NUCLEUS

NUCLEUS

Introduction

Nucleus is the most important organelle of cell, which controls all the activities of cell.

Discovery

Robert Brown discovered nucleus first time in 1831.

General Features

1. In animal cells, it generally occupies the central space, while in the case of plant cells it is pushed towards periphery due to the presence of a large vacuole.
2. Generally the cells have one nucleus and are called mononucleate. On the other hand, the cells with two nuclei are binucleated and with more than two as multinucleated.
3. Nucleus is only visible when the cell is in non-dividing stage. It contains chromatin network and soluble sap called nucleoplasm. In dividing cells, the nucleus disappears and chromosomes replace the chromatin material in it.
4. DNA, RNA and proteins including enzymes form the chemical composition of the nucleus.

Structure

Nucleus consists of three important components i.e.

(i) Nuclear membrane

(ii) Nucleolus

(iii) Chromosomes

Nuclear membrane

Nucleus is surrounded by a nuclear membrane, which separates the nuclear material from the cytoplasm.

• Nuclear membrane acts as nuclear envelope, which is composed of two membranes.
• Outer membrane is at places continuous with the endoplasmic reticulum.
• Inner membrane encloses the nuclear contents.

• Outer and inner membranes are continuous at several points, giving rise to nuclear pores.
• They allow exchange of materials between the nucleus and cytoplasm.
• Their number is highly variable. The undifferentiated cells e.g, eggs have numerous pores (about 30,000 per nucleus), whereas differentiated cells e.g, erythrocytes have only 3 or 4 pores per nucleus.
• Each pore has a specific structure, which controls the traffic of substances passing through them.
• There may be one or more nucleoli in the nucleus.

Nucleolus

It is a darkly stained body within the nucleus, and is without any membranous boundary to separate it from the rest of the nuclear material.

Physical Structure

It is composed of two regions; the peripheral granular area composed of precursors of ribosomal subunits and the central fibrillar consisting of large molecular weight RNA and rDNA.

Function

It is involved in the synthesis andstorage of ribosomal RNA. It is the site where ribosomes are assembled and exported to the cytoplasm via nuclear pores.

Chromosomes

Nucleus is deeply stained with basic dyes because of the chromatin material. During cell division chromatin material is converted into darkly stained thread like structures known as chromosomes.

Physical Structure

Under compound microscope, chromosomes appear to be made of arms and centromere. Each chromosome consists of.

• Two identical chromatids at the beginning of cell division (chromatid is exact replica of the chromosome), which are held together at centromere.

Chemical Structure

A chromosome is composed of DNA and proteins. All the information necessary to control the activities of the cell is located on the chromosomes in the form of genes, which are transferred from one generation to the other.

Number of Chromosomes

The number of chromosomes in all individuals of the same species remains constant generation after generation.

For example

The number of chromosomes in normal body cells is diploid (2n), whereas haploid number (n) is present in germ cells e.g. human sperm and eggs have 23, while those of Drosophila have 4 chromosomes.

• Centromere is the place on the chromosome where spindle fibers are attached during cell division.
• In man, each cell contains 46 chromosomes
• Frog cell has 26
• Chimpanzee has 48
• Fruitfly Drosophila has 8
• Onion cell has 16
• Potato cell has 48
• Garden pea has 14