The Cell Cycle # Lec4 Flashcards
What initiates DNA replication once per cell cycle?
S-Cdk initiates DNA replication once per cell cycle.
What are the two distinct steps in the initiation phase of DNA replication?
The first step occurs in late mitosis and early G1, and the second step occurs in S phase.
What happens during the first step of DNA replication initiation?
Two inactive DNA helicases (Mcm helicases) are loaded onto the DNA at the replication origin.
What is the purpose of licensing replication origins?
Initiation of DNA synthesis is permitted only at origins that are preloaded with Mcm helicases.
What triggers the activation of Mcm helicases in S phase?
Mcm helicases are activated primarily by S-Cdks.
What ensures that DNA is replicated only once per cell cycle?
Once a replication origin has been activated, it cannot be reused until new Mcm helicases are loaded.
What role does S-Cdk play in controlling chromosome duplication?
S-Cdk phosphorylates and inhibits ORC and Cdc6 proteins.
How does APC/C–Cdh1 affect Mcm helicase loading?
Inactivation of APC/C–Cdh1 in late G1 prevents Mcm helicase loading.
What happens to geminin in late mitosis and early G1?
APC/C–Cdh1 triggers the destruction of geminin, allowing Cdt1 to be active.
How does geminin affect Cdt1 during S phase?
Geminin accumulates and inhibits Cdt1 that is not associated with DNA.
What is the role of Cdt1 at active replication forks?
Cdt1 association stimulates its ubiquitylation by CRL4–Cdt2, leading to Cdt1 degradation.
What is required for chromosome duplication?
Chromosome duplication requires duplication of chromatin structure.
What increases during S phase to package newly synthesized DNA?
The production of chromatin proteins, including histones, increases.
What do S-Cdks stimulate in terms of histone synthesis?
S-Cdks stimulate a large increase in the synthesis of the four histone subunits.
What is the role of nucleosome assembly factors during DNA synthesis?
They associate with the replication fork and distribute nucleosomes on both strands of DNA.
What holds sister chromatids together?
Cohesins hold sister chromatids together.
How is cohesin loaded around unduplicated chromosomes?
Cohesin is loaded with assistance from a specialized loading complex.
What modifies cohesins during S phase?
An acetyltransferase modifies a subpopulation of cohesins to lock them around the sisters.
What enzyme disentangles sister DNAs between S phase and early mitosis?
Topoisomerase II gradually disentangles the sister DNAs.
What happens during M phase?
M phase includes mitosis and cytokinesis, separating the sister chromatids into daughter nuclei.
What are the five stages of mitosis?
The stages are prophase, prometaphase, metaphase, anaphase, and telophase.
