The structure of the DNA molecule was proposed. DNA structure: features, scheme
Molecular genetics – a branch of genetics that deals with the study of heredity at the molecular level.
Nucleic acids. DNA replication. Matrix synthesis reactions
Nucleic acids (DNA, RNA) were discovered in 1868 by the Swiss biochemist I.F. Misher. Nucleic acids are linear biopolymers consisting of monomers - nucleotides.
DNA - structure and function
The chemical structure of DNA was deciphered in 1953 by the American biochemist J. Watson and the English physicist F. Crick.
General structure of DNA. A DNA molecule consists of 2 chains, which are twisted into a helix (Fig. 11) one around the other and around a common axis. DNA molecules can contain from 200 to 2x10 8 base pairs. Along the helix of the DNA molecule, adjacent nucleotides are located at a distance of 0.34 nm from each other. A complete turn of the helix includes 10 base pairs. Its length is 3.4 nm.
Rice. 11 ... DNA structure diagram (double helix)
Polymerity of the DNA molecule. The DNA molecule - bioploimer consists of complex compounds - nucleotides.
DNA nucleotide structure. The DNA nucleotide consists of 3 units: one of the nitrogenous bases (adenine, guanine, cytosine, thymine); deoxyribose (monosaccharide); the remainder of phosphoric acid (Fig. 12).
There are 2 groups of nitrogenous bases:
purine - adenine (A), guanine (G), containing two benzene rings;
pyrimidine - thymine (T), cytosine (C), containing one benzene ring.
DNA contains the following types of nucleotides: adenine (A); guanine (G); cytosine (C); thymine (T). The names of the nucleotides correspond to the names of the nitrogenous bases that make up them: adenine nucleotide nitrogenous base adenine; guanine nucleotide nitrogenous base guanine; cytosine nucleotide nitrogenous base cytosine; thymine nucleotide nitrogenous base thymine.
Joining two strands of DNA into one molecule
Nucleotides A, G, C and T of one chain are connected, respectively, with nucleotides T, C, G and A of another chain hydrogen bonds... Two hydrogen bonds are formed between A and T, and three hydrogen bonds are formed between G and C (A = T, G≡C).
Pairs of bases (nucleotides) A - T and G - C are called complementary, that is, mutually corresponding. Complementarity Is the chemical and morphological correspondence of nucleotides to each other in paired DNA chains.
5 ’ 3 ’
1 2 3
3’ 5’
Rice. 12 Section of the DNA double helix. Nucleotide structure (1 - phosphoric acid residue; 2 - deoxyribose; 3 - nitrogenous base). The connection of nucleotides using hydrogen bonds.
Chains in a DNA molecule antiparallel, that is, directed in opposite directions, so that the 3'-end of one strand is opposite the 5'-end of the other strand. Genetic information in DNA is written from the 5 'end to the 3' end. This thread is called semantic DNA,
because the genes are located here. The second thread - 3'-5 'serves as a standard for storing genetic information.
The relationship between the number of different bases in DNA was established by E. Chargaff in 1949. Chargaff revealed that in DNA of various species the amount of adenine is equal to the amount of thymine, and the amount of guanine is equal to the amount of cytosine.
E. Chargaff's rule:
in a DNA molecule, the number of A (adenine) nucleotides is always equal to the number of T (thymine) nucleotides or the ratio of ∑ A to ∑ T = 1. The sum of G (guanine) nucleotides is equal to the sum of C (cytosine) nucleotides or the ratio ∑ G to ∑ C = 1;
the sum of purine bases (A + G) is equal to the sum of pyrimidine bases (T + C) or the ratio ∑ (A + G) to ∑ (T + C) = 1;
DNA synthesis method - replication... Replication is the process of self-duplication of a DNA molecule, carried out in the nucleus under the control of enzymes. Self-delight of the DNA molecule occurs based on complementarity- strict correspondence of nucleotides to each other in paired DNA chains. At the beginning of the replication process, the DNA molecule unwinds (despiralizes) in a certain area (Fig. 13), while hydrogen bonds are released. On each of the chains formed after the rupture of hydrogen bonds, with the participation of an enzyme DNA polymyrases, the daughter strand of DNA is synthesized. The material for synthesis is free nucleotides contained in the cytoplasm of cells. These nucleotides line up complementary to the nucleotides of the two maternal DNA strands. DNA polymerase enzyme attaches complementary nucleotides to the template DNA strand. For example, to the nucleotide A template chain polymerase attaches nucleotide T and, accordingly, nucleotide C to nucleotide G (Fig. 14). Crosslinking of complementary nucleotides occurs by an enzyme DNA ligases... Thus, by self-doubling, two daughter DNA chains are synthesized.
The resulting two DNA molecules from one DNA molecule are semi-conservative model because they consist of the old parent and new daughter chains and are an exact copy of the parent molecule (Fig. 14). The biological meaning of replication is the exact transfer of hereditary information from the parent molecule to the daughter one.
Rice. 13 ... Despiralization of a DNA molecule using an enzyme
1
Rice. 14 ... Replication - the formation of two DNA molecules from one DNA molecule: 1 - daughter DNA molecule; 2 - maternal (parental) DNA molecule.
The DNA polymerase enzyme can move along the DNA strand only in the 3 '-> 5' direction. Since the complementary strands in the DNA molecule are directed in opposite directions, and the DNA polymerase enzyme can move along the DNA strand only in the 3 '-> 5' direction, the synthesis of new strands is antiparallel ( anti-parallelism).
Place of DNA localization... DNA is contained in the nucleus of the cell, in the matrix of mitochondria and chloroplasts.
The amount of DNA in a cell is constant and amounts to 6.6x10 -12 g.
