of the parent strand.

Rice. 8. Replication (doubling) of the DNA molecule

DNA synthesis- this is not such a complicated process as it might seem at first glance. If you think about it, then first you need to figure out what synthesis is. It is the process of bringing something together. The formation of a new DNA molecule takes place in several stages:

1) DNA topoisomerase, located in front of the replication fork, cuts the DNA in order to facilitate its unwinding and unwinding.
2) DNA helicase, following topoisomerase, affects the process of "unwinding" the DNA helix.
3) DNA-binding proteins carry out the binding of DNA strands, and also carry out their stabilization, preventing them from sticking to each other.
4) DNA polymerase δ(delta) , coordinated with the speed of movement of the replication fork, performs the synthesisleadingchains subsidiary DNA in the direction 5" → 3" on the matrix maternal strands of DNA in the direction from its 3" end to the 5" end (speed up to 100 base pairs per second). These events on this maternal strands of DNA are limited.

Rice. 9. Schematic representation of the DNA replication process: (1) Lagging strand (lag strand), (2) Leading strand (leading strand), (3) DNA polymerase α (Polα), (4) DNA ligase, (5) RNA -primer, (6) Primase, (7) Okazaki fragment, (8) DNA polymerase δ (Polδ ), (9) Helicase, (10) Single-stranded DNA-binding proteins, (11) Topoisomerase.

The synthesis of the lagging daughter DNA strand is described below (see below). scheme replication fork and function of replication enzymes)

For more information on DNA replication, see

5) Immediately after the unwinding and stabilization of another strand of the parent molecule, it joinsDNA polymerase α(alpha)and in the direction 5 "→3" synthesizes a primer (RNA primer) - an RNA sequence on a DNA template with a length of 10 to 200 nucleotides. After that, the enzymeremoved from the DNA strand.

Instead of DNA polymeraseα attached to the 3" end of the primer DNA polymeraseε .

6) DNA polymeraseε (epsilon) as if continues to lengthen the primer, but as a substrate embedsdeoxyribonucleotides(in the amount of 150-200 nucleotides). As a result, a solid thread is formed from two parts -RNA(i.e. primer) and DNA. DNA polymerase εworks until it encounters the primer of the previousfragment Okazaki(synthesized a little earlier). This enzyme is then removed from the chain.

7) DNA polymerase β(beta) stands in place ofDNA polymerases ε,moves in the same direction (5" → 3") and removes primer ribonucleotides while inserting deoxyribonucleotides in their place. The enzyme works until the complete removal of the primer, i.e. until a deoxyribonucleotide (even more previously synthesizedDNA polymerase ε). The enzyme is not able to link the result of its work and the DNA in front, so it leaves the chain.

As a result, a fragment of the daughter DNA "lies" on the matrix of the mother thread. It is calledfragment of Okazaki.

8) DNA ligase ligates two adjacent fragments Okazaki , i.e. 5 "-end of the segment, synthesizedDNA polymerase ε,and 3" chain end built-inDNA polymeraseβ .

STRUCTURE OF RNA

Ribonucleic acid(RNA) is one of the three main macromolecules (the other two are DNA and proteins) that are found in the cells of all living organisms.

Just like DNA, RNA is made up of a long chain in which each link is called nucleotide. Each nucleotide is made up of a nitrogenous base, a ribose sugar, and a phosphate group. However, unlike DNA, RNA usually has one rather than two strands. Pentose in RNA is represented by ribose, not deoxyribose (ribose has an additional hydroxyl group on the second carbohydrate atom). Finally, DNA differs from RNA in the composition of nitrogenous bases: instead of thymine ( T) uracil is present in RNA ( U) , which is also complementary to adenine.

The sequence of nucleotides allows RNA to encode genetic information. All cellular organisms use RNA (mRNA) to program protein synthesis.

Cellular RNAs are formed in a process called transcription , that is, the synthesis of RNA on a DNA template, carried out by special enzymes - RNA polymerases.

Messenger RNAs (mRNAs) then take part in a process called broadcast, those. protein synthesis on the mRNA template with the participation of ribosomes. Other RNAs undergo chemical modifications after transcription, and after the formation of secondary and tertiary structures, they perform functions that depend on the type of RNA.

