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Research aimed at elucidating the chemical nature of hereditary material has irrefutably proven that the material substrate of heredity and variability arenucleic acids, which were discovered by F. Miescher (1868) in the nuclei of pus cells. Nucleic acids are macromolecules, i.e. have a high molecular weight. These are polymers consisting of monomers - nucleotides, including three components: sugar(pentose), phosphate And nitrogenous base(purine or pyrimidine). A nitrogenous base (adenine, guanine, cytosine, thymine or uracil) is attached to the first carbon atom in the C-1 pentose molecule, and a phosphate is attached to the fifth carbon atom C-5 using an ester bond; the third carbon atom C-3" always has a hydroxyl group - OH ( see diagram ).

The joining of nucleotides into a nucleic acid macromolecule occurs through the interaction of the phosphate of one nucleotide with the hydroxyl of another so that a phosphodiester bond(Fig. 3.2). As a result, a polynucleotide chain is formed. The backbone of the chain consists of alternating phosphate and sugar molecules. One of the nitrogenous bases listed above is attached to the pentose molecules at position C-1 (Fig. 3.3).

Rice. 3.1. Nucleotide structure diagram

The assembly of a polynucleotide chain is carried out with the participation of the enzyme polymerase, which ensures the attachment of the phosphate group of the next nucleotide to the hydroxyl group located in position 3", of the previous nucleotide (Fig. 3.3). Due to the noted specificity of the action of the named enzyme, the growth of the polynucleotide chain occurs only at one end: there , where the free hydroxyl is at position 3". The beginning of the chain always carries a phosphate group at position 5". This allows us to distinguish 5" and 3" - ends.

Among nucleic acids, two types of compounds are distinguished: deoxyribonucleic acid(DNA) And ribonucleic acid(RNA)acids. A study of the composition of the main carriers of hereditary material - chromosomes - discovered that their most chemically stable component is DNA, which is the substrate of heredity and variability.

DNA structure. Model by J. Watson et al. Scream

DNA consists of nucleotides, which include sugar - deoxyribose, phosphate and one of the nitrogenous bases - purine (adenine or guanine) or pyrimidine (thymine or cytosine).

A feature of the structural organization of DNA is that its molecules include two polynucleotide chains connected to each other in a certain way. In accordance with the three-dimensional model of DNA, proposed in 1953 by the American biophysicist J. Watson and the English biophysicist and geneticist F. Crick, these chains are connected to each other by hydrogen bonds between their nitrogenous bases according to the principle of complementarity. Adenine of one chain is connected by two hydrogen bonds to thymine of another chain, and three hydrogen bonds are formed between guanine and cytosine of different chains. This connection of nitrogenous bases ensures a strong connection between the two chains and maintaining an equal distance between them throughout.

Rice. 3.4. Diagram of the structure of a DNA molecule. The arrows indicate the antiparallelism of the circuits

Another important feature of the combination of two polynucleotide chains in a DNA molecule is their antiparallelism: the 5" end of one chain is connected to the 3" end of the other, and vice versa (Fig. 3.4).

X-ray diffraction data showed that a DNA molecule, consisting of two chains, forms a helix twisted around its own axis. The helix diameter is 2 nm, the pitch length is 3.4 nm. Each turn contains 10 pairs of nucleotides.

Most often, double helices are right-handed - when moving upward along the helix axis, the chains turn to the right. Most DNA molecules in solution are in the right-handed - B-form (B-DNA). However, left-handed forms (Z-DNA) also occur. How much of this DNA is present in cells and what its biological significance is has not yet been established (Fig. 3.5).

Rice. 3.5. Spatial models of left-handed Z-shape ( I)

and right-handed B-form ( II) DNA

Thus, in the structural organization of the DNA molecule we can distinguish primary structure - polynucleotide chain, secondary structure- two complementary and antiparallel polynucleotide chains connected by hydrogen bonds, and tertiary structure - a three-dimensional spiral with the above spatial characteristics.

One of the main properties of the material of heredity is its ability to self-copy - replication. This property is ensured by the peculiarities of the chemical organization of the DNA molecule, consisting of two complementary chains. During the replication process, a complementary chain is synthesized on each polynucleotide chain of the parent DNA molecule. As a result, two identical double helices are formed from one DNA double helix. This method of doubling molecules, in which each daughter molecule contains one parent and one newly synthesized chain, is called semi-conservative(see Fig. 2.12).

For replication to occur, the chains of maternal DNA must be separated from each other to become templates on which complementary chains of daughter molecules will be synthesized.

Initiation of replication occurs in special regions of DNA called ori (from the English origin - beginning). They include a sequence of 300 nucleotide pairs that is recognized by specific proteins. The DNA double helix in these loci is divided into two chains, and, as a rule, areas of divergence of polynucleotide chains are formed on both sides of the origin of replication - replication forks, which move in opposite directions from the locus ori directions. Between replication forks a structure called replication eye, where new polynucleotide chains are formed on two strands of maternal DNA (Figure 3.8, A).

The end result of the replication process is the formation of two DNA molecules, the nucleotide sequence of which is identical to that of the parent DNA double helix.

DNA replication in prokaryotes and eukaryotes is basically similar; however, the rate of synthesis in eukaryotes (about 100 nucleotides/s) is an order of magnitude lower than in prokaryotes (1000 nucleotides/s). The reason for this may be the formation of eukaryotic DNA in fairly strong compounds with proteins (see Chapter 3.5.2.), which complicates its despiralization necessary for replicative synthesis.

