DNA and Chromosomes

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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:


This is the second difference: eukaryotes have histones around which the DNA coils, while prokaryotes don't. So what does prokaryotic DNA look like?


The DNA above is stored as a small loop (the bacterial chromosome), and as a plasmid. A plasmid is even smaller, and may be copied and transferred to another bacterium of the same or different species by a process called conjugation (or, more colloquially, bacteria sex).

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Eukaryotic DNA is linear, rather than circular. That means the DNA, despite being coiled numerous times, has a distinguishable start point and end point, while prokaryotic DNA is a continuous circle (see above picture).


The Differences between Eukaryotic and Prokaryotic DNA

Size                                      large                  small

Shape                                   linear                 circular

Histones                              present                absent

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