Question

Generally describe what a Double Strand Break is, how it is repaired

Answer 1

A Double-Strand Break is a mutation where both strands of the DNA helix are severed. This creates a free-floating DNA molecule which could lead to loss of that chromosome fragment, chromosomal rearrangement or translocations. Double-stranded DNA breaks are often lethal to the cell.

Mechanisms like NHEJ (Non-Homologous End Joining) and Homologous Recombination (HR) are employed by the cell to mediate DSB repair. NHEJ usually utilizes the presence of single stranded overhangs on the DNA ends to be joined. When DSB's occur, they need not produce blunt ended products. DSB's can produce single stranded overhangs, which are used by NHEJ to align the two DNA molecules and join them together.

Homologous recombination is another mechanism utilized by the cells to repair double-stranded breaks. Homologous Recombination utilizes the sequence homology of either sister chromatids or homologous chromosomes to provide the sequence information necessary to fill in any lost sequences and restore the DNA molecule.