Final answer:
A "CpG island" is a DNA region with many CG pairs in gene promoters that can be methylated, affecting gene expression and regulation in complex ways.
Step-by-step explanation:
When a regulatory region is referred to as a "CpG island," it indicates a specific stretch of DNA that has a high frequency of cytosine (C) and guanine (G) dinucleotide pairs in the promoter region of genes. The cytosines in these CpG islands can be methylated, meaning a methyl group is added to them, which significantly alters the gene's expression by affecting how DNA interacts with various proteins, such as histones that regulate access to the gene. This methylation can either silence the gene or have other regulatory effects. Epigenetic factors like parental diet or environmental conditions may influence methylation patterns, thus impacting gene expression. The presence of CpG islands and their methylation status plays a crucial role in the complex process of gene regulation and expression, which becomes more intricate as organisms evolve.