Chemical composition of DNA and its macromolecular organization. Types of DNA helices. Molecular mechanisms of recombination, replication and DNA repair. The concept of nucleases and polymerases. DNA replication as a condition for the transmission of genetic information to descendants. General characteristics of the replication process. Actions that occur at a replication fork. Telomere replication, telomerase. The significance of underreplication of terminal chromosome fragments in the aging mechanism. Replication error correction systems. Corrective properties of DNA polymerases. Mechanisms of repair of damaged DNA. Concept of DNA repair diseases. Molecular mechanisms of general genetic recombination. Site-specific recombination. Gene conversion.
In 1865 Gregor Mendel discovered genes, and his contemporary Friedrich Miescher discovered them in 1869. discovered nucleic acids (in the nuclei of salmon pus and sperm cells). However, for a long time these discoveries were not connected with each other; for a long time the structure and nature of the substance of heredity were not known. The genetic role of NK was established after the discovery and explanation of the phenomena of transformation (1928, F. Griffiths; 1944, O. Avery), transduction (1951, Lederberg, Zinder) and reproduction of bacteriophages (1951, A. Hershey, M. Chase).
Transformation, transduction and reproduction of bacteriophages have convincingly proven the genetic role of DNA. In RNA viruses (AIDS, hepatitis B, influenza, TMV, murine leukemia, etc.), this role is performed by RNA.
Structure of nucleic acids. NCs are biopolymers involved in the storage and transmission of genetic information. NA monomers are nucleotides consisting of a nitrogenous base, a monosaccharide and one or more phosphate groups. All nucleotides in NA are monophosphates. A nucleotide without a phosphate group is called a nucleoside. The sugar contained in NA is the D-isomer and β-anomer of ribose or 2-deoxyribose. Nucleotides containing ribose are called ribonucleotides and are monomers of RNA, and nucleotides derived from deoxyribose are deoxyribonucleotides, and DNA consists of them. There are two types of nitrogenous bases: purines - adenine, guanine and pyrimidines - cytosine, thymine, uracil. The composition of RNA and DNA includes adenine, guanine, cytosine; Uracil is found only in RNA, and thymine only in DNA.
In some cases, NAs contain rare minor nucleotides, such as dihydrouridine, 4-thiouridine, inosine, etc. Their diversity is especially high in tRNA. Minor nucleotides are formed as a result of chemical transformations of NA bases that occur after the formation of the polymer chain. Various methylated derivatives are extremely common in RNA and DNA: 5-methyluridine, 5-methylcytidine, l-N-methyladenosine, 2-N-methylguanosine. In RNA, the object of methylation can also be the 2"-hydroxy groups of ribose residues, which leads to the formation of 2"-O-methylcytidine or 2"-O-methylguanosine.
Ribonucleotide and deoxyribonucleotide units are connected to each other using phosphodiester bridges, linking the 5"-hydroxyl group of one nucleotide with the 3"-hydroxyl group of the next. Thus, the regular backbone is formed by phosphate and ribose residues, and the bases are attached to sugars in the same way as side groups are attached to proteins. The order of the bases along the chain is called the primary structure of the NC. The sequence of bases is usually read in the direction from the 5" to the 3" carbon atom of the pentose.
DNA structure. The double helix model of DNA structure was proposed by Watson and Crick in 1953 (Fig. 7).
According to this three-dimensional model, the DNA molecule consists of two oppositely directed polynucleotide chains, which form a right-handed helix relative to the same axis. The nitrogenous bases are located inside the double helix, and their planes are perpendicular to the main axis, while the sugar phosphate residues are exposed outward. Specific H-bonds are formed between the bases: adenine - thymine (or uracil), guanine - cytosine, called Watson-Crick pairing. As a result, larger purines always interact with smaller pyrimidines, which ensures optimal backbone geometry. The antiparallel chains of the double helix are not identical either in base sequence or nucleotide composition, but they are complementary to each other precisely due to the presence of specific hydrogen bonding between the above bases.
Complementarity is very important for DNA copying (replication). The relationships between the number of different bases in DNA revealed
Fig.7. B - form of DNA
Chargraff et al. in the 50s, were of great importance for establishing the structure of DNA: it was shown that the number of adenine residues in the bases of the DNA chain, regardless of the organism, is equal to the number of thymine residues, and the number of guanine residues is equal to the number of cytosine residues. These equalities are a consequence of selective base pairing (Fig. 8).
The geometry of the double helix is such that adjacent base pairs are 0.34 nm apart and rotated 36° around the helix axis. Therefore, there are 10 base pairs per turn of the helix, and the helix pitch is 3.4 nm. The diameter of the double helix is 20 nm and two grooves are formed in it - large and small. This is due to the fact that the sugar phosphate backbone is located further from the helix axis than the nitrogenous bases.