Rice. 10. The difference between DNA and RNA in terms of the nitrogenous base: instead of thymine (T), RNA contains uracil (U), which is also complementary to adenine.

TRANSCRIPTION

This is the process of RNA synthesis on a DNA template. DNA unwinds at one of the sites. One of the chains contains information that needs to be copied onto the RNA molecule - this chain is called coding. The second strand of DNA, which is complementary to the coding strand, is called the template strand. In the process of transcription on the template chain in the 3'-5' direction (along the DNA chain), an RNA chain complementary to it is synthesized. Thus, an RNA copy of the coding strand is created.

Rice. 11. Schematic representation of transcription

For example, if we are given the sequence of the coding strand

3'-ATGTCCTAGCTGCTCG - 5',

then, according to the rule of complementarity, the matrix chain will carry the sequence

5'- TACAGGATCGACGAGC- 3',

and the RNA synthesized from it is the sequence

BROADCAST

Consider the mechanism protein synthesis on the RNA matrix, as well as the genetic code and its properties. Also, for clarity, at the link below, we recommend watching a short video about the processes of transcription and translation occurring in a living cell:

Rice. 12. Process of protein synthesis: DNA codes for RNA, RNA codes for protein

GENETIC CODE

Genetic code- a method of encoding the amino acid sequence of proteins using a sequence of nucleotides. Each amino acid is encoded by a sequence of three nucleotides - a codon or a triplet.

Genetic code common to most pro- and eukaryotes. The table lists all 64 codons and lists the corresponding amino acids. The base order is from the 5" to the 3" end of the mRNA.

Table 1. Standard genetic code

1st the foundation nie	2nd base								3rd the foundation nie
	U		C		A		G
U	U U U	(Phe/F)	U C U	(Ser/S)	U A U	(Tyr/Y)	U G U	(Cys/C)	U
	U U C		U C C		U A C		U G C		C
	U U A	(Leu/L)	U C A		U A A	Stop codon**	U G A	Stop codon**	A
	U U G		U C G		U A G	Stop codon**	U G G	(Trp/W)	G
C	C U U		C C U	(Pro/P)	C A U	(His/H)	C G U	(Arg/R)	U
	C U C		C C C		C A C		C G C		C
	C U A		C C A		C A A	(Gln/Q)	CGA		A
	C U G		C C G		C A G		C G G		G
A	A U U	(Ile/I)	A C U	(Thr/T)	A A U	(Asn/N)	A G U	(Ser/S)	U
	A U C		A C C		A A C		A G C		C
	A U A		A C A		A A A	(Lys/K)	A G A		A
	A U G	(Met/M)	A C G		A A G		A G G		G
G	G U U	(Val/V)	G C U	(Ala/A)	G A U	(Asp/D)	G G U	(Gly/G)	U
	G U C		G C C		G A C		G G C		C
	G U A		G C A		G A A	(Glu/E)	G G A		A
	G U G		G C G		G A G		G G G		G

Among the triplets, there are 4 special sequences that act as "punctuation marks":

• *Triplet AUG, also encoding methionine, is called start codon. This codon begins the synthesis of a protein molecule. Thus, during protein synthesis, the first amino acid in the sequence will always be methionine.

• **Triplets UAA, UAG and UGA called stop codons and do not code for any amino acids. At these sequences, protein synthesis stops.

Properties of the genetic code

1. Tripletity. Each amino acid is encoded by a sequence of three nucleotides - a triplet or codon.

2. Continuity. There are no additional nucleotides between the triplets, information is read continuously.

3. Non-overlapping. One nucleotide cannot be part of two triplets at the same time.

4. Uniqueness. One codon can code for only one amino acid.

5. Degeneracy. One amino acid can be encoded by several different codons.

6. Versatility. The genetic code is the same for all living organisms.

Example. We are given the sequence of the coding strand:

3’- CCGATTGCACGTCGATCGTATA- 5’.

The matrix chain will have the sequence:

5’- GGCTAACGTGCAGCTAGCATAT- 3’.

Now we “synthesize” informational RNA from this chain:

3’- CCGAUUGCACGUCGAUCGUAUA- 5’.

