Question

Which of the following groups of codons is called degenerate codons?
(a) UAA, UAG, and UGA
(b) GUA, GUG, GCA, GCG, and GAA
(c) UUA, UUG, CUU, CUC, CUA, and CUG
(d) AAC, AAG, GAG, and CGG

Answer 1

Hint: The genetic code is described as degenerate because one amino acid could also be coded by more than one codon. The genetic code doesn't overlap, meaning that every nucleotide is a component of just one codon.

Complete answer: The three nucleotides (triplet) base sequences in mRNA that act as code words for amino acids in protein constituting the genetic code or simply codons. The genetic code is considered a dictionary of nucleotide bases (A, G, C, and U) that determines the sequence of amino acids in proteins. The codons consist of the four nucleotide bases, the purines- adenine (A) and guanine (G), and the pyrimidines- cytosine (C) and uracil (U). These four bases construct 64 different combinations of three- base codons.
Degeneracy is the redundancy of the genetic code. A position of a codon is said to be a fourfold (having four units) degenerate site if any nucleotide at this location described the same amino acid. The codons encoding one amino acid may diverge in any of their three positions. The amino acid leucine is described by UUA, UUG, CUU, CUC, CUA, and CUG codons, which have differences in the first or third position, which is the degenerate codons.
The three codons UAA, UAG, and UGA don't have a code for amino acids. They act as stop signals in protein synthesis. These three codons are collectively known as terminal codons or non- sense codons. The codons UAG, UAA, and UGA are often referred to, respectively, as amber, ochre, and opal codons. The codons are the chain initiation codons, and sometimes GUG also is the chain initiation codon.
So, the correct answer is ‘UUA, UUG, CUU, CUC, CUA, and CUG’.

Note:
- The nucleotide sequence of the codon on mRNA is written from 5’ end to 3’ end.
- Sixty-one codons code for the 20 amino acids found in protein.
- The concept of codons was first described by Crick and his colleagues in 1961.
- Degenerate codons term was given by Bernfield and Nirenberg.