Question

UAA, UAG and UGA are:
A. Non overlapping codons
B. Nonsense codons
C. Degenerate codons
D. Chain initiator codons

Answer 1

Hint: The cells synthesize their specific proteins by the information flowing from the DNA. This information exists as the particular sequence of bases in the DNA strands and is called genetic code.

Complete answer:
A. Non overlapping codons:
(i) Means the successive triplets are read in order. Each nucleotide in the sequence is a part of only one codon.
(ii) The adjacent codons do not overlap, they do not share any base. Each single base is a part of only one codon.
(iii) The sequence CCUCAG is read only as CCU and CAG and not as CCU, CUC, UCA, CAG.
B. Nonsense codons: UAA, UAG, UGA codons, do not specify any amino acid. They are called nonsense or terminator codons.
C. Degenerate codons: It lacks specificity and one amino acid often has more than one code triplet. Only methionine and tryptophan have single triplet codons.
D. Chain initiator codons: AUG codon is called initiator or start codon as it begins the synthesis of polypeptide chains.

The correct option is (B) Nonsense codons.

Note: Out of 64 codons, three namely UAA, UAG and UGA do not specify any amino acid, they are nonsense codons. Either of these stops synthesis of the polypeptide chain. This only 61 of the 64 codons code for amino acids.