The stability of the DNA structure is due to different types of interactions, the main ones being H-bonds between bases and interplanar interaction (stacking). Thanks to the latter, not only favorable van der Waals contacts between atoms are ensured, but also
Fig.8. The principle of complementarity and antiparallelism of DNA chains
additional stabilization due to the overlap of p-orbitals of atoms of parallel bases. Stabilization is also facilitated by the favorable hydrophobic effect, which manifests itself in the protection of low-polar bases from direct contact with the aqueous environment. In contrast, the sugar phosphate backbone with its polar and ionized groups is exposed, which also stabilizes the structure.
Four polymorphic forms are known for DNA: A, B, C and Z. The usual structure is B-DNA, in which the planes of the base pairs are perpendicular to the axis of the double helix (Fig. 7.). In A-DNA, the planes of base pairs are rotated approximately 20° from the normal to the axis of the right-handed double helix; There are 11 base pairs per turn of the helix. In C-DNA there are 9 base pairs per turn of the helix. Z-DNA is a left-handed helix with 12 base pairs per turn; the planes of the bases are approximately perpendicular to the axis of the spiral. DNA in a cell is usually in the B form, but individual sections of it can be in A, Z, or even another conformation.
The DNA double helix is not a frozen formation, it is in constant motion:
· connections in circuits are deformed;
· complementary base pairs open and close;
DNA interacts with proteins;
· if the tension in the molecule is high, then it locally unravels;
· the right spiral turns into the left.
There are 3 fractions of DNA:
1. Frequently repeated (satellite) - up to 106 copies of genes (10% in mice). It is not involved in protein synthesis; separates genes; provides crossing over; contains transposons.
2. Weakly repeatable - up to 102 - 103 gene copies (15% in mice). Contains genes for t-RNA synthesis, genes for the synthesis of ribosomal proteins and chromatin proteins.
3. Unique (non-repeatable) – in mice 75% (in humans 56%). Consists of structural genes.
DNA localization: 95% of DNA is localized in the nucleus in chromosomes (linear DNA) and 5% in mitochondria, plastids and the cell center in the form of circular DNA.
Functions of DNA: storage and transmission of information; repair; replication.
The two DNA strands in the gene region are fundamentally different in their functional role: one of them is coding, or sense, and the second is template.
This means that in the process of “reading” a gene (transcription or pre-mRNA synthesis), the DNA template strand acts as a template. The product of this process, pre-mRNA, coincides in nucleotide sequence with the coding strand of DNA (with the replacement of thymine bases with uracil ones).
Thus, it turns out that with the help of the DNA template strand, the genetic information of the DNA coding strand is reproduced in the RNA structure during transcription.
The main matrix processes inherent in all living organisms are DNA replication, transcription and translation.
Replication- a process in which information encoded in the base sequence of a parent DNA molecule is transmitted with maximum accuracy to the daughter DNA. With semi-conservative replication, daughter cells of the first generation receive one strand of DNA from their parents, and the second strand is newly synthesized. The process is carried out with the participation of DNA polymerases, which belong to the class of transferases. The role of the template is played by the separated chains of double-stranded maternal DNA, and the substrates are deoxyribonucleoside-5"-triphosphates.
Transcription- the process of transferring genetic information from DNA to RNA. All types of RNA - mRNA, rRNA and tRNA - are synthesized according to the sequence of bases in DNA, which serves as a template. Only one, the so-called “+” DNA strand is transcribed. The process occurs with the participation of RNA polymerases. The substrates are ribonucleoside 5"-triphosphates.
The processes of replication and transcription in prokaryotes and eukaryotes differ significantly in speed and individual mechanisms.
Broadcast- the process of decoding mRNA, as a result of which information from the language of the base sequence of mRNA is translated into the language of the amino acid sequence of the protein. Translation takes place on ribosomes, the substrates being aminoacyl-tRNA.
Template DNA synthesis, catalyzed by DNA polymerases, performs two main functions: DNA replication - the synthesis of new daughter chains and the repair of double-stranded DNA that has breaks in one of the chains formed as a result of cutting out damaged sections of this chain by nucleases. There are three types of DNA polymerases in prokaryotes and eukaryotes. In prokaryotes, polymerases of types I, II and III are identified, designated as pol l, pol ll and pol III. The latter catalyzes the synthesis of the growing chain; pol plays an important role in the process of DNA maturation; the functions of pol ll are not fully understood. In eukaryotic cells, DNA polymerase ά is involved in chromosome replication, DNA polymerase β is involved in repair, and the γ variety is an enzyme that carries out mitochondrial DNA replication. These Enzymes, regardless of the type of cell in which replication occurs, attach a nucleotide to the OH group at the 3" end of one of the DNA strands, which grows in the 5"→3 direction. Therefore, they say that these Fs have 5"→3" polymerase activity. In addition, they all exhibit the ability to degrade DNA by cleaving off nucleotides in the 3"→5 direction, i.e. they are 3"→5" exonucleases.
In 1957, Meselson and Stahl, studying E. coli, found that on each free strand, the enzyme DNA polymerase builds a new, complementary strand. This is a semi-conservative way of replication: one strand is old - the other is new!