DNA functions:
Storage and transmission in a number of generations of genetic information to molecules and - RNA;
Structural. DNA is the structural basis of chromosomes (a chromosome is 40% DNA).
Species specificity of DNA... The nucleotide composition of DNA serves as a species criterion.
RNA, structure and function.
General structure.
RNA is a linear biopolymer consisting of one polynucleotide chain. Distinguish between primary and secondary structures of RNA. The primary structure of RNA is a single-stranded molecule, and the secondary structure is in the form of a cross and is characteristic of t-RNA.
Polymerity of the RNA molecule... An RNA molecule can range from 70 nucleotides to 30,000 nucleotides. The nucleotides that make up the RNA are as follows: adenyl (A), guanyl (G), cytidyl (C), uracil (U). In the RNA, the thymine nucleotide is substituted for the uracil (U) nucleotide.
RNA nucleotide structure.
The RNA nucleotide includes 3 links:
nitrogenous base (adenine, guanine, cytosine, uracil);
monosaccharide - ribose (ribose contains oxygen at each carbon atom);
the remainder of phosphoric acid.
RNA synthesis method - transcription... Transcription, like replication, is a matrix synthesis reaction. The matrix is a DNA molecule. The reaction proceeds according to the principle of complementarity on one of the DNA strands (Fig. 15). The transcription process begins with the despiralization of the DNA molecule at a specific site. On the transcribed DNA strand there is promoter - a group of DNA nucleotides, with which the synthesis of an RNA molecule begins. An enzyme attaches to the promoter RNA polymerase... The enzyme activates the transcription process. According to the principle of complementarity, the nucleotides coming from the cytoplasm of the cell to the transcribed DNA strand are completed. RNA polymerase activates the alignment of nucleotides into one strand and the formation of an RNA molecule.
In the process of transcription, four stages are distinguished: 1) binding of RNA polymerase with a promoter; 2) the beginning of the synthesis (initiation); 3) elongation - the growth of the RNA chain, that is, there is a sequential attachment of nucleotides to each other; 4) termination - completion of the synthesis of i-RNA.
Rice. 15 ... Transcription scheme
1 - DNA molecule (double strand); 2 - RNA molecule; 3 – codons; 4 - promoter.
In 1972, American scientists - virologist H.M. Temin and molecular biologist D. Baltimore discovered reverse transcription using viruses in tumor cells. Reverse transcription- rewriting genetic information from RNA to DNA. The process takes place with the help of an enzyme reverse transcriptase.
RNA types by function
Informational, or messenger RNA (i-RNA, or m-RNA) transfers genetic information from the DNA molecule to the site of protein synthesis - to the ribosome. It is synthesized in the nucleus with the participation of the RNA polymerase enzyme. It makes up 5% of all types of RNA in a cell. i-RNA comprises from 300 nucleotides to 30,000 nucleotides (the longest chain among RNA).
Transport RNA (t-RNA) transports amino acids to the site of protein synthesis, the ribosome. It has the shape of a cross (Fig. 16) and consists of 70 - 85 nucleotides. Its amount in the cell is 10-15% of the RNA of the cell.
Rice. 16. Scheme of the structure of t-RNA: A – D - pairs of nucleotides connected by means of hydrogen bonds; D - the place of attachment of the amino acid (acceptor site); E - anticodon.
3. Ribosomal RNA (r-RNA) is synthesized in the nucleolus and is part of the ribosomes. Includes approximately 3000 nucleotides. Makes 85% of the RNA of the cell. This type of RNA is found in the nucleus, in ribosomes, on the endoplasmic reticulum, in chromosomes, in the mitochondrial matrix, and also in plastids.
Fundamentals of Cytology. Solving typical tasks
Problem 1
How many thymine and adenine nucleotides are contained in DNA if 50 cytosine nucleotides are found in it, which is 10% of all nucleotides.
Solution. According to the rule of complementarity in the double strand of DNA, cytosine is always complementary to guanine. 50 cytosine nucleotides make up 10%, therefore, according to Chargaff's rule, 50 guanine nucleotides also make up 10%, or (if C = 10%, then ∑G = 10%).
The sum of the C + G nucleotide pair is 20%
The sum of a pair of nucleotides T + A = 100% - 20% (C + G) = 80%
In order to find out how many thymine and adenine nucleotides are contained in DNA, you need to make the following proportion:
50 cytosine nucleotides → 10%
X (T + A) → 80%
X = 50x80: 10 = 400 pieces
According to Chargaff's rule ∑А = ∑Т, therefore ∑А = 200 and ∑Т = 200.
Answer: the number of thymine, as well as adenine nucleotides in DNA, is 200.
Task 2
Thymine nucleotides in DNA make up 18% of the total number of nucleotides. Determine the percentage of the remaining types of nucleotides contained in the DNA.
Solution.∑T = 18%. According to Chargaff's rule T = ∑A, therefore, the share of adenine nucleotides also accounts for 18% (∑A = 18%).
The sum of the T + A nucleotide pair is 36% (18% + 18% = 36%). For a couple of nucleotides, GiC accounts for: G + C = 100% –36% = 64%. Since guanine is always complementary to cytosine, their content in DNA will be equal,
that is, ∑ Г = ∑Ц = 32%.
Answer: the content of guanine, like cytosine, is 32%.
Problem 3
20 cytosine DNA nucleotides make up 10% of the total nucleotides. How many adenine nucleotides are there in a DNA molecule?
Solution. In a double strand of DNA, the amount of cytosine is equal to the amount of guanine, therefore, their sum is: C + G = 40 nucleotides. Find the total number of nucleotides:
20 cytosine nucleotides → 10%
X (total number of nucleotides) → 100%
X = 20x100: 10 = 200 pieces
A + T = 200 - 40 = 160 pieces
Since adenine is complementary to thymine, their content will be equal,
i.e. 160 pieces: 2 = 80 pieces, or ∑A = ∑T = 80.
