Genetic Code

Genetic Code: DNA is the master molecule of the cell: it initiates, guides, regulates and controls protein synthesis, so it must carry the information for the synthesis of proteins within itself. The site of this information is the sequence of nucleotides (nitrogen bases), as evidenced by Yanofsky and Sarabhai (1964). About 20 amino acids take part in protein synthesis while DNA has only 4 kinds of nitrogen bases to identify them; how can 4 bases encode 20 amino acids? According to F.H.C. Crick the information is stored in a coded language (cryptogram) called the genetic code, whose code words (codons) each specify one amino acid; the genetic code is therefore the collection of base sequences that correspond to each amino acid.

A single base per codon (singlet code) could encode only 4 amino acids and two bases (doublet code) only 16, whereas three bases (triplet code) can specify 64. Hence G. Gamow (1954) suggested that a codon must be a combination of three consecutive bases, sufficient for 20 amino acids: the dictionary of the genetic code holds 64 triplet codons. That the code is a triplet code was first evidenced by Crick (1961) using frame-shift mutations. M. Nirenberg and Matthaei synthesised an artificial mRNA of only uracil (a poly-U homopolymer); added to a protein-synthesising system it produced a small polypeptide of phenylalanine, showing that UUU codes for phenylalanine. Other homopolymers, and codons of two or more bases, were then tried. Dr. Har Gobind Khorana devised a technique for artificially synthesising mRNA with repeated sequences of known nucleotides; using synthetic DNA he prepared polyribonucleotides with known repeats of two or three nucleotides, e.g. CUC UCU CUC UCU, which gave a polypeptide with two amino acids alternating (leucine and serine), while three-base repeats such as CUA CUA CUA gave a polypeptide of a single amino acid (leucine). Later Severo Ochoa established that the enzyme polynucleotide phosphorylase could polymerise RNA of defined sequence in a template-independent manner. Finally Nirenberg, Matthaei and Ochoa deciphered all 64 codons of the dictionary of the genetic code (Fig. 4.13).

Characteristics of Genetic code: During replication and transcription a nucleic acid is copied into another nucleic acid on the principle of complementarity; during translation the information passes from a polymer of nucleotides to a polymer of amino acids, where no complementarity exists, and a change in the nucleic acid results in a change in the amino acids of the protein. The genetic code of DNA has these fundamental characteristics:

i. Genetic code is a triplet code: three consecutive bases form a codon that specifies one particular amino acid; the base sequence in a codon is always read 5' to 3', and the code is a triplet code in every living organism.

ii. Genetic code has distinct polarity: the code has a definite direction and is always read 5' to 3', never 3' to 5', otherwise the message changes (e.g. 5' AUG 3').

iii. Genetic code is non-overlapping: each base is part of only one codon and adjacent codons do not overlap; with 6 consecutive bases a non-overlapping code gives only two amino acids, whereas an overlapping one would give four. Experimental evidence favours the non-overlapping nature.

iv. Genetic code is commaless: there is no gap or punctuation mark between successive codons.

v. Genetic code has degeneracy: usually one amino acid is encoded by one codon, but some are encoded by more than one codon, e.g. cysteine has two codons and isoleucine three. This degeneracy is explained by the wobble hypothesis: the first two bases of such codons are identical and only the third varies.

vi. Genetic code is universal: by and large a given codon specifies the same amino acid in all living organisms, e.g. AUG always specifies methionine, from bacteria to humans.

vii. Genetic code is non-ambiguous: a particular codon encodes one specific amino acid; two different amino acids are never encoded by the same codon.

viii. Initiation codon and termination codon: AUG is the initiation codon in every mRNA and codes for methionine. Of the 64 codons, three - UAA, UAG and UGA - are termination codons that stop the elongation of the polypeptide chain, as they do not code for any amino acid.

ix. Codon and anticodon: a codon is part of the mRNA transcribed from DNA, e.g. AUG, always written 5' AUG 3'; an anticodon is part of the tRNA and is always written 3' UAC 5'.