Degeneracy of the genetic code: general information
The genetic code expressed in codons isa system of coding information about the structure of proteins, inherent in all living organisms of the planet. His decoding took a decade, but the fact that he exists, science has understood almost a century. Universality, specificity, one-pointedness, and especially the degeneracy of the genetic code, are of great biological importance.
History of discoveries
The problem of encoding genetic informationhas always been key in biology. To the matrix structure of the genetic code, science moved rather slowly. Since the discovery of J. Watson and F. Crick in 1953, the double helical structure of DNA began the stage of unraveling the very structure of code that prompted faith in the greatness of nature. The linear structure of proteins and the same DNA structure implied the existence of a genetic code as a correspondence between two texts, but recorded using different alphabets. And if the alphabet of proteins was known, then the DNA marks became the subject of study of biologists, physicists and mathematicians.
There is no point in describing all the steps in thispuzzles. A direct experiment, which proved and confirmed that there is a clear and consistent correspondence between the codons of DNA and the amino acids of the protein, was carried out in 1964 by J. Janowski and S. Brenner. And then - the period of decoding the genetic code in vitro (in vitro) using the techniques of protein synthesis in cell-free structures.
The fully deciphered code E. Coli was unveiled 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, it was explained quite simply.
Obtaining data on the decipherment of the hereditarycode became one of the most significant events of the last century. Today, science continues to study in depth the mechanisms of molecular coding and its systemic features and the overabundance of signs, in which the degeneracy of the genetic code is expressed. A separate branch of study is the emergence and evolution of the coding system for hereditary material. Evidence of the connection of polynucleotides (DNA) and polypeptides (proteins) gave impetus to the development of molecular biology. And that, in turn, biotechnology, bioengineering, discoveries in breeding and plant growing.
Dogmas and rules
The main dogma of molecular biology is the information transferred from DNA to the information RNA, and then from it to the protein. In the opposite direction, transmission is possible with RNA on DNA and with RNA on another RNA.
But the matrix or base is always DNA. And all other fundamental features of information transfer are a reflection of this matrix nature of the transmission. Namely, the transfer by performing synthesis on the matrix of other molecules, which will become the structure of reproduction of hereditary information.
Linear coding of the structure of protein moleculesis carried out with the help of complementary codons (triplets) of nucleotides, of which 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 of such synthesis. But when a protein molecule is formed, there is no quality of the quantity and quality of monomers (DNA nucleotides - amino acids of the protein). This is the natural hereditary code - the system of recording in the sequence of nucleotides (codons) the sequence of amino acids in the protein.
The genetic code has several properties:
- The triplet.
- Redundancy (degeneracy) of the genetic code.
Here is a brief description, focusing on the biological significance.
Triplet, continuity and presence of stoplights
Each of the 61 amino acids corresponds to onesemantic triplet (triplet) of nucleotides. Three triplets do not carry information about the amino acid and are stop codons. Each nucleotide in the chain is part of the triplet, and does not exist by itself. At the end and at the beginning of the chain of nucleotides responsible for a single protein, there are stop codons. They start or stop the translation (synthesis of the protein molecule).
Specificity, non-overlapping and one-pointedness
Each codon (triplet) encodes only oneamino acid. Each triplet does not depend on the neighboring one and does not overlap. A single nucleotide can enter only one triplet in the chain. Synthesis of the protein is always only in one direction, which regulates the stop codons.
Redundancy of the genetic code
Each triplet of nucleotides encodes oneamino acid. A total of 64 nucleotides, 61 of them encode amino acids (semantic codons), and three - meaningless, that is, the amino acid is not coded (stop codons). Redundancy (degeneracy) of the genetic code lies in the fact that in each triplet there can be made substitutions - 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 the triplet (positions 1, 2 and 3), each nucleotide can be replaced by 4 - 1 = 3 other variants, then the total number of possible variants of nucleotide substitutions will be 61 to 9 = 549.
The degeneracy of the genetic code is manifested inThat 549 variants is much more than necessary for encoding information about 21 amino acids. Of the 549 variants, 23 replacements will lead to the formation of stop codons, 134 + 230 replacements are conservative, and 162 substitutions are radical.
The rule of degeneracy and exclusion
If two codons have two identical firstnucleotide, and the remaining ones 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 - AUA and UGA - the first encodes methionine, although there would have to be isoleucine, and the second - a stop codon, although it would have to encode tryptophan.
The meaning of degeneracy and universality
It is these two properties of the genetic code that havethe 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 hasan adaptive value, such as multiple duplication of one amino acid code. In addition, this means a decrease in the significance (degeneration) of the third nucleotide in the codon. This option minimizes mutational damage in DNA, which will lead to gross violations in the structure of the protein. This is the protective mechanism of living organisms of the planet.