Correct Answer: (C) Covalent DNA Adduct Formation
Reason:
Benzopyrene, a polycyclic aromatic hydrocarbon found in tobacco smoke and industrial pollutants, is metabolized into reactive benzopyrene diol epoxide (BPDE). BPDE forms covalent DNA adducts, particularly at guanine residues, leading to mutations in tumor suppressor genes (e.g., TP53), driving carcinogenesis.
Why Others Are Incorrect:
(A) Alkylation of DNA bases – Incorrect. Alkylating agents (e.g., methyl methanesulfonate) add alkyl groups, but benzopyrene primarily forms bulky adducts.
(B) Base pair deletions during replication – Incorrect. Benzopyrene adducts cause mispairing but do not directly induce base pair deletions.
(D) Hydrolysis of phosphodiester bonds – Incorrect. DNA backbone hydrolysis is caused by nucleases or extreme damage, not benzopyrene.
(E) UV-induced thymine dimer formation – Incorrect. Thymine dimers result from UV radiation, not chemical carcinogens.
Reason:
Ionizing radiation generates reactive oxygen species (ROS), which cause oxidative DNA damage. This leads to single-strand and double-strand breaks, base modifications (e.g., 8-oxo-dG), and increased genomic instability, contributing to acute tissue toxicity and carcinogenesis.
Why Others Are Incorrect:
(A) Base deamination leading to C→U transitions – Incorrect. Deamination is a spontaneous or enzymatic process, not primarily caused by ionizing radiation.
(B) Direct generation of apurinic (AP) sites – Incorrect. AP sites arise from base excision repair or spontaneous base loss, not directly from radiation.
(C) Induction of interstrand crosslinks – Incorrect. Crosslinks are typically caused by chemotherapy agents like cisplatin, not radiation.
(E) Single-base mismatch incorporation during replication – Incorrect. Replication errors occur due to polymerase mistakes, not ionizing radiation.