Typically, replication begins in strictly defined areas, called ori areas (from origin of replication), and from these areas it spreads in both directions. The ori regions are preceded by branch points of the mother DNA strands. The area adjacent to the branch point is called the replication fork (Fig. 9). During synthesis, the replication fork moves along the molecule, and more and more new sections of parental DNA are unraveled until the fork reaches the termination point. Chain separation is achieved using special F - helicases (topoisomerases). The energy required for this is released through the hydrolysis of ATP. Helicases move along polynucleotide chains in two directions.
To start DNA synthesis, a seed is needed - a primer. The role of the primer is performed by short RNA (10-60 nucleotides). It is synthesized complementary to a specific section of DNA with the participation of primase. After the primer is formed, DNA polymerase starts working. Unlike helicases, DNA polymerases can only move from the 3" to 5" end of the template. Therefore, elongation of the growing chain as the double-stranded parent DNA unwinds can occur only along one strand of the template, the one relative to which the replication fork moves from the 3" to the 5" end. The continuously synthesized chain is called the leading chain. Synthesis on the lagging strand also begins with the formation of a primer and proceeds in the direction opposite to the leading strand - from the replication fork. The lagging strand is synthesized in fragments (in the form of Okazaki fragments), since the primer is formed only when the replication fork releases the region of the template that has affinity for primase. Ligation (crosslinking) of Okazaki fragments to form a single chain is called the maturation process.
During strand maturation, the RNA primer is removed from both the 5" end of the leading strand and the 5" ends of the Okazaki fragments, and these fragments are stitched together. Removal of the primer is carried out with the participation of 3"→5" exonuclease. The same F, instead of the removed RNA, attaches deoxynucleotides using its 5"→3" polymerase activity. In this case, in the case of the addition of an “incorrect” nucleotide, “proofreading” is carried out - the removal of bases forming non-complementary pairs. This process provides extremely high replication accuracy, corresponding to one error per 109 base pairs.
Fig.9. DNA replication:
1 - replication fork, 2 - DNA polymerase (pol I - maturation);
3 - DNA polymerase (pol III - “proofreading”); 4-helicase;
5-gyrase (topoisomerase); 6-proteins that destabilize the double helix.
Correction is carried out in cases when an “incorrect” nucleotide is attached to the 3” end of the growing chain, unable to form the necessary hydrogen bonds with the matrix. When pol III mistakenly attaches the wrong base, its 3” - 5” exonuclease activity is “turned on”, and this base is immediately removed, after which polymerase activity is restored.This simple mechanism operates due to the fact that pol III is able to act as a polymerase only on a perfect DNA double helix with absolutely correct base pairing.
Another mechanism for removing RNA fragments is based on the presence in cells of a special ribonuclease, called RNase H. This F is specific to double-stranded structures built from one ribonucleotide and one deoxyribonucleotide chain, and it hydrolyzes the first of them.
RNase H is also capable of removing the RNA primer, followed by repair of the gap by DNA polymerase. At the final stages of assembling the fragments in the required order, DNA ligase acts, catalyzing the formation of a phosphodiester bond.
Unwinding of part of the DNA double helix by helicases in eukaryotic chromosomes leads to supercoiling of the rest of the structure, which inevitably affects the speed of the replication process. Supercoiling is prevented by DNA topoisomerases.
Thus, in addition to DNA polymerase, a large set of Ps takes part in DNA replication: helicase, primase, RNase H, DNA ligase and topoisomerase. This list of phosphorus proteins and proteins involved in template DNA biosynthesis is far from exhaustive. However, many of the participants in this process remain little studied to this day.
During the replication process, “proofreading” occurs - the removal of incorrect (forming non-complementary pairs) bases included in the newly synthesized DNA. This process provides extremely high replication accuracy, corresponding to one error per 109 base pairs.
Telomeres. In 1938 classic geneticists B. McClinton and G. Möller proved that at the ends of chromosomes there are special structures called telomeres (telos-end, meros-part).
Scientists have discovered that when exposed to X-ray radiation, only telomeres exhibit resistance. On the contrary, deprived of terminal sections, chromosomes begin to merge, which leads to severe genetic abnormalities. Thus, telomeres provide the individuality of chromosomes. Telomeres are densely packed (heterochromatin) and are inaccessible to enzymes (telomerase, methylase, endonucleases, etc.)
Functions of telomeres.
1. Mechanical: a) joining the ends of sister chromatids after the S-phase; b) fixation of chromosomes to the nuclear membrane, which ensures conjugation of homologues.
2. Stabilization: a) protection from underreplication of genetically significant DNA sections (telomeres are not transcribed); b) stabilization of the ends of broken chromosomes. In patients with α - thalassemia, chromosome 16d breaks occur in the α - globin genes and telomeric repeats (TTAGGG) are added to the damaged end.
