NUCLEUS

NUCLEUS
Brown in 1831 first discovered the nucleus. The presence of definite nucleus is the main feature which differentiates the eukaryotic cell from prokaryotic cell. Nucleus is present in almost all eukaryotic cells expect mature erythrocytes and platelets. Nucleus contains most of the hereditary units of the cell, called genes, which are coiled as chromatin fibers. During cell division, chromatin fibers condense and packed into compact structure known as chromosomes. DNA replication, transcription and RNA processing takes place within the nucleus. The nuclear envelope provides the structural framework of the nucleus and acts as a separates the contents of the nucleus from the cytoplasm. The nuclear envelop consist of two nuclear membranes, (inner and outer nuclear membrane), nuclear lamina and nuclear pore complexes. The inner membrane carries unique proteins that are specific to the nucleus and the outer membrane is continuous with the endoplasmic reticulum. Underlying the inner nuclear membrane is the nuclear lamina, a fibrous meshwork that provides structural support to the nucleus. The nuclear lamina is composed of proteins called lamins. They exist in diametric form and associate with each other to the filaments that make up lamina. The nuclear pores complexes allow selective traffic of ions, proteins and RNAs. Nucleolus is the most prominent substructure within the nucleus. Nucleolus is the site of rRNA transcription and processing.