Answer: DNA molecule contains 80 adenine nucleotides.
Problem 4
Add the nucleotides of the right DNA chain if the nucleotides of its left chain are known: AGA - TAT - GTG - TCT
Solution. The construction of the right DNA strand according to a given left strand is carried out according to the principle of complementarity - a strict correspondence of nucleotides to each other: adenonic - thymine (A – T), guanine - cytosine (G – C). Therefore, the nucleotides of the right DNA strand should be as follows: TCT - ATA - CAC - AGA.
Answer: nucleotides of the right DNA chain: TCT - ATA - TsAC - AGA.
Problem 5
Write down the transcription if the transcribed DNA strand has the following nucleotide order: AGA - TAT - THT - TCT.
Solution... The i-RNA molecule is synthesized according to the principle of complementarity on one of the strands of the DNA molecule. We know the order of the nucleotides in the transcribed DNA strand. Therefore, it is necessary to build a complementary strand of i-RNA. It should be remembered that instead of thymine, uracil is included in the RNA molecule. Hence:
DNA chain: AGA - TAT - THT - TCT
The i-RNA chain: UCU - AUA –ACA –AGA.
Answer: the sequence of nucleotides of i-RNA is as follows: UCU - AUA - ACA –AGA.
Problem 6
Write down the reverse transcription, i.e. build a fragment of a double-stranded DNA molecule from the proposed fragment of i-RNA, if the chain of i-RNA has the following nucleotide sequence:
ГЦГ - АТС - УУУ - УЦГ - ЦГУ - АГУ - АНА
Solution. Reverse transcription is the synthesis of a DNA molecule based on the m-RNA genetic code. The m-RNA encoding a DNA molecule has the following nucleotide order: GCG - ACA - UUU - UCG - TsGU - AGU - AGA. The DNA chain complementary to it: CHC - THT - AAA - AGC - HCA - TCA - TCT. The second DNA strand: GCG – ACA – TTT – TCG – CGT – AGT – AGA.
Answer: as a result of reverse transcription, two chains of the DNA molecule were synthesized: CGC - TGT - AAA - AGC - HCA - TCA and GCG – ACA – TTT – TCG – CGT – AGT – AGA.
Genetic code. Protein biosynthesis.
Gene- a section of a DNA molecule containing genetic information about the primary structure of one specific protein.
Exon-intron structure of the geneeukaryotes
promoter- a piece of DNA (up to 100 nucleotides in length) to which the enzyme is attached RNA polymerase required for transcription;
2) regulatory area- zone influencing gene activity;
3) structural part of a gene- genetic information about the primary structure of the protein.
A DNA nucleotide sequence that carries genetic information about the primary structure of a protein - exon... They are also part of i-RNA. DNA nucleotide sequence that does not carry genetic information about the primary structure of the protein - intron... They are not part of i-RNA. In the course of transcription with the help of special enzymes, intron copies are excised from i-RNA and the exon copies are stitched together during the formation of the i-RNA molecule (Fig. 20). This process is called splicing.
Rice. 20 ... Splicing scheme (formation of mature i-RNA in eukaryotes)
Genetic code - the system of the sequence of nucleotides in the DNA molecule, or m-RNA, which corresponds to the sequence of amino acids in the polypeptide chain.
Properties of the genetic code:
Tripletness(ACA - GTG - GTsG ...)
The genetic code is triplet, since each of the 20 amino acids is encoded by a sequence of three nucleotides ( triplet, codon).
There are 64 types of nucleotide triplets (4 3 = 64).
Unambiguity (specificity)
The genetic code is unambiguous, since each individual triplet of nucleotides (codon) encodes only one amino acid, or one codon always corresponds to one amino acid (Table 3).
Plurality (redundancy, or degeneracy)
One and the same amino acid can be encoded by several triplets (from 2 to 6), since there are 20 protein-forming amino acids and 64 triplets.
Continuity
The reading of genetic information occurs in one direction, from left to right. If there is a loss of one nucleotide, then during reading its place will be taken by the nearest nucleotide from the neighboring triplet, which will lead to a change in genetic information.
Versatility
The genetic code is characteristic of all living organisms, and the same triplets encode the same amino acid in all living organisms.
Has start and terminal triplets(starting triplet - AUG, terminal triplets UAA, UGA, UAG). These types of triplets do not code for amino acids.
Non-overlapping (discreteness)
The genetic code is non-overlapping, since the same nucleotide cannot be simultaneously included in two adjacent triplets. Nucleotides can belong to only one triplet, and if you rearrange them into another triplet, then the genetic information will change.
Table 3 - Table of the genetic code
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Codon bases |
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Note: Abbreviated amino acid names are given in accordance with international terminology.
Protein biosynthesis
Protein biosynthesis - type of plastic exchange substances in the cell, occurring in living organisms under the action of enzymes. Protein biosynthesis is preceded by matrix synthesis reactions (replication - DNA synthesis; transcription - RNA synthesis; translation - assembly of protein molecules on ribosomes). In the process of protein biosynthesis, 2 stages are distinguished:
transcription
broadcast
During transcription, the genetic information contained in the DNA found in the chromosomes of the nucleus is transferred to the RNA molecule. Upon completion of the transcription process, m-RNA enters the cytoplasm of the cell through pores in the nuclear membrane, is located between 2 ribosome subunits and participates in protein biosynthesis.
Translation is the process of translating a genetic code into a sequence of amino acids. The translation is carried out in the cytoplasm of the cell on the ribosomes, which are located on the surface of the EPS (endoplasmic reticulum). Ribosomes are spherical granules with an average diameter of 20 nm, consisting of large and small subunits. The i-RNA molecule is located between the two subunits of the ribosome. The translation process involves amino acids, ATP, i-RNA, t-RNA, the enzyme amino-acyl t-RNA synthetase.
