is a method characteristic of all living organisms of encoding the amino acid sequence of proteins using the sequence of nucleotides in a DNA molecule.
The implementation of genetic information in living cells (that is, the synthesis of a protein encoded in DNA) is carried out using two matrix processes: transcription (that is, the synthesis of mRNA on a DNA matrix) and translation (the synthesis of a polypeptide chain on an mRNA matrix).
DNA uses four nucleotides - adenine (A), guanine (G), cytosine (C), thymine (T). These “letters” make up the alphabet of the genetic code. RNA uses the same nucleotides, except for thymine, which is replaced by uracil (U). In DNA and RNA molecules, nucleotides are arranged in chains and, thus, sequences of “letters” are obtained.
The DNA nucleotide sequence contains code “words” for each amino acid of the future protein molecule - the genetic code. It consists in a certain sequence of arrangement of nucleotides in a DNA molecule.
Three consecutive nucleotides encode the “name” of one amino acid, that is, each of the 20 amino acids is encrypted by a significant unit of code - a combination of three nucleotides called a triplet or codon.
Currently, the DNA code has been completely deciphered, and we can talk about certain properties characteristic of this unique biological system, which ensures the translation of information from the “language” of DNA into the “language” of protein.
The carrier of genetic information is DNA, but since mRNA, a copy of one of the DNA strands, is directly involved in protein synthesis, the genetic code is most often written in the “RNA language.”
| Amino acid | RNA coding triplets |
|---|---|
| Alanin | GCU GCC GCA GCH |
| Arginine | TsGU TsGTs TsGA TsGG AGA AGG |
| Asparagine | AAU AAC |
| Aspartic acid | GAU GAC |
| Valin | GUU GUTS GUA GUG |
| Histidine | TsAU TsATs |
| Glycine | GGU GGC GGA YYY |
| Glutamine | CAA CAG |
| Glutamic acid | GAA GAG |
| Isoleucine | AUU AUC AUA |
| Leucine | TSUU TSUTS TSUA TSUG UUA UUG |
| Lysine | AAA AAG |
| Methionine | AUG |
| Proline | TsTsU TsTs TsTsTsG |
| Serin | UCU UCC UCA UCG ASU AGC |
| Tyrosine | UAU UAC |
| Threonine | ACU ACC ACA ACG |
| Tryptophan | UGG |
| Phenylalanine | UUU UUC |
| Cysteine | UGU UGC |
| STOP | UGA UAG UAA |
Properties of the genetic code
Three consecutive nucleotides (nitrogen bases) encode the “name” of one amino acid, that is, each of the 20 amino acids is encrypted with a significant code unit - a combination of three nucleotides called triplet or codon.
Triplet (codon)- a sequence of three nucleotides (nitrogen bases) in a DNA or RNA molecule that determines the inclusion of a certain amino acid in the protein molecule during its synthesis.
- Uniqueness (discreteness)
One triplet cannot encode two different amino acids; it encrypts only one amino acid. A specific codon corresponds to only one amino acid.
Each amino acid can be defined by more than one triplet. Exception - methionine And tryptophan. In other words, several codons can correspond to the same amino acid.
- Non-overlapping
The same base cannot appear in two adjacent codons at the same time.
Some triplets do not encode amino acids, but are a kind of “road signs” that determine the beginning and end of individual genes (UAA, UAG, UGA), each of which means the cessation of synthesis and is located at the end of each gene, so we can talk about the polarity of the genetic code.
In animals and plants, fungi, bacteria and viruses, the same triplet codes for the same type of amino acid, that is, the genetic code is the same for all living things. In other words, versatility - the ability of the genetic code to work equally in organisms of different levels of complexity from viruses to humans.The universality of the DNA code confirms the unity oforigin of all life on our planet. Genetic engineering methods are based on the use of the property of the universality of the genetic code.
From the history of the discovery of the genetic code
For the first time the idea of existence genetic code formulated by A. Down in 1952 - 1954. Scientists have shown that the nucleotide sequence that uniquely determines the synthesis of a particular amino acid must contain at least three units. It was later proven that such a sequence consists of three nucleotides called codon or triplet .
