Question

Describe how Nucleotide Excision Repair, Homologous Recombination Repair (HRR), and Non-homologous End Joining (NHEJ) are beneficial for repair of damaged DNA. If you could choose between having HRR and NHEJ performed, which would you choose and why?

Answer 1

Answer- 1) Nucleotide exvisexc repair: NER pathways repair "bulky"lesions in DNA that alters or distort the doble helix. These lesions include the uv induced pyrimidine dimers and DNA adducts (DNA adducts are a form of DNA damagecaused by covalent attachment of a chemical moiety to DNA) . One group of genes was designated uvr (ultraviolet repair) and included the uvrA, uvrB, and uvrC mutations.
In the NER pathway, the uvr gene products are involved in
recognizing and clipping out lesions in the DNA. Usually, a
specific number of nucleotides is clipped out around both
sides of the llesion.The repair is then completed by DNA polymerase I and DNA ligase ( gap is filled by DNA polymerase and DNA ligase).the undamaged strand opposite the lesion is used as a template for the replication, resulting in repair.   

Diagram of NER

2. Homologous recombination repair: HRR is a pathway that involved in double -strand break repair for example- when both strands of DNA helix are cleaved as a result of exposure to ionizing radiation.

The first step in this process involves the activity of an en-
zyme that recognizes the double-strand break, and
then digests back the 5'ends of the broken DNA he-
lix, leaving overhanging 3' ends . One
overhanging end searches for a region of sequence
complementarity on the sister chromatid and then
invades the homologous DNA duplex, aligning the
complementary sequences. Once aligned, DNA syn-
thesis proceeds from the 3' overhanging ends, using
the undamaged DNA strands as templates. The inter-
action of two sister chromatids is necessary because,
when both strands of one helix are broken, there is no
undamaged parental DNA strand available to use as a
source of the complementary template DNA sequence
during repair. After DNA repair synthesis, the resulting het-
eroduplex molecule is resolved and the two chromatids separate.

diagram of HRR

3) non- homologous end joining: it also repairs double-strand breaks. However, as the name implies, the mechanism does not recruit a homologous region of DNA during repair. This system is activated in G1,prior to DNA replication. End joining involves a complex of many proteins, including DNA-dependent protein kinase
and the breast cancer susceptibility gene product, BRCA1.
These and other proteins bind to the free ends of the bro-
ken DNA, trim the ends, and ligate them back together.

4) Between NHEJ and HRR I would choose HRR .Because in NHEJ some nucleotide sequences are lost in the process
of end joining, it is an error-prone repair system.