Question

Homologous Recombination

A. How are the repair of double strand breaks (DSBs) and production of DSBs for recombination connected?
B. How does the general model of strand invasion, branch migration, and resolution of the Holliday junction work?
C. How do RecA and RecBCD function to promote recombination?
D. Endonuclease activity of RecBCD is greater in the 3'-5' strand, rather than the 5'-3' strand. Describe how this affects strand invasion, branch migration and resolution.
E. How is recombination initiated during meiosis in eukaryotes?
F. What is the connection between nonhomologous end joining repair and cancer?

Answer 1

A. Double stranded breaks are the damages in DNA and this can lead to cancer and other chromosomal abberations to solve this there are many repair pathways like Non homologous repair, Nuclues excision repair, Base excisiom repair, Double stranded breaks repair and save from damages but in double stranded breaks of recombination nucleotide sequences of homologous dna is exchanged to create genetic variation and is very much importnat for the evolution and this is a kind of repair.

B. Homologous chromosome composed of duplex dna is cleaved by endonucleases and the broken ends dissociates and each single strand invades the other double stranded dna in a reciprocal manner thus a region formed that has starnds from both strands.The nick is sealed by ligases and the x shaped region formed by this is called holliday intermediate and branch migratiom expands this region and making the paired duplex to move around extending the region of paired duplex.This intermediate is resolved by nicking a strand in each duplex and ligation

C.Rec BCD has dual function it is helicase and also nuclease.Rec C recognise a special site in Dna called chi and recruit Rec D this signals Rec B to cut the DNA .Rec B cuts and unwinds the DNA and new 3' end us created and to this site Rec A is loaded.Rec A helps to exchnge the ssdna to the other strand for recombination.

D. Rec B and D are helicases that works in oposite direction .Rec B in 3'to 5' and Rec D in 5' to 3'.In case strand inversion is importnt for recombination.The rec A proteins being added to the single stranded dna and facilitates strand inversion from 5'to 3' end.Endonuckease activity is greater in 3 to 5 because chi is created at the 3' end.

E.Recombination is the process of exchanging genetic material by DNA molecules and is the important for genetic variation.In eukaryotes genetic recombination occurs in meiosis prophase1 by a process called crossing over.This process is mainly initiated by double stranded breaks and the DNAse are exchanged and later repaired by homologous recombination.This is intiated by spo 11 protein.

F. Non homologous end joing is the pathway that repairs double stranded DNA breaks without using any homologous strands or complementary strands so the pathway is error prone. If the double stranded breaks are not repaired this will lead to cancer and other chromosomal abberations and if there is damage in the repair will also lead to cancer.Thus this is a pathway which both supresses or induce cancer.