The nucleus is the largest organelle in eukaryotic cells and controls cell activity. It is surrounded by a double membrane and contains genetic material in the form of DNA. The nucleus has several structures including the nuclear envelope, nucleolus, chromatin, and chromosomes which contain the genes. It performs essential functions like DNA synthesis and repair, gene regulation, and cell division.

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Ned
What is Nucleus ? Explained its structure and
organelles in details.
easylearningwithned.blogspot.com/2022/11/what-is-nucleus-explained-its-structure.html
Nucleus is the largest organelle present in eukaryotic cells, who control the whole activity
of the cell. It contains genetic material and covered with the double layered membrane
known as nuclear envelope. Its detailed structure visible under electron microscope. In
1831, Robert Brown describes the nucleus from orchid cell and called it by this name. It is
composed with Protein, DNA, Lipids. The percentage of protein present in about 80%.
Percentage of DNA present in about 12% and the percentage of Lipids present in about
3%. The protein includes enzymes such as polymerase, which help in the synthesis of
DNA and RNA.
In most of cells the nucleus presents near to the center but in some cells it moves freely
anywhere. Most of the cells contain single nucleus known as uninucleate but some cells
contain double nucleus (Binucleate) and multiple nucleus (multinucleate). Binucleate cells
example is paramecium caudatum, a protozoan. Multinucleate cells are originated by
fusion of straight muscle fibres and latex vessels this term known as syncytia. Example of
multinucleate is Opalina, a protozoan.
Shape- Nucleus have different types of shape. Its shape depends on the shape of cell. In
squamous cell's nucleus is flattened shape, in cuboidal cell it is rounded shape, in
columnar cell nucleus is Oval shape, in paramecium the nucleus is Bean-Shaped, in
vorticella the nucleus is Horseshoe-shaped, in basophil the nucleus is 3-Lobed, in smooth
muscle fibre the nucleus is Elongated, in stentor the nucleus is Moniliform, in neutrophil
the nucleus is many lobed, in silk spinning cell of insect larva the nucleus is branched, in
Eosinophil the nucleus is 2-Lobed, in amoeba the nucleus is Lens-shaped.

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Shapes of nucleus
Size- the size of nucleus is varying with the cell type and species. But the general
nucleus varies from 5 to 25 micrometer in diameter. The size of nucleus is also
depended upon number of chromosomes presents in it.
Structure- the nucleus is covered with the double layer membrane known as nuclear
envelope. The first layer is known as outer layer and the other layer is known as inner
layer. The gap between the both layer is known as inter-membrane space. Ribosomes
are attached on outer layer. Nucleus is filled with the fluidic substance known as
karyolymph and have nuclear pores also. In fluidic media chromosomes present with
primary constriction and secondary constriction. It also contains rounded bodies called
Nucleolus present middle of the nucleus.
Nucleus
Function-
1) By synthesis of protein nucleus maintains the cells and their growth.

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2) By synthesis of enzymes nucleus regulates cell metabolism.
3) Nucleus takes part in the synthesis of ribosomes.
4) It contains genetic information.
5) Nucleus control the cytoplasmic activity.
6) Nucleus take part in cell replication.
Nuclear Envelope- it is also known as karyotheca. It formed during telophase by coming
together and fusion of small vesicles into which the nuclear envelope breaks up during
prophase.
Nuclear envelope consists two unit of membrane one is outer layer membrane and
second is inner layered membrane. The space between these two membranes is known
as intra-space membrane and it is filled with Fluid. The width of these membrane is 200-
400A wide. The ribosomes are attached on outer membrane because the perinuclear
space is continuous with the channels of endoplasmic reticulum. This inter-connection
allows to the nuclear envelop to shrink or expand rapidly. The inner membrane is smooth
because it is free from ribosomes but it has dense layered known as nuclear lamina
closely associated with inner membrane. It connects chromatin to the inner surface.
The nucleus is semi-permeable, contain nuclear pores. In one nucleus around 1000 to
10000 nuclear pores present. At the edge of nuclear pore both layers are continuing.
These pores fitted with complex protein structure called pore complex. These pores are
connected with both cytoplasm and nucleoplasm. It consists of two rings known as annuli
and each having 8 subunits, sending spoke into the pore. So, this Spoke enclose the
channel. The pore complex controls the passage of macromolecules like tRNA, mRNA,
enzymes so on. By these pores, the RNA molecule enter into the nucleus because due to
its size it cannot be pass through the membranes.

