Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter IV. MOLECULAR MECHANISMS OF HEREDITY AND REALIZATION OF GENETIC INFORMATION

CHAPTER 21. BIOSYNTHESIS OF PROTEINS IN RIBOSOMES

21.1. THE GENETIC CODE AND ITS PROPERTIES

Every living Organism is characterized by biochemical individuality, which is determined by a genetically programmed, specific set of protein molecules unique to that organism. At the same time, the flow of information that dictates The Structure of an organism's individual Proteins originates, According to the postulates of molecular biology, from genetic Nucleic Acids and consists of DNA Replication, RNA METABOLISM/31.html">Transcription, and Translation—that is, The conversion of information from the "language of NUCLEOTIDES" to the "language of Amino Acids".

In other words, establishing The Role of DNA as the carrier and repository of Genetic information, and RNA as the messenger carrying this information to the synthesized proteins, brought to the forefront The problem of the genetic (biological) code. This represents the set of signs, symbols, and a system of rules and algorithms by which the structural information contained in nucleic acids can be transformed into the specific Introduction/19.html">Primary Structure of Polypeptides, which in turn determines all the biological properties of protein molecules.

Class="center">Structure of The Genetic Code

The hypothesis that specific combinations of several different nucleotides (three out of four possible) in DNA molecules correspond to a single amino acid in protein and peptide molecules was first proposed in 1954 by the physicist George Gamow. According to this triplet theory, three consecutive nucleotides (triplets) in the polynucleotide chains of DNA encode the incorporation of a single specific amino acid residue into the polypeptide chain. Since 4 nucleotides (or nitrogenous bases) can yield 64 (4³) different combinations of 3 nucleotides (or nitrogenous bases, respectively), it was concluded that there are at least 64 "code words" for 20 amino acids.

The STRUCTURE OF THE genetic code was deciphered through direct biochemical and molecular genetic research conducted in the early 1960s. Since messenger (template) RNAs act as carriers of genetic information from DNA to proteins, the effort to crack the genetic code focused on solving the code words—triplets, or codons—within The nucleotide sequences of mRNA. The most prominent contribution to solving this problem was made in 1961 by the American biochemist M. Nirenberg.

Fig. 21.1. Marshall W. Nirenberg (born 1927), American biochemist. Made The most significant contribution to deciphering the genetic code. Nobel Prize laureate (1968).

M. Nirenberg's experiments utilized a Cell-free system from E. coli containing the Ribosomes, amino acids, tRNAs, cytosolic Enzymes, and Cofactors necessary for Protein Synthesis. It was found that when synthetic polyuridylic acid was introduced into the system as a template mRNA, the synthesis of a monotonous (consisting of a single amino acid residue) polypeptide—polyphenyla-lanine—took place. Accordingly, The Use of polyadenylic acid (poly-A) led to the synthesis of polylysine, polycytidylic acid (poly-C) to polyproline, and so on. Based on THE CONCEPT OF codons as triplets of nucleotides (or nitrogenous bases), these experiments meant the deciphering of the corresponding codons, specifically:

It was established that out of 64 nucleotide combinations, 61 codons are sense codons, meaning they specify the incorporation of a particular amino acid into a protein, while 3 codons are nonsense codons, meaning they do not encode any amino acid. These nonsense codons (UAA, UAG, UGA) act as translation termination signals (Table 21.1).

Table 21.1. The genetic code table

Properties of the genetic code:

(1) the code is universal for all biological systems—Viruses, Bacteria, and higher organisms;

(2) the code is unidirectional, meaning it is informative only when read from "left to right" (in the 5'→3' direction);

(3) the code is continuous, possessing a linear, uninterrupted reading order with no "punctuation marks" between codons;

(4) the code is non-overlapping—after information from one triplet is read, the "reading frame" shifts to the right by exactly three nucleotides;

(5) the code is degenerate, meaning each amino acid is encoded not by a single codon, but by several.