3.Influence on gene expression. The activity of genes located near telomeres is reduced. This is a manifestation of silencing – transcriptional silence.
4. "Counting function". Telomeres act as a clock device that counts the number of cell divisions. Each division shortens telomeres by 50-65 bp. And their total length in human embryonic cells is 10-15 thousand bp.
Telomeric DNA has recently come to the attention of biologists. The first objects of study are single-celled protozoa - ciliated ciliates (tetrahymena), which contain several tens of thousands of very small chromosomes and, therefore, many telomeres in one cell (in higher eukaryotes there are less than 100 telomeres per cell).
In the telomeric DNA of ciliates, blocks of 6 nucleotide residues are repeated many times. One strand of DNA contains a block of 2 thymine - 4 guanine (TTGGYG - G-chain), and the complementary chain - 2 adenine - 4 cytosine (AACCCC - C-chain).
Imagine the surprise of scientists when they discovered that human telomeric DNA differs from that of ciliates by just one letter and forms blocks 2 thymine - adenine - 3 guanine (TTAGGG). Moreover, it turned out that the telomeres (G - chain) of all mammals, reptiles, amphibians, birds and fish are built from TTAGGG blocks.
However, there is nothing surprising here, since telomeric DNA does not encode any proteins (it does not contain genes). In all organisms, telomeres perform universal functions, which were discussed above. A very important characteristic of telomeric DNA is its length. In humans, it ranges from 2 to 20 thousand base pairs, and in some species of mice it can reach hundreds of thousands of base pairs. It is known that there are special proteins near telomeres that ensure their functioning and are involved in the construction of telomeres.
It has been proven that for normal functioning, each linear DNA must have two telomeres: one telomere at each end.
Prokaryotes do not have telomeres - their DNA is closed in a ring.
We all know that a person’s appearance, some habits and even diseases are inherited. All this information about a living being is encoded in genes. So what do these notorious genes look like, how do they function and where are they located?
So, the carrier of all genes of any person or animal is DNA. This compound was discovered in 1869 by Johann Friedrich Miescher. Chemically, DNA is deoxyribonucleic acid. What does this mean? How does this acid carry the genetic code of all life on our planet?
Let's start by looking at where DNA is located. A human cell contains many organelles that perform various functions. DNA is located in the nucleus. The nucleus is a small organelle, which is surrounded by a special membrane, and in which all the genetic material - DNA - is stored.
What is the structure of a DNA molecule?
First of all, let's look at what DNA is. DNA is a very long molecule consisting of structural elements - nucleotides. There are 4 types of nucleotides - adenine (A), thymine (T), guanine (G) and cytosine (C). The chain of nucleotides schematically looks like this: GGAATTCTAAG... This sequence of nucleotides is the DNA chain.The structure of DNA was first deciphered in 1953 by James Watson and Francis Crick.
In one DNA molecule there are two chains of nucleotides that are helically twisted around each other. How do these nucleotide chains stay together and twist into a spiral? This phenomenon is due to the property of complementarity. Complementarity means that only certain nucleotides (complementary) can be found opposite each other in two chains. Thus, opposite adenine there is always thymine, and opposite guanine there is always only cytosine. Thus, guanine is complementary to cytosine, and adenine is complementary to thymine. Such pairs of nucleotides opposite each other in different chains are also called complementary.
It can be shown schematically as follows:
G - C
T - A
T - A
C - G
These complementary pairs A - T and G - C form a chemical bond between the nucleotides of the pair, and the bond between G and C is stronger than between A and T. The bond is formed strictly between complementary bases, that is, the formation of a bond between non-complementary G and A is impossible.
"Packaging" of DNA, how does a DNA strand become a chromosome?
Why do these DNA nucleotide chains also twist around each other? Why is this necessary? The fact is that the number of nucleotides is huge and a lot of space is needed to accommodate such long chains. For this reason, two strands of DNA twist around each other in a helical manner. This phenomenon is called spiralization. As a result of spiralization, DNA chains are shortened by 5-6 times.Some DNA molecules are actively used by the body, while others are rarely used. In addition to spiralization, such rarely used DNA molecules undergo even more compact “packaging.” This compact packaging is called supercoiling and shortens the DNA strand by 25-30 times!
How do DNA helices pack?
Supercoiling uses histone proteins, which have the appearance and structure of a rod or spool of thread. Spiralized strands of DNA are wound onto these “coils” - histone proteins. Thus, the long thread becomes very compactly packaged and takes up very little space. If it is necessary to use one or another DNA molecule, the process of “unwinding” occurs, that is, the DNA strand is “unwound” from the “spool” - the histone protein (if it was wound onto it) and unwinds from the spiral into two parallel chains. And when the DNA molecule is in such an untwisted state, then the necessary genetic information can be read from it. Moreover, genetic information is read only from untwisted DNA strands!
A set of supercoiled chromosomes is called heterochromatin, and the chromosomes available for reading information are euchromatin.
What are genes, what is their connection with DNA?
