Mutations

Question 1

xeroderma pigmentosum

Answer 1

• severe photosensitivity, poikiloderma, hyperpigmentation in sun-exposed areas, and squamous cell carcinoma of the skin
• autosomal recessive disorder characterized by defects in nucleotide excision repair.
• DNA can be damaged by ultraviolet rays, leading to formation of thymine dimers between 2 adjacent thymine residues
• These thymine dimers are repaired by UV-specific endonuclease, an enzyme that is frequently deficient in patients with xeroderma pigmentosum.
• endonuclease: recognizes distortions in the structure of DNA caused by thymine dimers and subsequently excises stretches of single-stranded DNA that contain these defects
• gap created following this excision is then filled by DNA polymerase, which uses the opposite DNA strand as a template
• the new strand of DNA is then joined on both ends to the existing DNA BY THE enzyme ligase