A team of scientists from the University of Virginia and University of North Carolina in the US have discovered a previously unidentified type of small circular DNA molecule occurring outside the chromosomes in mouse and human cells. The circular DNA is 200-400 base pairs in length and consists of non-repeating sequences. The new type of extra-chromosomal circular DNA (eccDNA) has been dubbed microDNA. Unlike other forms of eccDNA, in microDNA the sequences of base pairs are non-repetitive and are usually found associated with particular genes. This suggests they may be produced by micro-deletions of small sections of the chromosomal DNA.
Professor Anindya Dutta and colleagues pruified DNA taken from samples of mouse brain tissue and then digested away the linear DNA (which consists of millions of base pairs) to leave only circular DNA pieces, which they then sequenced using ultra-high-throughput sequencing. Circles were identified by a new bioinformatics program. They found the size of the circles was around the same length as the DNA on a nucleosome (a sub-unit of a chromosome). The small size of the circular DNA surprised them since extra-chromosomal DNA circles are larger. Their circular DNA was also dissimilar to the previously-known circles known as polydispersed DNA because the latter usually consist of repeating sequences of base pairs. Another interesting finding was that the circles are rich in the base pair GC (guanine-cytosine) with relatively little AT (adenine-thymine. The researchers repeated their experiments on other mouse tissues and on human cells.