Final answer:
Bases can be added during DNA synthesis because methylation occurs post-replication and serves gene regulatory functions, while DNA synthesis focusses on accurately copying the genetic code. Adenine methylation in bacteria differentiates parent and daughter strands during replication and repair.
Step-by-step explanation:
During DNA synthesis, the reason why bases are allowed to be added even though methylation occurs on A (adenine) bases is that the two processes serve different functions and occur during different stages of the DNA life cycle. Methylation often happens after DNA synthesis is complete and is mainly involved in gene regulation. Specifically, methylation within CpG islands can impact how DNA interacts with proteins, such as histones, and can lead to gene silencing or activation. Adenine methylation is more common in bacterial genomes and plays a role in DNA replication and repair by allowing for the differentiation between the parent and daughter strands. During DNA replication, the newly synthesized strand is not immediately methylated, which permits the addition of bases. Methylation may occur later as part of the cellular processes that recognize and modify the DNA molecule.