Action of ultraviolet rays on DNA is
(A) induction of thymidine dimers
(B) deletion of base pair
(C) addition of base pair
(D) methylation of base pair

Answer

To answer the question regarding the action of ultraviolet rays on DNA, we will analyze the options provided and determine the correct answer step by step.

### Step 1: Understanding Ultraviolet Rays
Ultraviolet (UV) rays are a form of electromagnetic radiation that can cause damage to DNA. When DNA is exposed to UV rays, it can lead to various types of mutations.

**Hint:** Consider what happens to DNA when it absorbs UV radiation.

### Step 2: Identifying the Effects of UV Rays on DNA
One of the primary effects of UV radiation on DNA is the formation of covalent bonds between adjacent thymine bases. This results in the formation of thymine dimers, where two thymine bases bond together instead of pairing with adenine.

**Hint:** Think about the specific type of mutation that occurs when two thymine bases are in close proximity.

### Step 3: Evaluating the Options
Now, let's evaluate the options provided in the question:
- (A) Induction of thymidine dimers: This is the correct answer, as UV rays cause adjacent thymine bases to bond together, forming dimers.
- (B) Deletion of base pair: While UV can cause mutations, it does not primarily result in the deletion of base pairs.
- (C) Addition of base pair: UV rays do not add base pairs; they cause damage to existing ones.
- (D) Methylation of base pair: Methylation is a different process that involves the addition of a methyl group, not directly caused by UV rays.

**Hint:** Compare each option with the known effects of UV radiation on DNA.

### Step 4: Conclusion
Based on the analysis, the action of ultraviolet rays on DNA is best described by option (A) induction of thymidine dimers. This is a well-documented effect of UV radiation on DNA, leading to potential mutations if not repaired.

**Final Answer:** (A) induction of thymidine dimers

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