Nucleus is an important unit of cell which control all activities of the cell. Nucleus holds the hereditary information. It is the largest among all cell organelles. It may be spherical, cuboidal, ellipsoidal or discoidal.
It is surrounded by a double membrane structure called nuclear envelope, which has the inner and outer membrane.
The inner membrane is smooth without ribosomes and the outer membrane is rough by the presence of ribosomes and is continues with irregular and infrequent intervals with the endoplasmic reticulum. The membrane is perforated by pores known as nuclear pores which allows materials such as mRNA, ribosomal units, proteins and other macromolecules to pass in and out of the nucleus. The pores enclosed by circular structures called annuli. The pore and annuli forms the pore complex . The space between two membranes is called perinuclear space.
Nuclear space is filled with nucleoplasm, a gelatinous matrix has uncondensed chromatin network and a conspicuous nucleoli. The chromatin network is the uncoiled, indistinct and remain thread like during the interphase. It has little amount of RNA and DNA bound to histone proteins in eukaryotic cells (Figure 6.25).
Chromatin is a viscous gelatinous substance that contains DNA, histone & non–histone proteins and RNA. H1, H2A, H2B, H3 and H4 are the different histones found in chromatin. It is formed by a series of repeated units called nucleosomes. Each nucleosome has a core of eight histone subunits.
During cell division chromatin is condensed into an organized form called chromosome. The portion of Eukaryotic chromosome which is transcribed into mRNA contains active genes that are not tightly condensed during interphase is called Euchromatin. The portion of a Eukaryotic chromosome that is not transcribed into mRNA which remains condensed during interphase and stains intensely is called Heterochromatin. I Nucleolus is a small, dense, spherical structure either present singly or in multiples inside nucleus and it’s not membrane bound. Nucleoli possesses genes for rRNA and tRNA.
• Controlling all the cellular activities
• Storing the genetic or hereditary information.
• Coding the information in the DNA for the production of enzymes and proteins.
· DNA duplication and transcription takes place in the nucleus.
· In nucleolus ribosomal biogenesis takes place.