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

Deoxyribonucleic acid. General information

DNA (deoxyribonucleic acid) is a kind of blueprint for 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 sets the entire development program of any living organism. Genetically determined factors predetermine the entire course of life of both a person and any other organism. Artificial or natural influence external environment are capable of only to a small extent influencing the overall expression of individual genetic traits or affecting the development of programmed processes.

Deoxyribonucleic acid(DNA) is a macromolecule (one of the three main ones, the other two are RNA and proteins) that ensures 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 cellular 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. In them and in lower eukaryotes (for example, yeast), small autonomous, predominantly circular DNA molecules called plasmids are also found.

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

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

In the vast majority of cases (except for some viruses containing single-stranded DNA), the DNA macromolecule consists of two chains oriented with nitrogenous bases towards each other. This double-stranded molecule is twisted along 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 DNA spiral “staircase” (see: Fig. 2, 3 and 4).

Rice. 2. Nitrogenous bases

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

Rice. 3. DNA replication

Location of basic combinations chemical compounds DNA and the quantitative relationships between these combinations provide the coding of hereditary information.

Education new DNA (replication)

1. Replication process: unwinding of the DNA double helix - synthesis of complementary strands by DNA polymerase - 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 an element of new DNA. New base pairs are connected in the same sequence as in the parent branch.

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

More detailed information:

STRUCTURE OF NUCLEIC ACIDS

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

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

NUCLEOTIDES consist of nitrogenous base, connected to a five-carbon carbohydrate (pentose) - deoxyribose(in 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. 5. Structure of nucleotides (left), location of the nucleotide in DNA (bottom) and types of nitrogenous bases (right): pyrimidine and purine

The carbon atoms in the pentose molecule are numbered from 1 to 5. The phosphate combines with the third and fifth carbon atoms. This is how nucleinotides are combined into a nucleic acid chain. Thus, we can distinguish the 3' and 5' ends of the DNA strand:

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

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

Complementarity rule:

A-T G-C

For example, if we are given a DNA strand with 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. 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 through template synthesis. In most cases of natural DNA replicationprimerfor DNA synthesis is short fragment (recreated). 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 according to a semi-conservative mechanism. This means that the double helix of DNA unwinds and a new chain is built 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 one. Replication occurs in the direction from the 3' to the 5' end of the mother strand.

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

DNA synthesis- this is not as complicated a process as it might seem at first glance. If you think about it, first you need to figure out what synthesis is. This is the process of combining something into one whole. The formation of a new DNA molecule occurs 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, influences the process of “unbraiding” of the DNA helix.
3) DNA-binding proteins bind DNA strands and also stabilize them, preventing them from sticking to each other.
4) DNA polymerase δ(delta) , coordinated with the speed of movement of the replication fork, carries out synthesisleadingchains subsidiary DNA in the 5"→3" direction on the matrix maternal DNA strands in the direction from its 3" end to the 5" end (speed up to 100 nucleotide pairs per second). These events at this maternal DNA strands are limited.

Rice. 9. Schematic representation of the DNA replication process: (1) Lagging strand (lagging 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 strand of daughter DNA is described below (see. Scheme replication fork and functions of replication enzymes)

For more information about DNA replication, see

5) Immediately after the other strand of the mother molecule is unraveled and stabilized, it is attached to itDNA polymerase α(alpha)and in the 5"→3" direction it synthesizes a primer (RNA primer) - an RNA sequence on a DNA template with a length of 10 to 200 nucleotides. After this the enzymeremoved from the DNA strand.

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

6) DNA polymeraseε (epsilon) seems to continue to extend the primer, but inserts it as a substratedeoxyribonucleotides(in the amount of 150-200 nucleotides). As a result, a single thread is formed from two parts -RNA(i.e. primer) and DNA. DNA polymerase εruns until it encounters the previous primerfragment of Okazaki(synthesized a little earlier). After this, this enzyme is removed from the chain.

7) DNA polymerase β(beta) stands insteadDNA polymerase ε,moves in the same direction (5"→3") and removes the primer ribonucleotides while simultaneously inserting deoxyribonucleotides in their place. The enzyme works until complete removal primer, i.e. until a deoxyribonucleotide (an even earlier synthesizedDNA polymerase ε). The enzyme is not able to connect the result of its work with the DNA in front, so it goes off the chain.

As a result, a fragment of daughter DNA “lies” on the matrix of the mother strand. It is calledfragment of Okazaki.

8) DNA ligase crosslinks two adjacent fragments of Okazaki , i.e. 5" end of the segment synthesizedDNA polymerase ε,and 3"-end chain 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 consists of a long chain in which each link is called nucleotide. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate group. However, unlike DNA, RNA usually has one strand rather than two. The pentose in RNA is 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) RNA contains uracil ( 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 RNA is produced through a process called transcription , that is, the synthesis of RNA on a DNA matrix, carried out by special enzymes - RNA polymerases.

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

Rice. 10. The difference between DNA and RNA in 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 strands contains information that needs to be copied onto an RNA molecule - this strand is called the coding strand. The second strand of DNA, complementary to the coding one, is called the template. During transcription, a complementary RNA chain is synthesized on the template strand in the 3’ - 5’ direction (along the DNA strand). This creates an RNA copy of the coding strand.

Rice. 11. Schematic representation of the transcription

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

3’- ATGTCCTAGCTGCTCG - 5’,

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

5’- TACAGGATCGACGAGC- 3’,

and the RNA synthesized from it is the sequence

BROADCAST

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

Rice. 12. Protein synthesis process: 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 triplet.

