7.7
DNA Damage can Stall the Cell Cycle
Q1: What happens when DNA damage is detected during the cell cycle?
When DNA damage is detected at cell cycle checkpoints, multiple enzymes probe the DNA and the cell cycle pauses until repairs are completed. This checkpoint mechanism ensures only intact, undamaged DNA progresses to the next generation. The pause prevents replication of damaged genetic material, maintaining genome integrity.
Q2: How does damaged DNA affect the replication fork during S phase?
During S phase, damaged DNA stalls the replication fork, causing it to become unstable and causing helicase and DNA polymerase to uncouple from the DNA. Replication protein A (RPA) then coats the single-stranded DNA to prevent reannealing. This complex is detected by the ATR protein, initiating the damage response pathway.
Q3: What is the role of ATM and ATR proteins in DNA damage response?
ATM and ATR are kinases that respond to different types of DNA damage. ATR detects single-strand breaks at stalled replication forks, while ATM responds to double-strand breaks via the MRN protein complex. Both phosphorylate downstream kinases Chk1 and Chk2, triggering cell cycle arrest and repair mechanisms.
Q4: How does p53 activation lead to cell cycle arrest?
When ATM phosphorylates p53, it becomes activated and binds directly to DNA, stimulating production of the p21 protein. p21 inhibits CDK2, preventing the cell from progressing to the next stage of cell division. This mechanism is critical to the G1/S checkpoint, and mutations in p53 can lead to uncontrolled cell division and malignant tumors.
Q5: Why do cells need to arrest at specific points during the cell cycle?
Cells must arrest at critical stages to prevent accumulating excessive DNA. If arrest occurs before DNA replication, cells contain twice the normal DNA amount. If arrest occurs after replication but before mitosis, cells contain four times the normal amount. Strategic checkpoint placement allows proper assessment and repair of damage.
Q6: What repair mechanisms are activated in response to double-strand breaks?
Double-strand breaks activate ATM protein, which initiates a cascade involving specialized repair pathways including Non-homologous End Joining, Homologous Repair, and Nucleotide Excision Repair. p53 can directly activate repair pathways such as Nucleotide Excision Repair by regulating factors that mediate these processes and induce dNTP synthesis.
Q7: How do ATM and ATR coordinate both fast and slow damage responses?
ATM and ATR operate on different timescales: fast post-translational modifications like phosphorylation of downstream kinases inhibit CDC25, preventing CDK activation and stalling the cell cycle. Slower transcriptional regulation through p53 controls expression of proteins involved in cell cycle arrest, apoptosis, or senescence, providing sustained damage response.