Nucleus is a specialized protoplasmic body which is usually spherical or oval in shape and is much denser than the cytoplasm. In the young cell, it occupies a median position. In the mature plant cell, with the formation of the vacuole, it lies in the lining layer of the cytoplasm. Usually, a single nucleus is present in each cell, but some cells may have more than one nucleus. In lower organisms like bacteria and blue green algae, true nucleus is absent but there is a corresponding nuclear material. The usual size of nuclei varies from 5 to 25 um.
Each nucleus is surrounded by a double membrane called nuclear envelope which separates the nucleus from the surrounding cytoplasm. Numerous pores are present in this membrane through which there is a transport of materials between the nucleus and the cytoplasm. Inside the nuclear membrane a colorless dense sap is present which is known as nuclear sap or karyolymph or nucleoplasm.
Inside the nucleus is a tangled mass of thread-like structures called chromatin. It is formed of deoxyribonucleic acid (DNA) and protein. When a cell starts to divide, the chromatin appears as long threads and finally, rod-like bodies called chromosomes. The chrosomes contain stretches of DNA which carry information for protein synthesis. These stretches of DNA are called genes. Genes are passed from parents to children through sperms and eggs. This is why a gene is called the hereditary unit and DNA is called the hereditary material.
The function of nucleus can be estimated from the following statement. Nucleus is the storehouse of genes (genetic endowments). Production of enzymes depends upon the activities of genes. Without the production of enzymes there cannot be any metabolic activity. Genes dictate the cell to perform specialized functions such as cell division, cell sectretion, cell communication, etc. Removal of nucleus from a cell means removal of guiding entities of that cell. Therefore, without nucleus, a cell can neither survive, nor show specialized activities.
Nucleolus is a spheroidal organelle found inside the nucleus. It is the densest organelle. Nucleolus is prominent in cells that synthesize proteins. The number of nucleoli may be one to several thousand. Some oocygtes have thousands of nucleoli. In the electron microscope, two regions in the nucleolus can be distinguished. The inner (confer) zone has fibrillar structure and the periphery bears granules. The core has DNA which codes for ribosomal RNA. The peripheral granules are complexes of proteins and ribosomal RNA.