Protein synthesis goes in the direction 5' → 3', therefore, we need to flip the sequence in order to "read" the genetic code:

5’- AUAUGCUAGCUGCACGUUAGCC- 3’.

Now find the start codon AUG:

5’- AU AUG CUAGCUGCACGUUAGCC- 3’.

Divide the sequence into triplets:

sounds like this: information from DNA is transferred to RNA (transcription), from RNA to protein (translation). DNA can also be duplicated by replication, and the process of reverse transcription is also possible, when DNA is synthesized from an RNA template, but such a process is mainly characteristic of viruses.

Rice. 13. Central dogma of molecular biology

GENOM: GENES AND CHROMOSOMES

(general concepts)

Genome - the totality of all the genes of an organism; its complete chromosome set.

The term "genome" was proposed by G. Winkler in 1920 to describe the totality of genes contained in the haploid set of chromosomes of organisms of the same biological species. The original meaning of this term indicated that the concept of the genome, in contrast to the genotype, is a genetic characteristic of the species as a whole, and not of an individual. With the development of molecular genetics, the meaning of this term has changed. It is known that DNA, which is the carrier of genetic information in most organisms and, therefore, forms the basis of the genome, includes not only genes in the modern sense of the word. Most of the DNA of eukaryotic cells is represented by non-coding (“redundant”) nucleotide sequences that do not contain information about proteins and nucleic acids. Thus, the main part of the genome of any organism is the entire DNA of its haploid set of chromosomes.

Genes are segments of DNA molecules that code for polypeptides and RNA molecules.

Over the past century, our understanding of genes has changed significantly. Previously, a genome was a region of a chromosome that encodes or determines one trait or phenotypic(visible) property, such as eye color.

In 1940, George Beadle and Edward Tatham proposed a molecular definition of a gene. Scientists processed fungus spores Neurospora crassa X-rays and other agents that cause changes in the DNA sequence ( mutations), and found mutant strains of the fungus that lost some specific enzymes, which in some cases led to disruption of the entire metabolic pathway. Beadle and Tatham came to the conclusion that a gene is a section of genetic material that defines or codes for a single enzyme. This is how the hypothesis "one gene, one enzyme". This concept was later extended to the definition "one gene - one polypeptide", since many genes encode proteins that are not enzymes, and a polypeptide can be a subunit of a complex protein complex.

On fig. 14 shows a diagram of how DNA triplets determine a polypeptide, the amino acid sequence of a protein, mediated by mRNA. One of the DNA strands plays the role of a template for the synthesis of mRNA, the nucleotide triplets (codons) of which are complementary to the DNA triplets. In some bacteria and many eukaryotes, coding sequences are interrupted by non-coding regions (called introns).

Modern biochemical definition of a gene even more specifically. Genes are all sections of DNA that encode the primary sequence of end products, which include polypeptides or RNA that have a structural or catalytic function.

Along with genes, DNA also contains other sequences that perform an exclusively regulatory function. Regulatory sequences may mark the beginning or end of genes, affect transcription, or indicate the site of initiation of replication or recombination. Some genes can be expressed in different ways, with the same piece of DNA serving as a template for the formation of different products.

We can roughly calculate minimum gene size coding for the intermediate protein. Each amino acid in a polypeptide chain is encoded by a sequence of three nucleotides; the sequences of these triplets (codons) correspond to the chain of amino acids in the polypeptide encoded by the given gene. A polypeptide chain of 350 amino acid residues (medium length chain) corresponds to a sequence of 1050 bp. ( bp). However, many eukaryotic genes and some prokaryotic genes are interrupted by DNA segments that do not carry information about the protein, and therefore turn out to be much longer than a simple calculation shows.

How many genes are on one chromosome?

Rice. 15. View of chromosomes in prokaryotic (left) and eukaryotic cells. Histones are a broad class of nuclear proteins that perform two main functions: they are involved in the packaging of DNA strands in the nucleus and in the epigenetic regulation of nuclear processes such as transcription, replication, and repair.

The DNA of prokaryotes is more simple: their cells do not have a nucleus, so the DNA is located directly in the cytoplasm in the form of a nucleoid.

