What combination of the following events normally prevents the origins of replication in the yeast Saccharomyces cerevisiae from "firing" more than once during the cell cycle? Your answer is a two-letter string composed of letters A to E only, e.g. AB. Order the letters in your answer alphabetically.
A) The helicase-loading proteins Cdc6 and Cdt1 are only active in S phase.
B) The helicase Mcm1 can be delivered to the origin recognition complex (ORC) only in S phase.
C) The ORC can only become active (by dephosphorylation) in G? phase.
D) The helicase can only become active (by phosphorylation) in S phase.
E) The ORC can bind to the origin only in S phase.

Question

What combination of the following events normally prevents the origins of replication in the yeast Saccharomyces cerevisiae from "firing" more than once during the cell cycle? Your answer is a two-letter string composed of letters A to E only, e.g. AB. Order the letters in your answer alphabetically.
A)	The helicase-loading proteins Cdc6 and Cdt1 are only active in S phase.
B)	The helicase Mcm1 can be delivered to the origin recognition complex (ORC) only in S phase.
C)	The ORC can only become active (by dephosphorylation) in G? phase.
D)	The helicase can only become active (by phosphorylation) in S phase.
E)	The ORC can bind to the origin only in S phase.

Quizplus · Accepted Answer

The helicase is delivered to the ORC by active Cdc6 and Cdt1 proteins in G1 phase, but is activated by phosphorylation only in S phase. At the same time, phosphorylation of ORC prevents its reactivation in S phase. ORC can be reactivated again later in G1.

What Combination of the Following Events Normally Prevents the Origins