Origins of replication tend to occur at segments rich in which base pairing?

• A. A=T rich segments
• B. G=C rich segments
• C. GC-rich segments
• D. ATG-rich segments

Answer: (A)

Origins of replication unwind more easily in regions that are rich in A and T base pairs. A–T pairs are held by only two hydrogen bonds, whereas G–C pairs have three, so AT-rich sequences have a lower melting temperature and separate more readily. This makes it easier for initiator proteins to load helicase and form the open complex needed to start replication. GC-rich segments are more stable and resist unwinding, so they’re less favorable as origins. ATG-rich sequences aren’t a known feature driving origin melting; ATG is a start codon in translation, not a factor in origin initiation. In bacteria, for example, the origin region often contains AT-rich motifs that promote initial unwinding by the replication machinery.