As you know, bacterial cells have a chromosome in the form of a DNA strand, packed into a compact structure - a nucleoid. prokaryotic chromosome Escherichia coli, whose genome is completely decoded, is a circular DNA molecule (in fact, this is not a regular circle, but rather a loop without beginning and end), consisting of 4,639,675 bp. This sequence contains approximately 4300 protein genes and another 157 genes for stable RNA molecules. V human genome approximately 3.1 billion base pairs corresponding to almost 29,000 genes located on 24 different chromosomes.

Prokaryotes (Bacteria).

Bacterium E. coli has one double-stranded circular DNA molecule. It consists of 4,639,675 b.p. and reaches a length of approximately 1.7 mm, which exceeds the length of the cell itself E. coli about 850 times. In addition to the large circular chromosome as part of the nucleoid, many bacteria contain one or more small circular DNA molecules that are freely located in the cytosol. These extrachromosomal elements are called plasmids(Fig. 16).

Most plasmids consist of only a few thousand base pairs, some contain more than 10,000 bp. They carry genetic information and replicate to form daughter plasmids, which enter the daughter cells during the division of the parent cell. Plasmids are found not only in bacteria, but also in yeast and other fungi. In many cases, plasmids offer no advantage to the host cells and their only job is to reproduce independently. However, some plasmids carry genes useful to the host. For example, genes contained in plasmids can confer resistance to antibacterial agents in bacterial cells. Plasmids carrying the β-lactamase gene confer resistance to β-lactam antibiotics such as penicillin and amoxicillin. Plasmids can pass from antibiotic-resistant cells to other cells of the same or different bacterial species, causing those cells to also become resistant. Intensive use of antibiotics is a powerful selective factor that promotes the spread of plasmids encoding antibiotic resistance (as well as transposons that encode similar genes) among pathogenic bacteria, and leads to the emergence of bacterial strains with resistance to several antibiotics. Doctors are beginning to understand the dangers of widespread use of antibiotics and prescribe them only when absolutely necessary. For similar reasons, the widespread use of antibiotics for the treatment of farm animals is limited.

See also: Ravin N.V., Shestakov S.V. Genome of prokaryotes // Vavilov Journal of Genetics and Breeding, 2013. V. 17. No. 4/2. pp. 972-984.

Eukaryotes.

Table 2. DNA, genes and chromosomes of some organisms

	shared DNA, b.s.	Number of chromosomes*	Approximate number of genes
Escherichia coli(bacterium)	4 639 675		4 435
Saccharomyces cerevisiae(yeast)	12 080 000	16**	5 860
Caenorhabditis elegans(nematode)	90 269 800	12***	23 000
Arabidopsis thaliana(plant)	119 186 200		33 000
Drosophila melanogaster(fruit fly)	120 367 260		20 000
Oryza sativa(rice)	480 000 000		57 000
Mus muscle(mouse)	2 634 266 500		27 000
Homo sapiens(Human)	3 070 128 600		29 000

Note. Information is constantly updated; For more up-to-date information, refer to individual genomic project websites.

* For all eukaryotes, except yeast, the diploid set of chromosomes is given. diploid kit chromosomes (from Greek diploos - double and eidos - view) - double set of chromosomes(2n), each of which has a homology to itself.
**Haploid set. Wild strains of yeast typically have eight (octaploid) or more sets of these chromosomes.
***For females with two X chromosomes. Males have an X chromosome, but no Y, i.e. only 11 chromosomes.

A yeast cell, one of the smallest eukaryotes, has 2.6 times more DNA than a cell E. coli(Table 2). fruit fly cells Drosophila, a classic object of genetic research, contains 35 times more DNA, and human cells contain about 700 times more DNA than cells E. coli. Many plants and amphibians contain even more DNA. The genetic material of eukaryotic cells is organized in the form of chromosomes. Diploid set of chromosomes (2 n) depends on the type of organism (Table 2).

For example, in a human somatic cell there are 46 chromosomes ( rice. 17). Each chromosome in a eukaryotic cell, as shown in Fig. 17, a, contains one very large double-stranded DNA molecule. Twenty-four human chromosomes (22 paired chromosomes and two sex chromosomes X and Y) differ in length by more than 25 times. Each eukaryotic chromosome contains a specific set of genes.