Genetic code common to most pro- and eukaryotes. The table shows all 64 codons and 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 basis tion	2nd base								3rd the basis tion
	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)	C GA		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 serve as “punctuation marks”:

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

• **Triplets UAA, UAG And U.G.A. are called stop codons and do not code for a single amino acid. At these sequences, protein synthesis stops.

Properties of the genetic code

1. Triplety. 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; the information is read continuously.

3. Non-overlapping. One nucleotide cannot be included in two triplets at the same time.

4. Unambiguity. 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 chain:

3’- CCGATTGCACGTCGATCGTATA- 5’.

The matrix chain will have the sequence:

5’- GGCTAACGTGCAGCTAGCATAT- 3’.

Now we “synthesize” information RNA from this chain:

3’- CCGAUUGCACGUCGAUCGUAUA- 5’.

Protein synthesis proceeds in the direction 5’ → 3’, therefore, we need to reverse the sequence to “read” the genetic code:

5’- AUAUGCUAGCUGCACGUUAGCC- 3’.

Now let's find the start codon AUG:

5’- AU AUG CUAGCUGCACGUUAGCC- 3’.

Let's divide the sequence into triplets:

sounds like this: information is transferred from DNA 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 this process is mainly characteristic of viruses.

Rice. 13. Central Dogma of Molecular Biology

GENOME: 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 set of genes contained in the haploid set of chromosomes of organisms of one biological species. The original meaning of this term indicated that the concept of a genome, in contrast to a 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 sections of DNA molecules that encode polypeptides and RNA molecules

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

In 1940, George Beadle and Edward Tatham proposed a molecular definition of the gene. Scientists processed fungal spores Neurospora crassa x-ray radiation and other agents that cause changes in the DNA sequence ( mutations), and discovered mutant strains of the fungus that had lost some specific enzymes, which in some cases led to disruption of the entire metabolic pathway. Beadle and Tatem concluded that a gene is a piece of genetic material that specifies or codes for a single enzyme. This is how the hypothesis appeared "one gene - one enzyme". This concept was later expanded to define "one gene - one polypeptide", since many genes encode proteins that are not enzymes, and the polypeptide may be a subunit of a complex protein complex.

In Fig. Figure 14 shows a diagram of how triplets of nucleotides in DNA determine a polypeptide - the amino acid sequence of a protein through the mediation of mRNA. One of the DNA chains 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 determination of the gene even more specific. 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 exclusively a regulatory function. Regulatory sequences may mark the beginning or end of genes, influence transcription, or indicate the site of initiation of replication or recombination. Some genes can be expressed in different ways, with the same DNA region serving as a template for the formation of different products.

We can roughly calculate minimum size gene, encoding the middle 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 that is encoded by this gene. Polypeptide chain of 350 amino acid residues (chain middle length) corresponds to a sequence of 1050 bp. ( base pairs). However, many eukaryotic genes and some prokaryotic genes are interrupted by DNA segments that do not carry protein information, 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 large class of nuclear proteins that perform two main functions: they participate in the packaging of DNA strands in the nucleus and in the epigenetic regulation of nuclear processes such as transcription, replication and repair.

As is known, bacterial cells have a chromosome in the form of a DNA strand arranged in a compact structure - a nucleoid. Prokaryotic chromosome Escherichia coli, whose genome has been completely deciphered, is a circular DNA molecule (in fact, it is not a perfect circle, but rather a loop without a beginning or end), consisting of 4,639,675 bp. This sequence contains approximately 4,300 protein genes and another 157 genes for stable RNA molecules. IN human genome approximately 3.1 billion base pairs corresponding to nearly 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 bp. and reaches a length of approximately 1.7 mm, which exceeds the length of the cell itself E. coli approximately 850 times. In addition to the large circular chromosome as part of the nucleoid, many bacteria contain one or several 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 provide no benefit to the host cells and their sole purpose is to reproduce independently. However, some plasmids carry genes beneficial to the host. For example, genes contained in plasmids can make bacterial cells resistant to antibacterial agents. Plasmids carrying the β-lactamase gene provide resistance to β-lactam antibiotics such as penicillin and amoxicillin. Plasmids can pass from cells that are resistant to antibiotics to other cells of the same or a different species of bacteria, 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, leading to the emergence of bacterial strains with resistance to multiple antibiotics. Doctors are beginning to understand the dangers of widespread use of antibiotics and prescribe them only in cases urgent need. For similar reasons, the widespread use of antibiotics to treat farm animals is limited.

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

Eukaryotes.

Table 2. DNA, genes and chromosomes of some organisms

	Shared DNA p.n.	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 musculus(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 genomics project websites

* For all eukaryotes, except yeast, the diploid set of chromosomes is given. Diploid kit chromosomes (from the Greek diploos - double and eidos - species) - a double set of chromosomes (2n), each of which has a homologous one.
**Haploid set. Wild yeast strains 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.

Yeast, one of the smallest eukaryotes, has 2.6 times more DNA than E. coli(Table 2). Fruit fly cells Drosophila, a classic subject of genetic research, contain 35 times more DNA, and human cells contain approximately 700 times more DNA than 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, a human somatic cell has 46 chromosomes ( rice. 17). Each chromosome of 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) vary in length by more than 25 times. Each eukaryotic chromosome contains a specific set of genes.

Rice. 17. Chromosomes of eukaryotes.A- a pair of linked 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.

