Nucleus

This slide describes the nucleus in detail.

1. Nucleus Prakash Dhakal M. Sc. Public Health Microbiology Tribhuvan University, Nepal

2. Nucleus Nucleus is the first organelle to be discovered . It was discovered by Anton von Leeuwenhoek . It is a double membrane enclosed organelle found only in eukaryotic cells. Eukaryotes usually have a single nucleus but a few cell types, such as mammalian red blood cells have no nucleus but others may have many. Nucleus contains most of the genetic material; DNA with a large variety of proteins such as histones to form chromosomes. Nucleus is the control centre of the cell and is the most prominent organelle in cell. Nucleus can be easily observed under light microscope.

3. Structure Generally the nucleus is spherical but it may be fusiform , ellipsoidal, flattened depending upon cell shape and function. The size of the nucleus is variable. In mammalian cells, it occupies about 10% of the total cell volume. The nucleus consists of four components: Nuclear membrane or nuclear envelope Nucleoplasm or Nuclear sap Nucleolus Chromatin fiber or chromatin network

4. Nuclear envelope or Nuclear membrane: It consists of two membranes; an inner and an outer membrane. The distance between two membrane is about 10 to 50 nm. The nuclear membrane completely encloses the nucleus and separates the cell’s genetic material from the surrounding cytoplasm. The outer membrane is continuous with the membrane of the rough endoplasmic reticulum ( RER ) and contains ribosomes. The space between two membrane is called the perinuclear space. Like the cell membrane, the nuclear envelope consists of phospholipids that form a lipid bilayer . The envelope helps to maintain the shape of the nucleus and assists in regulating the flow of molecules in and out of the nucleus through nuclear pore.

5. Nuclear pores: Nuclear pores are composed of several proteins. The pores are about 125 millions Dalton in weight. The pores are about 100 nm in total diameter but 9 nm is available to diffuse molecules freely. The small space selectively allows the small water soluble molecules while preventing larger molecules (e.g. DNA, RNA and proteins) from entering or exiting the nucleus. The larger molecules needs separate transport mechanisms. Generally a mammalian cell contains about 3000-4000 pores throughout its envelope.

6. Fibrous lamina or Nuclear lamina: The nuclear membrane of many cells contains an additional layer called fibrous or nuclear lamina. It is inner towards the inner membrane facing the interior of the nucleus. It is composed of fine filaments made up of proteins. They provide mechanical support to nuclear membranes. It is supposed that it influences the exchange of materials between nucleus and cytoplasm . 2) Nucleoplasm or Nuclear sap : Nucleoplasm is the viscous liquid that fills inside of the nucleus. It is similar in composition to the cytoplasm found outside the nucleus.

7. 3) Nucleolus : The nucleus contains a large, spherical and acidophilic dense granule known as the nucleolus. It is membrane less. It looks large dark spot within the nucleus when observed through microscope. A nucleus may contains up to four nucleoli, but within each species the number of nucleoli is fixed. During cell division, the nucleolus disappears. Some studies suggests that the nucleolus are involved in manufacturing ribosome , cell ageing and senescence.

8. 4) Chromatin fiber or chromatin network: The cell nucleus contains the cell’s genetic material in the form of DNA molecules organized into structures called chromosomes. Each human cell contains roughly six feet of the DNA. Chromosomes are organized in a DNA-protein complex known as chromatin. The chromatin appears as thread like, coiled and elongated structure. These are stained with basic dyes like Feulgen’s stain, Acetocarmine etc, that is why termed as chromatin fibers or chromatin substance (Greek, Chrome: color). These are visible during interphase stage of cell division.

9. Functions Nucleus contains all the genetic information in its chromatin. Nucleus takes part in transmission of genetic information from parent cell to its daughter cells and from one generations to the next. Nucleus forms ribosomes on its nucleolus . It is responsible for protein synthesis, cell division, growth and differentiation.