Question

Difference between homologous recombination and non homologous end joining

Answer 1

The two major pathways for repair of DNA double-strand breaks (DSBs) are homologous recombination (HR) and nonhomologous end joining (NHEJ). HR leads to accurate repair, while NHEJ is intrinsically mutagenic. To understand human somatic mutation it is essential to know the relationship between these pathways in human cells. Here we provide a comparison of the kinetics and relative contributions of HR and NHEJ in normal human cells. We used chromosomally integrated fluorescent reporter substrates for real-time in vivo monitoring of the NHEJ and HR. By examining multiple integrated clones we show that the efficiency of NHEJ and HR is strongly influenced by chromosomal location. Furthermore, we show that NHEJ of compatible ends (NHEJ-C) and NHEJ of incompatible ends (NHEJ-I) are fast processes, which can be completed in approximately 30 min, while HR is much slower and takes 7h or longer to complete. In actively cycling cells NHEJ-C is twice as efficient as NHEJ-I, and NHEJ-I is three times more efficient than HR. Our results suggest that NHEJ is a faster and more efficient DSB repair pathway than HR.

Answer 2

Non-homologous end-joining (NHEJ)

The NHEJ pathway is reviewed in [Lieber 2010]. The first protein to respond to double-strand breaks is Ku70/80, which is present at very high concentrations in cells and thus is recruited very quickly to DSBs. It in turn recruits DNA-PKcs (kinases) and then Artemis (which removes damaged ends) and end-processing enzymes. DNA pol λ and μ will then re-extend the ends. DNA pol λ can actually extend DNA without a template in order to give you sticky ends you can anneal.

Homologous recombination (HR)

We’ll introduce HR in the context of its ability to fix two-ended DSBs. At both of the double-stranded ends facing each other, the 5’ strand is resected (trimmed back). A recombinase (RecA in bacteria, Rad51 in eukaryotes) will then bind the single strand. This complex is called a presynaptic filament. The recombinase will then perform strand invasion whereby it base-pairs the single strand with the complementary strand of the homologous duplex