If you connect the DNA molecules of the human genome (22 chromosomes and chromosomes X and Y or X and X), 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 2 m. An adult human has approximately 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. This is how amazingly compact DNA is packed in our cells!

In eukaryotic cells there are other organelles containing DNA - 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 represent the rudiments of the chromosomes of ancient bacteria, which penetrated the cytoplasm of the host cells and became the precursors of these organelles. Mitochondrial DNA encodes mitochondrial tRNAs and rRNAs, as well as several mitochondrial proteins. More than 95% of mitochondrial proteins are encoded by nuclear DNA.

STRUCTURE OF GENES

Let's 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 that encodes only one protein or RNA, in addition to the immediate coding part, it also includes regulatory and other structural elements that have different structures in prokaryotes and eukaryotes.

Coding sequence- the main structural and functional unit of the gene, it is in it that the triplets of nucleotides encoding are locatedamino acid sequence. It begins with a start codon and ends with a stop codon.

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

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

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

At the beginning of the gene is regulatory region, which includes promoter And operator.

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

Gene structure in prokaryotes

The general plan of gene structure in prokaryotes and eukaryotes is no different - both contain a regulatory region with a promoter and operator, a transcription unit with coding and untranslated sequences, and a terminator. However, the organization of genes in prokaryotes and eukaryotes is different.

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

At the beginning and 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 areas withone protein is synthesized. Thus, Several protein molecules are synthesized from one mRNA molecule.

Prokaryotes are characterized by the combination of several genes into a single functional unit - operon. The operation of the operon can be regulated by other genes, which can be noticeably distant 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 genes contained in it at once.

Prokaryotes are also characterized by the phenomenon Transcription-translation interfaces.

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

Such coupling 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 RNA synthesis on a DNA template, a ribosome can immediately bind to the synthesized RNA molecule. Thus, translation begins even before transcription is completed. Moreover, several ribosomes can simultaneously bind to one RNA molecule, synthesizing several molecules of one protein at once.

Gene structure in eukaryotes

The genes and chromosomes of eukaryotes are very complexly organized

Many species of bacteria 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 found in multiple copies. Genes and regulatory sequences make up virtually the entire prokaryotic genome. Moreover, almost every gene strictly corresponds to the amino acid sequence (or RNA sequence) 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, brought many surprises. Many, if not most, eukaryotic genes have interesting feature: their nucleotide sequences contain one or more DNA regions that do not encode the amino acid sequence of the polypeptide product. Such untranslated insertions disrupt the direct correspondence between the nucleotide sequence of the gene and the amino acid sequence of the encoded polypeptide. These untranslated segments within 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, the combination of genes into operons practically does not occur, and the coding sequence of a eukaryotic gene is most often divided into translated sections - exons, and untranslated sections - introns.

In most cases, the function of introns is not 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 human DNA is 30% genes. Because genes make up a relatively small proportion of the human genome, a significant portion of DNA remains unaccounted for.

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

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

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

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

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

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

MUTATIONS AND MUTAGENESIS

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

The process that leads to mutations is called mutagenesis, and the body All whose cells carry the same mutation - mutant.

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

1. Mutations occur suddenly, spasmodically.

2. Mutations are passed on from generation to generation.

3. Mutations can be beneficial, harmful 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. There are mutations that arise under the influence of mutagenic impacts: physical (for example, ultraviolet or radiation), chemical (for example, colchicine or reactive oxygen species) and biological (for example, viruses). Mutations can also be caused replication errors.

Depending on the conditions under which mutations appear, mutations are divided into spontaneous- that is, mutations that arose 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 mitochondrial or plastid DNA. Accordingly, we can distinguish nuclear And cytoplasmic mutations.

As a result of mutations, new alleles can often appear. If a mutant allele suppresses the action of a normal one, the mutation is called dominant. If a normal allele suppresses a mutant one, this mutation is called recessive. Most mutations that lead to the emergence of new alleles are recessive.

Mutations are distinguished by effect adaptive leading to increased adaptability of the organism to the environment, neutral, which do not affect survival, harmful, reducing the adaptability of organisms to environmental conditions and lethal leading to the death of the organism early stages development.

According to the consequences, mutations leading to loss of protein function, mutations leading to emergence protein has a new function, as well as mutations that change gene dosage, 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 a mutation occurs in any other cell, it is called somatic. Such a mutation can manifest itself to one degree or another in the organism in which it arose, for example, leading to the formation of cancerous tumors. However, such a mutation is not inherited and does not affect descendants.

Mutations can affect regions of the genome of different sizes. Highlight genetic, chromosomal And genomic mutations.

Gene mutations

Mutations that occur on a scale smaller than one gene are called genetic, or point (point). Such mutations lead to changes in one or several nucleotides in the sequence. Among gene mutations there arereplacements, leading to the replacement of one nucleotide with 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 with another and can affect the structure of the synthesized protein, although they are often insignificant,nonsense mutations, leading to the replacement of the coding codon with a stop codon,mutations leading to splicing disorder:

Rice. 24. Mutation patterns

Also, according to the mechanism of action on the protein, mutations are distinguished that lead to frame shift reading, 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.

Rice. 29. Chromosome before and after duplication

Genomic mutations

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

Video on DNA

DNA REPLICATION, RNA CODING, PROTEIN SYNTHESIS

In 1869, Swiss biochemist Friedrich Miescher discovered compounds with acidic properties and even higher molecular weights than proteins in the nucleus of cells. Altman called them nucleic acids, from the Latin word “nucleus” - nucleus. Just like proteins, nucleic acids are polymers. Their monomers are nucleotides, and therefore nucleic acids can also be called polynucleotides.

