chromosome
A chromosome is, minimally, a very long, continuous piece of DNA, which contains many genes, regulatory elements and other intervening nucleotide sequences. In the chromosomes of eukaryotes, the uncondensed DNA exists in a quasi-ordered structure inside the nucleus, where it wraps around histones (structural proteins, Fig. 1), and where this composite material is called chromatin. During mitosis (nuclear division), the chromosomes are condensed and called metaphasic chromosomes. This is the only natural context in which DNA is visible with an optical microscope. Prokaryotes do not possess histones or nuclei. In its relaxed state, the DNA can be accessed for transcription, regulation, and replication.

Autosomes are chromosomes that carry genes not related to sex determination; sex chromosomes carry genes related to the determinance of sexual characteristics.