A geneticist searching for mutations uses the restriction endonucleases SmaI and PvuII to search for mutations that eliminate restriction sites. SmaI will not cleave DNA with CpG methylation. It cleaves DNA at the restriction digestion sequence ↓ 5′−CCC GGG−3′ 3′−GGG CCC−3′ ↑ PvuII is not sensitive to CpG methylation. It cleaves DNA at the restriction sequence ↓ 5′−CAG CTG−3′ 3′−GTC GAC−5′ ↑ Explain why CpG dinucleotides are hotspots of mutation.

Key Concepts

CpG Dinucleotides

CpG dinucleotides are pairs of cytosine (C) and guanine (G) nucleotides linked by a phosphate bond, often found in the promoter regions of genes. They are significant in gene regulation and are prone to methylation, which can affect gene expression. The methylation of cytosine in CpG sites can lead to mutations, particularly when the methylated cytosine is deaminated to thymine, resulting in a C-to-T transition mutation.

Methylation and Mutation

Methylation is a biochemical process where a methyl group is added to the DNA molecule, typically at cytosine bases in CpG dinucleotides. This modification can silence gene expression and is a key mechanism in epigenetics. However, when methylated cytosines undergo deamination, they can convert to thymine, creating a mismatch during DNA replication that can lead to mutations, making these sites hotspots for genetic changes.

Restriction Endonucleases

Restriction endonucleases, or restriction enzymes, are proteins that cut DNA at specific sequences, which are often palindromic. They are used in molecular biology for cloning and analyzing DNA. The sensitivity of these enzymes to methylation varies; for instance, SmaI is inhibited by methylation at CpG sites, while PvuII is not. This difference is crucial for geneticists when identifying mutations that affect restriction sites.