Codon- a section of a DNA molecule, or m-RNA, consisting of three sequentially located nucleotides, encoding one amino acid.
Anticodon- a region of the t-RNA molecule, consisting of three consecutive nucleotides and complementary to the codon of the i-RNA molecule. Codons are complementary to the corresponding anticodons and are connected to them using hydrogen bonds (Fig. 21).
Protein synthesis starts with start codon AUG... From him ribosome
moves along the i-RNA molecule, triplet by triplet. Amino acids come from a genetic code. Their insertion into the polypeptide chain on the ribosome occurs with the help of t-RNA. The primary structure of t-RNA (strand) is transformed into a secondary structure resembling a cross in shape, and at the same time the complementarity of nucleotides is preserved in it. In the lower part of the t-RNA, there is an acceptor site to which an amino acid is attached (Fig. 16). The amino acid is activated by an enzyme aminoacyl t-RNA synthetase... The essence of this process is that this enzyme interacts with an amino acid and with ATP. In this case, a triple complex is formed, represented by this enzyme, amino acid and ATP. The amino acid is enriched with energy, activated, and acquires the ability to form peptide bonds with a neighboring amino acid. Without the activation process of the amino acid, the polypeptide chain cannot be formed from amino acids.
The opposite, upper part of the t-RNA molecule contains a triplet of nucleotides anticodon, with the help of which t-RNA is attached to its complementary codon (Fig. 22).
The first t-RNA molecule, with an activated amino acid attached to it, attaches with its anticodon to the m-RNA codon, and one amino acid appears in the ribosome. Then the second t-RNA is attached with its anticodon to the corresponding codon of the m-RNA. In this case, there are already 2 amino acids in the ribosome, between which a peptide bond is formed. The first t-RNA leaves the ribosome as soon as it donates an amino acid to the polypeptide chain on the ribosome. Then the third amino acid is attached to the dipeptide, the third t-RNA brings it, etc. Protein synthesis stops at one of the terminal codons - UAA, UAH, UGA (Fig. 23).
1 - i-RNA codon; codonsUCG -UCH; CUA -CUA; CGU -CSU;
2 - t-RNA anticodon; anticodon GAT - GAT
Rice. 21 ... Translation phase: the m-RNA codon is attracted to the t-RNA anticodon by the corresponding complementary nucleotides (bases)
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 the DNA is located. In a human cell, there are many organelles that perform various functions. DNA is located in the nucleus. The nucleus is a small organelle that 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 made up of building blocks - nucleotides. There are 4 types of nucleotides - adenine (A), thymine (T), guanine (G) and cytosine (C). The nucleotide chain schematically looks like this: GGAATCTAAG ... This is the sequence of nucleotides that 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 stick together and twist into a spiral? This phenomenon is due to the property of complementarity. Complementarity means that only certain nucleotides (complementary) can be located opposite each other in two strands. So, opposite to adenine there is always thymine, and opposite to guanine there is always only cytosine. Thus, guanine is complementary to cytosine, and adenine is complementary to thymine. Such pairs of nucleotides facing each other in different strands are also called complementary.
It can be schematically depicted 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.
DNA packaging, how does a DNA strand become a chromosome?
Why are these DNA nucleotide chains also twisting around each other? Why is this needed? The fact is that the number of nucleotides is enormous and it takes a lot of space to accommodate such long chains. For this reason, there is a spiral twisting of two DNA strands around the other. This phenomenon is called spiralization. As a result of spiralization, DNA strands are shortened by 5-6 times.Some DNA molecules are actively used by the body, while others are rarely used. Such rarely used DNA molecules, in addition to spiralization, undergo an even more compact "packing". This compact package is called supercoiling and shortens the DNA strand by 25-30 times!
How does DNA strand packing take place?
For supercoiling, histone proteins are used, which have the appearance and structure of a rod or thread spool. Spiralized DNA strands are wound on these "coils" - histone proteins. Thus, the long thread becomes very compactly packed and takes up very little space. If it is necessary to use this or that DNA molecule, the process of "unwinding" occurs, that is, the DNA strand is "unwound" from the "coil" - a histone protein (if it was wound on it) and unwound from a spiral into two parallel chains. And when the DNA molecule is in such an untwisted state, the necessary genetic information can be read from it. Moreover, the reading of genetic information occurs only from untwisted DNA strands!
The set of supercoiled chromosomes is called heterochromatin, and chromosomes available for reading information - euchromatin.
What are genes, what is their relationship to DNA?
Now let's look at what genes are. It is known that there are genes that determine the blood type, color of eyes, 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 located in a strictly defined combination. Location in a strictly defined area of DNA means that a specific gene has been assigned its place, and it is impossible to change this place. It is appropriate to make such a comparison: a person lives on a certain street, in a certain house and apartment, and a person cannot arbitrarily 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 cannot become more or less. For example, the gene for insulin production is 60 base pairs in length; the gene encoding the production of the hormone oxytocin - of 370 base pairs. The strict sequence of nucleotides is unique for each gene and is strictly defined. For example, the AATTAATA sequence is a fragment of a gene that codes for insulin production. In order to obtain insulin, just such a 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 is the unique combination of a certain number of nucleotides, standing in "its place" - this is gene.
In addition to genes, the so-called "non-coding sequences" are located in the DNA chain. Such non-coding nucleotide sequences regulate the work of genes, help the spiralization of chromosomes, and mark the beginning and end of a gene. However, to date, the role of most non-coding sequences remains unclear.
