In eukaryotes, DNA is associated with and bound to proteins called histones.The DNA double helix is wound round the histones to fix it in position and then coiled. The DNA-histone complex is then looped and further coiled into a chromosome.The number of chromosomes varies from species to species but is always the same for each member of a species.Chloroplasts and mitochondria contain short circular pieces of DNA not associated with histones.Usually the DNA and histones are loosely coiled into chromatin. At the start of cell division they coil further and become visible as chromosomes. By the start of nuclear division the chromosomes have been replicated into two identical copies. Two identical chromatids are visible, joined at the centromere.