DNA and chromosomes may seem like completely separate things. Well, they’re not. In fact, all chromosomes are individual DNA molecules coiled and twisted around, because DNA is huge. At least in eukaryotes it is. That’s one of the first differences between eukaryotes and prokaryotes in their DNA – prokaryotes have less DNA.
Eukaryotic DNA is stored within the nucleus of each cell (apart from cells without one, e.g. red blood cells). Because of its sheer size, it must be organised well. Proteins called histones help do just that:
The S phase in the cell cycle involves the replication of DNA in chromosomes. Checkpoints ensure that all DNA is copied accurately, and once all chromosomes have been duplicated, they must be properly attached to the cell spindle fibres that will separate them during mitosis into the two new cells.