The double-stranded structure of DNA has two major advantages. First, it serves as a safe repository of genetic information where one strand serves as the back-up in case the other strand is damaged. Second, the double-helical structure can be wrapped around proteins called histones to form nucleosomes, which can then be tightly wound to form chromosomes. This way, DNA chains up to 2 inches long can be contained within microscopic structures in a cell. A double-stranded break not only damages both copies of genetic information but also disrupts the continuity of DNA, making the chromosome fragile.
In a cell, there are an estimated ten double-strand breaks (DSBs) per day. The primary source of damage is metabolic by-products such as Reactive Oxygen Species and environmental factors such as ionizing radiations. Although less common, malfunctioning nuclear enzymes can also cause DSBs. Failure of enzymes like type II topoisomerases, which cut both strands of DNA and rejoin them while disentangling chromosomes, can inadvertently result in DSBs. Mechanical stress on the DNA duplex can also lead to DSBs. In prokaryotes, prolonged desiccation strains DNA, causing DSBs.
Of the two mechanisms for DNA repair, homologous recombination depends on a sister chromatid being nearby, which happens during the S and G2 phases. Due to this restriction, in the absence of a homology donor, cells have to resort to Nonhomologous end joining (NHEJ), even though it is much less accurate. It has been hypothesized that the reason higher eukaryotes can afford to preferentially utilize NHEJ for DSB repairs is that they have abundant non-coding DNA, which permits nucleotide substitutions, deletions or additions without grievous consequences.
When both strands of DNA are damaged, there is no intact template left for accurate repair but if left unrepaired, this scenario can lead to cell death.
There are two mechanisms to repair double-strand breaks. The first type, non-homologous end joining, permits joining of ends even if there is no sequence similarity between them - and takes place before DNA duplication when the DNA needs quick repair.
In mammalian cells, this is carried out by the DNA end-binding heterodimeric protein Ku which forms a complex with the catalytic subunits of DNA-dependent protein kinase.
This complex holds the broken chromosome ends in place while a DNA polymerase inserts nucleotides to bridge the gap between these ends. Next, DNA ligase IV forms a complex with its cofactor XRCC and another protein called XLF, and rejoins and seals these ends.
Quick fixes like these can lead to mutations at the repair site, or genomic rearrangements including deletions, translocations of genetic material, and fusions which may result in chromosomes with two centromeres, or lacking centromeres altogether.
Mutations are widespread, and human somatic cells can tolerate as many as 2000 of these. Genomic rearrangements, on the other hand, are rare - but can be found in cancerous cells.
Most DNA double-strand breaks lead to single-stranded overhangs and the second repair type, homologous recombination, fixes these breaks. This form of recombination is much more accurate than non-homologous end joining and requires DNA from a sister chromatid as a template– so it typically occurs after gene duplication during cell division.
