The genetic code, expressed in codons, is a system for encoding information about the structure of proteins, inherent in all living organisms on the planet. Its decoding took a decade, but the fact that it exists, science 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 encoding genetic information has always been a key one in biology. Science moved rather slowly towards the matrix structure of the genetic code. Since the discovery by J. Watson and F. Crick in 1953 of the double helical structure of DNA, the stage of unraveling the very 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 of two texts, but written using different alphabets. And ifthe alphabet of proteins was known, then the signs of DNA became the subject of study for biologists, physicists and mathematicians.
It makes no sense to describe all the steps in solving this riddle. A direct experiment, which proved and confirmed that there is a clear and consistent correspondence between DNA codons and protein amino acids, was carried out in 1964 by C. Janowski and S. Brenner. And then - the period of deciphering the genetic code in vitro (in vitro) using the techniques of protein synthesis in cell-free structures.
The fully deciphered E. coli code was made public in 1966 at a symposium of biologists in Cold Spring Harbor (USA). Then the redundancy (degeneracy) of the genetic code was discovered. What this means was explained quite simply.
Decoding continues
Obtaining data on the decoding of the hereditary 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 property of the degeneracy of the genetic code. A separate branch of study is the emergence and evolution of the coding system for hereditary material. Evidence of the relationship between polynucleotides (DNA) and polypeptides (proteins) gave impetus to the development of molecular biology. And that, in turn, to biotechnology, bioengineering, discoveries in selection and crop production.
Dogmas and rules
The main dogma of molecular biology - information is transferred from DNA to informationRNA, and then from it to protein. In the opposite direction, transmission is possible from RNA to DNA and from RNA to another RNA.
But the matrix or basis is always DNA. And all the other fundamental features of the transmission of information are a reflection of this matrix nature of the transmission. Namely, transmission by means of synthesis on the matrix of other molecules, which will become the structure for the reproduction of hereditary information.
Genetic code
Linear coding of the structure of protein molecules is carried out using complementary codons (triplets) of nucleotides, of which there are only 4 (adeine, guanine, cytosine, thymine (uracil)), which spontaneously leads to the formation of another chain of nucleotides. The same number and chemical complementarity of nucleotides is the main condition for such a synthesis. But during the formation of a protein molecule, there is no correspondence between the quantity and quality of monomers (DNA nucleotides are protein amino acids). This is the natural hereditary code - a system of recording in the nucleotide sequence (codons) the amino acid sequence in the protein.
The genetic code has several properties:
- Tripletity.
- Uniqueness.
- Orientation.
- Non-overlapping.
- Redundancy (degeneracy) of the genetic code.
- Versatility.
Let's give a brief description, focusing on the biological significance.
Tripletity, continuity and stoplights
Each of the 61 amino acids corresponds to one semantic triplet (triple) of nucleotides. Three triplets do not carry information about the amino acid and are stop codons. Each nucleotide in the chain is part of a triplet, and does not exist on its own. 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 (the synthesis of a protein molecule).
Specific, non-overlapping and unidirectional
Each codon (triplet) codes for only one amino acid. Each triplet is independent of the neighboring one and does not overlap. One nucleotide can be included in only one triplet in the chain. Protein synthesis always goes in only one direction, which is regulated by stop codons.
Redundancies of the genetic code
Each triplet of nucleotides encodes one amino acid. There are 64 nucleotides in total, of which 61 encode amino acids (sense codons), and three are meaningless, that is, they do not encode an amino acid (stop codons). The redundancy (degeneracy) of the genetic code lies in the fact that in each triplet substitutions can be made - radical (lead to amino acid replacement) and conservative (do not change the amino acid class). It is easy to calculate that if 9 substitutions can be made in a triplet (positions 1, 2 and 3), each nucleotide can be replaced by 4 - 1=3 other options, then the total number of possible nucleotide substitution options will be 61 x 9=549.
The degeneracy of the genetic code is manifested in the fact that 549 variants are much more thannecessary to encode information about 21 amino acids. At the same time, out of 549 variants, 23 substitutions will lead to the formation of stop codons, 134 + 230 substitutions are conservative, and 162 substitutions are radical.
The rule of degeneracy and exclusion
If two codons have two identical first nucleotides, and the rest are 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 - AUA and UGA - the first encodes methionine, although it should be isoleucine, and the second is a stop codon, although it should encode tryptophan.
The meaning 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, like multiple 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 in DNA, which will lead to gross violations in the protein structure. This is the defense mechanism of the living organisms of the planet.
