Question

Out of 64 codons, 61 codons are for 20 types of amino acids. It is due to

• A. Overlapping genes
• B. Degeneracy of genetic code
• C. Wobbling of codons
• D. Universality of codons

Answer 1

Answer: (B)

Degeneracy of genetic code

The genetic code is universal which means that a particular codon specifies a particular amino acid in all the organisms. The mRNA codon "ACU" codes for threonine, irrespective of its source. It does not explain the presence of 61 codons for 20 amino acids. The genetic code is nonoverlapping means a base in mRNA is not shared by two different codons; if it was the case, six bases could code for four amino acids which are not true. Wobble pairing refers to loose base pairing between first base of the anticodon (5’-3’direction) and the third base of the codon on mRNA. The presence of wobble bases (inosine, I) at the first (wobble) nucleotide of an anticodon allows three different codons to be recognized by a tRNA. This is because it contains the uncommon base hypoxanthine and can pair with three different nucleotides (U, C, and A). Wobble base allows rapid dissociation of from mRNA but does not explain 61 codons for 20 amino acids. Genetic code show degeneracy which means that multiple codons specify particular amino acid; asparagine is encoded by "GAU and GAC". Degeneracy allows one amino acid to be encoded by multiple codons; thus 61 codons for 20 amino acids. The correct option is B.