NUCLEUS
Nucleus is the most prominent and the largest cellular organelle. It has a diameter of 10 μ to 22 μ and occupies about 10% of total volume of the cell. Nucleus is present in all the cells in the body except the red blood cells. The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes. Presence of nucleus is necessary for cell division. Most of the cells have only one nucleus (uninucleated cells). Few types of cells like skeletal muscle cells have many nuclei (multinucleated cells). Generally, the nucleus is located in the center of the cell. It is mostly spherical in shape. However, the shape and situation of nucleus vary in some cells.
STRUCTURE OF NUCLEUS
Nucleus is covered by a membrane
called nuclear
membrane and contains many components. Major components of nucleus are nucleoplasm, chromatin
and
nucleolus.
Nuclear Membrane
Nuclear membrane is double layered and
porous in nature. This allows the nucleoplasm to communicate with the
cytoplasm. The outer layer of nuclear membrane is continuous with the membrane
of endoplasmic reticulum. The space between the two layers of nuclear membrane is
continuous with the lumen of endoplasmic reticulum. Pores of the nuclear
membrane are guarded (lined) by protein molecules. Diameter of the
pores is about
80 to 100 nm. However, it is decreased to about 7 to 9 nm
because of the attachment of protein molecules with the periphery of the pores.
Exchange of materials
between nucleoplasm and cytoplasm
occurs through these pores.
Nucleoplasm
Nucleoplasm is a highly
viscous fluid that forms the ground substance of the nucleus. It is similar to
cytoplasm present
outside the nucleus. Nucleoplasm surrounds chromatin and nucleolus. It
contains dense fibrillar network of proteins called the nuclear matrix and
many substances such as nucleotides
and enzymes. The nuclear matrix forms
the structural framework for organizing chromatin. The soluble liquid part of
nucleoplasm is known as nuclear hyaloplasm.
Chromatin
Chromatin is a thread-like material
made up of large molecules of DNA. The DNA molecules are compactly packed with the
help of a specialized basic protein called histone. So, chromatin is referred
as DNA-histone complex. It
forms the major bulk of nuclear material. DNA is a double helix which
wraps around central
core of eight histone molecules to form the fundamental packing unit of
chromatin called nucleosome.
Nucleosomes
are packed together tightly with the help of a histone molecule to
form a chromatin fiber.
Just before cell division, the chromatin condenses to form chromosome.
Chromosomes
Chromosome is the rod-shaped nuclear
structure that carries a complete blueprint of all the hereditary
characteristics of that species. A
chromosome is formed from a single DNA molecule coiled around histone molecules.
Each DNA contains many genes. Normally, the chromosomes are not visible in the
nucleus under microscope. Only during
cell division, the chromosomes are visible under microscope. This is because
DNA becomes more tightly packed just before cell division, which makes the chromosome
visible during cell division. All the dividing cells of the body except reproductive
cells contain 23 pairs of chromosomes. Each pair consists of one chromosome inherited
from mother and one from father. The cells with 23 pairs of chromosomes are
called diploid
cells. The
reproductive cells called gametes or
sex cells contain only 23 single
chromosomes. These cells are called haploid cells.
Nucleolus
Nucleolus is a small, round granular
structure of the nucleus. Each nucleus contains one or more nucleoli.
The nucleolus contains RNA and some
proteins, which are similar to those found in ribosomes. The RNA is synthesized
by five different pairs of chromosomes and stored in the nucleolus. Later, it is
condensed to form the subunits of ribosomes. All the subunits formed in the
nucleolus are transported to cytoplasm through the pores of nuclear membrane.
In the cytoplasm, these subunits fuse to form ribosomes, which play an
essential role in the formation of proteins.
FUNCTIONS OF NUCLEUS
Major functions of
nucleus are the control of cellular activities and storage of hereditary
material. Several
processes are involved in the nuclear
functions.
Functions of nucleus:
1. Control of all the cell activities
that include metabolism, protein synthesis, growth and reproduction (cell
division)
2. Synthesis of RNA
3. Formation of subunits of ribosomes
4. Sending genetic instruction to the
cytoplasm for protein synthesis through messenger RNA (mRNA) 5. Control of the
cell division through genes
6. Storage of hereditary information
(in genes) and transformation of this information from one
generation of the species to the next.
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