What is the primary function of DNA methylation in eukaryotic cells?

• A. It activates gene expression by loosening chromatin structure.
• B. It silences gene expression by tightening chromatin structure.
• C. It repairs damaged DNA by replacing methylated bases.
• D. It facilitates DNA replication by unwinding the double helix.

Answer: (B)

Verified step by step guidance

1. Understand the concept of DNA methylation: DNA methylation involves the addition of a methyl group (CH₃) to the DNA molecule, typically at cytosine bases in the context of CpG dinucleotides.
2. Recognize the impact of DNA methylation on gene expression: Methylation generally leads to the repression of gene expression by affecting the chromatin structure.
3. Explore how methylation affects chromatin: Methylation can lead to a tighter chromatin structure, making it less accessible for transcription factors and RNA polymerase, thus silencing gene expression.
4. Differentiate DNA methylation from other processes: DNA methylation is distinct from DNA repair mechanisms and does not directly facilitate DNA replication or activate gene expression.
5. Conclude the primary function: The primary function of DNA methylation in eukaryotic cells is to silence gene expression by tightening the chromatin structure, preventing transcription.