What is a chromosome? Sex chromosomes
The collection of an individual's genes is called a genome. Naturally, it is impossible to fit the entire genome into one DNA. The genome is broken down into 46 pairs of DNA molecules. One pair of DNA molecules is called a chromosome. So it is these chromosomes that a person has 46 pieces. Each chromosome carries a strictly defined set of genes, for example, chromosome 18 contains genes that code for eye color, etc. Chromosomes differ from each other in length and shape. The most common forms are X or Y, but there are others as well. A person has two chromosomes of the same shape, which are called paired (pairs). Due to such differences, all paired chromosomes are numbered - there are 23 pairs of them. This means that there is a pair of chromosomes # 1, pair # 2, # 3, etc. Each gene responsible for a particular trait is located on the same chromosome. In modern manuals for specialists, the localization of the gene can be indicated, for example, as follows: 22 chromosome, long arm.What are the differences between chromosomes?
How else are chromosomes different? What does the term long shoulder mean? Let's take chromosomes of 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 crossing point, some ends are longer, others, respectively, are shorter. Such long ends are usually called the long arm of the chromosome, and the short ones, respectively, are called the short arm. In chromosomes of the Y form, long shoulders occupy most of them, and short ones are very small (they are not even indicated in the schematic image). The size of chromosomes varies: the largest are chromosomes of pairs # 1 and # 3, the smallest are chromosomes of pairs # 17, # 19.
In addition to the shape and size, chromosomes differ in their functions. Of the 23 couples, 22 are somatic and 1 are sexual. What does it mean? Somatic chromosomes determine all the external signs of an individual, features of his behavioral reactions, hereditary psychotype, that is, all the traits and characteristics of each individual person. A pair of sex chromosomes determines the sex of a person: 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 do occur, and then genetic diseases are detected in people. For example, when there are three chromosomes on 21 pairs of chromosomes instead of two, a person is born with Down syndrome.There are many smaller “breakdowns” of the genetic material that do not lead to the onset of disease, but, on the contrary, impart good properties. All "breakdowns" of genetic material are called mutations. Mutations leading to disease or deterioration of the body's properties are considered negative, and mutations leading to the formation of new beneficial properties are considered positive.
However, in relation to most of the diseases that people suffer today, it is not a disease that is inherited, but only a predisposition. For example, the father of a child assimilates 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 mellitus.
Today, the so-called predicative medicine. Within the framework of this medical practice, predispositions are identified in a person (based on the identification of the corresponding genes), and then recommendations are given to him - what diet to follow, how to correctly alternate the mode of work and rest so as not to get sick.
How to read information encoded in DNA?
How can you read the information contained in DNA? How does her 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, and DNA is deprived of this ability (it can only be in the nucleus). The encoded information is used in the cell itself. So, RNA is the carrier of encoded information from the nucleus to the cell.How is RNA synthesized, how is protein synthesized with the help of RNA?
The DNA strands, from which it is necessary to "read" information, unwind, a special enzyme - "builder" approaches them and synthesizes a complementary RNA strand in parallel to the DNA strand. The RNA molecule also consists of 4 types of nucleotides - adenine (A), uracil (Y), 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 attaches C, etc. Thus, from each active gene during transcription, a template is formed - a copy of RNA that can pass through the nuclear membrane. How is the synthesis of a protein encoded by a specific gene occurs?
After leaving the nucleus, RNA enters the cytoplasm. Already in the cytoplasm, RNA can be, as a matrix, embedded in special enzyme systems (ribosomes), which can synthesize, guided by RNA information, the corresponding protein amino acid sequence. As you know, a protein molecule is composed of amino acids. How does the ribosome manage to find out which amino acid needs to be attached to the growing protein chain? This is done on the basis of a triplet code. Triplet code means that the sequence of three nucleotides of the RNA chain ( triplet, for example, HGH) encode one 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 attached next as it reads information in RNA. When a chain of amino acids is formed, it takes on a certain spatial form and becomes a protein capable of performing the enzymatic, building, hormonal and other functions assigned to it.Protein for any living organism is a product of a gene. It is proteins that determine all the various properties, qualities and external manifestations of genes.
The DNA molecule consists of two strands that form a double helix. Its structure was first deciphered by Francis Crick and James Watson in 1953.
At first, the DNA molecule, consisting of a pair of nucleotide chains twisted around each other, raised questions about why it has exactly this shape. Scientists have called this phenomenon complementarity, which means that only certain nucleotides can be located in its strands opposite each other. For example, adenine is always opposite thymine, and guanine is opposite cytosine. These nucleotides of the DNA molecule are called complementary.
This is schematically depicted as follows:
T - A
C - G
These pairs form a chemical nucleotide bond that determines the order in which the amino acids are arranged. In the first case, it is slightly weaker. The connection between C and G is stronger. Non-complementary nucleotides do not form pairs with each other.
About the structure
So, the structure of the DNA molecule is special. It has such a shape for a reason: the fact is that the number of nucleotides is very large, and a lot of space is needed to accommodate long chains. It is for this reason that spiral twisting is inherent in chains. This phenomenon is called spiralization, it allows the filaments to shorten about five to six times.
The body uses some molecules of this kind very actively, others rarely. The latter, in addition to spiralization, also undergo such "compact packaging" as supercoiling. And then the length of the DNA molecule decreases 25-30 times.
What is the "packing" of a molecule?
In the process of supercoiling, histone proteins are involved. They have the structure and appearance of a thread spool or rod. Spiralized threads are wound on them, which immediately become "compactly packed" and take up little space. When it becomes necessary to use this or that thread, it is unwound from a coil, for example, of a histone protein, and the spiral unwinds into two parallel chains. When the DNA molecule is in this state, the necessary genetic data can be read from it. However, there is one condition. Obtaining information is possible only if the structure of the DNA molecule is untwisted. The chromosomes available for reading are called euchromatins, and if they are supersypiralized, then these are already heterochromatins.