The questions of which nucleotides are responsible for the inclusion of a particular amino acid in a protein molecule and how many nucleotides determine this inclusion remained unresolved until 1961. Theoretical analysis showed that the code cannot consist of one nucleotide, since in this case only 4 amino acids can be encoded. However, the code cannot be a doublet, that is, a combination of two nucleotides from a four-letter “alphabet” cannot cover all amino acids, since only 16 such combinations are theoretically possible (4 2 = 16).
To encode 20 amino acids, as well as a stop signal indicating the end of the protein sequence, three consecutive nucleotides are sufficient, when the number of possible combinations is 64 (4 3 = 64).
They line up in chains and thus produce sequences of genetic letters.
The proteins of almost all living organisms are built from only 20 types of amino acids. These amino acids are called canonical. Each protein is a chain or several chains of amino acids connected in a strictly defined sequence. This sequence determines the structure of the protein, and therefore all its biological properties.
CUU (Leu/L)Leucine
CUC (Leu/L)Leucine
CUA (Leu/L)Leucine
CUG (Leu/L)Leucine
In some proteins, nonstandard amino acids, such as selenocysteine and pyrrolysine, are inserted by a ribosome reading the stop codon, depending on the sequences in the mRNA. Selenocysteine is now considered to be the 21st, and pyrrolysine the 22nd, amino acids that make up proteins.
Despite these exceptions, all living organisms have a genetic code common features: a codon consists of three nucleotides, where the first two are decisive; codons are translated by tRNA and ribosomes into an amino acid sequence.
| Example | Codon | Normal meaning | Reads like: |
|---|---|---|---|
| Some types of yeast Candida | C.U.G. | Leucine | Serin |
| Mitochondria, in particular in Saccharomyces cerevisiae | CU(U, C, A, G) | Leucine | Serin |
| Mitochondria of higher plants | CGG | Arginine | Tryptophan |
| Mitochondria (in all studied organisms without exception) | U.G.A. | Stop | Tryptophan |
| Mitochondria in mammals, Drosophila, S. cerevisiae and many protozoa | AUA | Isoleucine | Methionine = Start |
| Prokaryotes | G.U.G. | Valin | Start |
| Eukaryotes (rare) | C.U.G. | Leucine | Start |
| Eukaryotes (rare) | G.U.G. | Valin | Start |
| Prokaryotes (rare) | UUG | Leucine | Start |
| Eukaryotes (rare) | A.C.G. | Threonine | Start |
| Mammalian mitochondria | AGC, AGU | Serin | Stop |
| Drosophila mitochondria | A.G.A. | Arginine | Stop |
| Mammalian mitochondria | AG(A, G) | Arginine | Stop |
History of ideas about the genetic code
However, in the early 60s of the 20th century, new data revealed the inconsistency of the “code without commas” hypothesis. Then experiments showed that codons, considered meaningless by Crick, could provoke protein synthesis in vitro, and by 1965 the meaning of all 64 triplets was established. It turned out that some codons are simply redundant, that is, a whole series of amino acids are encoded by two, four or even six triplets.
see also
Notes
- Genetic code supports targeted insertion of two amino acids by one codon. Turanov AA, Lobanov AV, Fomenko DE, Morrison HG, Sogin ML, Klobutcher LA, Hatfield DL, Gladyshev VN. Science. 2009 Jan 9;323(5911):259-61.
- The AUG codon encodes methionine, but at the same time serves as a start codon - translation usually begins with the first AUG codon of mRNA.
- NCBI: "The Genetic Codes", Compiled by Andrzej (Anjay) Elzanowski and Jim Ostell
- Jukes TH, Osawa S, The genetic code in mitochondria and chloroplasts., Experience. 1990 Dec 1;46(11-12):1117-26.
- Osawa S, Jukes TH, Watanabe K, Muto A (March 1992). "Recent evidence for evolution of the genetic code." Microbiol. Rev. 56 (1): 229–64. PMID 1579111.
- SANGER F. (1952). "The arrangement of amino acids in proteins." Adv Protein Chem. 7 : 1-67. PMID 14933251.
- M. Ichas Biological code. - World, 1971.
- WATSON JD, CRICK FH. (April 1953). “Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid." Nature 171 : 737-738. PMID 13054692.
