Effect of ultraviolet radiation on DNA is formation of
## **Core Concept**
Ultraviolet (UV) radiation can cause damage to the DNA molecule, leading to the formation of specific types of lesions. This damage primarily occurs due to the absorption of UV light by the nucleotide bases in DNA, particularly by pyrimidines (cytosine and thymine). The most common type of DNA damage caused by UV radiation is the formation of covalent bonds between adjacent pyrimidine bases.
## **Why the Correct Answer is Right**
The correct answer, **pyrimidine dimers**, is accurate because UV radiation leads to the formation of covalent bonds between adjacent pyrimidine bases (thymine or cytosine) on the same DNA strand. This results in the creation of cyclobutane pyrimidine dimers (CPDs) and (6,4) photoproducts, with CPDs being the most common. These dimers distort the DNA double helix and can lead to mutations if not properly repaired.
## **Why Each Wrong Option is Incorrect**
- **Option A:** While purines (adenine and guanine) are also nucleotide bases in DNA, UV radiation primarily causes damage leading to the formation of pyrimidine dimers, not purine dimers.
- **Option B:** 8-oxoguanine is a type of DNA lesion caused by oxidative stress, not directly by UV radiation. It is a marker of oxidative DNA damage.
- **Option D:** DNA strand breaks are a type of DNA damage but are not the direct result of UV radiation. UV radiation primarily causes the formation of pyrimidine dimers.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **pyrimidine dimers** are a major cause of UV-induced DNA damage. The formation of these dimers can lead to skin cancers, such as melanoma and non-melanoma skin cancers, if not adequately repaired by the cell's DNA repair mechanisms, notably nucleotide excision repair (NER).
## **Correct Answer:** . pyrimidine dimers