This document discusses DNA damage caused by ultraviolet light in the form of pyrimidine dimers, as well as the DNA repair process of photoreactivation. It notes that UV light induces the formation of covalent bonds between thymine and cytosine bases, creating cyclobutane pyrimidine dimers and 6,4 photoproducts. These lesions inhibit DNA polymerase and replication. The document then describes the two main types of dimers and explains that the photoreactivation repair mechanism uses the photolyase enzyme to bind to DNA and, upon absorption of visible light, monomerize thymine dimers.

1. TOPIC : DNA Damage- Pyrimidine Dimer and
DNA Repair- Photoreactivation
NAME – Vivek kumar
COURSE –Microbiology
Bangalore University

2. • Pyrimidine dimers are molecular lesions formed from thymine or
cytosine bases in DNA via photochemical reactions.
• Ultraviolet light induces the formation of covalent linkages by
reactions localized on the C=C double bonds.
• Two common UV products are cyclobutane pyrimidine dimers (CPDs,
including thymine dimers) and 6,4 photoproducts.
• These lesions alter the structure of DNA and consequently inhibit
polymerases and arrest replication.

4. Types of dimers
• Cyclobutane – A cyclobutane pyrimidine dimer (CPD) contains a four
membered ring arising from the coupling of the C=C double bonds of
pyrimidines. Such dimers interfere with base pairing during DNA
replication, leading to mutations.
• 6,4-photoproducts – 6,4-photoproducts or 6,4 pyrimidine-
pyrimidones, results from the formation of a bond between C6 of one
pyrimidine base and C4 of the another pyrimidine base. Spore
photoproduct lyase provides another enzymatic pathway for repair of
thymine photodimers.

6. DNA Repair - Photoreactivation
• Photoreactivation repair mechanism can be used to repair a specific
DNA damage, called as Pyrimidine dimers.
• Those DNA who have their thymines as 3H- labelled are ultraviolated-
irradiated damaged.
• By this rule a repairable DNA is identified.

7. Initiation
• Photolyase enzyme binds to the DNA and creates
enzyme-DNA complex.
Repairing
• Absorption of visible light and activation of the
enzyme and then the thymine dimer is monomerized.
Release
• After repairing release of enzyme.

9. Reference
• Molecular Biology by David Freifelder , Narosa Publishing House, Page
no.277-284
• David S. Goodsell (2001). “The Molecular Perspective: Ultraviolet
Light and Pyrimidine Dimers” PMID 11423677

10. THANK
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