Nucleus is the central processing unit of the cell. It is called the brain of the cell. Using the same analogy, just like the brain stores all the information about our body and life, and facilitates the bodily functions centrally, nucleus stores the information about the life of the cell and that of the cell itself. It is the largest organelle in a cell. Most cells usually have a single nucleus. However, there are exceptions where a cell may not have a nucleus at all or may have more than one nucleus. The red blood cells are anucleated cells, which mean they have no nucleus, while some cells like those of the skeletal muscle are polynucleated, which means they have more than one nucleus. Nucleus contains the cell’s genome. It has a vital role in cell division and inheritance as it controls the transmission and expression of this genetic information. It stores this information in the form of DNA or Deoxyribose Nucleic Acid molecules super coiled to form a chromosome. There are twenty three pairs of chromosomes, including the X and Y chromosomes (the sex chromosomes) within the nucleus and are collectively known as the “karyotype”. It normally contains chromatin, nucleoli and the nucleoplasm. Chromatin mainly consists of the DNA and histone proteins in mammals.
It is oval or spherical in shape and usually about four to six micro meters in diameter. It is surrounded by a double membrane known as the nuclear envelope. The nuclear envelope is linked to the endomembrane system through the endoplasmic reticulum. At various points, the inner and outer membranes of the nuclear envelopes are fused together by structures called the “nuclear pores”. They control the traffic of molecules and other substances in and out of the nucleus. Normally there are about three thousand nuclear pores per nucleus. Nucleolus is a structure within the nucleus of a cell. It is a non-membrane bound structure in which the ribosomal RNA (rRNA) is transcribed. It contains densely staining granules in the outer layer and fibrils in the centre. Usually, there are one or two nucleoli per cell, however, there could be up to four in certain cells. Nucleoli are formed around specific genetic loci’ called the Nucleolar Organizing Regions’ and due to this non-random organization, the nucleolous is determined as a genetically determined element of a cell. Nucleoli produce the ribosomal sub-units from proteins and the rRNA. Sources: Principles of Anatomy and Physiology, by Tortora and other lecture notes
