Question

The amino acid sequences of a bacterial protein and a human protein carrying out similar function are found to be 60% identical. However, the DNA sequences of the genes coding for these proteins are only 45% identical. This is because

• A. Protein sequence does not depend on DNA sequence.
• B. DNA codons having different nucleotides in the third position can code for the same amino acids.
• C. DNA codons having different nucleotides in the second position can code for the same amino acids.
• D. Same DNA codons can code for multiple amino acids.

Answer 1

The correct option is B DNA codons having different nucleotides in the third position can code for the same amino acids.

The nucleotide sequence of DNA serves as the template for RNA synthesis during transcription. The mRNA sequence of triplet serves as the template for amino acid sequence in proteins; thus an amino acid sequence of the protein is indirectly determined by DNA; option A is wrong. In genetic code table, each box is specified by the first and second positions (for example the AAX box, in which X is any of the four bases) therefore eight of the sixteen boxes contain just one amino acid per box. This means codons need only be read in the first two positions for these eight amino acids because the same amino acid will be represented regardless of the third base of the codon. That’s why multiple codons specify single amino acid and make the genetic code degenerate. Thus, DNA segments with 45% homology can code for the protein with 60% homology; option B is correct. There is a specific base pairing between first two bases of mRNA codon and tRNA anticodon, thus two triplets differing in the second base cannot code for same amino acid; option C is wrong. Codons are specific and same codons do not specify multiple amino acids; option D is incorrect. The correct answer is B.