The genetic code is a biological program. Thanks to her, the amino acid sequences of proteins are encoded using the corresponding nucleotide sequences. This encoding is triplet. That is, one amino acid corresponds to a sequence of 3 nucleotides of mRNA. Such a triplet of nucleotides is called a codon. The biological text written in mRNA is read by the ribosome. She does it consistently. It starts with an initiation codon, that is, the initial, and then moves on to other codons. An explanatory diagram is shown below.
In the scheme, the letters "a" denote the amino acid residues of the protein. There are 20 types of them. And there are 64 types of codons. This shows that not every codon has an amino acid. Such insignificant codons perform a special function. They are responsible for marking the ends of the protein chains. They are called termination codons. Other codons correspond to some amino acid residues.
Thus, it can be seen that the considered code is triplet, non-overlapping (reading occurs sequentially, codon by codon) and contains termination and initiation codons.
How did the specialists manage to establish the correspondence of each amino acid residue to specific codons and determine which codons indicate the beginning and end of the protein chain synthesis? To do this, it was necessary to read 2 parallel biological texts - the genome and the amino acid corresponding to a specific protein gene. Since the cells know the code, they were asked to recognize different nucleotide sequences.
To do this, we took cell extracts that had the ability to synthesize protein into RNA, but did not contain enzymes capable of cleaving RNA. Such extracts are called acellular system.
The extract was obtained from the bacterium E. coli, and then artificial RNA, consisting only of uracils, was added to it. In this way, the cell-free system was asked the question: what amino acid does the UUU codon correspond to? It turned out that phenylalanine corresponds to it. So the decryption of the code was found. Then the corresponding translation was made for other amino acids.
The fully deciphered genetic code is shown below. In the central circle, the first nucleotides of the codons are indicated, in the second circle - the second, and in the third - the third. On the outside are indicated amino acid residues corresponding to codons.
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Image of the genetic code
Termination codons are denoted by the symbol TEP. What are the symbols for initiation codons? There are no such special codons. This role under certain conditions is assumed by the codons AUG and GUG. They usually correspond to methionine and valine.
The figure clearly shows a certain pattern: which acid a particular codon will correspond to is determined by the first 2 nucleotides. The third nucleotide does not play an important role. The main load is borne by the doublet located at the beginning of the codon. In other words, we can say that the code is quasi-doublet.
This main feature was noted at the earliest stage of its decoding. Naturally, it is impossible to encode all 20 amino acids with doublets, since the number of doublets is 16. Hence, the third nucleotide in the codon carries a certain semantic load.
However, there is a universal rule based on the fact that 4 nucleotides - adenine, cytosine, guanine and uracil in their structure are combined into 2 different classes. These are pyrimidine (U and C) and purine (A and D).
Hence, the code degeneracy rule is formulated as follows: if 2 codons with 2 identical first nucleotides, and the third belong to the same class (purine or pyrimidine), then they encode the same amino acid.
The figure shows that the rule is strictly followed. But there are 2 exceptions to it. The AUA codon is responsible for isoleucine, not methionine. The UGA codon signals the end of the synthesis, and in theory it should have responded to tryptophan. These are the surprises the genetic code has. They must be taken into account and at the same time it must be understood that the given rule is universal.
Vyacheslav Markin