- WATSON JD, CRICK FH. (May 1953). "Genetic implications of the structure of deoxyribonucleic acid." Nature 171 : 964-967. PMID 13063483.
- Crick FH. (April 1966). “The genetic code - yesterday, today, and tomorrow.” Cold Spring Harb Symp Quant Biol.: 1-9. PMID 5237190.
- G. GAMOW (February 1954). "Possible Relation between Deoxyribonucleic Acid and Protein Structures." Nature 173 : 318. DOI:10.1038/173318a0. PMID 13882203.
- GAMOW G, RICH A, YCAS M. (1956). "The problem of information transfer from the nucleic acids to proteins." Adv Biol Med Phys. 4 : 23-68. PMID 13354508.
- Gamow G, Ycas M. (1955). "STATISTICAL CORRELATION OF PROTEIN AND RIBONUCLEIC ACID COMPOSITION. " Proc Natl Acad Sci U S A. 41 : 1011-1019. PMID 16589789.
- Crick FH, Griffith JS, Orgel LE. (1957). “CODES WITHOUT COMMAS. " Proc Natl Acad Sci U S A. 43 : 416-421. PMID 16590032.
- Hayes B. (1998). "The Invention of the Genetic Code." (PDF reprint). American Scientist 86 : 8-14.
Literature
- Azimov A. Genetic code. From the theory of evolution to deciphering DNA. - M.: Tsentrpoligraf, 2006. - 208 pp. - ISBN 5-9524-2230-6.
- Ratner V. A. Genetic code as a system - Soros educational journal, 2000, 6, No. 3, pp. 17-22.
- Crick FH, Barnett L, Brenner S, Watts-Tobin RJ. General nature of the genetic code for proteins - Nature, 1961 (192), pp. 1227-32
Links
- Genetic code- article from the Great Soviet Encyclopedia
Wikimedia Foundation.
2010.
In this lesson we will learn about the importance of protein biosynthesis for living organisms, about the two stages of protein biosynthesis in a cell, transcription and translation, and we will show how the sequence of nucleotides in DNA encodes the sequence of amino acids in a polypeptide. We will also characterize the genetic code and its main properties from the standpoint of the unity of origin of all living organisms on Earth, and consider the features of transcription in eukaryotes. Transcription
- a mechanism by which a sequence of bases in one of the chains of a DNA molecule is “rewritten” into a complementary sequence of mRNA bases. Transcription requires the presence of the enzyme RNA polymerase. Since one DNA molecule can contain many genes, it is very important that RNA polymerase begins the synthesis of messenger RNA from a strictly defined place in the DNA, otherwise information about a protein that does not exist in nature (not needed by the cell) will be recorded in the structure of the mRNA. Therefore, at the beginning of each gene there is a special specific sequence of nucleotides called promoter (see Fig. 7). RNA polymerase “recognizes” the promoter, interacts with it and, thus, begins the synthesis of the mRNA chain from the right place. The enzyme continues to synthesize mRNA, adding new nucleotides to it, until it reaches the next “punctuation mark” in the DNA molecule - terminator
. This is a sequence of nucleotides indicating that mRNA synthesis should be stopped.
Rice. 7. mRNA synthesis
In prokaryotes, synthesized mRNA molecules can immediately interact with ribosomes and participate in protein synthesis. In eukaryotes, mRNA first interacts with nuclear proteins and enters the cytoplasm through nuclear pores, where it interacts with ribosomes and protein biosynthesis occurs.
There are antibiotics that selectively affect one of the stages of protein synthesis, for example transcription. These include rifamycins, which are produced by actinomycetes of the genus Streptomyces. The best antibiotic in this class is Rifampicin.
Bibliography
- Kamensky A.A., Kriksunov E.A., Pasechnik V.V. General biology 10-11 grade Bustard, 2005.
- Biology. Grade 10. General biology. A basic level of/ P.V. Izhevsky, O.A. Kornilova, T.E. Loshchilina and others - 2nd ed., revised. - Ventana-Graf, 2010. - 224 pp.
- Belyaev D.K. Biology 10-11 grade. General biology. A basic level of. - 11th ed., stereotype. - M.: Education, 2012. - 304 p.
