DNA methylation patterns in eukaryotes In eukaryotic genomes, what is the predominant and most common form of DNA methylation under normal physiological conditions?

Introduction:
Epigenetic DNA methylation regulates gene expression, chromatin state, and genome stability. The most widespread and studied methyl mark in vertebrate eukaryotes occurs at specific cytosine contexts and is maintained through DNA replication by dedicated enzymes.

Given Data / Assumptions:

• Focus on canonical DNA methylation in animals, particularly vertebrates.
• Exclude specialized or rare modifications that occur in certain species or developmental stages.
• Consider enzymatic pathways responsible for establishing and maintaining methylation patterns.

Concept / Approach:

DNA methyltransferases add a methyl group to carbon 5 of cytosine, primarily in CpG dinucleotides. This modification is symmetrical in double stranded DNA, enabling copying of the pattern after replication by maintenance methyltransferases.

Step-by-Step Solution:

Verification / Alternative check:

Genome wide bisulfite sequencing maps demonstrate that the bulk of methylation resides at CpG sites in vertebrates, with islands, shores, and gene bodies showing context dependent patterns.

Why Other Options Are Wrong:

Adenosine or guanosine methylation is not the predominant canonical mark in vertebrate DNA. Thymine methylation is not the standard epigenetic mechanism in these organisms. None of the above cannot be correct because cytosine in CpG is well established.

Common Pitfalls:

Conflating RNA methylation with DNA methylation, or generalizing plant specific contexts to vertebrates. Another pitfall is mixing histone methylation with DNA methylation.

Final Answer:

Cytosine methylation in CpG dinucleotides