DNA repair mechanisms directly revert damage through single step systems like ligase, photolyase and methyl transferase, or remove damaged DNA sections and replace them through multistep systems. Single step repair enzymes like ligase simply join single strand breaks, while photolyase recognizes pyrimidine dimers and breaks the bond to restore the pyrimidines. Methyl transferase is a suicidal enzyme that repairs alkylation damage. Multistep repair more broadly removes damaged DNA and replaces it with undamaged DNA.

1. DNA Repair Mechanism
Dr. Madhura Mukadam
Associate Professor,
Dept. of Zoology,
Gogate Jogalekar College, Ratnagiri
Dt. 13.01.2018

3. • Directly to revert the damage (Single step repair system)
• To remove the damaged section of DNA and fill it with new
DNA
• Single step repair system include repair mediated by
enzyme ligase, photolyase and methyl transferase etc.
• Ligase- single strand breaks are simply joined.
• Photolyase- recognizes pyrimidine dimer and activates the
enzynme to break the bond between two pyrimidines and
restores them.
• Methyl transferase- suicidal enzyme. This enzyme is
induced when cell is exposed to alkylating agents. These
are refered as multistep repair system and involve
removing damged part of DNA and replacing it with new
undamaged DNA.

4. Mechanisms that repairs damaged DNA
• A. Direct repair (Alkyl transferase)