I'd be happy to help you with that question! Here's a clear and concise answer to your question:
A substitution mutation is when a nucleotide is replaced with another nucleotide. This means that one base in the DNA sequence is swapped out for a different base.
For example, let's consider the following DNA sequence:
Original: A T C G A
After substitution mutation: A T G G A
In this case, the third nucleotide (C) was replaced with a different nucleotide (G) resulting in a change in the DNA sequence due to a substitution mutation.
Therefore, the correct answer to the question "Which sentence describes a substitution mutation?" is:
B. A nucleotide is replaced with another nucleotide.