Now let's look at what genes are. It is known that there are genes that determine blood type, eye color, hair, skin and many other properties of our body. A gene is a strictly defined section of DNA, consisting of a certain number of nucleotides arranged in a strictly defined combination. Location in a strictly defined DNA section means that a specific gene is assigned its place, and it is impossible to change this place. It is appropriate to make the following comparison: a person lives on a certain street, in a certain house and apartment, and a person cannot voluntarily move to another house, apartment or to another street. A certain number of nucleotides in a gene means that each gene has a specific number of nucleotides and they cannot become more or less. For example, the gene encoding insulin production consists of 60 nucleotide pairs; the gene encoding the production of the hormone oxytocin - of 370 nucleotide pairs. The strict nucleotide sequence is unique for each gene and strictly defined. For example, the sequence AATTAATA is a fragment of a gene that codes for insulin production. In order to obtain insulin, exactly this sequence is used; to obtain, for example, adrenaline, a different combination of nucleotides is used. It is important to understand that only a certain combination of nucleotides encodes a certain “product” (adrenaline, insulin, etc.). Such a unique combination of a certain number of nucleotides, standing in “its place” - this is gene.
In addition to genes, the DNA chain contains so-called “non-coding sequences”. Such non-coding nucleotide sequences regulate the functioning of genes, help in the spiralization of chromosomes, and mark the starting and ending point of a gene. However, to date, the role of most non-coding sequences remains unclear.
What is a chromosome? Sex chromosomes
The collection of genes of an individual is called the genome. Naturally, the entire genome cannot be contained in one DNA. The genome is divided into 46 pairs of DNA molecules. One pair of DNA molecules is called a chromosome. So, humans have 46 of these chromosomes. Each chromosome carries a strictly defined set of genes, for example, chromosome 18 contains genes encoding eye color, etc. Chromosomes differ from each other in length and shape. The most common shapes are X or Y, but there are others as well. Humans have two chromosomes of the same shape, which are called pairs. Due to such differences, all paired chromosomes are numbered - there are 23 pairs. This means that there is chromosome pair No. 1, pair No. 2, No. 3, etc. Each gene responsible for a specific trait is located on the same chromosome. Modern guidelines for specialists may indicate the location of the gene, for example, as follows: chromosome 22, long arm.What are the differences between chromosomes?
How else do chromosomes differ from each other? What does the term long shoulder mean? Let's take chromosomes of the form X. The intersection of DNA strands can occur strictly in the middle (X), or it can occur not centrally. When such an intersection of DNA strands does not occur centrally, then relative to the point of intersection, some ends are longer, others, respectively, shorter. Such long ends are usually called the long arm of the chromosome, and short ends are called the short arm. In chromosomes of the Y shape, most of the arms are occupied by long arms, and the short ones are very small (they are not even indicated in the schematic image). The size of the chromosomes varies: the largest are chromosomes of pairs No. 1 and No. 3, the smallest chromosomes are pairs No. 17, No. 19.
In addition to their shape and size, chromosomes differ in the functions they perform. Of the 23 pairs, 22 pairs are somatic and 1 pair is sexual. What does it mean? Somatic chromosomes determine all the external characteristics of an individual, the characteristics of his behavioral reactions, hereditary psychotype, that is, all the traits and characteristics of each individual person. A pair of sex chromosomes determines a person’s gender: male or female. There are two types of human sex chromosomes: X (X) and Y (Y). If they are combined as XX (x - x) - this is a woman, and if XY (x - y) - we have a man.
Hereditary diseases and chromosome damage
However, “breakdowns” of the genome occur, and then genetic diseases are detected in people. For example, when there are three chromosomes in the 21st pair of chromosomes instead of two, a person is born with Down syndrome.There are many smaller “breakdowns” of genetic material that do not lead to disease, but on the contrary, impart good properties. All “breakdowns” of genetic material are called mutations. Mutations leading to diseases or deterioration of the body's properties are considered negative, and mutations leading to the formation of new beneficial properties are considered positive.
However, with most of the diseases that people suffer from today, it is not the disease that is inherited, but only a predisposition. For example, the father of a child absorbs sugar slowly. This does not mean that the child will be born with diabetes, but the child will have a predisposition. This means that if a child abuses sweets and flour products, he will develop diabetes.
Today, the so-called predicative medicine. As part of this medical practice, a person’s predispositions are identified (based on the identification of the corresponding genes), and then he is given recommendations - what diet to follow, how to properly alternate between work and rest so as not to get sick.
How to read the information encoded in DNA?
How can you read the information contained in DNA? How does its own body use it? DNA itself is a kind of matrix, but not simple, but encoded. To read information from the DNA matrix, it is first transferred to a special carrier - RNA. RNA is chemically ribonucleic acid. It differs from DNA in that it can pass through the nuclear membrane into the cell, while DNA lacks this ability (it can only be found in the nucleus). The encoded information is used in the cell itself. So, RNA is a carrier of encoded information from the nucleus to the cell.How does RNA synthesis occur, how is protein synthesized using RNA?
