Which of the following is the most common site of spontaneous DNA mutation in humans?

Explanation

## Most Common Site of Spontaneous DNA Mutation ### CpG Dinucleotides: Mutation Hotspots **Key Point:** CpG dinucleotides (cytosine followed by guanine) are the **most common sites of spontaneous point mutations** in the human genome, accounting for approximately **30–40% of all point mutations** despite comprising only ~1% of the genome. ### Mechanism: Methylation and Deamination 1. **Cytosine methylation:** In vertebrates, cytosines in CpG dinucleotides are frequently methylated to 5-methylcytosine (5mC) for gene regulation and chromatin silencing. 2. **Spontaneous deamination:** 5-methylcytosine spontaneously deaminates to thymine at a much higher rate than unmethylated cytosine deaminates to uracil. 3. **Repair evasion:** Uracil (from unmethylated cytosine deamination) is recognized and removed by base excision repair (BER). However, thymine (from 5-methylcytosine deamination) is a normal DNA base and is NOT recognized as abnormal, so it escapes repair. 4. **Result:** C→T transition mutation at CpG sites (CpG → TpG on the coding strand; CpG → CpA on the complementary strand). **High-Yield:** This is why CpG dinucleotides are called "mutation hotspots" and why CpG islands (unmethylated CpG-rich regions near gene promoters) are evolutionarily depleted in the genome. ### Comparison with Other Mutation Sites | Site Type | Mutation Frequency | Mechanism | Clinical Relevance | | --- | --- | --- | --- | | **CpG dinucleotides** | ~30–40% of point mutations | 5-methylcytosine → thymine deamination | Most common; hotspot for germline and somatic mutations | | **Homopolymeric runs** | ~5–10% | Slippage during replication (microsatellite instability) | Common in mismatch repair defects; not point mutations | | **Inverted repeats** | <5% | Secondary structure formation; replication errors | Rare; mainly associated with deletions/rearrangements | | **Purine-rich sequences** | <5% | No specific intrinsic mutability | Not a hotspot | ### Clinical Pearl **Key Point:** Many human genetic diseases show a bias toward CpG mutations. Examples include: - **Hemophilia B Leyden:** CpG mutation in Factor IX gene (c.−20C>T) - **Cystic fibrosis:** CpG mutations in CFTR gene - **Familial hypercholesterolemia:** CpG mutations in LDLR gene **Mnemonic:** **"CpG = Cancer Prone Gene"** — CpG dinucleotides are the most frequent sites of both germline and somatic mutations, contributing to cancer development.