Homology directed repair (Homologous recombination)

Homology directed repair (HDR) is a mechanism in cells to repair double-strand DNA lesions.


The most common form of HDR is homologous recombination. 

Homologous recombination refers to the process through which genetic information is exchanged between two similar or identical DNA molecules, such as during meiosis to generate genetic diversity.

The HDR mechanism can only be utilized  by cells when there is a homologous piece of DNA present in the nucleus.

Homologous recombination factors are involved in the repair of DNA double strand breaks and repair of DNA interstrand crosslinks, and the recovery of stalled and broken replication forks.

HR is one of the two major pathways for the repair of double strand brakes induced by both endogenous and exogenous sources.

It is a high-fidelity, template dependent repair process during the S and G2 phases of the cell cycle, when a donor DNA molecule with extensive sequence homologous to the broken DNA is available as a template for repair.

This occurs mostly in G2 and S phase of the cell cycle. 


HDR suppresses the formation of cancer. 


It maintains genomic stability by repairing broken DNA strands.


Every piece of single stranded DNA is covered by the Replication Protein A.


Replication Protein A keep the single stranded DNA pieces stable until the complementary piece is resynthesized by a polymerase. 


The polymerase synthesizes the missing part of the broken strand. 


HDR and other mechanisms repair double strand breaks. 

Germline mutations in genes encoding homologous recombination proteins, are associated with cancer predisposition, developmental disorders, and premature aging.
Specifically, the germline pathogenic variants in ATM, BRCA1 , BRCA2, and PALB2, are known to cause a predisposition to breast, ovarian, prostate, pancreas and gastric cancer (in the presence of H.pylori).

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