The DNA strands from which information needs to be “read” unwind, a special “builder” enzyme approaches them and synthesizes a complementary RNA chain parallel to the DNA strand. The RNA molecule also consists of 4 types of nucleotides - adenine (A), uracil (U), guanine (G) and cytosine (C). In this case, the following pairs are complementary: adenine - uracil, guanine - cytosine. As you can see, unlike DNA, RNA uses uracil instead of thymine. That is, the “builder” enzyme works as follows: if it sees A in the DNA strand, then it attaches Y to the RNA strand, if G, then it attaches C, etc. Thus, a template is formed from each active gene during transcription - a copy of RNA that can pass through the nuclear membrane. How does the synthesis of a protein encoded by a specific gene occur?
After leaving the nucleus, RNA enters the cytoplasm. Already in the cytoplasm, RNA can be embedded as a matrix into special enzyme systems (ribosomes), which can synthesize, guided by RNA information, the corresponding sequence of protein amino acids. As you know, a protein molecule consists of amino acids. How does the ribosome know which amino acid to add to the growing protein chain? This is done based on the triplet code. The triplet code means that the sequence of three nucleotides of the RNA chain ( triplet, for example, GGU) code for a single amino acid (in this case glycine). Each amino acid is encoded by a specific triplet. And so, the ribosome “reads” the triplet, determines which amino acid should be added next as it reads the information in the RNA. When a chain of amino acids is formed, it takes on a certain spatial shape and becomes a protein capable of performing the enzymatic, construction, hormonal and other functions assigned to it.Protein for any living organism is the product of a gene. It is proteins that determine all the various properties, qualities and external manifestations of genes.
Structure and functions of DNA
| Parameter name | Meaning |
| Article topic: | Structure and functions of DNA |
| Rubric (thematic category) | Education |
DNA- a polymer whose monomers are deoxyribonucleotides. A model of the spatial structure of the DNA molecule in the form of a double helix was proposed in 1953. J. Watson and F. Crick (to build this model they used the works of M. Wilkins, R. Franklin, E. Chargaff).
DNA molecule formed by two polynucleotide chains, helically twisted around each other and together around an imaginary axis, ᴛ.ᴇ. is a double helix (with the exception of some DNA-containing viruses have single-stranded DNA). The diameter of the DNA double helix is 2 nm, the distance between neighboring nucleotides is 0.34 nm, and there are 10 nucleotide pairs per turn of the helix. The length of the molecule can reach several centimeters. Molecular weight - tens and hundreds of millions. The total length of DNA in the nucleus of a human cell is about 2 m. In eukaryotic cells, DNA forms complexes with proteins and has a specific spatial conformation.
DNA monomer - nucleotide (deoxyribonucleotide)- consists of residues of three substances: 1) a nitrogenous base, 2) a five-carbon monosaccharide (pentose) and 3) phosphoric acid. The nitrogenous bases of nucleic acids belong to the classes of pyrimidines and purines. DNA pyrimidine bases(have one ring in their molecule) - thymine, cytosine. Purine bases(have two rings) - adenine and guanine.
The DNA nucleotide monosaccharide is deoxyribose.
The name of a nucleotide is derived from the name of the corresponding base. Nucleotides and nitrogenous bases are indicated by capital letters.
The polynucleotide chain is formed as a result of nucleotide condensation reactions. In this case, between the 3"-carbon of the deoxyribose residue of one nucleotide and the phosphoric acid residue of another, phosphoester bond(belongs to the category of strong covalent bonds). One end of the polynucleotide chain ends with a 5" carbon (called the 5" end), the other ends with a 3" carbon (3" end).
Opposite one strand of nucleotides is a second strand. The arrangement of nucleotides in these two chains is not random, but strictly defined: thymine is always located opposite the adenine of one chain in the other chain, and cytosine is always located opposite guanine, two hydrogen bonds arise between adenine and thymine, and between guanine and cytosine - three hydrogen bonds. The pattern according to which the nucleotides of different DNA chains are strictly ordered (adenine - thymine, guanine - cytosine) and selectively connect with each other is usually called the principle of complementarity. It should be noted that J. Watson and F. Crick came to understand the principle of complementarity after familiarizing themselves with the works of E. Chargaff. E. Chargaff, having studied a huge number of samples of tissues and organs of various organisms, found that in any DNA fragment the content of guanine residues always exactly corresponds to the content of cytosine, and adenine to thymine ( "Chargaff's rule"), but he is unable to explain this fact.
From the principle of complementarity it follows that the nucleotide sequence of one chain determines the nucleotide sequence of the other.
The DNA strands are antiparallel (multidirectional), ᴛ.ᴇ. nucleotides of different chains are located in opposite directions, and, therefore, opposite the 3" end of one chain is the 5" end of the other. The DNA molecule is sometimes compared to a spiral staircase. The “railing” of this staircase is a sugar-phosphate backbone (alternating residues of deoxyribose and phosphoric acid); “Steps” are complementary nitrogenous bases.
