DNA repair

DNA repair - homologous recombination & non-homologous end joining

DNA is similar to a fragile blueprint. It contains all the instructions for life, yet it is not indestructible. There are various ways it can be damaged and break, Such as doublestrand breaks, when both strands of the DNA helix are broken. Imagine cutting both sides of a ladder simultaneously. The cell needs to repair it, or else it will lose vital information.

Cells have two ways to repair this: homologous recombination (HR) and non-homologous end joining (NHEJ).

Homologous Recombination (HR):

Homologous recombination is the more precise mechanism. This is how it happens:

The cell finds an intact sister chromatid, a perfect copy of the broken DNA, which is usually available during or near the time of DNA replication (S and G2 phases).

Broken DNA ends are processed to create single-stranded overhangs, which invade the sister strand and use it as a template.

The missing information is replicated from the other strand, and the strands are stitched back together almost perfectly.

HR is like having a backup copy, if one copy is damaged, you can restore it from the backup.

Non-Homologous End Joining (NHEJ):

NHEJ is faster, but messier. It doesn't wait around for a sister chromatid. It:

Takes the broken DNA ends,

Cleans them up if need be,

And simply glues them back together.

It's present throughout the cell cycle, particularly in G1 phase when there's no identical copy available.

But NHEJ has one drawback: because it doesn't use a template, it can lose or add a few nucleotides at the join. So while it saves the cell from disaster, occasionally it leaves a typo in the genetic text.

When each method is used

HR is slow but accurate. The cell prefers it when it can afford to be accurate — mainly after DNA has been replicated.

NHEJ is quick and available at any time, especially when speed is more important than perfection.

Summary:

Homologous recombination (HR): accurate, with a copy being used as a template.

Non-homologous end joining (NHEJ): fast, direct ligation of DNA ends.

References

1. Alberts B et al. Molecular Biology of the Cell, 6th ed. Garland Science, 2014.
2. Watson JD et al. Molecular Biology of the Gene, 7th ed. Pearson, 2013.
3. Kornberg RD. The molecular basis of eukaryotic transcription. PNAS. 2007.