Question

Match each DNA repair mechanism with its description.

photoactive repair

base excision repair

nucleotide excision repair

mismatch repair  

SOS system

nonhomologous end-joining

A.	recognizes newly synthesized but improperly paired DNA and nicks the strand to replace
B.	cuts a piece out of distorted DNA to be filled in by polymerase
C.	an emergency, error-prone effort to salvage replication of damaged DNA
D.	removes and replaces depurinated or deaminated bases from sugars
E.	repairs pyrimidine dimers in the presence of light
F.	binds together two broken strands of DNA

Answer 1

1. Photoactive Repair - Repairs pyrimidine dimers in presence of light

Photoactive repair or photoreactivation is a light dependent direct repair system which acts directly on damaged nucleotides (usually pyrimidine dimers). The enzyme involved in the process is DNA photolyase and direct repair system is stimulated with the light of wavelength between 300 to 500 nm.

2. Base excision repair - removes and replaces depurinated or deaminated bases from sugars

In base excision repair system enzyme DNA glycosylase cleaves N glycosidic bonds which result in liberation of altered or damaged bases, which therefore generate a apyrimidinic or apurinic sites known as AP sites and AP sites are then repaired by AP endonucleases. This system is used for repair of minor damages including deamination and alkylation of bases which is usually caused on exposure to to mutagens.

3. Nucleotide Excision Repair - cuts a piece out of distorted DNA to be filled in by DNA polymerase

In nucleotide excision repair system breakage of phosphodiester bond on either side of lesion takes place which results in excision of damaged oligonucleotide, leaving a gap which is repaired by DNA polymerases and breaks formed are sealed by ligase.

Exposure to UV radiation results in formation of thymine dimers which causes of distortion of DNA inhibiting DNA polymerase, therefore hampering replication, transcription and translation. Damaged DNA is recognised and excised by proteins UvrA,B and C. The gap formed is filled by DNA polymerase l.

4. Mismatch Repair - Recognize newly synthesized but improperly paired DNA and nicks the strand to replace

DNA mismatch repair system rectifies errors such as deletions, insertions or incorporation of mismatched bases which are caused during DNA replication or recombination. Newly formed strand is recognised due to absence of methylation whereas the parent strand (error free) is methylated. MutH, MutL and MutS proteins recognize the mismatched DNA base pairs and unmethylated strand is cleaved by MutH. Complete removal of the unmethylated mismatched sequence is catalyzed by DNA helicase II and exonuclease l. Gap formed is filled by DNA polymerase and nicks are sealed by DNA ligase.

5. SOS system - an emergency, error prone effort to salvage replication of damaged DNA

SOS system of DNA repair is activated on mutagenesis (caused by UV radiation, chemical reagents, alkylating agents etc.) resulting in cell cycle arrest. In SOS response protein RecA inactivates LexA ( repressor of SOS response genes) therefore activating SOS response genes. SOS repair is an error prone repair system which causes variation in DNA sequences.

6. Non-Homologous end joining (NHEJ) - binds together two broken strands of DNA

NHEJ is the primary pathway for repair of double stranded breaks of DNA. In this repair system, broken ends are ligated directly without any homologous template. Direct religation of broken strands usually results in small deletions or insertions making it a error prone DNA repair system.