This DNA is different from the DNA in bacteria, mitochondria and plastids such as chloroplasts. Nuclear DNA is very long, and is bound up in chromosomes which, apart from the DNA, are protein structures. The second difference is that each eukaryote cell has two sets of DNA, one set from each parent: it is diploid. Mitochondrial and plastid DNA is relatively short, is in a circle, and there is only one set (haploid), not two. It is supposed that these organelles were once independent bacteria (see endosymbiosis).