Extrachromosomal Circular DNA are composed of chromatin and replicate in the nucleus of the cell during cell division. Unlike typical chromosomes, they are composed of circular fragments of DNA, up to only a few million base pairs in size and contain no centromere or telomere. Extrachromosomal Circular DNA Extrachromosomal circular DNA (eccDNA) is circular DNA found in the nuclei of plant and animal cells, including human cells, in addition to chromosomal DNA. eccDNA can be from 200 up to 5 million base pairs in length. eccDNA has been found in the nuclei of human cancer cells and shown to carry many copies of
DNA methylation in cancer and predisposes the cell and its descendants to progression to cancer. Another review also indicated an early role for hypermethylation of DNA repair genes in cancer. If a gene necessary for DNA repair is hypermethylated, resulting in deficient DNA repair, DNA damages will accumulate. Increased DNA damage tends to cause increased errors during DNA synthesis, leading to mutations that can give rise to cancer. If hypermethylation of a DNA repair gene is an early step in carcinogenesis, then it may also occur in the normal-appearing tissues surrounding the cancer from which the cancer arose (the field defect). See the table