What occurs during a substitution mutation?

A substitution mutation is characterized by the replacement of one nucleotide in the DNA sequence with a different nucleotide. This modification can occur in various forms, such as transitioning one base to another or transversion where a purine is swapped for a pyrimidine or vice versa. While the overall length of the DNA strand does not change with a substitution, the specific sequence of nucleotides is altered, potentially impacting the resulting protein if the mutation occurs within a coding region.

In contrast, the other options involve different types of mutations. Adding a nucleotide indicates an insertion mutation, while duplicating a DNA segment refers to a duplication mutation. Removing a segment of DNA describes a deletion mutation. These processes alter the DNA sequence and potentially impact gene function and protein expression but do not define a substitution. Thus, a substitution mutation specifically pertains to the replacement of one nucleotide with another.