Fixing Double-strand Breaks

When both DNA strands are damaged, no intact template remains for accurate repair. If left unrepaired, this scenario leads to cell death. Cells must employ specialized repair mechanisms to fix double-strand breaks and prevent lethal consequences. Without repair, the chromosome becomes fragile and genetic information is lost.

Non-homologous end joining joins broken DNA ends without requiring sequence similarity, occurring before DNA replication when quick repair is needed. The Ku protein complex holds chromosome ends in place while DNA polymerase bridges the gap. DNA ligase IV then seals the ends. This mechanism is fast but error-prone, potentially causing mutations or genomic rearrangements.

Inaccurate repair can cause mutations at repair sites or genomic rearrangements including deletions, translocations, and chromosomal fusions. Human somatic cells tolerate approximately 2,000 mutations, but genomic rearrangements are rare in normal cells. However, these rearrangements are frequently found in cancerous cells, linking repair errors to cancer development.

Homologous recombination uses a sister chromatid as a template for accurate repair, ensuring correct nucleotide insertion. Non-homologous end joining joins ends without a template, leading to errors. Homologous recombination occurs after gene duplication during cell division when sister chromatids are available, making it the preferred but timing-restricted repair pathway.

Double-strand breaks result from metabolic byproducts like reactive oxygen species, environmental factors such as ionizing radiation, and malfunctioning nuclear enzymes. Type II topoisomerases, which cut and rejoin DNA strands during chromosome disentanglement, can inadvertently cause breaks if they malfunction. Mechanical stress on DNA also contributes to double-strand breaks.

Cells resort to non-homologous end joining when homologous recombination is unavailable, particularly outside S and G2 cell cycle phases when sister chromatids are absent. Higher eukaryotes can tolerate this less accurate mechanism because they possess abundant non-coding DNA, which permits nucleotide substitutions, deletions, or additions without severe consequences.

An estimated ten double-strand breaks occur per day in a cell. Despite this frequency, cells survive through efficient repair mechanisms. The primary sources are metabolic byproducts and environmental damage, with malfunctioning enzymes and mechanical stress contributing less commonly to break formation.