Rice. 17. eukaryotic chromosomes.a- a pair of connected and condensed sister chromatids from the human chromosome. In this form, eukaryotic chromosomes remain after replication and in metaphase during mitosis. b- a complete set of chromosomes from a leukocyte of one of the authors of the book. Each normal human somatic cell contains 46 chromosomes.

The size and function of DNA as a matrix for storing and transmitting hereditary material explains the presence of special structural elements in the organization of this molecule. In higher organisms, DNA is distributed between chromosomes.

The set of DNA (chromosomes) of an organism is called the genome. Chromosomes are located in the cell nucleus and form a structure called chromatin. Chromatin is a complex of DNA and basic proteins (histones) in a 1:1 ratio. The length of DNA is usually measured by the number of pairs of complementary nucleotides (bp). For example, the 3rd human chromosomecentury is a DNA molecule with a size of 160 million bp. has a length of approximately 1 mm, therefore, a linearized molecule of the 3rd human chromosome would be 5 mm in length, and the DNA of all 23 chromosomes (~ 3 * 10 9 bp, MR = 1.8 * 10 12) of a haploid cell - egg or sperm cell - in a linearized form would be 1 m. With the exception of germ cells, all cells of the human body (there are about 1013 of them) contain a double set of chromosomes. During cell division, all 46 DNA molecules replicate and reorganize into 46 chromosomes.

If you connect the DNA molecules of the human genome (22 chromosomes and chromosomes X and Y or X and X) to each other, you get a sequence about one meter long. Note: In all mammals and other heterogametic male organisms, females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY).

Most human cells, so the total DNA length of such cells is about 2m. An adult human has about 10 14 cells, so the total length of all DNA molecules is 2・10 11 km. For comparison, the circumference of the Earth is 4・10 4 km, and the distance from the Earth to the Sun is 1.5・10 8 km. That's how amazingly compactly packaged DNA is in our cells!

In eukaryotic cells, there are other organelles containing DNA - these are mitochondria and chloroplasts. Many hypotheses have been put forward regarding the origin of mitochondrial and chloroplast DNA. The generally accepted point of view today is that they are the rudiments of the chromosomes of ancient bacteria that penetrated into the cytoplasm of the host cells and became the precursors of these organelles. Mitochondrial DNA codes for mitochondrial tRNA and rRNA, as well as several mitochondrial proteins. More than 95% of mitochondrial proteins are encoded by nuclear DNA.

STRUCTURE OF GENES

Consider the structure of the gene in prokaryotes and eukaryotes, their similarities and differences. Despite the fact that a gene is a section of DNA encoding only one protein or RNA, in addition to the direct coding part, it also includes regulatory and other structural elements that have a different structure in prokaryotes and eukaryotes.

coding sequence- the main structural and functional unit of the gene, it is in it that the triplets of nucleotides encodingamino acid sequence. It starts with a start codon and ends with a stop codon.

Before and after the coding sequence are untranslated 5' and 3' sequences. They perform regulatory and auxiliary functions, for example, ensure the landing of the ribosome on mRNA.

Untranslated and coding sequences make up the unit of transcription - the transcribed DNA region, that is, the DNA region from which mRNA is synthesized.

Terminator A non-transcribed region of DNA at the end of a gene where RNA synthesis stops.

At the beginning of the gene is regulatory area, which includes promoter and operator.

promoter- the sequence with which the polymerase binds during transcription initiation. Operator- this is the area to which special proteins can bind - repressors, which can reduce the activity of RNA synthesis from this gene - in other words, reduce it expression.

The structure of genes in prokaryotes

The general plan for the structure of genes in prokaryotes and eukaryotes does not differ - both of them contain a regulatory region with a promoter and operator, a transcription unit with coding and non-translated sequences, and a terminator. However, the organization of genes in prokaryotes and eukaryotes is different.

