DNA Damage can Stall 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.

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.

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.

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.

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.

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.

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.