The genetic code, expressed in codons, is a coding system for protein structure information common to all living organisms on the planet. Deciphering it took a decade, but the fact that it exists, science has understood for almost a century. Universality, specificity, unidirectionality, and especially the degeneracy of the genetic code are of great biological importance.
Discovery history
The problem of coding genetic information has always been a key one in biology. Science progressed rather slowly to the matrix structure of the genetic code. Since the discovery of the double helical structure of DNA by J. Watson and F. Crick in 1953, the stage of unraveling the structure of the code began, which prompted faith in the greatness of nature. The linear structure of proteins and the same structure of DNA implied the presence of a genetic code as a correspondence between two texts, but written using different alphabets. And if the alphabet of proteins was known, then the signs of DNA became the subject of study by biologists, physicists and mathematicians.
It makes no sense to describe all the steps in solving this puzzle. A direct experiment, which proved and confirmed that there is a clear and consistent correspondence between the codons of DNA and amino acids of the protein, was carried out in 1964 by C. Janowski and S. Brenner. And then - the period of decoding the genetic code in vitro (in vitro) using protein synthesis techniques in cell-free structures.
The fully decrypted E. Coli code was published in 1966 at a bioscience symposium in Cold Spring Harbor (USA). Then the redundancy (degeneracy) of the genetic code was discovered. What this means is explained quite simply.
Decoding continues
Obtaining data on decoding the inheritance code has become one of the most significant events of the last century. Today, science continues to study in depth the mechanisms of molecular encodings and its systemic features and an overabundance of signs, which expresses the degeneracy of the genetic code. A separate branch of study is the emergence and evolution of the coding system of hereditary material. Evidence of the association of polynucleotides (DNA) and polypeptides (proteins) gave impetus to the development of molecular biology. And that, in turn, biotechnology, bioengineering, discoveries in breeding and crop production.
Dogmas and rules
The main dogma of molecular biology is that information is transferred from DNA to messenger RNA, and then from it to protein. In the opposite direction, transfer is possible from RNA to DNA and from RNA to another RNA.
But DNA is always the matrix or basis. And all the other fundamental features of information transfer are a reflection of this matrix nature of transmission. Namely, the transfer through the synthesis on the matrix of other molecules, which will become the structure of reproduction of hereditary information.
Genetic code
The linear coding of the structure of protein molecules is carried out using complementary codons (triplets) of nucleotides, of which there are only 4 (adein, guanine, cytosine, thymine (uracil)), which spontaneously leads to the formation of another nucleotide chain. The same number and chemical complementarity of nucleotides is the main condition for this synthesis. But with the formation of a protein molecule, there is no quality correspondence between the quantity and quality of monomers (DNA nucleotides - protein amino acids). This is a natural hereditary code - a system of recording in the sequence of nucleotides (codons) of the amino acid sequence in a protein.
Genetic code has several properties:
- Tripletness.
- Unambiguity.
- Directivity.
- Non-overlap.
- Redundancy (degeneracy) of the genetic code.
- Universality.
We give a brief description, focusing on the biological value.
Tripletness, continuity and the presence of brake lights
Each of the 61 amino acids corresponds to one semantic triplet (triple) of nucleotides. Three triplets do not carry amino acid information and are stop codons. Each nucleotide in the chain is part of a triplet, but does not exist by itself. At the end and at the beginning of the chain of nucleotides responsible for one protein, there are stop codons. They start or stop translation (synthesis of a protein molecule).
Specificity, non-overlapping and unidirectional
Each codon (triplet) encodes only one amino acid. Each triplet is independent of the neighboring one and does not overlap. One nucleotide can enter only one triplet in a chain. Protein synthesis always goes in only one direction, which is regulated by stop codons.
Genetic code redundancies
Each triplet of nucleotides encodes one amino acid. A total of 64 nucleotides, of which 61 encode amino acids (sense codons), and three are senseless, that is, they do not encode an amino acid (stop codons). The redundancy (degeneracy) of the genetic code lies in the fact that substitutions can be made in each triplet - radical (lead to the replacement of amino acids) and conservative (do not change the class of amino acids). It is easy to calculate that if 9 substitutions can be made in a triplet (1, 2 and 3 positions), each nucleotide can be replaced by 4 - 1 = 3 other options, then the total number of possible nucleotide substitutions will be 61 by 9 = 549.
The degeneracy of the genetic code is manifested in the fact that 549 variants are much more than necessary for encoding information on 21 amino acids. Moreover, out of 549 options, 23 substitutions will lead to the formation of stop codons, 134 + 230 substitutions will be conservative, and 162 substitutions will be radical.
The degeneracy rule and exceptions
If two codons have two identical first nucleotides, and the remaining are represented by nucleotides of the same class (purine or pyrimidine), then they carry information about the same amino acid. This is the rule of degeneracy or redundancy of the genetic code. Two exceptions - ACA and CAA - the first encodes methionine, although Isoleucine should have been, and the second is the stop codon, although Tryptophan should have been encoded.
The significance of degeneracy and universality
It is these two properties of the genetic code that have the greatest biological significance. All the properties listed above are characteristic of the hereditary information of all forms of living organisms on our planet.
The degeneracy of the genetic code has an adaptive value, such as repeated duplication of the code of one amino acid. In addition, this means a decrease in the significance (degeneracy) of the third nucleotide in the codon. This option minimizes mutational damage to the DNA, which will entail gross violations in the structure of the protein. This is the protective mechanism of living organisms on the planet.