Nucleic acids
Nucleic acids, like proteins, are biopolymers. The main function is the storage, implementation and transmission of hereditary (genetic information). They are of two types: DNA and RNA (deoxyribonucleic and ribonucleic). Monomers in them are nucleotides, each of which contains a phosphoric acid residue, a five-carbon sugar (deoxyribose / ribose), and a nitrogenous base. The DNA code includes 4 types of nucleotides - adenine (A) / guanine (G) / cytosine (C) / thymine (T). They differ in the nitrogenous base they contain.
In a DNA molecule, the number of nucleotides can be enormous - from several thousand to tens and hundreds of millions. Such giant molecules can be viewed through an electron microscope. In this case, it will be possible to see a double strand of polynucleotide strands, which are interconnected by hydrogen bonds of the nitrogenous bases of nucleotides.
Research
In the course of research, scientists have found that the types of DNA molecules in different living organisms differ. It was also found that guanine of one chain can bind only with cytosine, and thymine - with adenine. The arrangement of the nucleotides of one strand strictly corresponds to the parallel one. Due to this complementarity of polynucleotides, the DNA molecule is capable of duplication and self-reproduction. But first, the complementary chains diverge under the influence of special enzymes that destroy paired nucleotides, and then synthesis of the missing chain begins in each of them. This is due to the free nucleotides present in large quantities in each cell. As a result, instead of the "parent molecule", two "daughter" molecules are formed, identical in composition and structure, and the DNA code becomes the original one. This process is a precursor to cell division. It ensures the transfer of all hereditary data from mother cells to daughter cells, as well as to all subsequent generations.
How is the gene code read?
Today, not only the mass of the DNA molecule is calculated - it is possible to find out more complex data that were previously not available to scientists. For example, you can read information about how the body uses its own cell. Of course, at first these information are encoded and have the form of some kind of matrix, and therefore it must be transported to a special carrier, which is RNA. Ribonucleic acid is able to infiltrate into the cell through the nuclear membrane and read the encoded information inside. Thus, RNA is a carrier of hidden data from the nucleus to the cell, and it differs from DNA in that it contains ribose instead of deoxyribose, and uracil instead of thymine. In addition, RNA is single stranded.
RNA synthesis
A deep analysis of DNA showed that after RNA leaves the nucleus, it enters the cytoplasm, where it can be incorporated as a matrix into ribosomes (special enzyme systems). Based on the information received, they can synthesize the appropriate sequence of protein amino acids. The ribosome learns from the triplet code what kind of organic compound must be attached to the forming protein chain. Each amino acid has its own specific triplet, which encodes it.
After the formation of the chain is completed, it acquires a specific spatial form and turns into a protein capable of performing its hormonal, building, enzymatic and other functions. For any organism, it is a gene product. It is from it that all kinds of qualities, properties and manifestations of genes are determined.
Genes
First of all, sequencing processes were developed in order to obtain information about how many genes the structure of a DNA molecule has. And, although the research has allowed scientists to make great headway in this matter, it is not yet possible to know the exact number of them.
A few years ago, it was assumed that DNA molecules contain approximately 100 thousand genes. A little later, the figure decreased to 80 thousand, and in 1998 geneticists announced that only 50 thousand genes are present in one DNA, which is only 3% of the entire length of DNA. But the latest conclusions of geneticists were amazed. Now they claim that the genome includes 25-40 thousand of these units. It turns out that only 1.5% of chromosomal DNA is responsible for coding proteins.
The research did not stop there. A parallel team of genetic engineering specialists found that the number of genes in one molecule is exactly 32 thousand. As you can see, it is still impossible to get a definitive answer. There are too many contradictions. All researchers rely only on their own results.
Has there been an evolution?
Despite the fact that there is no evidence of the evolution of the molecule (since the structure of the DNA molecule is fragile and small in size), scientists have made one suggestion. Based on laboratory data, they voiced a version of the following content: at the initial stage of its appearance, the molecule looked like a simple self-replicating peptide, which included up to 32 amino acids found in ancient oceans.
After self-replication, thanks to the forces of natural selection, the molecules acquired the ability to protect themselves from the influence of external elements. They began to live longer and reproduce in large numbers. The molecules that found themselves in the lipid bladder got every chance for self-reproduction. As a result of a series of successive cycles, lipid bubbles acquired the form of cell membranes, and then - all known particles. It should be noted that today any part of the DNA molecule is a complex and clearly functioning structure, all the features of which have not yet been fully studied by scientists.
Modern world
Recently, Israeli scientists have developed a computer that can perform trillions of operations per second. Today it is the fastest car on Earth. The whole secret is that the innovative device is powered by DNA. The professors say that in the short term, such computers will even be able to generate power.
Specialists from the Weizmann Institute in Rehovot (Israel) a year ago announced the creation of a programmable molecular computer consisting of molecules and enzymes. They replaced silicon microchips with them. By now, the team is still moving forward. Now only one DNA molecule can provide the computer with the necessary data and provide the necessary fuel.
Biochemical "nanocomputers" are not fiction, they already exist in nature and are manifested in every living being. But often they are not controlled by humans. A person cannot yet operate on the genome of any plant in order to calculate, say, the number "pi".
The idea of using DNA to store / process data first hit the bright minds of scientists in 1994. It was then that a molecule was used to solve a simple math problem. Since then, a number of research groups have proposed various projects related to DNA computers. But here all attempts were based only on the energy molecule. You cannot see such a computer with the naked eye; it looks like a transparent solution of water in a test tube. There are no mechanical parts in it, but only trillions of biomolecular devices - and that's just in one drop of liquid!