Rice. 18. Scheme of the structure of the gene in prokaryotes (bacteria) -the image is enlarged

At the beginning and at the end of the operon, there are common regulatory regions for several structural genes. From the transcribed region of the operon, one mRNA molecule is read, which contains several coding sequences, each of which has its own start and stop codon. From each of these areasone protein is synthesized. In this way, Several protein molecules are synthesized from one i-RNA molecule.

Prokaryotes are characterized by the combination of several genes into a single functional unit - operon. The work of the operon can be regulated by other genes, which can be noticeably removed from the operon itself - regulators. The protein translated from this gene is called repressor. It binds to the operator of the operon, regulating the expression of all the genes contained in it at once.

Prokaryotes are also characterized by the phenomenon transcription and translation conjugations.

Rice. 19 The phenomenon of conjugation of transcription and translation in prokaryotes - the image is enlarged

This pairing does not occur in eukaryotes due to the presence of a nuclear envelope that separates the cytoplasm, where translation occurs, from the genetic material, on which transcription occurs. In prokaryotes, during the synthesis of RNA on a DNA template, a ribosome can immediately bind to the synthesized RNA molecule. Thus, translation begins even before transcription is complete. Moreover, several ribosomes can simultaneously bind to one RNA molecule, synthesizing several molecules of one protein at once.

The structure of genes in eukaryotes

The genes and chromosomes of eukaryotes are very complexly organized.

Bacteria of many species have only one chromosome, and in almost all cases there is one copy of each gene on each chromosome. Only a few genes, such as rRNA genes, are contained in multiple copies. Genes and regulatory sequences make up almost the entire genome of prokaryotes. Moreover, almost every gene strictly corresponds to the amino acid sequence (or RNA sequence) that it encodes (Fig. 14).

The structural and functional organization of eukaryotic genes is much more complex. The study of eukaryotic chromosomes, and later the sequencing of complete eukaryotic genome sequences, has brought many surprises. Many, if not most, eukaryotic genes have an interesting feature: their nucleotide sequences contain one or more DNA regions that do not encode the amino acid sequence of the polypeptide product. Such non-translated inserts disrupt the direct correspondence between the nucleotide sequence of the gene and the amino acid sequence of the encoded polypeptide. These untranslated segments in the genes are called introns, or built-in sequences, and the coding segments are exons. In prokaryotes, only a few genes contain introns.

So, in eukaryotes, there is practically no combination of genes into operons, and the coding sequence of a eukaryotic gene is most often divided into translated regions. - exons, and untranslated sections - introns.

In most cases, the function of introns has not been established. In general, only about 1.5% of human DNA is "coding", that is, it carries information about proteins or RNA. However, taking into account large introns, it turns out that 30% of human DNA consists of genes. Since genes make up a relatively small proportion of the human genome, a significant amount of DNA remains unaccounted for.

Rice. 16. Scheme of the structure of the gene in eukaryotes - the image is enlarged

From each gene, an immature, or pre-RNA, is first synthesized, which contains both introns and exons.

After that, the splicing process takes place, as a result of which the intron regions are excised, and a mature mRNA is formed, from which a protein can be synthesized.

Rice. 20. Alternative splicing process - the image is enlarged

Such an organization of genes allows, for example, when different forms of a protein can be synthesized from one gene, due to the fact that exons can be fused in different sequences during splicing.

Rice. 21. Differences in the structure of genes of prokaryotes and eukaryotes - the image is enlarged

MUTATIONS AND MUTAGENESIS

mutation called a persistent change in the genotype, that is, a change in the nucleotide sequence.

The process that leads to mutation is called mutagenesis, and the organism all whose cells carry the same mutation mutant.

mutation theory was first formulated by Hugh de Vries in 1903. Its modern version includes the following provisions:

1. Mutations occur suddenly, abruptly.

2. Mutations are passed down from generation to generation.

3. Mutations can be beneficial, deleterious or neutral, dominant or recessive.

4. The probability of detecting mutations depends on the number of individuals studied.

5. Similar mutations can occur repeatedly.

6. Mutations are not directed.

Mutations can occur under the influence of various factors. Distinguish between mutations caused by mutagenic impacts: physical (eg ultraviolet or radiation), chemical (eg colchicine or reactive oxygen species) and biological (eg viruses). Mutations can also be caused replication errors.

