Eukaryotic licensing factors prevent DNA replication from being initiated at ori

Question

Eukaryotic licensing factors prevent DNA replication from being initiated at origins more than once in the cell cycle. After replication has begun at an origin, a protein called Geminin inhibits licensing factors that are required for MCM2-7 to bind to an origin and initiate replication. Thus, when Geminin is present, MCM2-7 will not bind to an origin. At the end of mitosis, Geminin is degraded, allowing MCM2-7 to bind once again to DNA and relicense the origin. Marina Melixetian and her colleagues suppressed the expression of Geminin protein inhuman cells by treating the cells with small interfering RNAs (siRNAs) complementary to Geminin messenger RNA (M. Melixetian et al. 2004. Journal of Cell Biology 165:473–482). (Small interfering RNAs form a complexwith proteins and pair with complementary sequences on mRNAs; the complex then cleaves the mRNA, so there
is no translation of the mRNA; see pp. 418–419 in Chapter 14). Forty-eight hours after treatment with siRNA, the Geminin-depleted cells were enlarged and contained a single giant nucleus. Analysis of DNA content showed that many of these Geminin-depleted cells were 4 n or greater. Explain these results.

Explanation / Answer

Eukaryotic licensing factors prevent DNA replication from being initiated at ori

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