Function of DNA- storage and transmission of hereditary information.
Structure and functions of DNA - concept and types. Classification and features of the category "Structure and functions of DNA" 2017, 2018.
In this article you can learn the biological role of DNA. So, this abbreviation is familiar to everyone since school, but not everyone has an idea what it is. After a school biology course, only minimal knowledge of genetics and heredity remains in the memory, since children are taught this complex topic only superficially. But this knowledge (the biological role of DNA, the effect it has on the body) can be incredibly useful.
Let's start with the fact that nucleic acids perform an important function, namely, they ensure the continuity of life. These macromolecules come in two forms:
- DNA (DNA);
- RNA (RNA).
They are transmitters of the genetic plan for the structure and functioning of the body's cells. Let's talk about them in more detail.
DNA and RNA
Let's start with what branch of science deals with such complex issues as:
- studying the principles of storage;
- its implementation;
- broadcast;
- study of the structure of biopolymers;
- their functions.
All this is studied by molecular biology. It is in this branch of biological sciences that one can find the answer to the question of what is the biological role of DNA and RNA.
These high molecular weight compounds formed from nucleotides are called “nucleic acids”. It is here that information about the body is stored, which determines the development of the individual, growth and heredity.
The discovery of deoxyribonucleic acid dates back to 1868. Then scientists were able to detect them in the nuclei of leukocytes and moose sperm. Subsequent research showed that DNA can be found in all plant and animal cells. The DNA model was presented in 1953, and the Nobel Prize for the discovery was awarded in 1962.
DNA
Let's start this section with the fact that there are 3 types of macromolecules:
- Deoxyribonucleic acid;
- ribonucleic acid;
- proteins.
Now we will take a closer look at the structure and biological role of DNA. So, this biopolymer transmits data about heredity, developmental characteristics not only of the carrier, but also of all previous generations. - nucleotide. Thus, DNA is the main component of chromosomes, containing the genetic code.
How is the transfer of this information possible? The whole point is the ability of these macromolecules to reproduce themselves. Their number is infinite, which can be explained by their large size, and as a consequence - by a huge number of various nucleotide sequences.
DNA structure
In order to understand the biological role of DNA in a cell, it is necessary to become familiar with the structure of this molecule.
Let's start with the simplest, all nucleotides in their structure have three components:
- nitrogenous base;
- pentose sugar;
- phosphate group.
Each individual nucleotide in a DNA molecule contains one nitrogenous base. It can be absolutely any of four possible:
- A (adenine);
- G (guanine);
- C (cytosine);
- T (thymine).
A and G are purines, and C, T and U (uracil) are pyramidins.
There are several rules for the ratio of nitrogenous bases, called Chargaff's rules.
- A = T.
- G = C.
- (A + G = T + C) we can move all the unknowns to the left side and get: (A + G)/(T + C) = 1 (this formula is the most convenient when solving problems in biology).
- A + C = G + T.
- The value (A + C)/(G + T) is constant. In humans it is 0.66, but, for example, in bacteria it is from 0.45 to 2.57.
The structure of each DNA molecule resembles a twisted double helix. Please note that the polynucleotide chains are antiparallel. That is, the arrangement of nucleotide pairs on one chain has the opposite sequence than on the other. Each turn of this helix contains as many as 10 nucleotide pairs.
How are these chains connected to each other? Why is the molecule strong and does not disintegrate? It's all about the hydrogen bond between nitrogenous bases (between A and T - two, between G and C - three) and hydrophobic interaction.
To conclude this section, I would like to mention that DNA is the largest organic molecules, the length of which varies from 0.25 to 200 nm.
Complementarity
Let's take a closer look at pair connections. We have already said that pairs of nitrogenous bases are not formed in a chaotic manner, but in a strict sequence. Thus, adenine can only bind to thymine, and guanine can only bind to cytosine. This sequential arrangement of pairs in one chain of the molecule dictates their arrangement in the other.
When replicating or doubling to form a new DNA molecule, this rule, called “complementarity,” must be observed. You can notice the following pattern, which was mentioned in the summary of Chargaff’s rules - the number of the following nucleotides is the same: A and T, G and C.
Replication
Now let's talk about the biological role of DNA replication. Let's start with the fact that this molecule has this unique ability to reproduce itself. This term refers to the synthesis of a daughter molecule.
In 1957, three models of this process were proposed:
- conservative (the original molecule is preserved and a new one is formed);
- semi-conservative (breaking the original molecule into monochains and adding complementary bases to each of them);
- dispersed (decay of the molecule, replication of fragments and collection in random order).
The replication process has three stages:
- initiation (unbraiding of DNA sections using the helicase enzyme);
- elongation (chain lengthening by adding nucleotides);
- termination (achieving the required length).