Depending on the conditions for the appearance of mutations are divided into spontaneous- that is, mutations that have arisen under normal conditions, and induced- that is, mutations that arose under special conditions.

Mutations can occur not only in nuclear DNA, but also, for example, in the DNA of mitochondria or plastids. Accordingly, we can distinguish nuclear and cytoplasmic mutations.

As a result of the occurrence of mutations, new alleles can often appear. If the mutant allele overrides the normal allele, the mutation is called dominant. If the normal allele suppresses the mutated one, the mutation is called recessive. Most mutations that give rise to new alleles are recessive.

Mutations are distinguished by effect adaptive, leading to an increase in the adaptability of the organism to the environment, neutral that do not affect survival harmful that reduce the adaptability of organisms to environmental conditions and lethal leading to the death of the organism in the early stages of development.

According to the consequences, mutations are distinguished, leading to loss of protein function, mutations leading to emergence the protein has a new function, as well as mutations that change the dose of a gene, and, accordingly, the dose of protein synthesized from it.

A mutation can occur in any cell of the body. If a mutation occurs in a germ cell, it is called germinal(germinal, or generative). Such mutations do not appear in the organism in which they appeared, but lead to the appearance of mutants in the offspring and are inherited, so they are important for genetics and evolution. If the mutation occurs in any other cell, it is called somatic. Such a mutation can manifest itself to some extent in the organism in which it arose, for example, lead to the formation of cancerous tumors. However, such a mutation is not inherited and does not affect offspring.

Mutations can affect parts of the genome of different sizes. Allocate genetic, chromosomal and genomic mutations.

Gene mutations

Mutations that occur on a scale smaller than one gene are called genetic, or dotted (dotted). Such mutations lead to a change in one or more nucleotides in the sequence. Gene mutations includesubstitutions, leading to the replacement of one nucleotide by another,deletions leading to the loss of one of the nucleotides,insertions, leading to the addition of an extra nucleotide to the sequence.

Rice. 23. Gene (point) mutations

According to the mechanism of action on the protein, gene mutations are divided into:synonymous, which (as a result of the degeneracy of the genetic code) do not lead to a change in the amino acid composition of the protein product,missense mutations, which lead to the replacement of one amino acid by another and can affect the structure of the synthesized protein, although often they are insignificant,nonsense mutations, leading to the replacement of the coding codon with a stop codon,mutations leading to splicing disorder:

Rice. 24. Mutation schemes

Also, according to the mechanism of action on the protein, mutations are isolated, leading to frame shift readings such as insertions and deletions. Such mutations, like nonsense mutations, although they occur at one point in the gene, often affect the entire structure of the protein, which can lead to a complete change in its structure. when a segment of a chromosome rotates 180 degrees Rice. 28. Translocation

Rice. 29. Chromosome before and after duplication

Genomic mutations

Finally, genomic mutations affect the entire genome, that is, the number of chromosomes changes. Polyploidy is distinguished - an increase in the ploidy of the cell, and aneuploidy, that is, a change in the number of chromosomes, for example, trisomy (the presence of an additional homologue in one of the chromosomes) and monosomy (the absence of a homolog in the chromosome).

Video related to DNA

DNA REPLICATION, RNA CODING, PROTEIN SYNTHESIS

(If the video is not displayed, it is available on

Many people, faced with the need to conduct a DNA examination, are wondering: can a DNA examination be wrong? Considering that often the test result is required not only for personal reasons, but also for submission to the court, the question seems reasonable.

Experts from the Lab-DNK genetic research center explain that the analysis cannot be questioned if it was done in a medical center with a solid reputation. It doesn’t matter at all whether the DNA test is informational (for oneself), or it is a pre-trial / judicial genetic examination by court order - both analyzes are performed using exactly the same technology, and a modern professional approach eliminates the possibility of a DNA test error. In Lab-DNA laboratories, each
the genetic material accepted by the registration department is completely depersonalized and only after that it is transferred to two separate groups of geneticists, after receiving laboratory results they are compared. Since hundreds of studies are conducted every day, this helps to eliminate the human factor and bias.

