What type of mutation occurs when there is a change in the number of cytidine residues during DNA replication?

The situation described involves a change in the number of cytidine residues during DNA replication, which is indicative of slipped-strand mispairing. Slipped-strand mispairing occurs during DNA replication when the DNA polymerase encounters repeated sequences, particularly in regions rich in homopolymers (like cytidine). This can lead to the insertion or deletion of nucleotides, causing shifts in the reading frame and changes in the length of the DNA sequence.

In essence, during replication, the new strand can loop out, resulting in either the addition of extra nucleotides (in the case of insertion) or the omission of nucleotides (in the case of deletion), specifically involving the cytidine residues in this scenario. This type of mutation is a common mechanism behind microsatellite instability, which is particularly relevant in various genetic disorders and cancers.

Other processes, like crossing over, DNA excision repair, and thymidine dimer formation, involve different mechanisms and outcomes, and do not directly lead to a change in the number of specific nucleotides as described in the question.