Chromosomal origins of replication are frequently found in adenine-thymine (AT)-
rich regions. From a chemical and molecular perspective, why is this feature
advantageous for DNA replication initiation? Could a guanine-cytosine (GC)-rich
sequence serve the same purpose? Select the best answer.
AT-rich origins are recognized by specific initiator proteins that bind
preferentially to A-T base steps. GC-rich sequences lack the same sequence
motifs, preventing the correct assembly of the pre-replication complex.
AT-rich regions facilitate origin activity because the A-T base pairs form only two
hydrogen bonds, requiring less energy to unwind the DNA helix. In contrast, G-C
base pairs form three hydrogen bonds, making strand separation more
thermodynamically difficult.
AT-rich regions promote replication initiation because they adopt a bent DNA
structure that favors protein loading. GC-rich regions maintain a stiffer helical
shape that resists conformational changes necessary for initiation complex
assembly.
AT-rich sequences are preferred because they lower the melting temperature of
the DNA duplex, making it easier for helicases to locally denature the strands.
GC-rich regions have higher melting points that impede efficient unwinding.