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Complete Cytology Notes for Nucleus
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This document provides in-depth knowledge with a step-by-step explanation of each and every concept regarding the Nucleus to enhance your knowledge
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NUCLEUS1,GENERAL INTRODUCTION
1. Nucleus was first seen by Anton Van Leeuwenhoek in RBC of fish.
2. The nucleus is the most important and essential part of the cell. It directs and controls all the
cellular activities and carries the activity information of the cell.
3. It is called "Director of the cell"
4. It was discovered by Robert brown (1831)
2. OCCURENCE
1. The nucleus is found in all eukaryotic cells.
2. In some cells’ nucleus are present during early stage of development
3. In prokaryotic cell the nucleus is without distinct nuclear membrane and nucleolus.
3. POSITION
Generally Nucleus is located in the center but change according to metabolic activities of the
cell.
Example
(1) In embryonic cells nucleus is generally found in the center of the cell.
(2) In some glandular cell nucleus located in the basal portion of the cell.
4. NUMBER
Normally cell contains only one nucleus but it may vary in different cells.
1. Two nuclei are found in paramecium (protozoa) liver cell, cartilage cell.
2. Many nuclei are found in Osteoblast, Rhizopus. Vaucheria etc.
5. SHAPE AND SIZE
1. It differs from cell to cell. It could be spherical,cuboidal,ellipsoidal, discoidal or
irregular. 0
2. The size depends on the volume of the cell. Polypoids generally have bigger nuclei.
They are about 10 micron in length
Formula - Np = Np/Cv - Nv
Np = Nucleoplasmic Index
Nv = Volume of nucleus
Cv = Volume of cytoplasm
The size of the nucleus depend on the volume of cellan DNA, protein and metabolic activity of
cell.
6. STRUCTURE
The interphase nucleus is composed of the following structure
(6.1) 1 Nuclear membrane
(6.2) Nucleoplasm
, (6.3) Chromatin Network
(6.4) Nucleolus
6.1. Nuclear membrane
1. It is nuclear envelope composed of two membrane outer membrane and inner membrane.
2. Both membranes have a unit membrane structure
3. Each membrane is about 50 Ao thick.
4. Both membranes are separated by a space called perinuclear space. Perinuclear space is
100-300 A° wide.
5. Outer membrane is rough due to presence of ribosome and is continuous with the unit
membrane of endoplamic reticulum
6.1.1. Nuclear pore
1. Nuclear membrane is perforated by pores called nuclear pore. Each pore is 600 Ao to
1000 A° in diameter The pore may be om or polygonal.
2. Pores allow exchange of substance between nucleus and cytoplasm.
3. They allow exit of mRNA and ribosomes unit and entry of nucleotides, and many other
macromolecules
4. Nuclear pore possess ring like cylindrical structure called as annulus.
5. The inner nuclear membrane is smooth as it lacks ribosome.
. 6.1.2 Origin of Nuclear Membrane
1. Nuclear membrane is derived from the membrane of endoplasmic reticulum
2. During telophase cisternae of endoplasmic reticulum get collected around the
chromosome and fuse with the another and result in formation of nuclear membrane.
Note: Nuclear membrane disappear in prophase stage of mitosis.
6.1.3. Function of Nuclear Membrane
(i) It provides definite shape to the nucleus.
(ii) Exchange of genetic material takes place through nuclear membrane in between nucleoplasm
and cytoplasm.
(iii) Nuclear pore also exchange the macromolecule.
6.3. Chromatin network
1. Nucleoplasm contain many thread like coiled and elongated structure, known as
chromatin fibre.
2. During cell division mitosis & meiosis chromatin fibre become thick rod like structure
known as chromosome.
3. The fibre of chromatin are twisted in nature and two type of chromatin.
6.3.1. Heterochromatin 6.3.2. Euchromatin
6.3.1. Heterochromatin
Chromatin which remain condensed during interphase is called Heterochromatin
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