Human DNA
What kind of human DNA, people became aware in 1953, when scientists were for the first time able to demonstrate to the world a double-stranded model of DNA. For this, Kirk and Watson received the Nobel Prize, since this discovery became fundamental in the 20th century.
Over time, of course, they proved that a structured human molecule can look not only like in the proposed version. After a more detailed analysis of DNA, the A-, B- and left-handed forms of Z- were discovered. Form A- is often an exception, since it is formed only if there is a lack of moisture. But this is possible only in laboratory studies, for the natural environment it is abnormal, in a living cell such a process cannot occur.
The B- shape is classic and is known as a double right-handed chain, but the Z- shape is not only twisted in the opposite direction, to the left, but also has a more zigzag appearance. Scientists have also identified the G-quadruplex form. There are not 2, but 4 threads in its structure. According to geneticists, this form occurs in those areas where there is an excess amount of guanine.
Artificial DNA
Artificial DNA already exists today, which is an identical copy of the real one; it perfectly repeats the structure of the natural double helix. But, unlike the primordial polynucleotide, in the artificial one there are only two additional nucleotides.
Since dubbing was created on the basis of information obtained in the course of various studies of real DNA, it can also be copied, self-replicating and evolving. Experts have been working on the creation of such an artificial molecule for about 20 years. The result is an amazing invention that can use the genetic code in the same way as natural DNA.
To the four available nitrogenous bases, genetics added an additional two, which they created by the method of chemical modification of natural bases. Unlike natural DNA, artificial DNA is quite short. It contains only 81 base pairs. However, it also reproduces and evolves.
Replication of an artificially obtained molecule takes place thanks to the polymerase chain reaction, but so far this does not happen on its own, but through the intervention of scientists. They independently add the necessary enzymes to the mentioned DNA, placing it in a specially prepared liquid medium.
Final result
The process and the final result of DNA development can be influenced by various factors, for example, mutations. This necessitates the study of samples of matter, so that the result of the analyzes is reliable and reliable. An example is the paternity test. But it is good news that such incidents as mutation are rare. Nevertheless, samples of matter are always rechecked in order to obtain more accurate information based on the analysis.
Plant DNA
Thanks to high technology sequencing (HTS), a revolution has been made in the field of genomics - the extraction of DNA from plants is also possible. Of course, obtaining high-quality DNA molecular weight from plant material causes some difficulties due to the large number of copies of DNA mitochondria and chloroplasts, as well as the high level of polysaccharides and phenolic compounds. To isolate the structure we are considering, in this case, a variety of methods are used.
Hydrogen bond in DNA
The hydrogen bond in the DNA molecule is responsible for the electromagnetic attraction created between a positively charged hydrogen atom, which is attached to an electronegative atom. This dipole interaction does not meet the chemical bond criterion. But it can be realized intermolecularly or in different parts of the molecule, that is, intramolecularly.
The hydrogen atom is attached to the electronegative atom, which is the donor of this bond. The electronegative atom can be nitrogen, fluorine, oxygen. He - through decentralization - attracts an electron cloud from the hydrogen core and makes the hydrogen atom charged (partially) positively. Since the size of H is small compared to other molecules and atoms, the charge is also small.
DNA decoding
Before decoding a DNA molecule, scientists first take a huge number of cells. For the most accurate and successful work, they need about a million. The results obtained in the course of the study are constantly compared and recorded. Today, genome decoding is no longer a rarity, but an affordable procedure.
Of course, deciphering the genome of one cell is an inappropriate exercise. The data obtained in the course of such studies are not of any interest to scientists. But it is important to understand that all currently existing decoding methods, despite their complexity, are not effective enough. They will only allow 40-70% of the DNA to be read.
However, Harvard professors recently announced a way to decipher 90% of the genome. The technique is based on the addition of primer molecules to the isolated cells, with the help of which DNA replication begins. But even this method cannot be considered successful; it still needs to be improved before being openly used in science.
Nucleic acid molecules of all types of living organisms are long unbranched polymers of mononucleotides. The role of the bridge between the nucleotides is played by the 3 ", 5" -phosphodiester bond connecting the 5 "-phosphate of one nucleotide and the 3" -hydroxyl residue of ribose (or deoxyribose) of the next one. In this regard, the polynucleotide chain turns out to be polar. At one end, a free 5 "-phosphate group remains, at the other end, a 3" -OH group.
DNA is like proteins, has primary, secondary and tertiary structures.
Primary DNA structure ... This structure defines the information encoded in it, representing the sequence of alternation of deoxyribonucleotides in the polynucleotide chain.
A DNA molecule consists of two spirals having the same axis and opposite directions. The sugar-phosphate backbone is located along the periphery of the double helix, and the nitrogenous bases are inside. The skeleton contains covalent phosphodiester bonds, and both spirals between the bases are connected hydrogen bonds and hydrophobic interactions.
These connections were first discovered and studied by E. Chargaff in 1945 and received the name principle of complementarity, and the features of the formation of hydrogen bonds between bases are called Chargaff rules:
- the purine base always binds to the pyrimidine base: adenine - with thymine (A®T), guanine - with cytosine (G®C);
- the molar ratio of adenine to thymine and guanine to cytosine is 1 (A = T, or A / T = 1 and G = C, or G / C = 1);
- the sum of residues A and G is equal to the sum of residues T and C, i.e. A + G = T + C;
- in DNA isolated from different sources, the ratio (G + C) / (A + T), called the coefficient of specificity, is not the same.
Chargaff's rules are based on the fact that adenine makes two bonds with thymine, and guanine makes three bonds with cytosine:
Based on Chargaff's rules, you can imagine the double-stranded structure of DNA, which is shown in the figure.