Nucleic acids have been found in the cells of all organisms, from the simplest to the highest. The most amazing thing is that chemical composition, the structure and basic properties of these substances turned out to be similar in a variety of living organisms. But if about 20 types of amino acids take part in the construction of proteins, then there are only four different nucleotides that make up nucleic acids.

Nucleic acids are divided into two types - deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA contains nitrogenous bases (adenine (A), guanine (G), thymine (T), cytosine (C)), deoxyribose C5H10O4 and a phosphoric acid residue. RNA contains uracil (U) instead of thymine, and ribose (C5H10O5) instead of deoxyribose. The monomers of DNA and RNA are nucleotides, which consist of nitrogenous, purine (adenine and guanine) and pyrimidine (uracil, thymine and cytosine) bases, a phosphoric acid residue and carbohydrates (ribose and deoxyribose).

DNA molecules are found in the chromosomes of the cell nucleus of living organisms, in the equivalent structures of mitochondria, chloroplasts, in prokaryotic cells and in many viruses. The structure of the DNA molecule is similar to a double helix. Structural model of DNA in
form of a double helix was first proposed in 1953 by the American biochemist J. Watson and the English biophysicist and geneticist F. Crick, who together with the English biophysicist M. Wilkinson, who received the X-ray diffraction pattern of DNA, were awarded the 1962 Nobel Prize. Nucleic acids are biopolymers, the macromolecules of which consist of repeatedly repeating units - nucleotides. Therefore they are also called polynucleotides. The most important characteristic of nucleic acids is their nucleotide composition. The composition of a nucleotide - a structural unit of nucleic acids - includes three components:

nitrogenous base - pyrimidine or purine. Nucleic acids contain four different types of bases: two of them belong to the class of purines and two to the class of pyrimidines. The nitrogen contained in the rings gives the molecules their basic properties.

monosaccharide - ribose or 2-deoxyribose. The sugar that is part of the nucleotide contains five carbon atoms, i.e. is a pentose. Depending on the type of pentose present in the nucleotide, two types of nucleic acids are distinguished - ribonucleic acids (RNA), which contain ribose, and deoxyribonucleic acids (DNA), which contain deoxyribose.

phosphoric acid residue. Nucleic acids are acids because their molecules contain phosphoric acid.

The method for determining the composition of PC is based on the analysis of hydrolysates formed during their enzymatic or chemical breakdown. Three methods of chemical cleavage of NC are commonly used. Acid hydrolysis under severe conditions (70% perchloric acid, 100°C, 1h or 100% formic acid, 175°C, 2h), used for the analysis of both DNA and RNA, leads to the cleavage of all N- glycosidic bonds and the formation of a mixture of purine and pyrimidine bases.

Nucleotides are linked into a chain through covalent bonds. The nucleotide chains formed in this way are combined into one DNA molecule along the entire length by hydrogen bonds: the adenine nucleotide of one chain is connected to the thymine nucleotide of the other chain, and the guanine nucleotide to the cytosine one. In this case, adenine always recognizes only thymine and binds to it and vice versa. A similar pair is formed by guanine and cytosine. Such base pairs, like nucleotides, are called complementary, and the principle of the formation of a double-stranded DNA molecule is called the principle of complementarity. The number of nucleotide pairs, for example, in the human body is 3 - 3.5 billion.

DNA is a material carrier of hereditary information, which is encoded by a sequence of nucleotides. The location of the four types of nucleotides in the DNA chains determines the sequence of amino acids in protein molecules, i.e. their primary structure. The properties of cells and the individual characteristics of organisms depend on the set of proteins. A certain combination of nucleotides that carry information about the structure of the protein and the sequence of their location in the DNA molecule form the genetic code. A gene (from the Greek genos - genus, origin) is a unit of hereditary material responsible for the formation of any trait. It occupies a section of the DNA molecule that determines the structure of one protein molecule. The set of genes contained in a single set of chromosomes of a given organism, is called a genome, and the genetic constitution of an organism (the totality of all its genes) is called a genotype. Violation of the nucleotide sequence in the DNA chain, and therefore in the genotype, leads to hereditary changes in the body - mutations.

DNA molecules are characterized by the important property of duplication - the formation of two identical double helices, each of which is identical to the original molecule. This process of doubling a DNA molecule is called replication. Replication involves the breaking of old and the formation of new hydrogen bonds that unite nucleotide chains. At the beginning of replication, the two old strands begin to unwind and separate from each other. Then, according to the principle of complementarity, new chains are attached to the two old chains. This creates two identical double helices. Replication ensures accurate copying of genetic information contained in DNA molecules and passes it on from generation to generation.

1. DNA composition

DNA (deoxyribonucleic acid)- a biological polymer consisting of two polynucleotide chains connected to each other. The monomers that make up each DNA strand are complex organic compounds, including one of four nitrogenous bases: adenine (A) or thymine (T), cytosine (C) or guanine (G); pentaatomic sugar pentose - deoxyribose, from which DNA itself is named, as well as a phosphoric acid residue. These compounds are called nucleotides. In each chain, nucleotides are joined by forming covalent bonds between the deoxyribose of one nucleotide and the phosphoric acid residue of the next nucleotide. Two chains are combined into one molecule using hydrogen bonds that arise between nitrogenous bases that are part of the nucleotides that form different chains.

