Which mutation type results in a change from cytosine to thymine?

• A. Pyrimidine to purine
• B. Pyrimidine to pyrimidine
• C. Purine to purine
• D. Purine to pyrimidine

Answer: (B)

A change from cytosine to thymine represents a transition mutation where one pyrimidine base is replaced by another pyrimidine base. Both cytosine and thymine are members of the pyrimidine class of nucleobases, which are characterized by their single-ring structure. Transitions occur when a base is substituted by another base of the same category, either purine to purine or pyrimidine to pyrimidine.

In this case, since cytosine and thymine are both pyrimidines, the mutation type that encompasses this change is indeed a pyrimidine to pyrimidine transition. Understanding this classification is essential in genetics, as it helps in predicting the potential effects of mutations on codons and ultimately on protein function.