Q.50 Which of the following statements about initiation of DNA replication in
eukaryotes is/are true?
(A) DNA replication is initiated at the origin of replication licensed by loading
of Mcm helicase complex
(B) Loading of Mcm helicase complex takes place in S phase
(C) Mcm helicase complex are activated by S-Cdks
(D) Mcm helicase complex is responsible for loading of origin recognition complex
Eukaryotic DNA Replication Initiation
DNA replication in eukaryotes begins at origins licensed by the Mcm helicase complex during G1 phase. Only specific statements accurately describe this regulated process.
Correct Answer
Statement (A) is true: DNA replication initiates at origins licensed through Mcm helicase loading by the origin recognition complex (ORC), Cdc6, and Cdt1.
Option Breakdown
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(A) True: ORC binds origins in G1 phase, recruiting Cdc6 and Cdt1 to load inactive Mcm2-7 double hexamers, licensing origins for S-phase firing.
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(B) False: Mcm loading occurs in G1 phase at low CDK levels, not S phase, preventing re-licensing after replication starts.
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(C) True in context, but phrasing issue: S-Cdks (S-phase cyclin-dependent kinases) activate loaded Mcm via CMG helicase assembly with Cdc45 and GINS, though DDK also contributes.
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(D) False: ORC enables Mcm loading; Mcm does not load ORC, which binds DNA first.
Key Process Stages
Eukaryotic replication ensures once-per-cycle copying through licensing and activation:
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G1: ORC-Cdc6-Cdt1 loads Mcm double hexamers.
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S phase: S-CDKs and DDK activate Mcm into CMG for unwinding.
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Prevents re-replication via high CDK blocking new loading.
