Nucleus | Definition, Structure & its Function
It is cell organelles was first described by Robert Brown as early as 1831. Later, the material is stained by the basic dyes was given the name Chromatin by Flemming.
It is a relatively large organelle controlling all the activities of the eukaryotic cell. Some cells have more than one nucleus, e.g., Opalina. However, some cells lack nucleus (anucleate) at maturity, such as, mammalian RBCs & sieve tube cells.
Structure of Nucleus
A nucleus in non-dividing phase is called interphase nucleus and a typical interphase nucleus is differentiated into nuclear envelope, nucleoplasm or nuclear matrix, nucleolus and chromatin.
It bounds the nucleus on the outside & separates it from the cytoplasm. It is made up of two membranes – outer & inner. The inner membrane is smooth and the outer membrane may be smooth or its cytoplasmic surface may bear ribosomes like the RER. The outer membrane is often connected to ER. These two membrane of the nucleus envelope are separated by a space is known as Perinuclear space. The space between the membrane 10 to 50 nm in width.
It is a transparent, semi-fluid & colloidal substance which fills the nucleus. It contains nucleolus & highly extended & elaborate nucleoprotein fibres called chromatin.
(i) It is a spherical structure found in the nucleoplasm.
(ii) It is not separated from rest of the nucleoplasm as it is not bounded by a membrane.
(iii) It is the site for ribosomal RNA (rRNA) synthesis. Thus nucleoli are larger & more numerous in cells that are actively involved in protein synthesis.
Chromatin means – (Gk. chroma – colour). The interphase nucleus contains a loose, extended & diffused network of nucleoprotein fibres called chromatin. These are named so because of their ability to get stained with contain basic dyes by Flemming in 1879. Chromatin fibres condense to form chromosomes.
Chromatin is essentially composed of DNA & basic proteins called histones. It also contain RNA & some non – histone proteins. The histone proteins are the packaging proteins that are associated with packaging of DNA into compact structures called chromosomes.
Heterochromatin & Euchromatin : It was observed that when chromosomes are stained with basic dyes like acetocarmine or fuelgen stain, then two types of regions can be observed :
(a) Heterochromatin region : This region gets dark stain during interphase. This portion is genetically inactive & highly condensed region with tightly packed DNA.
(b) Eurochromatin region : This region gets light stain during interphase. This portion is genetically active & rich in loosely packed DNA. Transcription occurs here.