Final answer:
Cytosine methylation at CpG islands leads to gene silencing by attracting proteins that compact the DNA and make it inaccessible for transcription. This tightly packed, transcriptionally inactive chromatin structure is central to several cellular processes and is targeted in cancer therapy. Heritable epigenetic changes such as these also play a role in how organisms respond to environmental factors.
Step-by-step explanation:
Cytosine methylation, particularly within CpG islands of gene promoter regions, serves as an epigenetic modification that can repress transcription. When the cytosine within these CG pairs is methylated, it commonly leads to gene silencing. This is because methylated DNA tends to be bound by proteins that compact the DNA into a configuration that is inaccessible to transcription factors.
Additionally, methylated DNA can attract DNA methyltransferases that further propagate the silencing signal. This results in a chromatin state that is highly methylated and with histones that are deacetylated, causing the nucleosomes to pack tightly together. As a consequence, genes within these regions are often transcriptionally inactive. This is integral to biological processes such as X-chromosome inactivation and genomic imprinting.
Understanding these mechanisms is also crucial in cancer research and treatment, as drugs that result in decreased DNA methylation or prevent removal of acetyl groups from histones can lead to reactivation of tumor suppressor genes, aiding in cancer therapy. Such epigenetic changes are key to regulating gene expression in response to environmental factors and can be heritable.