By examining the nucleotide composition of DNA of various origins, Chargaff discovered the following patterns.

1. All DNA, regardless of their origin, contains the same number of purine and pyrimidine bases. Consequently, in any DNA there is one pyrimidine nucleotide for every purine nucleotide.

2. Any DNA always contains equal amounts in pairs of adenine and thymine, guanine and cytosine, which are usually denoted as A=T and G=C. The third follows from these regularities.

3. The number of bases containing amino groups in position 4 of the pyrimidine nucleus and 6 of the purine nucleus (cytosine and adenine) is equal to the number of bases containing an oxo group in the same positions (guanine and thymine), i.e. A+C=G+T . These patterns are called Chargaff's rules. Along with this, it was found that for each type of DNA the total content of guanine and cytosine is not equal to the total content of adenine and thymine, i.e. that (G+C)/(A+T), as a rule, differs from unity (maybe both more and less of it). Based on this feature, two main types of DNA are distinguished: A T-type with a predominant content of adenine and thymine and G C-type with a predominant content of guanine and cytosine.

The ratio of the content of the sum of guanine and cytosine to the sum of the content of adenine and thymine, characterizing the nucleotide composition of a given type of DNA, is usually called specificity coefficient. Each DNA has a characteristic specificity coefficient, which can vary from 0.3 to 2.8. When calculating the specificity coefficient, the content of minor bases is taken into account, as well as the replacement of major bases with their derivatives. For example, when calculating the specificity coefficient for wheat germ EDNA, which contains 6% 5-methylcytosine, the latter is included in the sum of the content of guanine (22.7%) and cytosine (16.8%). The meaning of Chargaff's rules for DNA became clear after its spatial structure was established.

1. Macromolecular structure of DNA

In 1953, Watson and Crick, relying on known data on the conformation of nucleoside residues, the nature of internucleotide bonds in DNA and the regularities of the nucleotide composition of DNA (Chargaff's rules), deciphered x-ray diffraction patterns of the paracrystalline form of DNA [the so-called B-form, formed at a humidity above 80 % and at a high concentration of counterions (Li+) in the sample]. According to their model, the DNA molecule is a regular helix formed by two polydeoxyribonucleotide chains twisted relative to each other and around a common axis. The diameter of the helix is ​​almost constant along its entire length and is equal to 1.8 nm (18 A).

Macromolecular structure of DNA.

(a)-Watson-Crick model;

(6) parameters of the B-, C- and T-form DNA helices (projections perpendicular to the helix axis);

(c) - cross-section of a DNA helix in B-form (shaded rectangles represent base pairs);

(G)-parameters of the DNA helix in A-form;

(d)- cross section of a DNA helix in A-shape.
The length of the helix turn, which corresponds to its identity period, is 3.37 nm (33.7 A). For one turn of the helix there are 10 base residues in one chain. The distance between the base planes is thus approximately 0.34 nm (3.4 A). The planes of the base residues are perpendicular to the long axis of the helix. The planes of carbohydrate residues deviate somewhat from this axis (initially Watson and Crick suggested that they were parallel to it).

The figure shows that the carbohydrate-phosphate backbone of the molecule faces outward. The spiral is twisted in such a way that two grooves of different sizes can be distinguished on its surface (they are often also called grooves) - a large one, about 2.2 nm wide (22 A), and a small one, about 1.2 nm wide (12 A). The spiral is dextrorotatory. The polydeoxyribonucleotide chains in it are antiparallel: this means that if we move along the long axis of the helix from one end to the other, then in one chain we will pass phosphodiester bonds in the 3"à5" direction, and in the other - in the 5"à3 direction ". In other words, at each end of a linear DNA molecule there is a 5" end of one strand and a 3" end of another strand.

The regularity of the helix requires that a purine base residue on one chain be opposite a pyrimidine base residue on the other chain. As already emphasized, this requirement is implemented in the form of the principle of the formation of complementary base pairs, i.e., adenine and guanine residues in one chain correspond to thymine and cytosine residues in the other chain (and vice versa).

Thus, the sequence of nucleotides in one chain of a DNA molecule determines the nucleotide sequence of the other chain.

This principle is the main consequence of the Watson and Crick model, since it explains in surprisingly simple chemical terms the main functional purpose of DNA - to be the storehouse of genetic information.

Concluding the consideration of the Watson and Crick model, it remains to add that neighboring pairs of base residues in DNA, which is in the B-form, are rotated relative to each other by 36° (the angle between the straight lines connecting the C 1 atoms in adjacent complementary pairs).
4.1 Isolation of deoxyribonucleic acids
Living cells, with the exception of sperm, normally contain significantly more ribonucleic acid than deoxyribonucleic acid. Methods for isolating deoxyribonucleic acids have been greatly influenced by the fact that, while ribonucleoproteins and ribonucleic acids are soluble in a dilute (0.15 M) solution of sodium chloride, deoxyribonucleoprotein complexes are actually insoluble in it. Therefore, the homogenized organ or organism is thoroughly washed with a dilute saline solution, and deoxyribonucleic acid is extracted from the residue using a strong saline solution, which is then precipitated by adding ethanol. On the other hand, elution of the same residue with water gives a solution from which the deoxyribonucleoprotein precipitates when salt is added. Cleavage of the nucleoprotein, which is basically a salt-like complex between polybasic and polyacid electrolytes, is easily achieved by dissolution in a strong saline solution or treatment with potassium thiocyanate. Most protein can be removed either by adding ethanol or by emulsifying with chloroform and amyl alcohol (the protein forms a gel with chloroform). Detergent treatments were also widely used. Later, deoxyribonucleic acids were isolated by extraction with aqueous n-aminosalicylate-phenolic solutions. Using this method, deoxyribonucleic acid preparations were obtained, some of which contained residual protein, while others were virtually free of protein, indicating that the nature of the protein-nucleic acid association differs in different tissues. A convenient modification is to homogenize the animal tissue in 0.15 M phenolphthaleine diphosphate solution, followed by the addition of phenol to precipitate DNA (free of RNA) in good yield.

