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Literally the nucleus is present in all kinds of living cells, but a well-organised nucleus with a definite nuclear membrane that separates the DNA from cytoplasm, is absent in bacteria, PPLO (Pleuropneumonia like organisms), Spirochaeta, blue-green algae etc.
On the basis of presence or absence of true nucleus, living organisms are divided sharply into two groups- prokaryotes, which do not have true nucleus, and eukaryotes, which have a well-organised nucleus.
The membrane less nuclear area which contains the most important genetic material, DNA, within a cell of prokaryotic organism is termed as ‘prokaryon’ or nucleoid, rather than a true nucleus.
The prokaryon or nucleoid is the nucleus-like nuclear region which is visibly distinct from cytoplasm but virtually remains in direct contact with the rest of protoplasm due to lack of nuclear membrane. Functionally it is now known to play a similar role like that of true nucleus.
Source: Biology discussion.com
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