In homologous end joining, which molecule serves as a repair template?

In the process of homologous end joining (HEJ), the sister chromatid serves as the repair template. This mechanism is crucial for repair of double-strand breaks in DNA, particularly in eukaryotic cells.

During the cell cycle, after DNA replication has occurred, each chromosome has a sister chromatid. This sister chromatid is nearly identical to the broken chromatid, making it an ideal template for accurate repair. The cell utilizes the homologous region of the sister chromatid to guide the repair of the damaged DNA, ensuring that the correct genetic information is restored and that mutations are minimized.

When a double-strand break occurs, the broken ends are processed, and proteins involved in HEJ facilitate strand invasion where the broken DNA strand pairs with the homologous sequence on the sister chromatid. After aligning the sequences, DNA synthesis can occur, filling in any gaps and allowing for the restoration of the fragmented DNA.

This process is distinct from other repair mechanisms, which could involve different templates or methods. For instance, the parent strand is not used as a direct template in this specialized form of repair. Plasmids are extrachromosomal elements and would not serve as templates in this context. DNA polymerase, while crucial for synthesizing new