Deoxyribonucleic acids, no matter how they are isolated, are mixtures of polymers of different molecular weights, with the exception of samples obtained from certain types of bacteriophages.
4.2 Fractionation
Early method separation involved the fractional dissociation of deoxyribonucleoprotein gels (for example, nucleohistone) through extraction aqueous solutions sodium chloride of increasing molarity. In this way, deoxyribonucleic acid preparations were divided into a number of fractions characterized by different ratios of adenine and thymine to the sum of guanine and cytosine, with fractions enriched in guanine and cytosine being more easily isolated. Similar results were obtained by chromatographic separation of deoxyribonucleic acid from histone adsorbed on kieselguhr using gradient elution with sodium chloride solutions. In an improved version of this method, purified histone fractions were combined with n-aminobenzylcellulose to form diazo bridges from the tyrosine and histidine groups of the protein. Fractionation of nucleic acids on methylated serum albumin (with diatomaceous earth as a carrier) has also been described. Column elution rate saline solutions increasing concentration depends on molecular weight, composition (nucleic acids with a high content of guanine with cytosine elute more easily) and secondary structure (denatured DNA is more firmly retained by the column than native DNA). In this way, a natural component, polydeoxyadenylic-thymidylic acid, was isolated from the DNA of the sea crab Cancer borealis. Fractionation of deoxyribonucleic acids was also carried out by gradient elution from a column filled with calcium phosphate.

1. Functions of DNA

In the DNA molecule, the sequence of amino acids in peptides is encrypted using a biological code. Each amino acid is encoded by a combination of three nucleotides, in this case 64 triplets are formed, of which 61 encode amino acids, and 3 are meaningless and serve as punctuation marks (ATT, ACT, ATC). The encryption of one amino acid by several triplets is called triplet code degeneracy. Important properties The genetic code is its specificity (each triplet is capable of encoding only one amino acid), universality (indicating the unity of origin of all life on Earth) and non-overlapping codons when reading.

DNA performs the following functions:

storage of hereditary information occurs with the help of histones. The DNA molecule folds, first forming a nucleosome, and then heterochromatin, which makes up chromosomes;

transmission of hereditary material occurs through DNA replication;

implementation of hereditary information in the process of protein synthesis.

Which of the above structural and functional features of the DNA molecule allow it to store and transmit hereditary information from cell to cell, from generation to generation, to provide new combinations of characteristics in the offspring?

1. Stability. It is provided by hydrogen, glycosidic and phosphodiester bonds, as well as by the mechanism of repair of spontaneous and induced damage;

2. Replication ability. Thanks to this mechanism, the diploid number of chromosomes is maintained in somatic cells. All of the listed features of DNA as a genetic molecule are shown schematically in the figure.

3. Presence of genetic code. The sequence of bases in DNA is converted through the processes of transcription and translation into the sequence of amino acids in a polypeptide chain;
4. Capacity for genetic recombination. Thanks to this mechanism, new combinations of linked genes are formed.

Also in maternity hospital Having seen her baby, any mother worries: is everything normal with him, are his fingers and toes intact, are there any other serious abnormalities. And you will be terribly worried and worried if you see at least some incomprehensible spot on your child’s body.

Often, attentive mothers discover that white spots have appeared on the baby’s gums, plaque, and some other suspicious growths. And then they begin to ask questions: what is this? Where did they come from? What to do?

Swelling of the gums, white plaque – mother should be alerted

Normal healthy gums in an infant have a pinkish tint, without abrasions, bumps, or bumps. But often they take on a whitish color in the form of plaque, which cannot but cause concern for the parent.

What can cause abnormalities in the condition of a baby's gums? Let's give some examples.

1. Failure to comply with hygiene standards and rules. After each feeding, the mother should carefully care for the baby's mouth. Carry out daily cleaning procedures to remove milk residues. Missing teeth in a child still leaves plaque on the gums after drinking milk.
2. Lack of vitamins in the body. Mother's milk should ideally contain a whole range of vitamins necessary for an infant. However, in life this is not always the case. Milk from mothers who are still very young, and from those who are not very well behaved healthy image life, smoke or abuse alcohol, and often lack calcium and vitamins. Therefore, the baby does not receive enough milk necessary for the body vitamins and elements.
3. Stomatitis. For certain disorders in the body, which can be caused by specific reasons, and sometimes without visible reasons, the child develops a disease of the oral mucosa. This is often caused by a lack of iron and vitamins. But it can contribute to the disease viral infection. Low rate folic acid, poisoning and even emotional stress all play a role. Diseases of the oral mucosa must be taken seriously and under no circumstances should they be neglected.

Self-medication should not be done. It is necessary to consult a pediatrician who will determine what caused the disease. And, based on this, he will give recommendations and prescribe the necessary treatment.

