Final answer:
Apurinic sites result from the loss of a purine base such as adenine (A) or guanine (G) from DNA, which can cause mutations if the DNA repair mechanisms do not correct the error.
Step-by-step explanation:
Apurinic sites (AP sites) involve a spontaneous loss of a purine base (A or G) in an intact double-helix DNA molecule. These sites can be a consequence of depurination, which is the hydrolytic removal of guanine or adenine from the DNA strand. If not repaired, this can lead to a mutation during DNA replication. For instance, the replisome may ignore the missing base and lead to a single base-pair deletion in the daughter DNA. Additionally, spontaneous deamination of 5-methylcytosine can cause a change in the DNA sequence, further highlighting the importance of the DNA repair mechanisms to maintain the integrity of genetic information.