cell cycle entry and DNA replication Flashcards
lecture 19
What happens during the G1 phase of the cell cycle?
Deciding if conditions are right for a full cell cycle.
Growing and preparing for DNA synthesis.
What happens during the S phase of the cell cycle?
DNA and centrosomes are replicated.
What happens during the G2 phase of the cell cycle?
Deciding if conditions are right for mitosis.
What happens during the M phase of the cell cycle?
Chromosome segregation and cytokinesis.
What is G0, and what types of cells are found here?
G0 is the resting state where cells are not in the cell cycle.
Examples: terminally differentiated cells (e.g., neurons), senescent cells, or quiescent cells that can re-enter the cycle.
What are mitogens, and how do they influence the G1/S transition?
Mitogens are signalling molecules that promote G1/S-cyclin synthesis by activating pathways such as Ras-Raf-MAPK, leading to Cyclin D expression.
What role does Cyclin D play in cell cycle progression?
Cyclin D binds to Cdk4/6 to form G1-Cdk, which drives the cell past the G1 checkpoint.
How does Retinoblastoma protein (Rb) regulate the G1-S transition?
In early G1, Rb binds and inactivates E2F.
Phosphorylation of Rb by G1-Cdk inactivates Rb, freeing E2F to upregulate Cyclins E and A.
What cyclins are associated with the G1/S and S phases?
Cyclin E and Cyclin A form complexes with Cdk2 to drive DNA replication.
How does p53 respond to DNA damage during G1?
DNA damage activates ATM/ATR and Chk1/Chk2 kinases, which phosphorylate and stabilize p53.
p53 upregulates CKIs like p21, inhibiting G1/S-Cdk activity.
What happens if the G1 checkpoint fails?
Cells may enter S phase with damaged DNA, potentially leading to mutations and diseases such as cancer.
What are the two main tasks of the S-phase?
1.Replication of genomic DNA.
2. Duplication of centrosomes.
Why is licensing important in DNA replication?
Licensing ensures each origin is used only once. ORCs recruit pre-replicative proteins in G1 but deactivate after firing in S phase.
What is the role of DNA helicase?
Separates the DNA double helix at replication forks.
How is the lagging strand synthesized?
Synthesized discontinuously as Okazaki fragments, which are later joined by DNA ligase.
What is the role of the sliding clamp (PCNA)?
Ensures DNA polymerase remains attached to the DNA during replication.
How does DNA polymerase maintain high fidelity?
Incorporates proofreading exonuclease activity to remove incorrectly paired nucleotides.
What is the function of telomerase?
Extends the 3’ end of chromosomes with GGGTTA repeats, allowing lagging strand synthesis without losing genetic information.
What protects the free ends of DNA?
Telomere binding proteins and the T-loop structure prevent degradation.
What is strand-directed mismatch repair?
Corrects replication errors by recognizing mismatched nucleotides on the newly synthesized strand.
What is base excision repair?
Recognizes and removes damaged or chemically altered bases using specialized enzymes.
How is a double-stranded DNA break repaired?
- Non-homologous end joining (NHEJ): error-prone, directly ligates free ends.
- Homologous recombination (HR): accurate, uses a sister chromatid template.
What prevents entry into mitosis if DNA replication is incomplete?
p53 and p21 inhibit M-Cdk, and Cdc25 is inactivated to prevent mitosis.
What questions are asked at the G1 checkpoint?
Are nutritional conditions suitable?
Is the cell receiving proliferation signals?
Has DNA damage been repaired?
What questions are asked at the G2 checkpoint?
Is DNA replication complete?
Has DNA damage been repaired?
What happens at the metaphase-anaphase transition checkpoint?
Ensures all chromosomes are properly attached to the spindle before triggering anaphase.
What happens if the G1 checkpoint is defective?
Aberrant mitogen signalling can drive uncontrolled proliferation (e.g., cancer caused by EGFR or Ras mutations).
What happens if the G2 checkpoint fails?
Cells accumulate DNA damage, often due to p53 mutations.
What happens if the mitotic checkpoint fails?
Aneuploidy (incorrect chromosome number) can result, leading to conditions like cancer predisposition syndromes.
How do mitogens promote S-phase entry?
Mitogens activate pathways that increase G1-Cdk activity, which inactivates Rb to release E2F and drive the synthesis of S-phase cyclins.
How is the DNA replication process initiated and regulated?
ORC licensing in G1 ensures replication origins fire only once. Replication is initiated in S phase and involves a complex of enzymes for elongation.
How is DNA damage handled during replication?
DNA polymerase proofreading, strand-directed mismatch repair, and base/nucleotide excision repair maintain fidelity.
What ensures complete replication of linear DNA?
Telomeres and telomerase allow lagging strand synthesis at the chromosome ends.