• if the white coating is caused by non-compliance with certain hygiene rules, they should be eliminated. It is necessary to remove plaque from the gums of an infant with a special brush. And perform this procedure regularly after feeding;
• flaw necessary elements, vitamins in the body should be replenished in consultation with the pediatrician. He will advise which this moment the child needs nutritional formulas, vitamins and will write out a prescription;
• when the reason white plaque is a disease of the oral mucosa, treatment should be started immediately. As a rule, it is carried out medicinally and depending on the complexity of the disease.

The most best medicines are not always able to overcome the disease. Therefore, we must try to do everything to ensure that the child grows up healthy. As you know, a disease is always much easier and cheaper to prevent than to treat.

Therefore, quite an excellent plaque prevention for your child will be simple procedures, which do not require any additional labor or costs. For the first procedure, you only need boiled water. After you feed your baby milk, give him a teaspoon of this water.

The next procedure is also simple. Here you will need, in addition to water, 1 teaspoon baking soda. Dissolve it in a glass of water and dip your finger with a bandage wrapped around it. Don't forget to wash your hands before the procedure! And carefully remove plaque from the baby.

Causes of White Spots or Dots

In addition to white plaque, children often develop white spots on their gums. What is it, and what reasons contribute to its occurrence?

• There can be many reasons for this. Small cysts are quite common in children. These are formations from residual tissue of the salivary glands. They are very similar to congenital teeth and are often confused because of this. They resemble the shape of beads, there can be from one to several, in common parlance they are called pearl oysters. A more correct name is Bon's nodes. They do not cause concern to the child. After some time they disappear without a trace.
• There are another “pearls” that can also be found quite often in infants – Epstein’s pearls. They are located on the baby's palate. They also do not pose a danger and dissolve without a trace over time.
• Another reason that causes a white spot or spot is congenital teeth. In this case, you should definitely consult a doctor and undergo an examination. Such a tooth may need to be removed if the doctor determines that it is supernumerary to allow room for the normal development of baby teeth. This happens quite often. And you need to monitor the baby’s oral cavity.

The above-mentioned reasons that caused a white dot to appear on a baby’s gum, in most cases do not pose a threat to the child’s life, but there are other, more serious reasons. Here are some of them.

1. Stomatitis. This disease occurs not only in children; adults also suffer from it. It must be remembered that stomatitis is easier to fight before ulcers form and you can quickly get rid of the disease. If the disease has become advanced, then you will have to make a lot of effort to recover from it. As a rule, stomatitis is accompanied by anxious behavior in the child, which may be caused by pain; symptoms are expressed by fever, loss of appetite. In most cases, the disease progresses.
2. Cyst. Recognize it dangerous disease possible using X-ray examination. She is an abscess large sizes. There are many reasons for this, but in most cases it is caused by acute respiratory disease or infection in the dental tissues. Self-medication is unlikely to bring any results; only doctors can defeat the disease.
3. Thrush. Fungal disease, which destroys the oral mucosa. You need to make sure that the child does not lack water, try to give him to drink more often. Control the air humidity in the room where the baby is. All this will contribute to the good secretion of saliva, which has excellent antiseptic properties. In a dry room and lack of moisture in the body, the mucous membranes of the baby will dry out, saliva production will begin to be weak, which will contribute to the development of thrush.
4. Periodontitis. The white dots are shaped like bubbles. They cannot be destroyed, otherwise the infection may find a loophole and infect the body. If these symptoms occur, you should not postpone your visit to the dentist.
5. It happens that a white dot in the form of a ball appears on the baby’s gum. Perhaps it's a wen. Despite the fact that the neoplasm itself does not pose a great threat to the child’s life and does not cause inconvenience, it should not be overlooked. With a stable increase in size of the wen, the question of its removal will most likely arise. Similar formations occur in both young children and adults.
6. Cancer. White gums should alert parents. Such symptoms may indicate a possible harbinger cancerous tumor. If you notice even the slightest suspicion, you should immediately seek medical advice. A piece of tissue will be taken from the diseased area and sent for a biopsy. The results of the study will show the presence or absence of the disease.

As can be seen from the above examples, the appearance of various plaques, balls, and ulcers can be caused by quite a few different reasons. Only a doctor can determine the nature of the tumor.

Easier to warn

Any disease is easier to prevent, so prevention should be given the closest attention.

What should you do to avoid various white spots, plaques and other things in infants:

• It is necessary to provide the child with adequate nutrition. Then oral cavity, gums will not be susceptible to various diseases;
• It is important to keep the baby’s mouth clean. Therefore, one should not forget about the basics of hygiene, make sure that the child does not put dirty fingers or sharp objects into the mouth;
• try not to buy him toys that can cause injury to the oral cavity. Often, from contact with such toys, a child develops not only white spots, but bruises;
• A person with unhealthy teeth should not be allowed to come into close contact with the child. This must not be forgotten - the child’s body is very susceptible to various infectious diseases. The baby's health should always come first;
• The baby's eating and drinking utensils must always be clean, and no one else should use them except him. The same applies to hygiene items. Then the child’s chances of being healthy and strong will increase significantly.

Summarize

As we can see, very different reasons cause the appearance of white plaque, dots, ulcers, and bumps. They may not be dangerous or dangerous. But, in any case, they all have one thing in common - insufficient care for the child, lack of adequate nutrition, and, as a result, a lack of nutrition in the baby’s body. important elements and vitamins.

A healthy baby is joy and happiness. Take care of him and he will grow up strong and healthy!

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