- Agafonova I.B., Zakharova E.T., Sivoglazov V.I. Biology 10-11 grade. General biology. A basic level of. - 6th ed., add. - Bustard, 2010. - 384 p.
- Bio-faq.ru ().
- Biouroki.ru ().
- Youtube.com().
- Sbio.info().
Homework
- Questions 1, 2 at the end of paragraph 26 (p. 101) Kamensky A.A., Kriksunov E.A., Pasechnik V.V. "General Biology", grades 10-11 ()
- What is the role of the enzyme RNA polymerase in the process of mRNA synthesis?
- What is a promoter and what is its role in mRNA synthesis?
- What is a terminator and what is its role in mRNA synthesis?
- What is further fate synthesized mRNA in the cell of prokaryotes and eukaryotes?
Each living organism has a special set of proteins. Certain nucleotide compounds and their sequence in the DNA molecule form the genetic code. It conveys information about the structure of the protein. A certain concept has been accepted in genetics. According to it, one gene corresponded to one enzyme (polypeptide). It should be said that research on nucleic acids and proteins has been carried out over a fairly long period. Later in the article we will take a closer look at the genetic code and its properties. A brief chronology of the research will also be provided.
Terminology
The genetic code is a way of encoding the sequence of amino acid proteins involving the nucleotide sequence. This method of generating information is characteristic of all living organisms. Proteins are natural organic substances with high molecularity. These compounds are also present in living organisms. They consist of 20 types of amino acids, which are called canonical. Amino acids are arranged in a chain and connected in a strictly established sequence. It determines the structure of the protein and its biological properties. There are also several chains of amino acids in a protein.
DNA and RNA
Deoxyribonucleic acid is a macromolecule. She is responsible for the transmission, storage and implementation of hereditary information. DNA uses four nitrogenous bases. These include adenine, guanine, cytosine, thymine. RNA consists of the same nucleotides, except that it contains thymine. Instead, there is a nucleotide containing uracil (U). RNA and DNA molecules are nucleotide chains. Thanks to this structure, sequences are formed - the “genetic alphabet”.
Realization of information
Protein synthesis, which is encoded by the gene, is realized by combining mRNA on a DNA template (transcription). The genetic code is also transferred into the amino acid sequence. That is, the synthesis of the polypeptide chain on mRNA takes place. To encrypt all amino acids and the signal for the end of the protein sequence, 3 nucleotides are enough. This chain is called a triplet.
History of the study
The study of proteins and nucleic acids has been carried out for a long time. In the middle of the 20th century, the first ideas about the nature of the genetic code finally appeared. In 1953, it was discovered that some proteins consist of sequences of amino acids. True, at that time they could not yet determine their exact number, and there were numerous disputes about this. In 1953, two works were published by the authors Watson and Crick. The first stated about the secondary structure of DNA, the second spoke about its permissible copying using template synthesis. In addition, emphasis was placed on the fact that a specific sequence of bases is a code that carries hereditary information. American and Soviet physicist Georgiy Gamow assumed the coding hypothesis and found a method for testing it. In 1954, his work was published, during which he proposed to establish correspondences between amino acid side chains and diamond-shaped “holes” and use this as a coding mechanism. Then it was called rhombic. Explaining his work, Gamow admitted that the genetic code could be a triplet. The physicist’s work was one of the first among those that were considered close to the truth.
Classification
Over the years, various models of genetic codes have been proposed, of two types: overlapping and non-overlapping. The first was based on the inclusion of one nucleotide in several codons. It includes a triangular, sequential and major-minor genetic code. The second model assumes two types. Non-overlapping codes include combination code and comma-free code. The first option is based on the encoding of an amino acid by triplets of nucleotides, and the main thing is its composition. According to the "code without commas", certain triplets correspond to amino acids, but others do not. In this case, it was believed that if any significant triplets were arranged sequentially, others located in a different reading frame would be unnecessary. Scientists believed that it was possible to select a nucleotide sequence that would satisfy these requirements, and that there were exactly 20 triplets.
Although Gamow and his co-authors questioned this model, it was considered the most correct over the next five years. At the beginning of the second half of the 20th century, new data appeared that made it possible to discover some shortcomings in the “code without commas”. It was found that codons are capable of inducing protein synthesis in vitro. Closer to 1965, the principle of all 64 triplets was comprehended. As a result, redundancy of some codons was discovered. In other words, the amino acid sequence is encoded by several triplets.
