Nucleus in prokaryotes and eukaryotes

Physical Basis of Heredity 1.  The Nucleus and the Chromosome
The Nucleus 
Significance of nucleus : Hammerling's experiment
Number, shape and size of nucleus
Nucleus in prokaryotes and eukaryotes
Nuclear envelope
Nuclear pore complex and nucleocytoplasmic traffic
Number, size and shape of chromosomes
Morphology of chromosomes
Euchromatin and heterochromatin
Constitutive and facultative heterochromatin
Single-stranded and multi-stranded hypotheses for chromosomes
Chemical composition of chromosomes
Infrastructure of chromosomes
Function of chromosomes
Special types of chromosomes 
Lampbrush chromosomes
Salivary gland chromosomes
Prokaryotic Nucleoids

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.