Practice question

Mammalian gene promoters frequently overlap CpG islands, stretches of 300 to 3000 base pairs with elevated GC content, high CpG dinucleotide frequency and low DNA methylation when active. Most housekeeping and many regulated promoters reside within islands, providing platforms for Sp1 and other transcription factors and H3K4 trimethylation deposition protecting from silencing. Bulk genome shows CpG depletion due to spontaneous deamination of 5-methylcytosine to thymine over evolution, but islands remain unmethylated via TET dioxygenase activity and CXXC domain proteins. Satellite DNA and centromeres distinct AT-rich repeats, not GC islands.