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
Mitochondrial DNA organisms (e.g. in some species of Cnidaria) the mtDNA is found as linearly organized DNA. Most of these linear mtDNAs possess telomerase-independent telomeres (i.e. the ends of the linear DNA) with different modes of replication, which have made them interesting objects of research, as many of these unicellular organisms with linear mtDNA are known pathogens. For human mitochondrial DNA (and probably for that of metazoans in general), 100–10,000 separate copies of mtDNA are usually present per somatic cell (egg and sperm cells are exceptions). In mammals, each double-stranded circular mtDNA molecule consists of 15,000–17,000 base pairs. The two strands of