Distinctive features
The properties of the genetic code include:
Variations
The first deviation of the genetic code from the standard was discovered in 1979 during the study of mitochondrial genes in the human body. Further similar variants were further identified, including many alternative mitochondrial codes. These include the decoding of the UGA stop codon, which is used to determine tryptophan in mycoplasmas. GUG and UUG in archaea and bacteria are often used as starting options. Sometimes genes encode a protein with a start codon that differs from that normally used by the species. Additionally, in some proteins, selenocysteine and pyrrolysine, which are nonstandard amino acids, are inserted by the ribosome. She reads the stop codon. This depends on the sequences found in the mRNA. Currently, selenocysteine is considered the 21st and pyrrolysane the 22nd amino acid present in proteins.
General features of the genetic code
However, all exceptions are rare. In living organisms, the genetic code mainly has a number of common features. These include the composition of a codon, which includes three nucleotides (the first two belong to the defining ones), the transfer of codons by tRNA and ribosomes into the amino acid sequence.
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 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 structural information various types 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 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)
- Replication process: unwinding of the DNA double helix - synthesis of complementary strands by DNA polymerase - formation of two DNA molecules from one.
- The double helix "unzips" into two branches when enzymes break the bond between the base pairs of chemical compounds.
- 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(cm. principle of complementarity).
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. It means that double helix The DNA unwinds and a new chain is built on each of its strands 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 the primer is completely removed, 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 transcription after transcription. chemical modifications, 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, at the link below, we recommend watching a 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 tion |
2nd base |
3rd 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 the same 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 development 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-rays 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 came to the conclusion that a gene is a region genetic material, which defines 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 final 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 gene size, 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. A polypeptide chain of 350 amino acid residues (medium length chain) 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 are involved in the packaging of DNA strands in the nucleus and in the epigenetic regulation of such nuclear processes, such as transcription, replication and repair.
The DNA of prokaryotes is simpler: their cells do not have a nucleus, so the DNA is located directly in the cytoplasm in the form of a nucleoid.
As is known, bacterial cells have a chromosome in the form of a DNA strand arranged in a compact structure - a nucleoid. Chromosome of a prokaryote 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 of 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
|
Total 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) - 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, in a human somatic cell there are 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.
The size and function of DNA as a matrix for storing and transmitting hereditary material explains the presence of special structural elements in the organization of this molecule. In higher organisms, DNA is distributed between chromosomes.
The collection of DNA (chromosomes) of an organism is called the genome. Chromosomes are found in the cell nucleus and form a structure called chromatin. Chromatin is a complex of DNA and basic proteins (histones) in a 1:1 ratio. DNA length is usually measured by the number of complementary nucleotide pairs (bp). For example, the 3rd human chromosomecentury is a DNA molecule measuring 160 million bp. Isolated linearized DNA measuring 3*10 6 bp. has a length of approximately 1 mm, therefore, the linearized molecule of the 3rd human chromosome would be 5 mm in length, and the DNA of all 23 chromosomes (~3 * 10 9 bp, MR = 1.8 * 10 12) of a haploid cell - an egg or sperm - in linearized form would be 1 m. With the exception of germ cells, all cells of the human body (there are about 1013 of them) contain a double set of chromosomes. At cell division all 46 DNA molecules are replicated and reorganized into 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 are therefore total length The DNA of such cells is about 2m. 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 having different structure in prokaryotes and eukaryotes.
Coding sequence- the main structural and functional unit of the gene, it is in it that the triplets of nucleotides 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 coupling.
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 almost 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 regions - 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. The process of alternative splicing - 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 the splicing process, 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 that do not affect survival, harmful, reducing the adaptability of organisms to environmental conditions and lethal, leading to the death of the organism in the early stages of development.
According to the consequences, mutations 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 originated, 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.
when a section of a chromosome rotates 180 degrees, Rice. 28. Translocation
Rice. 29. Chromosome before and after duplication
Genomic mutations
Finally, genomic mutations affect the entire genome, that is, the number of chromosomes changes. 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).
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DNA REPLICATION, RNA CODING, PROTEIN SYNTHESIS
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