A-form B-form
A-adenine, G-guanine, C-cytosine, T-thymine
Schematic representation of a double-stranded
DNA molecules
Secondary structure of DNA ... In accordance with the model proposed in 1953 by J. Watson and F. Crick, the secondary structure of DNA is double-stranded right handed helix from complementary to each other antiparallel polynucleotide chains.
For the secondary structure of DNA, two structural features of the nitrogenous bases of nucleotides are decisive. The first is the presence of groups capable of forming hydrogen bonds. The second feature is that the pairs of complementary bases A-T and G-C are the same not only in size, but also in shape.
Due to the ability of nucleotides to mate, a rigid, well-stabilized double-stranded structure is formed. The main elements and parametric characteristics of such a structure are clearly shown in the figure.
Based on a thorough analysis of the X-ray diffraction patterns of the isolated DNA, it has been established that the double helix of DNA can exist in the form of several forms (A, B, C, Z, etc.). These forms of DNA differ in the diameter and pitch of the spiral, the number of base pairs in a turn, and the angle of inclination of the plane of the bases with respect to the axis of the molecule.
Tertiary structure of DNA. In all living organisms, double-stranded DNA molecules are tightly packed to form complex three-dimensional structures. Double-stranded DNA of prokaryotes, which have a circular covalently closed shape, form left (-) super spirals... The tertiary structure of DNA of eukaryotic cells is also formed by supercoiling, but not free DNA, but its complexes with chromosome proteins (histone proteins of the classes H1, H2, H3, H4 and H5).
Several levels can be distinguished in the spatial organization of chromosomes. First level- nucleosomal. As a result of the nucleosomal organization of chromatin, the DNA double helix 2 nm in diameter acquires a diameter of 10-11 nm and is shortened by about 7 times.
Second level the spatial organization of chromosomes is the formation of a chromatin fibril from a nucleosome strand with a diameter of 20-30 nm (a decrease in the linear dimensions of DNA by another 6-7 times).
Tertiary level the organization of chromosomes is due to the packing of chromatin fibrils into loops. Non-histone proteins are involved in the formation of loops. The region of DNA corresponding to one loop contains from 20,000 to 80,000 base pairs. As a result of such packaging, the linear dimensions of DNA are reduced by about 200 times. The loop-shaped domain organization of DNA, called interphase chromonema, can undergo further compaction, the degree of which varies depending on the phase of the cell cycle.
Self-reproduction of genetic material. Replication.
Principles of recording genetic information. Genetic code and its properties.
Genetic code- inherent in all living organisms, a method of encoding the amino acid sequence of proteins using a sequence of nucleotides. In nature, 20 different amino acids are used to build proteins. Each protein is a chain or several chains in a strictly defined sequence. This sequence determines the structure of the protein, and hence its properties. The set of amino acids is universal for almost all living organisms.
Gene properties. code:
Triplet - a combination of 3 nucleotides
Continuity - there are no punctuation marks between triplets, i.e. information is read continuously
Non-overlapping - the same nucleotide cannot be part of several triplets at the same time
Specificity - a certain codon corresponds to only 1 amino acid
Degeneracy - several codons can correspond to the same amino acid
Versatility - the genetic code works the same in organisms of different levels of complexity
Immunity
In the process of replication of genetic material, hydrogen bonds between nitrogenous bases are broken, and two DNA strands are formed from the double helix. Each of them becomes a template for the synthesis of another complementary DNA strand. The latter, through a hydrogen bond, combines with the template DNA. So, any daughter DNA molecule consists of one old and one new polynucleotide chain. As a result, the daughter cells receive the same genetic information as the parent cells. The maintenance of such a situation is provided by a self-correction mechanism carried out by DNA polymerase. The ability of genetic material, DNA, to reproduce itself (replicate) underlies the reproduction of living organisms, the transfer of hereditary properties from generation to generation and the development of a multicellular organism from a zygote.
Uncorrected changes in the chemical structure of genes reproduced in successive replication cycles and manifested in offspring in the form of new variants of traits are called gene mutations.
Changes in the structure of DNA can be divided into 3 groups: 1. Replacement of some bases with others.
2. shift of the reading frame with a change in the number of nucleotide pairs in the composition of the gene.
3. reordering of nucleotide sequences within a gene.
1. Replacing some bases with others. May occur accidentally or under the influence of specific chemical agents. If the altered form of the base remains unnoticed during repair, then during the next cycle of replication it can attach another nucleotide to itself.
Another reason may be the erroneous inclusion in the synthesized DNA strand of a nucleotide carrying a modified form of the base or its analogue. If this error goes unnoticed during repair, then the changed base is included in the replication process, which leads to the replacement of one pair for another.
As a result, a new triplet is formed in DNA. If this triplet encodes the same amino acid, then the changes will not affect the structure of the peptide (degeneracy of the genetic code). If the newly formed triplet encodes a different amino acid, the structure of the peptide chain and the properties of the protein change.
2. shift of the reading frame. These mutations occur due to the loss (deletion) or insertion of one or more pairs of complementary nucleotides into the DNA nucleotide sequence. This may be due to exposure of the genetic material to certain chemicals (acridine compounds). A large number of mutations occur due to the inclusion of movable genetic elements - transposons - into DNA. Errors during recombination in case of unequal intragenic crossing over may also be the reason.
With such mutations, the meaning of the biological information recorded in this DNA changes.
3... changing the order of nucleotide sequences. This type of mutation occurs due to a 180ᵒ rotation of the DNA region (inversion). This is due to the fact that the DNA molecule forms a loop within which replication goes in the wrong direction. Within the inverted region, information reading is disrupted and the amino acid sequence of the protein is disrupted.
Causes:- unequal crossing over between homologous chromosomes
Intrachromosomal crossing over
Chromosome breaks
Breaks followed by joining of chromosome elements
Copying a gene and transferring it to another part of the chromosome
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