Which of the following is the most common type of DNA damage caused by ultraviolet (UV) radiation?

Explanation

## Most Common UV-Induced DNA Lesion ### UV Radiation and DNA Damage Ultraviolet radiation, particularly UV-B (280–320 nm) and UV-C (200–280 nm), causes direct photochemical damage to DNA by inducing covalent cross-links between adjacent pyrimidine bases on the same DNA strand. ### Thymine Dimers: The Predominant Lesion **Key Point:** Thymine dimers (also called cyclobutane pyrimidine dimers, CPDs) account for **~75–80% of all UV-induced DNA lesions**. They form when two adjacent thymine residues (or cytosine residues) undergo a photochemical reaction, creating a four-membered cyclobutane ring. **High-Yield:** Thymine dimers are the most frequent and most mutagenic UV lesion. They distort the DNA helix and block DNA polymerase, causing replication fork stalling and error-prone bypass synthesis. ### Other UV Lesions (Less Common) | Lesion Type | Frequency | Mechanism | Clinical Significance | | --- | --- | --- | --- | | **Thymine dimers (CPDs)** | ~75–80% | Cyclobutane ring formation between adjacent pyrimidines | Most mutagenic; main cause of skin cancer | | **6-4 Photoproducts** | ~15–20% | Covalent adduct between C6 of one pyrimidine and C4 of the next | Highly distorting; recognized by XPA | | **Double-strand breaks** | <1% | Rare; indirect effect of multiple lesions | Not primary UV lesion | | **Oxidative damage** | Minimal | Requires ROS; not direct UV effect | Secondary damage | ### Repair Pathways **Key Point:** Thymine dimers are removed by **nucleotide excision repair (NER)**, which is defective in xeroderma pigmentosum (XP) patients, leading to extreme UV sensitivity and skin cancer predisposition. ### Clinical Pearl Patients with xeroderma pigmentosum cannot repair thymine dimers efficiently and develop multiple skin cancers (basal cell carcinoma, squamous cell carcinoma, melanoma) by age 20–30 years, even with minimal sun exposure. **Mnemonic:** **TDimer = Top Damage** — Thymine Dimers are the top (most common) UV-induced DNA damage.