8.7 The Genetic Code

The genetic code : DNA is genetic material and contains genetic information. The expression of a gene takes place through specific enzymes. Each gene produces a specific (one-gene one-protien hypothesis). In other words, formation of each specific protein is controlled by a particular gene. A gene is (almost always) a segment of DNA strand and so, the information for the formation of a protein is contained in the DNA strand.

Further, each protein is a long polypeptide chain molecule formed by joining amino acid molecules. From the cell pool, only 20 different types of amino acids are used for protein synthesis.

The sequence of the nitrogen bases in the DNA determines the sequence of amino acids in a protein molecule through the mRNA. This sequence is copied down by the mRNA (transcription). It is present on the mRNA strand in the form of coded language (cyptogram or mRNA language or genetic code). The mRNA bases (A, U, C and G) serve as the four alphabets of the coded language.

Codon : The smallest sequence of the nitrogen bases (nucleotides) on the mRNA which can specify one amino acid is called a codon . Each codon consists of three successive bases on the mRNA.

Why should each codon in the genetic code consist of 3 bases (triplet codon) and not of one base each or 2 bases each? This is because there are 20 different amino acids which can be used in the synthesis of proteins in the cells. There must be at least one specific codon for each amino acid. Thus, there has to be at least 20 different codons in the genetic code. There are only four bases. A minimum of 3 bases per codon is necessary to have (a minimum of) 20 codes.

The wobble hypothesis (Crick, 1966) : The anticodon on tRNA is complementary to the codon on the mRNA as per the A = U, G = C base pairing rule. However, it has been observed that the 3rd base position may vary and yet still code for the same amino acid. For example, both codons TTA and TTG code for the amino acid, leucine. Thus, the third position is called the wobble position.

Thus, Crick’s (1966) wobble hypothesis explains the degeneracy of the genetic code at the third position of the codon.

Table of Contents

8.0 Introduction
8.1 Packaging of Hereditary Material
8.2 The Structure of DNA
8.3 Replication Of DNA In Eukaryotes
8.4 Replicatin of Pokaryotic Chromosome
8.5 Plasmids
8.6 RNA: Structure and Types
8.7 The Genetic Code
8.8 The Central Theme of Protein Synthesis

Chapter 9

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