It is not uncommon for the judiciary to recommend a reliable research laboratory known for its highly qualified staff and
observance of medical secrecy. It is worth paying attention to whether the clinic concludes an agreement, whether it provides a guarantee of the result

Important information
The court may recommend an expert organization to you, but according to the federal law “On Protection of Competition”, you have the right to choose the contractor yourself by submitting to the court the licenses and qualifications of the laboratory you have chosen. What for? The average cost of establishing paternity in
state laboratory - 30,000 ₽. The average cost of a similar study in modern commercial laboratories is 13,800 rubles.

TEST RESULTS OR CAN A DNA TEST BE ERROR

A DNA test conducted in an innovative company confirms the fact of paternity by 99.9999%. But people are constantly wondering if it is possible to fake a DNA test? Often the result is given with a long tail of numbers, which further confirms the fact that a particular man is the biological father of the child. By the way, the specialist conducting the examination is criminally responsible for the result. The urgent problem, whether genetic expertise can be wrong, has long worried the minds of scientists and ordinary people.

Is it possible to fake a DNA test - To answer this question, you need to immerse yourself a little in the process and highlight the places where you can "make a mistake".

collection of material for DNA extraction

Already at this stage, unscrupulous companies can replace the envelopes with the material or the probes themselves - how to avoid this? The answer is simple - do not chase the price, since such companies usually make tests cheaper than anyone else (and it’s not a fact that they do), so trust only professionals, the data obtained at the Lab-DNA genetic research center can be checked in any independent laboratory.

Important information
As for the quality of DNA sampling, you don’t have to worry, if the procedure is performed negligently, then the material will simply not be accepted for work. So
either our geneticists will receive a mixed profile (from more than 1 person), or they will not be able to isolate the profile completely (in the case when the material is not enough, or it has degraded). In both cases, the genetic examination will be denied, and the patients will be sent for retake.

CONFIRMING OR DISCLAIMER OF THE RESULTS OF A DNA PATERNITY TEST

From our practice, the result of a paternity test varies on average from 99.95…. up to 99.9999 (if the study passes through 25 loci), or 100% if there is a refutation. If skeptics believe that the absence of a 100% result allows for the possibility of error, then they are mistaken. The paternity index is calculated as the sum of the indexes for each locus and, based on the formula, simply cannot reach 100 percent.

Therefore, those who think about the question of whether DNA examination can be wrong should just understand the intricacies of the process in more detail.

An interesting fact (can a DNA test be wrong?)
Yes maybe. A small fraction of a percent is set aside for the fact that each putative father has a theoretical identical twin brother. Identical twins are completely genetically identical, so the twins' paternity test results will be exactly the same.

CAN DNA EXAMINATION BE ERROR?

First you need to understand how DNA examination differs from a DNA test. Technically, these are two absolutely identical tests. The difference is only in the procedure for taking biological material for DNA extraction and the design / form of the report.

When we talk about ordinary DNA testing, it is understood that the material can be collected both by the center's employees and usually by a person. The material can be either standard (buccal epithelium) or non-standard. Such a DNA test can be done both officially and completely anonymously. The result of the study is issued on a company numbered letterhead (stamped paper with watermarks, with the personal signature of an expert geneticist).

When we mean genetic examination in order to establish paternity / motherhood (or any other relationship) - everything is strict here! All participants in the study are required to have identification documents, the collection of biological material for

Important!
If one of the participants is underage, they must be accompanied by their parents (confirmed by a birth certificate). If this is not possible, the child must be accompanied by an official guardian, as evidenced by a document of guardianship.

DNA extraction is carried out in the presence of witnesses with a signature under 307 Art. Criminal Code of the Russian Federation. All documents are photocopied and certified, and if minors are present in the DNA study, they are photographed to identify the person. Envelopes with biosamples are sealed and stamped. Envelopes are opened in the laboratory for photo fixation, and the result of the report is provided on 25-30 sheets and sealed.

In principle, it is almost impossible to deceive a DNA test, especially if it is a genetic DNA examination! Sample conclusions look like this!

sample information
paternity DNA test

Sample conclusion on the conduct of judicial (pre-trial)
genetic testing to establish paternity