Genetic Code

Structure and basic principles

Although DNA is the initial carrier of information, the genetic code is traditionally represented in the form of mRNA (messenger RNA) codons. The basic unit of this system is the triplet (or codon)—a combination of three nucleotides.

The total capacity of the genetic code is 64 possible combinations. Of these, the vast majority—61 codons—are sense (coding) codons. They are responsible for incorporating one of the 20 standard amino acids into the protein chain: alanine, arginine, asparagine, aspartic acid, valine, histidine, glycine, glutamine, glutamic acid, isoleucine, leucine, lysine, methionine, proline, serine, tyrosine, threonine, tryptophan, phenylalanine, and cysteine.

The remaining 3 triplets (UAA, UGA, UAG) do not code for any amino acids. They act as specific punctuation marks—stop codons (termination codons)—which signal the end of protein synthesis.

Main properties of the genetic code

Deciphering the genetic code became one of the major biological breakthroughs of the 20th century. Today, several fundamental properties of this system ensure the accuracy of hereditary information transfer:

• Triplet nature. Each amino acid is always encoded by a sequence of three nucleotides.
• Degeneracy (redundancy). Since there are significantly more sense codons (61) than encoded amino acids (20), many amino acids are specified by two or more codons. For example, there are 4 codons for glycine and as many as 6 for serine. Exceptions to this rule are methionine and tryptophan—each of these amino acids corresponds to only one specific triplet.
• Specificity. Despite redundancy, each individual triplet codes for strictly one specific amino acid and can never code for another.
• Non-overlapping. During protein synthesis, codons in mRNA are read in a strictly sequential manner. The same nucleotide cannot simultaneously belong to two adjacent codons.
• Presence of punctuation marks. Translation is tightly regulated: the start codon AUG initiates assembly, while stop codons halt it.
• Universality. The code is virtually identical across the vast majority of living organisms on the planet, proving the common origin of life. Rare exceptions are found only in mitochondrial DNA and certain specific species.

Role of the start codon

The AUG codon plays a dual role in the cell. First, it serves as the initiation start codon: located at the very beginning of a gene, it signals the start of translation. Second, this triplet is a sense codon and encodes the amino acid methionine.

It is important to note that if the AUG codon occurs in the middle of a gene rather than at the beginning, it loses its start signal function and simply codes for the incorporation of methionine into the growing polypeptide chain.

History of discovery and deciphering

The path to understanding how nucleic acids direct protein synthesis took about twenty years of intense work by scientists worldwide:

1. In the 1950s, physicist G. Gamow formulated the theoretical proposition that the system must be a triplet code. He mathematically proved that to encode 20 amino acids using 4 types of nucleotides, combinations of at least three elements are required.
2. In the 1960s, a group of researchers (F. Crick, S. Brenner, G. Wittmann, and others) experimentally confirmed Gamow's theory. They proved that the code is indeed triplet and that the mRNA molecule is read continuously by the cellular machinery.
3. In 1966, researchers M. Nirenberg, S. Ochoa, and H.G. Khorana completed the final deciphering of the genetic code, establishing the exact correspondence of all sense triplets to the twenty amino acids.

Stages of genetic information expression

Genetic information is expressed through a complex cascade of processes. In eukaryotes, this pathway consists of the following stages:

1. Transcription (RNA synthesis on a DNA template).
2. Processing (maturation of the RNA molecule).
3. Translation (direct assembly of the polypeptide on the ribosome).
4. Post-translational protein modifications (acquisition of the functional protein conformation).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of genetic information transcription
• Processing and splicing of pre-mRNA in eukaryotes
• Translation: initiation, elongation, and termination
• Structure and functions of transfer RNAs (tRNA)
• Types of post-translational protein modifications

Other topics in "Molecular Genetics"

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