Question 1
Multiple choiceMedium1 markTwo versions of a gene occur in a population. In version X, one nucleotide is replaced by another; in version Y, one nucleotide is inserted within the coding sequence, before its final codon. No other sequence changes are present. Which statements are correct?
I. Version X is a single-nucleotide polymorphism (SNP) resulting from a base substitution.
II. The replacement in X must change the amino acid encoded at that position.
III. The insertion in Y shifts the reading frame of subsequent codons, making it likely that the polypeptide ceases to function.
Show answer and explanation
Answer: B. A base substitution can give rise to an SNP, but degeneracy of the genetic code means it may or may not change the amino acid. A one-nucleotide insertion within a coding sequence shifts the reading frame of subsequent codons, making loss of polypeptide function likely. Thus I and III are correct; II is not.
A: accepts the misconception in II that every substitution changes an amino acid, and omits the consequence of the insertion.
C: accepts II and fails to identify the base substitution as an SNP.
D: includes II; degeneracy means the replacement need not change the amino acid.
Remember: substitutions may be silent, while a one-nucleotide insertion can shift the reading frame.