#photolyase

Repair Pathway

DNA photolyase requires which light for activation?

DNA photolyase contains light-harvesting cofactors such as methenyltetrahydrofolate and flavin adenine dinucleotide in reduced FADH- form. Enzyme flips out thymine dimer into active pocket where FADH- absorbs photons in blue region 350-450 nanometer. Upon photon absorption, excited FADH- donates electron to dimer splitting cyclobutane ring via radical mechanism returning dimers to monomeric thymines and restoring FAD radical which recaptures electron completing catalytic cycle. Absorption maximum around 384 nanometer and blue light requirement distinguishes photolyase from UV-C which itself creates dimers, explaining why sunlight can both damage and provide substrate for repair.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 12: Photolyase Blue Light Activation Mechanism

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Repair Pathway

Which repair pathway directly reverses thymine dimers?

Pyrimidine dimers induced by UV form cyclobutane rings linking adjacent thymines distorting helix. Cells possess multiple repair strategies. Photoreactivation, also called direct reversal, utilizes DNA photolyase enzyme that specifically binds cyclobutane pyrimidine dimers and uses energy from visible blue light to break cyclobutane ring restoring intact pyrimidines in single step without excision or synthesis. This light-dependent direct repair differs from nucleotide excision repair which cuts and replaces damaged segment, base excision which removes single base, and mismatch repair which corrects replication errors, making photolyase rapid and error-free.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Photoreactivation Direct Reversal of Thymine Dimers

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cryptochrome

Despite homology to photolyase, cryptochromes lack:

Answer: C) Photolyase activity. For cryptochrome, remember the key idea and you’ll land on this option; the distractors usually swap cause and effect or confuse similar terms. Avoid: A) DNA-binding ability; B) Chromophore; D) Nuclear localization. Name the process first, then pick the option that describes it — don’t reverse cause and effect.

Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.

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cryptochrome

Cryptochrome proteins show sequence homology to which enzyme?

Answer: B) DNA photolyase. For cryptochrome, remember the key idea and you’ll land on this option; the distractors usually swap cause and effect or confuse similar terms. Avoid: A) Rubisco; C) ATP synthase; D) Nitrate reductase. Name the process first, then pick the option that describes it — don’t reverse cause and effect.

Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.

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