On the basis of presence or absence of well-defined nucleus, living organisms have been classified into two groups by molecular biologists in recent years. These groups are (i) prokaryotes, the individuals which do not have a well-organized nucleus and will therefore include viruses, bacteria and blue-green algae; and (ii) eukaryotes, which would include the remaining types, which have a well-organized nucleus. The nuclear equivalent of a prokaryotic organism is known as prokaryon or more commonly as nucleoid rather than a nucleus. The 'prokaryon' or 'nucleoid' does not have a true chromosome, is not enclosed in a nuclear envelope and does not divide by regular mitosis. The nuclei may even be absent in some specialized cells of eukaryotes. For instance mature mammalian red blood cells are also without any nuclei. This is why they are often called as red blood corpuscles rather than cells.
A nucleus may be described as having three important parts, namely, nuclear membrane or nuclear envelope, nucleolus and chromosomes. The fluid, in which nucleolus and chromosomes are present and which is enclosed in nuclear membrane, is called nucleoplasm.