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A Pore Complex
Functions-
1) Nuclear envelope maintains the shape of the nucleus.
2) It maintains the flow of material inside and outside the nucleus.
3) It keeps the nucleus content in place and distinct from the cytoplasm.
4) Its pores allow the exit of macromolecules.
5) It preserves the stability of the genetic material.
Nuclear sap- it is known as Karyolymph. Nuclear sap filled with clear and fluidic material.
It contains raw materials, metal ions, enzymes for the synthesis of DNA and RNA. It
contains numbers of protein also. Histone and non-histone protein for combination with
DNA and other proteins for the formation of ribosomal subunits. The ribosomes and RNA
synthesis occur in the nucleoplasm.
Functions-

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1) Nuclear sap is the set of synthesis of DNA, RNAs and Ribosomal subunits.
2) It supports the chromatin material and nucleoli.
3) It provides turgidity to the nucleus.
Nuclear Matrix- is known as nuclear skeleton because it is a network of fine,
crisscrossing, protein containing fibril which are joined to nuclear envelop by their ends.
By these it maintains the shape of nucleus.
Function-
1) Nuclear matrix is the machinery for various nuclear activities.
2) It maintains the shape of the nucleus.
Chromatin- is the combination of DNA and Protein. It is extended, uncondensed form of
deoxyribonucleoprotein chains. Proteins are two types: - histones and non-histones.
Histone are basic in nature and non-histone is acid or sometimes neutral in nature. The
Histone is structural protein of chromatin and the non-histone is functional protein of the
chromatin. The chromatin occurs in interphase as fine filaments known as chromatin
fibres. After that, these fibres lie crisscross and make a diffuse network called the
chromatin. Chromatin is a colored material so it is easy to take up and stain for
microscopic examination.
Chromatins are two types: - Euchromatin and Heterochromatin.
Euchromatin- consists thin, uncoiled, extended, scattered chromatin fibres.
Euchromatins are present in most of nucleus. It stains lightly and its genes are active.
Euchromatin permits crossing over.

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Heterochromatin- consists thick, coiled, compact, localized chromatin fibres.
Heterochromatins are present close to the nuclear lamina. It stains deeply and its genes
are inactive. Heterochromatin inhibits the crossover.
Chromosomes- present in the nucleus, it made up by protein and single molecule of
deoxyribonucleic acid DNA. It is a tiny thread like structure. chromosomes contain the
genes. These genes are responsible for passing genetic information to the daughter cells.
In a single cell 23 pairs of chromosomes present. These chromosomes give unique
feature to our body.
Chromosomes have two long arms and two short arms. It consists
two identical halves known as chromatids and these chromatids
held together at one point known as centromere. The chromosome
have single centromere known as primary constriction but if
chromosome have two centromere it known as secondary
constriction. The part of chromosomes beyond the secondary
constriction is known as Statellite. The Secondary constriction
always stay in one position so it also known as marker
chromosomes. Chromosomes are visible in the M phase. These
appear as short, thick, rod-like organelle.
Chromosome
Types of chromosomes- chromosomes are four types.

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i) Metacentric chromosome- in this chromosome, the centromere present near to the
center and the arms of both sided chromosomes are equal.
ii) Submetacentric chromosome- in this chromosome, the centromere presents slightly
away from the center and the arm of one-sided chromosome is short and other sided arm
is long.
iii) Acrocentric chromosome- in this the centromere present near to the end and the
one arm of chromosome is very short and other is very long.
iv) Telocentric chromosome- in this type of chromosome, the centromere is present on
the tip of chromosome and there is one sided arm present.
Types of Chromosomes
Functions-
1) Chromosomes contain the genetic material.
2) It is responsible for gene regulation.
3) Chromosomes are also responsible for protein synthesis.

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Nucleolus- Present in mid of the nucleus. It is rounded, dense, dark staining, granular
sub-organelle. Each cell contains usually 1-4 nucleolus. The nucleolus is formed at
specific site known as nucleolar organizer. These nucleoli contain largely of RNAs and
Proteins. It also contains DNA which produce precursor RNAs for the formation of
ribosomes. For the maintaining configuration of the nucleolus the calcium is necessary.
In electron microscope the nucleolus shows four regions-
i) Matrix- the matrix is amorphous, homogeneous, proteinaceous ground substance of
the nucleolus.
ii) Fibrillar Region- this region contains fibrils that represent RNA precursor molecules in
early stage of processing.
iii) Granular region- by their name it contains spherical granules. These granules
represent ribosomes subunits.
iv) Nucleolar Chromatin- it consists chromatin fibre. It synthesizes rRNA.
Function-
1) Nucleolus store the ribosomal protein.
2) It synthesizes and stores RNA.
3) It also plays a role in cell division.
Some important questions-

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1) Who discovered the nucleus?
Ans- Robert Brown discovered the nucleus.
2) What is the main site of rRNA synthesis?
Ans- The main site of rRNA synthesis is Nucleolus.
Reference book- