They understand what property of the genetic code is universal. Biosynthesis of protein and nucleic acids

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Genetic code is a way of encoding the amino acid sequence of proteins using the sequence of nucleotides in the DNA molecule, characteristic of all living organisms.

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, mRNA synthesis on a DNA matrix) and translation (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 line up in chains and, thus, sequences of “letters” are obtained.

In the nucleotide sequence of DNA there are code "words" for each amino acid of the future protein molecule - the genetic code. It consists in a certain sequence of nucleotides in the 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 code unit - a combination of three nucleotides called a triplet or codon.

At present, the DNA code has been completely deciphered, and we can talk about certain properties that are characteristic of this unique biological system, which provides the translation of information from the "language" of DNA to 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	Coding RNA triplets
Alanine	GCU GCC GCA GCG
Arginine	TsGU TsGTs TsGA TsGG AGA AGG
Asparagine	AAU AAC
Aspartic acid	GAU GAC
Valine	GUU GUTS GUA GUG
Histidine	CAU CAC
Glycine	GSU GGC GGA GYY
Glutamine	CAA CAG
Glutamic acid	GAA GAG
Isoleucine	AAU AUC AUA
Leucine	TSUU TSUT TSUA TSUG UUA UUG
Lysine	AAA AAG
Methionine	AUG
Proline	CCC CCC CCA CCG
Serene	UCU UCC UCA UCG ASU AGC
Tyrosine	UAU UAC
Threonine	ACC ACC ACA ACG
tryptophan	UGG
Phenylalanine	uuu uuc
Cysteine	UGU UHC
STOP	UGA UAG UAA

Properties of the genetic code

Three consecutive nucleotides (nitrogenous bases) encode the "name" of one amino acid, that is, each of the 20 amino acids is encrypted by a significant code unit - a combination of three nucleotides called triplet or codon.

Triplet (codon)- a sequence of three nucleotides (nitrogenous bases) in a DNA or RNA molecule, which determines the inclusion of a certain amino acid in the protein molecule during its synthesis.

• Unambiguity (discreteness)

One triplet cannot encode two different amino acids; it encodes only one amino acid. A certain 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 be present 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, in fungi, bacteria and viruses, the same triplet encodes the same type of amino acid, that is, the genetic code is the same for all living beings. In other words, universality is the ability of the genetic code to work in the same way in organisms of different levels of complexity, from viruses to humans. The universality of the DNA code confirms the unity of the origin of all life on our planet. Genetic engineering methods are based on the use of the universality property 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 and G. Gamow in 1952-1954. Scientists have shown that a nucleotide sequence that uniquely determines the synthesis of a particular amino acid must contain at least three links. Later it was proved that such a sequence consists of three nucleotides, called codon or triplet.

The questions of which nucleotides are responsible for incorporating a certain amino acid into 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 either, 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).

Three consecutive nucleotides are enough to encode 20 amino acids, as well as a “stop” signal, which means the end of the protein sequence, when the number of possible combinations is 64 (4 3 = 64).

Hereditary information is information about the structure of a protein (information about which amino acids in what order combine during the synthesis of the primary structure of the protein).

Information about the structure of proteins is encoded in DNA, which in eukaryotes is part of the chromosomes and is located in the nucleus. The section of DNA (chromosome) that encodes information about one protein is called gene.

Transcription- this is the rewriting of information from DNA to mRNA (messenger RNA). mRNA carries information from the nucleus to the cytoplasm, to the site of protein synthesis (to the ribosome).

Broadcast is the process of protein biosynthesis. Inside the ribosome, tRNA anticodons are attached to mRNA codons according to the principle of complementarity. The ribosome links the amino acids brought by the tRNA with a peptide bond to form a protein.

The reactions of transcription, translation, and replication (doubling of DNA) are reactions matrix synthesis. DNA serves as a template for mRNA synthesis, mRNA serves as a template for protein synthesis.

Genetic code is the way in which information about the structure of a protein is recorded in DNA.

Genecode Properties

1) Tripletity: one amino acid is encoded by three nucleotides. These 3 nucleotides in DNA are called a triplet, in mRNA - a codon, in tRNA - an anticodon (but in the exam there may be a “code triplet”, etc.)

2) Redundancy(degeneracy): there are only 20 amino acids, and there are 61 triplets encoding amino acids, so each amino acid is encoded by several triplets.

3) Unambiguity: each triplet (codon) codes for only one amino acid.

4) Versatility: the genetic code is the same for all living organisms on Earth.

Tasks

Tasks for the number of nucleotides / amino acids
3 nucleotides = 1 triplet = 1 amino acid = 1 tRNA

Tasks at ATHC
DNA mRNA tRNA
A U A
T A U
G C G
C G C

Choose one, the most correct option. mRNA is a copy
1) one gene or group of genes
2) chains of a protein molecule
3) one protein molecule
4) parts of the plasma membrane

Answer

Choose one, the most correct option. The primary structure of a protein molecule, given by the mRNA nucleotide sequence, is formed in the process
1) broadcasts
2) transcriptions
3) reduplication
4) denaturation

Answer

Choose one, the most correct option. Which sequence correctly reflects the way of realization of genetic information
1) gene --> mRNA --> protein --> trait
2) trait --> protein --> mRNA --> gene --> DNA
3) mRNA --> gene --> protein --> trait
4) gene --> DNA --> trait --> protein

Answer

Choose one, the most correct option. Choose the correct sequence of information transfer in the process of protein synthesis in the cell
1) DNA -> messenger RNA -> protein
2) DNA -> transfer RNA -> protein
3) ribosomal RNA -> transfer RNA -> protein
4) ribosomal RNA -> DNA -> transfer RNA -> protein

Answer

Choose one, the most correct option. The same amino acid corresponds to the CAA anticodon on transfer RNA and the triplet on DNA
1) CAA
2) TSUU
3) GTT
4) GAA

Answer

Choose one, the most correct option. AAU anticodon on transfer RNA corresponds to a triplet on DNA
1) TTA
2) AAT
3) AAA
4) TTT

Answer

Choose one, the most correct option. Each amino acid in a cell is encoded
1) one DNA molecule
2) several triplets
3) multiple genes
4) one nucleotide

Answer

Choose one, the most correct option. Functional unit of the genetic code
1) nucleotide
2) triplet
3) amino acid
4) tRNA

Answer

Choose three options. As a result of reactions of the matrix type, molecules are synthesized
1) polysaccharides
2) DNA
3) monosaccharides
4) mRNA
5) lipids
6) squirrel

Answer

1. Determine the sequence of processes that provide protein biosynthesis. Write down the corresponding sequence of numbers.
1) the formation of peptide bonds between amino acids
2) attachment of the tRNA anticodon to the complementary mRNA codon
3) synthesis of mRNA molecules on DNA
4) movement of mRNA in the cytoplasm and its location on the ribosome
5) delivery of amino acids to the ribosome using tRNA

Answer

2. Establish the sequence of protein biosynthesis processes in the cell. Write down the corresponding sequence of numbers.
1) the formation of a peptide bond between amino acids
2) interaction of mRNA codon and tRNA anticodon
3) release of tRNA from the ribosome
4) connection of mRNA with a ribosome
5) release of mRNA from the nucleus into the cytoplasm
6) mRNA synthesis

Answer

3. Set the sequence of processes in protein biosynthesis. Write down the corresponding sequence of numbers.
1) mRNA synthesis on DNA
2) amino acid delivery to the ribosome
3) formation of a peptide bond between amino acids
4) attachment of an amino acid to tRNA
5) mRNA connection with two ribosome subunits

Answer

4. Set the sequence of steps in protein biosynthesis. Write down the corresponding sequence of numbers.
1) separation of a protein molecule from a ribosome
2) attachment of tRNA to the start codon
3) transcription
4) elongation of the polypeptide chain
5) release of mRNA from the nucleus into the cytoplasm

Answer

5. Set the correct sequence of protein biosynthesis processes. Write down the corresponding sequence of numbers.
1) attachment of an amino acid to a peptide
2) mRNA synthesis on DNA
3) codon recognition of anticodon
4) association of mRNA with a ribosome
5) release of mRNA into the cytoplasm

Answer

Choose one, the most correct option. Which transfer RNA anticodon corresponds to the TGA triplet in the DNA molecule
1) ACU
2) ZUG
3) UGA
4) AHA

Answer

Choose one, the most correct option. The genetic code is universal because
1) each amino acid is encoded by a triplet of nucleotides
2) the place of an amino acid in a protein molecule is determined by different triplets
3) it is the same for all creatures living on Earth
4) several triplets code for one amino acid

Answer

Choose one, the most correct option. The section of DNA containing information about one polypeptide chain is called
1) chromosome
2) triplet
3) genome
4) code

Answer

Choose one, the most correct option. Translation is the process by which
1) the number of DNA strands doubles
2) mRNA is synthesized on the DNA template
3) proteins are synthesized on the mRNA template in the ribosome
4) hydrogen bonds between DNA molecules are broken

Answer

Choose three options. Protein biosynthesis, unlike photosynthesis, occurs
1) in chloroplasts
2) in mitochondria
3) in plastic exchange reactions
4) in reactions of the matrix type
5) in lysosomes
6) in leukoplasts

Answer

Choose one, the most correct option. The translation matrix is ​​the molecule
1) tRNA
2) DNA
3) rRNA
4) mRNA

Answer

All but two of the features below can be used to describe the functions of nucleic acids in a cell. Identify two signs that “fall out” from the general list, and write down the numbers under which they are indicated in the table.
1) carry out homeostasis
2) transfer hereditary information from the nucleus to the ribosome
3) participate in protein biosynthesis
4) are part of the cell membrane
5) transport amino acids

Answer

AMINO ACIDS - CODONS mRNA
How many mRNA codons encode information about 20 amino acids? Write down only the appropriate number in your answer.

Answer

AMINO ACIDS - NUCLEOTIDES mRNA
1. The polypeptide region consists of 28 amino acid residues. Determine the number of nucleotides in the mRNA region containing information about the primary structure of the protein.

Answer

2. How many nucleotides does mRNA contain if the protein synthesized from it consists of 180 amino acid residues? Write down only the appropriate number in your answer.

Answer

AMINO ACIDS - DNA NUCLEOTIDES
1. Protein consists of 140 amino acid residues. How many nucleotides are in the region of the gene in which the primary structure of this protein is encoded?

Answer

2. Protein consists of 180 amino acid residues. How many nucleotides are in the gene that encodes the sequence of amino acids in this protein. Write down only the appropriate number in your answer.

Answer

3. A fragment of a DNA molecule encodes 36 amino acids. How many nucleotides does this DNA fragment contain? Write down the corresponding number in your answer.

Answer

4. The polypeptide consists of 20 amino acid units. Determine the number of nucleotides in the gene region encoding these amino acids in the polypeptide. Write your answer as a number.

Answer

5. How many nucleotides in the gene region encode a protein fragment of 25 amino acid residues? Write down the correct number for your answer.

Answer

6. How many nucleotides in a fragment of the DNA template chain encode 55 amino acids in a polypeptide fragment? Write down only the appropriate number in your answer.

Answer

AMINO ACIDS - tRNA
1. How many tRNAs took part in protein synthesis, which includes 130 amino acids? Write the correct number in your answer.

Answer

2. A fragment of a protein molecule consists of 25 amino acids. How many tRNA molecules were involved in its creation? Write down only the appropriate number in your answer.

Answer

AMINO ACIDS - TRIPLETS
1. How many triplets does a fragment of a DNA molecule contain, encoding 36 amino acids? Write down the corresponding number in your answer.

Answer

2. How many triplets encode 32 amino acids? Write down the correct number for your answer.

Answer

NUCLEOTIDES - AMINO ACIDS
1. What is the number of amino acids encoded in the gene section containing 129 nucleotide residues?

Answer

2. How many amino acids does 900 nucleotides encode? Write down the correct number for your answer.

Answer

3. What is the number of amino acids in a protein if its coding gene consists of 600 nucleotides? Write down the correct number for your answer.

Answer

4. How many amino acids does 1203 nucleotides encode? In response, write down only the number of amino acids.

Answer

5. How many amino acids are needed for the synthesis of a polypeptide if the mRNA encoding it contains 108 nucleotides? Write down only the appropriate number in your answer.

Answer

mRNA NUCLEOTIDES - DNA NUCLEOTIDES
An mRNA molecule takes part in protein synthesis, the fragment of which contains 33 nucleotide residues. Determine the number of nucleotide residues in the region of the DNA template chain.

Answer

NUCLEOTIDES - tRNA
How many transport RNA molecules were involved in translation if the gene section contains 930 nucleotide residues?

Answer

TRIPLETS - NUCLEOTIDES mRNA
How many nucleotides are in a fragment of an mRNA molecule if the fragment of the DNA coding chain contains 130 triplets? Write down only the appropriate number in your answer.

Answer

tRNA - AMINO ACIDS
Determine the number of amino acids in a protein if 150 tRNA molecules were involved in the translation process. Write down only the appropriate number in your answer.

Answer

SIMPLY
How many nucleotides make up one mRNA codon?

Answer

How many nucleotides make up one mRNA stop codon?

Answer

How many nucleotides make up a tRNA anticodon?

Answer

DIFFICULT
The protein has a relative molecular weight of 6000. Determine the number of amino acids in a protein molecule if the relative molecular weight of one amino acid residue is 120. In your answer, write down only the corresponding number.

Answer

There are 3,000 nucleotides in two strands of a DNA molecule. Information about the protein structure is encoded on one of the chains. Count how many amino acids are encoded on one strand of DNA. In response, write down only the number corresponding to the number of amino acids.

Answer

Choose one, the most correct option. The same amino acid corresponds to a UCA anticodon on transfer RNA and a triplet in a gene on DNA
1) GTA
2) ACA
3) TGT
4) TCA

Answer

Choose one, the most correct option. The synthesis of hemoglobin in the cell controls a certain segment of the DNA molecule, which is called
1) codon
2) triplet
3) genetic code
4) genome

Answer

In which of the following cell organelles do matrix synthesis reactions take place? Identify three true statements from the general list, and write down the numbers under which they are indicated.
1) centrioles
2) lysosomes
3) Golgi apparatus
4) ribosomes
5) mitochondria
6) chloroplasts

Answer

Consider the picture depicting the processes occurring in the cell, and indicate A) the name of the process, indicated by the letter A, B) the name of the process, indicated by the letter B, C) the name of the type of chemical reactions. For each letter, select the appropriate term from the list provided.
1) replication
2) transcription
3) broadcast
4) denaturation
5) exothermic reactions
6) substitution reactions
7) matrix synthesis reactions
8) cleavage reactions

Answer

Look at the picture and write (A) the name of process 1, (B) the name of process 2, (c) the end product of process 2. For each letter, select the appropriate term or concept from the list provided.
1) tRNA
2) polypeptide
3) ribosome
4) replication
5) broadcast
6) conjugation
7) ATP
8) transcription

Answer

Establish a correspondence between the processes and stages of protein synthesis: 1) transcription, 2) translation. Write the numbers 1 and 2 in the correct order.
A) t-RNA amino acid transfer
B) DNA is involved
C) i-RNA synthesis
D) formation of a polypeptide chain
D) occurs on the ribosome

Answer

All of the features listed below, except for two, are used to describe the process depicted in the figure. Identify two features that “fall out” of the general list, and write down the numbers under which they are indicated.
1) according to the principle of complementarity, the nucleotide sequence of a DNA molecule is translated into a nucleotide sequence of molecules of various types of RNA
2) the process of translating a nucleotide sequence into an amino acid sequence
3) the process of transferring genetic information from the nucleus to the site of protein synthesis
4) the process takes place in ribosomes
5) the result of the process - RNA synthesis

Answer

The molecular weight of the polypeptide is 30,000 USD. Determine the length of the gene encoding it if the molecular weight of one amino acid is on average 100, and the distance between nucleotides in DNA is 0.34 nm. Write down only the appropriate number in your answer.

Answer

Choose from the reactions listed below two related to the reactions of matrix synthesis. Write down the numbers under which they are indicated.
1) cellulose synthesis
2) ATP synthesis
3) protein biosynthesis
4) glucose oxidation
5) DNA replication

Answer

Choose three correct answers from six and write down the numbers under which they are indicated in the table. Matrix reactions in the cell include
1) DNA replication
2) photolysis of water
3) RNA synthesis
4) chemosynthesis
5) protein biosynthesis
6) ATP synthesis

Answer

All of the following features, except for two, can be used to describe the process of protein biosynthesis in a cell. Identify two features that “fall out” of the general list, and write down in response the numbers under which they are indicated.
1) The process occurs in the presence of enzymes.
2) The central role in the process belongs to RNA molecules.
3) The process is accompanied by the synthesis of ATP.
4) Amino acids serve as monomers for the formation of molecules.
5) The assembly of protein molecules is carried out in lysosomes.

Answer

Find three errors in the given text. Specify the numbers of proposals in which they are made.(1) During protein biosynthesis, matrix synthesis reactions occur. (2) Matrix synthesis reactions include only replication and transcription reactions. (3) As a result of transcription, mRNA is synthesized, the template for which is the entire DNA molecule. (4) After passing through the pores of the nucleus, mRNA enters the cytoplasm. (5) Messenger RNA is involved in the synthesis of tRNA. (6) Transfer RNA provides amino acids for protein assembly. (7) The energy of ATP molecules is spent on the connection of each of the amino acids with tRNA.

Answer

All but two of the following concepts are used to describe translation. Identify two signs that “fall out” from the general list, and write down the numbers under which they are indicated.
1) matrix synthesis
2) mitotic spindle
3) polysome
4) peptide bond
5) higher fatty acids

Answer

© D.V. Pozdnyakov, 2009-2019

The series of articles describing the origins of the Civil Code can be regarded as an investigation of events about which we have very few traces. However, understanding these articles requires a bit of effort to understand the molecular mechanisms of protein synthesis. This article is the introductory article for a series of auto-publications devoted to the origin of the genetic code, and it is the best place to start acquaintance with this topic.
Usually genetic code(GC) is defined as a method (rule) of encoding a protein on the primary structure of DNA or RNA. In the literature, it is most often written that this is a one-to-one correspondence of a sequence of three nucleotides in a gene to one amino acid in the synthesized protein or the end point of protein synthesis. However, there are two errors in this definition. This implies 20 so-called canonical amino acids, which are part of the proteins of all living organisms without exception. These amino acids are protein monomers. The errors are the following:

1) Canonical amino acids are not 20, but only 19. We can call an amino acid a substance that simultaneously contains an amino group -NH 2 and a carboxyl group - COOH. The fact is that the protein monomer - proline - is not an amino acid, since it contains an imino group instead of an amino group, so it is more correct to call proline an imino acid. However, in the future, in all articles on HA, for convenience, I will write about 20 amino acids, implying the indicated nuance. The amino acid structures are shown in fig. one.

Rice. 1. Structures of canonical amino acids. Amino acids have constant parts, marked in black in the figure, and variable (or radicals), marked in red.

2) The correspondence of amino acids to codons is not always unambiguous. See below for the violation of uniqueness cases.

The occurrence of HA means the occurrence of encoded protein synthesis. This event is one of the key ones for the evolutionary formation of the first living organisms.

The structure of the HA is presented in a circular form in fig. 2.

Rice. 2. Genetic code in a circular shape. The inner circle is the first letter of the codon, the second a circle - the second letter of the codon, the third circle - the third letter of the codon, the fourth circle - designations of amino acids in a three-letter abbreviation; P - polar amino acids, NP - non-polar amino acids. For clarity of symmetry, the chosen order of symbols is important U-C-A-G.

So, let's proceed to the description of the main properties of HA.

1. Tripletity. Each amino acid is encoded by a sequence of three nucleotides.

2. The presence of intergenetic punctuation marks. Intergenic punctuation marks include nucleic acid sequences on which translation begins or ends.

Translation I can not begin with any codon, but only with a strictly defined - starting. The start codon is the AUG triplet, which starts translation. In this case, this triplet encodes either methionine or another amino acid, formylmethionine (in prokaryotes), which can only be switched on at the beginning of protein synthesis. At the end of each gene encoding a polypeptide is at least one of the 3 termination codons, or brake lights: UAA, UAG, UGA. They terminate translation (the so-called protein synthesis on the ribosome).

3. Compactness, or the absence of intragenic punctuation marks. Within a gene, each nucleotide is part of a significant codon.

4. Non-overlapping. Codons do not overlap with each other, each has its own ordered set of nucleotides, which does not overlap with similar sets of neighboring codons.

5. Degeneracy. The reverse correspondence in the amino acid-codon direction is ambiguous. This property is called degeneracy. Series is a set of codons encoding one amino acid, in other words, it is a group equivalent codons. Think of a codon as XYZ. If XY defines “meaning” (i.e. amino acid), then the codon is called strong. If a certain Z is needed to determine the meaning of a codon, then such a codon is called weak.

The degeneracy of the code is closely related to the ambiguity of the codon-anticodon pairing (an anticodon means a sequence of three nucleotides on a tRNA that can complementarily pair with a codon on messenger RNA (see two articles on this in more detail: Molecular Mechanisms for Ensuring Code Degeneracy and Lagerquist's rule. Physico-chemical substantiation of symmetries and Rumer's relations). One anticodon per tRNA can recognize one to three codons per mRNA.

6.Unambiguity. Each triplet encodes only one amino acid or is a translation terminator.

There are three known exceptions.

First. In prokaryotes, in the first position (capital letter), it encodes formylmethionine, and in any other - methionine. At the beginning of the gene, formylmethionine is encoded both by the usual AUG methionine codon, and also by the GUG valine codon or UUG leucine codon, which inside the gene encode valine and leucine, respectively .

In many proteins, formylmethionine is cleaved off or the formyl group is removed, as a result of which formylmethionine is converted to ordinary methionine.

Second. In 1986, several groups of researchers at once discovered that the UGA termination codon on mRNA can encode selenocysteine ​​(see Fig. 3) provided that it is followed by a special nucleotide sequence.

Rice. 3. The structure of the 21st amino acid - selenocysteine.

At E. coli(this is the Latin name for Escherichia coli) selenocysteyl-tRNA during translation and recognizes the UGA codon in mRNA, but only in a certain context e: for the recognition of the UGA codon as meaningful, the sequence of 45 nucleotides long, located after the UGA codon, is important.

The considered example shows that, if necessary, a living organism can change the meaning of the standard genetic code. In this case, the genetic information contained in the genes is encoded in a more complex way. The meaning of the codon is determined in the context of e with a certain long sequence of nucleotides and with the participation of several highly specific protein factors. It is important that selenocysteine ​​tRNA was found in representatives of all three branches of life (archaea, eubacteria and eukaryotes), which indicates the antiquity of the origin of selenocysteine ​​synthesis, and possibly its presence in the last universal common ancestor (it will be discussed in other articles). Most likely, selenocysteine ​​is found in all living organisms without exception. But in each individual organism, selenocysteine ​​is found in no more than a couple of dozens of proteins. It is part of the active sites of enzymes, in a number of homologues of which ordinary cysteine ​​can function at a similar position.

Until recently, it was believed that the UGA codon could be read either as selenocysteine ​​or as a terminal, but recently it has been shown that in ciliates Euplotes The UGA codon codes for either cysteine ​​or selenocysteine. Cm. " Genetic code allows for inconsistencies"

Third exception. In some prokaryotes (5 species of archaea and one eubacterium - the information on Wikipedia is very outdated) there is a special acid - pyrrolysine (Fig. 4). It is encoded by the UAG triplet, which in the canonical code serves as a translation terminator. It is assumed that in this case, like the case with coding for selenocysteine, the reading of UAG as a pyrrolysine codon occurs due to a special structure on the mRNA. Pyrrolysine tRNA contains the anticodon CTA and is aminoacylated by class 2 APCases (for the classification of APCases, see the article "Codases help to understand how genetic code ").

UAG is rarely used as a stop codon, and if it is, it is often followed by another stop codon.

Rice. 4. Structure of the 22nd amino acid of pyrrolysine.

7. Versatility. After the decoding of the GC was completed in the mid-60s of the last century, for a long time it was believed that the code is the same in all organisms, which indicates the unity of the origin of all life on Earth.

Let's try to understand why the GC is universal. The fact is that if at least one coding rule were changed in the body, this would lead to the fact that the structure of a significant part of the proteins changed. Such a change would be too dramatic and therefore almost always lethal, since a change in the meaning of a of only one codon can affect, on average, 1/64 of all amino acid sequences.

One very important thought follows from this - the HA has hardly changed since its formation more than 3.5 billion years ago. And, therefore, its structure bears a trace of its occurrence, and the analysis of this structure can help to understand how exactly the GC could arise.

In reality, HA may differ slightly in bacteria, mitochondria, the nuclear code of some ciliates and yeasts. Now there are at least 17 genetic codes that differ from the canonical one by 1-5 codons. In total, in all known variants of deviations from the universal GC, 18 different substitutions for the meaning of a codon are used. Most deviations from the standard code are known in mitochondria - 10. It is noteworthy that the mitochondria of vertebrates, flatworms, echinoderms are encoded by different codes, and mold fungi, protozoa and coelenterates - by one.

The evolutionary closeness of species is by no means a guarantee that they have similar GCs. Genetic codes can differ even between different types of mycoplasmas (some species have a canonical code, while others are different). A similar situation is observed for yeast.

It is important to note that mitochondria are descendants of symbiotic organisms that have adapted to live inside cells. They have a highly reduced genome, some of the genes have moved to the cell nucleus. Therefore, changes in the HA in them are no longer so dramatic.

The exceptions discovered later are of particular interest from an evolutionary point of view, as they can help shed light on the mechanisms of code evolution.

Table 1.

Mitochondrial codes in various organisms.

codon	Universal code	Mitochondrial codes
		Vertebrates	Invertebrates	Yeast	Plants
UGA	STOP	trp	trp	trp	STOP
AUA	ile	Met	Met	Met	ile
CUA	Leu	Leu	Leu	Thr	Leu
AGA	Arg	STOP	Ser	Arg	Arg
AGG	Arg	STOP	Ser	Arg	Arg

Three mechanisms for changing the amino acid encoded by the code.

The first is when some codon is not used (or almost not used) by some organism due to the uneven occurrence of some nucleotides (GC-composition), or combinations of nucleotides. As a result, such a codon may completely disappear from use (for example, due to the loss of the corresponding tRNA), and in the future it can be used to code for another amino acid without causing significant damage to the body. This mechanism is probably responsible for the appearance of some dialects of codes in mitochondria.

The second is the transformation of the stop codon into the meaning of the new one. In this case, some of the translated proteins may have additions. However, the situation is partially saved by the fact that many genes often end with not one, but two stop codons, since translation errors are possible, in which stop codons are read as amino acids.

The third is the possible ambiguous reading of certain codons, as occurs in some fungi.

8 . Connectivity. Groups of equivalent codons (that is, codons that code for the same amino acid) are called series. The GC contains 21 series, including stop codons. In what follows, for definiteness, any group of codons will be called liaison, if from each codon of this group it is possible to pass to all other codons of the same group by successive nucleotide substitutions. Of the 21 series, 18 are connected. 2 series contain one codon each, and only 1 series for the amino acid serine is unconnected and splits into 2 connected subseries.

Rice. 5. Connectivity graphs for some code series. a - connected series of valine; b - connected series of leucine; the serine series is unrelated, splitting into two connected subseries. The figure is taken from an article by V.A. Ratner " Genetic code like a system."

The property of connectivity can be explained by the fact that during the period of formation, the HA captured new codons that minimally differed from those already used.

9. Regularity properties of amino acids by the roots of triplets. All amino acids encoded by U triplets are non-polar, not of extreme properties and sizes, and have aliphatic radicals. All C-root triplets have strong bases, and the amino acids encoded by them are relatively small. All triplets with root A have weak bases and encode non-small polar amino acids. G-root codons are characterized by extreme and abnormal variants of amino acids and series. They encode the smallest amino acid (glycine), the longest and flattest (tryptophan), the longest and "clumsiest" (arginine), the most reactive (cysteine), and form an abnormal subset for serine.

10. Blockiness. The universal CC is a "block" code. This means that amino acids with similar physicochemical properties are encoded by codons that differ from each other by one base. The blockiness of the code is clearly visible in the following figure.

Rice. 6. Block structure of the Civil Code. White color indicates amino acids with an alkyl group.

Rice. 7. Color representation of the physicochemical properties of amino acids based on the values ​​described in the bookStyers "Biochemistry". Left - hydrophobicity. On the right, the ability to form an alpha helix in a protein. Red, yellow and blue colors indicate amino acids with high, medium and low hydrophobicity (left) or the corresponding degree of ability to form an alpha helix (right).

The property of blockiness and regularity can also be explained by the fact that during the period of formation, the HA captured new codons that minimally differed from those already used.

Codons with the same first base (codon prefix) code for amino acids with similar biosynthetic pathways. The codons of amino acids belonging to the shikimate, pyruvate, aspartate, and glutamate families have prefixes U, G, A, and C, respectively. For the pathways of the ancient biosynthesis of amino acids and its connection with the properties of the modern code, see "Ancient doublet genetic code was predetermined by the pathways for the synthesis of amino acids. "Based on these data, some researchers conclude that the formation of the code was greatly influenced by the biosynthetic relationships between amino acids. However, the similarity of biosynthetic pathways does not at all mean the similarity of physicochemical properties.

11. Noise immunity. In its most general form, the noise immunity of HA means that, with random point mutations and translation errors, the physicochemical properties of amino acids do not change very much.

The replacement of one nucleotide in a triplet in most cases either does not lead to a replacement of the encoded amino acid, or leads to a replacement with an amino acid with the same polarity.

One of the mechanisms that ensure the noise immunity of the GK is its degeneracy. The average degeneracy is - number of encoded signals/total number of codons, where encoded signals include 20 amino acids and the translation termination sign. The average degeneracy for all amino acids and the termination sign is three codons per encoded signal.

In order to quantify noise immunity, we introduce two concepts. Mutations of nucleotide substitutions that do not lead to a change in the class of the encoded amino acid are called conservative. Nucleotide substitution mutations that change the class of the encoded amino acid are called radical .

Each triplet allows 9 single substitutions. There are 61 triplets encoding amino acids in total. Therefore, the number of possible nucleotide substitutions for all codons is

61 x 9 = 549. Of these:

23 nucleotide substitutions result in stop codons.

134 substitutions do not change the encoded amino acid.
230 substitutions do not change the class of the encoded amino acid.
162 substitutions lead to a change in the amino acid class, i.e. are radical.
Of the 183 substitutions of the 3rd nucleotide, 7 lead to the appearance of translation terminators, and 176 are conservative.
Of the 183 substitutions of the 1st nucleotide, 9 lead to the appearance of terminators, 114 are conservative and 60 are radical.
Of the 183 substitutions of the 2nd nucleotide, 7 lead to the appearance of terminators, 74 are conservative, and 102 are radical.

Based on these calculations, we obtain a quantitative estimate of the noise immunity of the code, as the ratio of the number of conservative replacements to the number of radical replacements. It is equal to 364/162=2.25

In a real assessment of the contribution of degeneracy to noise immunity, it is necessary to take into account the frequency of occurrence of amino acids in proteins, which varies in different species.

What is the reason for the noise immunity of the code? Most researchers believe that this property is a consequence of the selection of alternative HAs.

Stephen Freeland and Lawrence Hurst generated random such codes and found out that only one of the hundred alternative codes has no less noise immunity than the universal GC.
An even more interesting fact came to light when these investigators introduced an additional constraint to take into account actual trends in DNA mutation patterns and translational errors. Under such conditions, ONLY ONE CODE FROM A MILLION POSSIBLE turned out to be better than the canonical code.
Such an unprecedented vitality of the genetic code is most easily explained by the fact that it was formed as a result of natural selection. Perhaps at one time in the biological world there were many codes, each with its own sensitivity to errors. The organism that coped better with them was more likely to survive, and the canonical code simply won the struggle for existence. This assumption seems quite realistic - after all, we know that alternative codes do exist. For more information about noise immunity, see Coded Evolution (S. Freeland, L. Hurst "Code Evolution". / / In the world of science. - 2004, No. 7).

In conclusion, I propose to count the number of possible genetic codes that can be generated for 20 canonical amino acids. For some reason this number never came across to me. So, we need to have 20 amino acids and a stop signal encoded by AT LEAST ONE CODON in the generated GCs.

Mentally, we will number the codons in some order. We will reason as follows. If we have exactly 21 codons, then each amino acid and stop signal will occupy exactly one codon. In this case, there will be 21 possible GCs!

If there are 22 codons, then an extra codon appears, which can have one of any 21 meanings, and this codon can be located in any of the 22 places, while the remaining codons have exactly one different meaning y, as in the case of 21 codons. Then we get the number of combinations 21!x(21x22).

If there are 23 codons, then arguing similarly, we get that 21 codons have exactly one different meaning of s (21! options), and two codons have 21 different meanings of a (21 2 meanings of s at a FIXED position of these codons). The number of different positions for these two codons will be 23x22. Total number of GK variants for 23 codons - 21!x21 2x23x22

If there are 24 codons, then the number of GCs will be 21!x21 3 x24x23x22, ...

....................................................................................................................

If there are 64 codons, then the number of possible GCs will be 21!x21 43x64!/21! = 21 43 x64! ~ 9.1x10 145

In any cell and organism, all features of the anatomical, morphological and functional nature are determined by the structure of the proteins that are included in them. The hereditary property of an organism is the ability to synthesize certain proteins. Amino acids are located in a polypeptide chain, on which biological characteristics depend.
Each cell has its own sequence of nucleotides in the DNA polynucleotide chain. This is the genetic code of DNA. Through it, information about the synthesis of certain proteins is recorded. About what the genetic code is, about its properties and genetic information is described in this article.

A bit of history

The idea that perhaps a genetic code exists was formulated by J. Gamow and A. Down in the middle of the twentieth century. They described that the nucleotide sequence responsible for the synthesis of a particular amino acid contains at least three links. Later they proved the exact number of three nucleotides (this is a unit of the genetic code), which was called a triplet or codon. There are sixty-four nucleotides in total, because the acid molecule, where or RNA occurs, consists of residues of four different nucleotides.

What is the genetic code

The method of coding the protein sequence of amino acids due to the sequence of nucleotides is characteristic of all living cells and organisms. That's what the genetic code is.
There are four nucleotides in DNA:

• adenine - A;
• guanine - G;
• cytosine - C;
• thymine - T.

They are indicated by capital letters in Latin or (in Russian-language literature) Russian.
RNA also has four nucleotides, but one of them is different from DNA:

• adenine - A;
• guanine - G;
• cytosine - C;
• uracil - U.

All nucleotides line up in chains, and in DNA a double helix is ​​obtained, and in RNA it is single.
Proteins are built on twenty amino acids, where they, located in a certain sequence, determine its biological properties.

Properties of the genetic code

Tripletity. The unit of the genetic code consists of three letters, it is triplet. This means that the twenty existing amino acids are coded for by three specific nucleotides called codons or trilpets. There are sixty-four combinations that can be created from four nucleotides. This amount is more than enough to encode twenty amino acids.
Degeneracy. Each amino acid corresponds to more than one codon, with the exception of methionine and tryptophan.
Unambiguity. One codon codes for one amino acid. For example, in the gene of a healthy person with information about the beta target of hemoglobin, the triplet of GAG and GAA codes for A in everyone who has sickle cell anemia, one nucleotide is changed.
Collinearity. The amino acid sequence always corresponds to the nucleotide sequence that the gene contains.
The genetic code is continuous and compact, which means that it does not have "punctuation marks". That is, starting at a certain codon, there is a continuous reading. For example, AUGGUGTSUUAAAUGUG will be read as: AUG, GUG, CUU, AAU, GUG. But not AUG, UGG, and so on, or in any other way.
Versatility. It is the same for absolutely all terrestrial organisms, from humans to fish, fungi and bacteria.

Table

Not all available amino acids are present in the presented table. Hydroxyproline, hydroxylysine, phosphoserine, iodo derivatives of tyrosine, cystine, and some others are absent, since they are derivatives of other amino acids encoded by mRNA and formed after protein modification as a result of translation.
From the properties of the genetic code, it is known that one codon is able to code for one amino acid. The exception is the genetic code that performs additional functions and codes for valine and methionine. RNA, being at the beginning with a codon, attaches a t-RNA that carries formyl methion. Upon completion of the synthesis, it splits off itself and takes the formyl residue with it, transforming into a methionine residue. Thus, the above codons are the initiators of the synthesis of a chain of polypeptides. If they are not at the beginning, then they are no different from others.

genetic information

This concept means a program of properties that is transmitted from ancestors. It is embedded in heredity as a genetic code.
Implemented during protein synthesis genetic code:

• information and RNA;
• ribosomal rRNA.

Information is transmitted by direct communication (DNA-RNA-protein) and reverse (environment-protein-DNA).
Organisms can receive, store, transfer it and use it most effectively.
Being inherited, information determines the development of an organism. But due to interaction with the environment, the reaction of the latter is distorted, due to which evolution and development take place. Thus, new information is laid in the body.

The calculation of the laws of molecular biology and the discovery of the genetic code illustrated the need to combine genetics with Darwin's theory, on the basis of which a synthetic theory of evolution emerged - non-classical biology.
Heredity, variability and Darwin's natural selection are complemented by genetically determined selection. Evolution is implemented at the genetic level through random mutations and inheritance of the most valuable traits that are most adapted to the environment.

Deciphering the human code

In the nineties, the Human Genome Project was launched, as a result of which, in the 2000s, fragments of the genome containing 99.99% of human genes were discovered. Fragments that are not involved in protein synthesis and are not encoded remained unknown. Their role is still unknown.

Chromosome 1, last discovered in 2006, is the longest in the genome. More than three hundred and fifty diseases, including cancer, appear as a result of disorders and mutations in it.

The role of such research can hardly be overestimated. When they discovered what the genetic code is, it became known according to what patterns development occurs, how the morphological structure, the psyche, predisposition to certain diseases, metabolism and vices of individuals are formed.

The genetic code is a system for recording hereditary information in nucleic acid molecules, based on a certain alternation of nucleotide sequences in DNA or RNA that form codons corresponding to amino acids in a protein.

Properties of the genetic code.

The genetic code has several properties.

Tripletity.

Degeneracy or redundancy.

Unambiguity.

Polarity.

Non-overlapping.

Compactness.

Versatility.

It should be noted that some authors also offer other properties of the code related to the chemical features of the nucleotides included in the code or to the frequency of occurrence of individual amino acids in the proteins of the body, etc. However, these properties follow from the above, so we will consider them there.

a. Tripletity. The genetic code, like many complexly organized systems, has the smallest structural and smallest functional unit. A triplet is the smallest structural unit of the genetic code. It consists of three nucleotides. A codon is the smallest functional unit of the genetic code. As a rule, mRNA triplets are called codons. In the genetic code, a codon performs several functions. First, its main function is that it codes for one amino acid. Second, a codon may not code for an amino acid, but in this case it has a different function (see below). As can be seen from the definition, a triplet is a concept that characterizes elementary structural unit genetic code (three nucleotides). codon characterizes elementary semantic unit genome - three nucleotides determine the attachment to the polypeptide chain of one amino acid.

The elementary structural unit was first deciphered theoretically, and then its existence was confirmed experimentally. Indeed, 20 amino acids cannot be encoded by one or two nucleotides. the latter are only 4. Three out of four nucleotides give 4 3 = 64 variants, which more than covers the number of amino acids present in living organisms (see Table 1).

The combinations of nucleotides presented in Table 64 have two features. First, of the 64 variants of triplets, only 61 are codons and encode any amino acid, they are called sense codons. Three triplets do not encode

amino acids a are stop signals marking the end of translation. There are three such triplets UAA, UAG, UGA, they are also called "meaningless" (nonsense codons). As a result of a mutation, which is associated with the replacement of one nucleotide in a triplet with another, a meaningless codon can arise from a sense codon. This type of mutation is called nonsense mutation. If such a stop signal is formed inside the gene (in its informational part), then during protein synthesis in this place the process will be constantly interrupted - only the first (before the stop signal) part of the protein will be synthesized. A person with such a pathology will experience a lack of protein and will experience symptoms associated with this lack. For example, this kind of mutation was found in the gene encoding the hemoglobin beta chain. A shortened inactive hemoglobin chain is synthesized, which is rapidly destroyed. As a result, a hemoglobin molecule devoid of a beta chain is formed. It is clear that such a molecule is unlikely to fully fulfill its duties. There is a serious disease that develops according to the type of hemolytic anemia (beta-zero thalassemia, from the Greek word "Talas" - the Mediterranean Sea, where this disease was first discovered).

The mechanism of action of stop codons is different from the mechanism of action of sense codons. This follows from the fact that for all the codons encoding amino acids, the corresponding tRNAs were found. No tRNAs were found for nonsense codons. Therefore, tRNA does not take part in the process of stopping protein synthesis.

codonAUG (sometimes GUG in bacteria) not only encodes the amino acid methionine and valine, but is alsobroadcast initiator .

b. Degeneracy or redundancy.

61 of the 64 triplets code for 20 amino acids. Such a threefold excess of the number of triplets over the number of amino acids suggests that two coding options can be used in the transfer of information. Firstly, not all 64 codons can be involved in encoding 20 amino acids, but only 20, and secondly, amino acids can be encoded by several codons. Studies have shown that nature used the latter option.

His preference is clear. If only 20 out of 64 triplet variants were involved in coding amino acids, then 44 triplets (out of 64) would remain non-coding, i.e. meaningless (nonsense codons). Earlier, we pointed out how dangerous for the life of the cell is the transformation of the coding triplet as a result of mutation into a nonsense codon - this significantly disrupts the normal operation of RNA polymerase, ultimately leading to the development of diseases. There are currently three nonsense codons in our genome, and now imagine what would happen if the number of nonsense codons increased by about 15 times. It is clear that in such a situation the transition of normal codons to nonsense codons will be immeasurably higher.

A code in which one amino acid is encoded by several triplets is called degenerate or redundant. Almost every amino acid has several codons. So, the amino acid leucine can be encoded by six triplets - UUA, UUG, CUU, CUC, CUA, CUG. Valine is encoded by four triplets, phenylalanine by two and only tryptophan and methionine encoded by one codon. The property that is associated with the recording of the same information with different characters is called degeneracy.

The number of codons assigned to one amino acid correlates well with the frequency of occurrence of the amino acid in proteins.

And this is most likely not accidental. The higher the frequency of occurrence of an amino acid in a protein, the more often the codon of this amino acid is present in the genome, the higher the probability of its damage by mutagenic factors. Therefore, it is clear that a mutated codon is more likely to code for the same amino acid if it is highly degenerate. From these positions, the degeneracy of the genetic code is a mechanism that protects the human genome from damage.

It should be noted that the term degeneracy is used in molecular genetics in another sense as well. Since the main part of the information in the codon falls on the first two nucleotides, the base in the third position of the codon turns out to be of little importance. This phenomenon is called “degeneracy of the third base”. The latter feature minimizes the effect of mutations. For example, it is known that the main function of red blood cells is the transport of oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. This function is carried out by the respiratory pigment - hemoglobin, which fills the entire cytoplasm of the erythrocyte. It consists of a protein part - globin, which is encoded by the corresponding gene. In addition to protein, hemoglobin contains heme, which contains iron. Mutations in globin genes lead to the appearance of different variants of hemoglobins. Most often, mutations are associated with substitution of one nucleotide for another and the appearance of a new codon in the gene, which can code for a new amino acid in the hemoglobin polypeptide chain. In a triplet, as a result of a mutation, any nucleotide can be replaced - the first, second or third. Several hundred mutations are known to affect the integrity of globin genes. Near 400 of which are associated with the replacement of single nucleotides in the gene and the corresponding amino acid substitution in the polypeptide. Of these, only 100 substitutions lead to instability of hemoglobin and various kinds of diseases from mild to very severe. 300 (approximately 64%) substitution mutations do not affect hemoglobin function and do not lead to pathology. One of the reasons for this is the “degeneracy of the third base” mentioned above, when the replacement of the third nucleotide in the triplet encoding serine, leucine, proline, arginine, and some other amino acids leads to the appearance of a synonym codon encoding the same amino acid. Phenotypically, such a mutation will not manifest itself. In contrast, any replacement of the first or second nucleotide in a triplet in 100% of cases leads to the appearance of a new hemoglobin variant. But even in this case, there may not be severe phenotypic disorders. The reason for this is the replacement of an amino acid in hemoglobin with another one similar to the first in terms of physicochemical properties. For example, if an amino acid with hydrophilic properties is replaced by another amino acid, but with the same properties.

Hemoglobin consists of an iron porphyrin group of heme (oxygen and carbon dioxide molecules are attached to it) and a protein - globin. Adult hemoglobin (HbA) contains two identical - chains and two -chains. Molecule -chain contains 141 amino acid residues, - chain - 146, - and -chains differ in many amino acid residues. The amino acid sequence of each globin chain is encoded by its own gene. The gene encoding - the chain is located on the short arm of chromosome 16, -gene - in the short arm of chromosome 11. Change in the gene encoding - hemoglobin chain of the first or second nucleotide almost always leads to the appearance of new amino acids in the protein, disruption of hemoglobin functions and serious consequences for the patient. For example, replacing “C” in one of the CAU (histidine) triplets with “U” will lead to the appearance of a new UAU triplet encoding another amino acid - tyrosine. Phenotypically, this will manifest itself in a serious illness .. A similar replacement in position 63 -chain of the histidine polypeptide to tyrosine will destabilize hemoglobin. The disease methemoglobinemia develops. Change, as a result of mutation, of glutamic acid to valine in the 6th position chain is the cause of a severe disease - sickle cell anemia. Let's not continue the sad list. We only note that when replacing the first two nucleotides, an amino acid may appear similar in physicochemical properties to the previous one. Thus, the replacement of the 2nd nucleotide in one of the triplets encoding glutamic acid (GAA) in -chain on “Y” leads to the appearance of a new triplet (GUA) encoding valine, and the replacement of the first nucleotide with “A” forms an AAA triplet encoding the amino acid lysine. Glutamic acid and lysine are similar in physicochemical properties - they are both hydrophilic. Valine is a hydrophobic amino acid. Therefore, the replacement of hydrophilic glutamic acid with hydrophobic valine significantly changes the properties of hemoglobin, which ultimately leads to the development of sickle cell anemia, while the replacement of hydrophilic glutamic acid with hydrophilic lysine changes the function of hemoglobin to a lesser extent - patients develop a mild form of anemia. As a result of the replacement of the third base, the new triplet can encode the same amino acids as the previous one. For example, if uracil was replaced by cytosine in the CAH triplet and a CAC triplet arose, then practically no phenotypic changes in a person will be detected. This is understandable, because Both triplets code for the same amino acid, histidine.

In conclusion, it is appropriate to emphasize that the degeneracy of the genetic code and the degeneracy of the third base from a general biological position are protective mechanisms that are incorporated in evolution in the unique structure of DNA and RNA.

in. Unambiguity.

Each triplet (except for meaningless ones) encodes only one amino acid. Thus, in the direction of codon - amino acid, the genetic code is unambiguous, in the direction of amino acid - codon - it is ambiguous (degenerate).

unambiguous

codon amino acid

degenerate

And in this case, the need for unambiguity in the genetic code is obvious. In another variant, during the translation of the same codon, different amino acids would be inserted into the protein chain and, as a result, proteins with different primary structures and different functions would be formed. The cell's metabolism would switch to the "one gene - several polypeptides" mode of operation. It is clear that in such a situation the regulatory function of genes would be completely lost.

g. Polarity

Reading information from DNA and from mRNA occurs only in one direction. Polarity is essential for defining higher order structures (secondary, tertiary, etc.). Earlier we talked about the fact that structures of a lower order determine structures of a higher order. The tertiary structure and structures of a higher order in proteins are formed immediately as soon as the synthesized RNA chain moves away from the DNA molecule or the polypeptide chain moves away from the ribosome. While the free end of the RNA or polypeptide acquires a tertiary structure, the other end of the chain still continues to be synthesized on DNA (if RNA is transcribed) or ribosome (if polypeptide is transcribed).

Therefore, the unidirectional process of reading information (in the synthesis of RNA and protein) is essential not only for determining the sequence of nucleotides or amino acids in the synthesized substance, but for the rigid determination of secondary, tertiary, etc. structures.

e. Non-overlapping.

The code may or may not overlap. In most organisms, the code is non-overlapping. An overlapping code has been found in some phages.

The essence of a non-overlapping code is that the nucleotide of one codon cannot be the nucleotide of another codon at the same time. If the code were overlapping, then the sequence of seven nucleotides (GCUGCUG) could encode not two amino acids (alanine-alanine) (Fig. 33, A) as in the case of a non-overlapping code, but three (if one nucleotide is common) (Fig. 33, B) or five (if two nucleotides are common) (see Fig. 33, C). In the last two cases, a mutation of any nucleotide would lead to a violation in the sequence of two, three, etc. amino acids.

However, it has been found that a mutation of one nucleotide always disrupts the inclusion of one amino acid in a polypeptide. This is a significant argument in favor of the fact that the code is non-overlapping.

Let us explain this in Figure 34. Bold lines show triplets encoding amino acids in the case of non-overlapping and overlapping code. Experiments have unambiguously shown that the genetic code is non-overlapping. Without going into the details of the experiment, we note that if we replace the third nucleotide in the nucleotide sequence (see Fig. 34)At (marked with an asterisk) to some other then:

1. With a non-overlapping code, the protein controlled by this sequence would have a replacement for one (first) amino acid (marked with asterisks).

2. With an overlapping code in option A, a replacement would occur in two (first and second) amino acids (marked with asterisks). Under option B, the substitution would affect three amino acids (marked with asterisks).

However, numerous experiments have shown that when one nucleotide in DNA is broken, the protein always affects only one amino acid, which is typical for a non-overlapping code.

ГЦУГЦУГ ГЦУГЦУГ ГЦУГЦУГ

HCC HCC HCC UHC CUG HCC CUG UGC HCU CUG

*** *** *** *** *** ***

Alanine - Alanine Ala - Cys - Lei Ala - Lei - Lei - Ala - Lei

A B C

non-overlapping code overlapping code

Rice. 34. Scheme explaining the presence of a non-overlapping code in the genome (explanation in the text).

The non-overlapping of the genetic code is associated with another property - the reading of information begins from a certain point - the initiation signal. Such an initiation signal in mRNA is the codon encoding AUG methionine.

It should be noted that a person still has a small number of genes that deviate from the general rule and overlap.

e. Compactness.

There are no punctuation marks between codons. In other words, the triplets are not separated from each other, for example, by one meaningless nucleotide. The absence of "punctuation marks" in the genetic code has been proven in experiments.

and. Versatility.

The code is the same for all organisms living on Earth. Direct proof of the universality of the genetic code was obtained by comparing DNA sequences with corresponding protein sequences. It turned out that the same sets of code values ​​are used in all bacterial and eukaryotic genomes. There are exceptions, but not many.

The first exceptions to the universality of the genetic code were found in the mitochondria of some animal species. This concerned the terminator codon UGA, which read the same as the UGG codon encoding the amino acid tryptophan. Other rarer deviations from universality have also been found.

DNA code system.

The genetic code of DNA consists of 64 triplets of nucleotides. These triplets are called codons. Each codon codes for one of the 20 amino acids used in protein synthesis. This gives some redundancy in the code: most amino acids are encoded by more than one codon.
One codon performs two interrelated functions: it signals the beginning of translation and encodes the incorporation of the amino acid methionine (Met) into the growing polypeptide chain. The DNA code system is designed so that the genetic code can be expressed either as RNA codons or as DNA codons. RNA codons occur in RNA (mRNA) and these codons are able to read information during the synthesis of polypeptides (a process called translation). But each mRNA molecule acquires a nucleotide sequence in transcription from the corresponding gene.

All but two amino acids (Met and Trp) can be coded for by 2 to 6 different codons. However, the genome of most organisms shows that certain codons are favored over others. In humans, for example, alanine is encoded by GCC four times more often than in GCG. This probably indicates a greater translation efficiency of the translation apparatus (eg, the ribosome) for some codons.

The genetic code is almost universal. The same codons are assigned to the same stretch of amino acids and the same start and stop signals are overwhelmingly the same in animals, plants, and microorganisms. However, some exceptions have been found. Most of these involve assigning one or two of the three stop codons to an amino acid.