Day 111 MCAT Practice Question

Answer D is correct because the DNA code is non-overlapping in the

sense that a set of three nucleotide base pairs correspond to a particular codon

(and that codon’s particular corresponding amino acid). If a DNA segment has the

base pairs CCCAACGGA, the code is read as three discrete codons (CCC-AACGGA), with none of the nucleotide base pairs contributing to more than one codon.

Each base pair contributes to one and only one codon. The triplet nature of the

DNA code refers to the fact that it requires three nucleotide base pairs to form a

single codon.

Missense mutations refer to mutations that result in a different amino acid being

joined to the growing peptide chain. For example, an A to U mutation that causes

GAC (aspartic acid) to become GUC (glycine) results in the transfer of a different

amino acid to the growing peptide chain (glycine rather than aspartic acid). In

contrast, nonsense mutations refer to those that create a stop codon (UAA, UAG,

or UGA) and therefore terminate the growing peptide chain altogether