Until recently, much of genetic research has been directed towards determining the sequences of DNA that code for proteins. Little was known about non-coding regions, although these played an important role in the human genome project.
CpG islands are regions of non-coding DNA, where the number of cytosine and guanine residues is greater than 50% (the remainder consisting of adenine and thymine). These areas of DNA are unmethylated and often found near the 5' ends of genes. They often overlap the promoter region of genes and may even extend into areas where transcription starts. Because of this, and their unique sequences, they make useful tools for gene sequencing and were used in the human genome sequencing project in a process called CpG island tagging.
CpG islands are now being studied for another reason. Although the human genome has been sequenced, we still don't know everything about the gene sequences of origins of replication (ORIs), the starting points from which DNA strands are copied. It appears that a large percent of ORI sequences may lie within GC-rich regions of DNA, and often close to CpG islands. Deciphering the details of where ORIs are found and how they control replication and chromatin formation is part of the study of epigenetics.
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