The following points highlight the six main components of the nucleus found in cells. The components are: 1. Nuclear Envelope 2. Nuclear Lamina 3. Nucleoplasm (Nuclear Sap) 4. Nucleolus 5. Chromatin Fibres 6. Nuclear Matrix.
Component # 1. Nuclear Envelope:
The nuclear envelope is made up of two concentric membranes, each of 7-8 nm thickness. The space between the two membranes is known as perinuclear apace; it measures 15-30 nm across. The outer membrane is adjacent to the cytoplasmic compartment, while the inner membrane is adjacent to the nucleoplasm, the interior of the nucleus (Fig. 2.22).
The outer membrane contains ribosomes attached to the outer surface and is connected with the ER. Thus the perinuclear space is continuous with the cisternae of ER.
Nuclear envelope has several pore complexes (Fig. 2.22, 2.23) about 3000 pores are present in an animal cell nuclear envelope. The outer and inner membranes are connected with each other at the “nuclear pores”. The Pore complex is 120 nm in diameter; it is composed of a protein complex joined to the nuclear membrane.
The pore contains two annuli of which one faces the cytosol, while the other faces the nucleoplasm (Fig. 2.22).
The protein complex appear to be made up of 8 subunits which form an octagonal symmetry. These 8 subunits form 8 spokes and a coaxial ring of about 120 nm diameter. Outside the coaxial ring, there occur 8 radial arms which enter the membrane and hold the pore complex in place.
Together with the radial arms, the diameter of the pore complex is 150 nm (Fig. 2.23). The pore complex has a “central pore” which form a channel of about 10 nm diameter. It is surrounded by a “tansporter”. The central pore can be opened up to 20 nm diameter through which proteins and nucleoproteins can pass freely.
In developing spermatocytes and oocytes, some stacks of cytoplasmic membranes are found associated with the nuclear envelope. They are called annulate lamellae and are rich in RNA content. A large amount of mRNA may be stored in these lamellae of such cell types.
Component # 2. Nuclear Lamina:
On the nucleoplasm side, adhered to the bilayer of inner membrane, there is a dense layer of tough proteinaceous fibrous material; this structure is called nuclear lamina (Fig. 2.22, 2.24). Lamina is absent from the nuclear pore.
It is composed of three protein subunits called lamin A, lamin B and lamin C. The main function of the nuclear lamina is to organize the structure of nucleus. The lamins become phosphorylated during mitosis, due to which the lamin proteins get solubilized resulting in the breakdown of lamina.
Breakage and reconstruction of nuclear envelope and lamina during cell division cycle:
A separate factor is involved in the breakdown of the nuclear envelope during mitosis. After breakage, the nuclear envelope forms small vesicles (Fig. 2.24). Phosphorylation of the Lamina proteins causes the breakage of lamina into its protein subunits. One of the lamina protein subunits remains attached with the fragments of nuclear envelope, while the other two subunits are free.
ER and Golgi complex membranes also break into small vesicles, and the cytoskeleton is dissociated. After the chromosomes reach the two poles (telophase), dephosphorylation of lamina proteins permits the reassembly of lamina. Lamina surrounds the chromatin and the membrane vesicles accumulate around the lamina. Then the vesicles fuse to form the nuclear envelope (Fig. 2.24).
Component # 3. Nucleoplasm (Nuclear Sap):
It contains the soluble materials of nucleus, the main substances being enzymatic proteins, involved in the replication of DNA and transcription, RNA and ribonucleoproteins, enzymes, ions, precursors of nucleosides and nucleotides etc.
Component # 4. Nucleolus:
It appears as a large spherical, dense structure within the nucleus (Fig. 2.22). The number of nucleoli varies from 1 to many per nucleus. Nucleolus is formed in association with the nucleolar organizer region (NOR) of the SAT-chromosomes. It is the site of synthesis of ribosomal RNA.
The rRNA genes are amplified in large number of copies to produce large amounts of rRNA. Thin section electron microscopy has revealed that the nucleolus has a fibrous region (core) surrounded by a granular region (nucleolar matrix). The core of the nucleolus has thin fibrils of 5 nm diameter. The granular region contains granules of ribonucleoproteins (RNP) of about 15 nm diameter.
Component # 5. Chromatin Fibres:
DNA is the main component of the nucleus. It is complexed with proteins to form the nucleoprotein complex known as chromatin, which account for about 80-90% of the total mass of the nucleus. Chromatin fibres are dispersed in the nucleus during interphase; their condensation occurs during mitosis so that individual chromosomes become visible under the light microscope.
The chief proteins associated with DNA are histones, although non-hostone proteins are also present. Certain sperm cells contain protamines instead of histones.
Component # 6. Nuclear Matrix:
When the chromatin is removed from nucleus, the overall shape of the nucleus is retained; the structures remaining within the nucleus constitute the nuclear matrix. It provides the skeletal structure of the nucleus. A small amount of tightly bound DNA and RNA is found associated with the matrix. Newly synthesized DNA is found preferentially associated with the matrix.