This complex process has a special function, that is, a biological role - ensuring the accurate transmission of genetic information.
RNA
We have told you what the biological role of DNA is, now we propose to move on to consideration (that is, RNA).
Let's start this section with the fact that this molecule is no less important than DNA. We can detect it in absolutely any organism, prokaryotic and eukaryotic cells. This molecule is even observed in some viruses (we are talking about RNA viruses).
A distinctive feature of RNA is the presence of a single chain of molecules, but, like DNA, it consists of four nitrogenous bases. In this case it is:
- adenine (A);
- uracil (U);
- cytosine (C);
- guanine (G).
All RNAs are divided into three groups:
- matrix, which is usually called informational (abbreviation is possible in two forms: mRNA or mRNA);
- ribosomal (rRNA).
Functions
Having understood the biological role of DNA, its structure and the characteristics of RNA, we propose to move on to the special missions (functions) of ribonucleic acids.
Let's start with mRNA or mRNA, the main task of which is to transfer information from the DNA molecule to the cytoplasm of the nucleus. Also, mRNA is a template for protein synthesis. As for the percentage of this type of molecules, it is quite low (about 4%).
And the percentage of rRNA in the cell is 80. They are necessary because they are the basis of ribosomes. Ribosomal RNA takes part in protein synthesis and polypeptide chain assembly.
The adapter that builds the amino acid chain is tRNA, which transfers amino acids to the area of protein synthesis. The percentage in the cell is about 15%.
Biological role
To summarize: what is the biological role of DNA? At the time of the discovery of this molecule, they could not provide obvious information on this matter, but even now not everything is known about the significance of DNA and RNA.
If we talk about general biological significance, then their role is to transfer hereditary information from generation to generation, protein synthesis and coding of protein structures.
Many people also express this version: these molecules are connected not only with the biological, but also with the spiritual life of living beings. According to metaphysicians, DNA contains past life experiences and divine energy.
According to its chemical structure, DNA ( Deoxyribonucleic acid) is biopolymer, whose monomers are nucleotides. That is, DNA is polynucleotide. Moreover, a DNA molecule usually consists of two chains twisted relative to each other along a helical line (often called “helically twisted”) and connected to each other by hydrogen bonds.
The chains can be twisted both to the left and to the right (most often) side.
Some viruses have single strand DNA.
Each DNA nucleotide consists of 1) a nitrogenous base, 2) deoxyribose, 3) a phosphoric acid residue.
Double right-handed DNA helix
The composition of DNA includes the following: adenine, guanine, thymine And cytosine. Adenine and guanine are purins, and thymine and cytosine - to pyrimidines. Sometimes DNA contains uracil, which is usually characteristic of RNA, where it replaces thymine. 
The nitrogenous bases of one chain of a DNA molecule are connected to the nitrogenous bases of another strictly according to the principle of complementarity: adenine only with thymine (form two hydrogen bonds with each other), and guanine only with cytosine (three bonds).
The nitrogenous base in the nucleotide itself is connected to the first carbon atom of the cyclic form deoxyribose, which is a pentose (a carbohydrate with five carbon atoms). The bond is covalent, glycosidic (C-N). Unlike ribose, deoxyribose lacks one of its hydroxyl groups. The deoxyribose ring is formed by four carbon atoms and one oxygen atom. The fifth carbon atom is outside the ring and is connected through an oxygen atom to a phosphoric acid residue. Also, through the oxygen atom at the third carbon atom, the phosphoric acid residue of the neighboring nucleotide is attached.
Thus, in one strand of DNA, adjacent nucleotides are linked to each other by covalent bonds between deoxyribose and phosphoric acid (phosphodiester bond). A phosphate-deoxyribose backbone is formed. Directed perpendicular to it, towards the other DNA chain, are nitrogenous bases, which are connected to the bases of the second chain by hydrogen bonds.
The structure of DNA is such that the backbones of the chains connected by hydrogen bonds are directed in different directions (they say “multidirectional”, “antiparallel”). On the side where one ends with phosphoric acid connected to the fifth carbon atom of deoxyribose, the other ends with a “free” third carbon atom. That is, the skeleton of one chain is turned upside down relative to the other. Thus, in the structure of DNA chains, 5" ends and 3" ends are distinguished.
During DNA replication (doubling), the synthesis of new chains always proceeds from their 5th end to the third, since new nucleotides can only be added to the free third end.
Ultimately (indirectly through RNA), every three consecutive nucleotides in the DNA chain code for one protein amino acid.
The discovery of the structure of the DNA molecule occurred in 1953 thanks to the work of F. Crick and D. Watson (which was also facilitated by the early work of other scientists). Although DNA was known as a chemical substance back in the 19th century. In the 40s of the 20th century, it became clear that DNA is the carrier of genetic information.
The double helix is considered the secondary structure of the DNA molecule. In eukaryotic cells, the overwhelming amount of DNA is located in chromosomes, where it is associated with proteins and other substances, and is also more densely packaged.