The nucleus is the largest organelle seen clearly when the cell is not dividing. It stains deeply, is mostly spherical, WBC have lobed nuclei.

It is mostly one in each cell (uninucleate), some cells have many nuclei (multinucleate).

Double layered nuclear membrane having fine nuclear pores encloses nucleoplasm which contains chromatin network and a nucleolus.

Functions of Nucleus

  • Maintains the cell in a working order.
  • Co-ordinates the activities of other cell organelles.
  • Takes care of repair work.
  • Participates directly in cell division to produce genetically identical daughter cells. This division is called mitotic cell division.
  • Participates in production of meio-gametes and meiospores through another type of cell division called meiotic cell division.

Nuclear Membrane

Double layered membrane is interrupted by large number of nuclear pores. Membrane is made up of lipids and proteins (like plasma membrane) and has ribosomes attached on the outer membrane which make the outer membrane rough.

The pores allow the transport of large molecules in and out of nucleus, and the membranes keep the hereditary material in contact with the rest of the cell.


Within the nuclear membrane there is jelly like substance (karyolymph or nucleoplasm) rich in proteins. In the karyolymph, fibrillar structures form a network called chromatin fibrils, which gets condensed to form distinct bodies called chromosomes during cell division.

On staining the chromosomes, two regions can be identified in the chromatin material - heterochromatin (dark) and euchromatin (light). Heterochromatin has highly coiled DNA and genetically less active than euchromatin which has highly uncoiled DNA and genetically more active.

The number of chromosomes is fixed in an organism. During mitotic cell division chromosomes divide in a manner that the daughter cells receive identical amounts of hereditary matter.


Membraneless, spheroidal bodies present in all eukaryotic cells except in sperms and in some algae. Their number varies from one to few, they stain uniformly and deeply. It has DNA, RNA and proteins.

It is store house for RNA and proteins. It disappears during early phase of cell cycle and reappears after telophase in the newly formed daughter nuclei.