Genetics with Basics of Selection - M.P. Myhun - 2008

CHAPTER II. Material Basis and Molecular Mechanisms of Heredity

2.5. The Genetic Code and Its Main Properties

After it was established that Nucleic Acids are the direct material carriers of heredity and The Structure of the DNA model was deciphered, scientists set out to uncover the mechanism by which Genetic information is recorded in nucleic acid molecules, particularly in DNA.

One of the early hypotheses was based on The Role of the structure of polypeptide chain molecules. The core of this dogma was that the informational content of any Gene lies in the arrangement of the Introduction/19.html">Primary Structure of its corresponding polypeptide. It follows that The nucleotide sequences of purine and pyrimidine nitrogenous bases must determine the corresponding sequences of amino acid residues during polypeptide synthesis. Therefore, only a Genetic Code of extreme functional simplicity could promptly and unerringly determine which amino acid should come next during polypeptide chain assembly. Examining the architecture of the double-stranded DNA model, it is easy to see that the simplest mechanism for METABOLISM/28.html">The Genetic Code would be for each nucleotide to encode one of the 20 Amino Acids that make up Proteins; however, there are only 4 nitrogenous bases.

To build a polypeptide chain from the 20 Essential Amino Acids found in nature, an mRNA molecule must have at least 20 different codons. If a codon consisted of 2 NUCLEOTIDES, their combinations would yield only 16 different codons, which is insufficient for the synthesis of 20 amino acids. A combination of 3 nucleotides yields 64 possible codon variants, which is more than enough.

Hence, a triplet formed by three consecutively arranged nitrogenous bases serves as a discrete unit encoding a single amino acid.

Thus, the genetic code is the recording of genetic information in nucleic acid molecules (DNA, RNA) as a complete sequence of nucleotides that encodes The sequence of amino acids in the corresponding polypeptide chains.

The code corresponding to a single amino acid consists of three nucleotides and is called a codon.

Consequently, a gene encoding a specific polypeptide can be viewed as a defined sequence of codons (triplets, nucleotides) (Table 2.1).

Class="center">Table 2.1.

The Genetic Code

Out of the 64 triplets, 61 are sense codons, meaning they encode amino acids. Two sense codons in mRNA — AUG and GUG — are called initiation codons because the ribosome starts the Synthesis of the gene product (the polypeptide) precisely with them. Three triplets in mRNA — UAA, UAG, and UGA — do not encode amino acids, but instead signal the termination of Polypeptide chain synthesis; thus, they carry a crucial biological meaning and are referred to as "termination triplets" (stop codons).

The hypothesis that the code consists of three nucleotides per codon was proposed in 1954 by the American theoretical physicist G. Gamow, who performed theoretical calculations for the genetic code, and was proven experimentally in 1961 by F. Crick using T₄ Bacteriophages. Crick practically confirmed the triplet Nature of the genetic code. Later, this Conclusion was verified by directly comparing nucleotide sequences in specific genes with Amino acid sequences in their polypeptide products. It turned out that the sequences of codons (triplets) in a gene and amino acids in the corresponding polypeptide are colinear, meaning they correspond directly to one another.

The structure of individual triplets encoding specific amino acids was first elucidated by M. Nirenberg and J. Matthaei (1961) and S. Ochoa (1962), and the code was subsequently fully deciphered.

From the four nucleotides that make up nucleic acids, 64 triplets (4³) or codons can be formed, which encode the 20 amino acids found in proteins. This means there are more codons than amino acids, and Most amino acids are encoded by two or more triplets. It should be noted that all codons are read consecutively, starting from a specific nucleotide, without any punctuation marks between them; in other words, reading occurs within a specific reading frame. A shift in the reading frame by A number of nucleotides not divisible by three disrupts the genetic meaning of the entire nucleotide sequence.

Until recently, the code was considered universal, meaning identical for All living organisms. However, it turned out that this property is not absolute, and at least in Mitochondrial DNA, the meaning of certain codons differs entirely from that in nuclear DNA.

Main Properties of the Genetic Code

1. Triplet nature (each amino acid is encoded by three nucleotides);

2. Non-overlapping (each triplet is independent and they do not share common nucleotides);

3. Colinearity (the arrangement of codons corresponds to the Amino Acid Sequence of the encoded polypeptide);

4. Degeneracy (all amino acids except Methionine and Tryptophan are encoded by two or more nucleotides);

5. Specificity (a single codon corresponds to only one amino acid);

6. Largely universal (identical for all living organisms, though the code differs slightly in Cell/35.html">Mitochondria and Plastids).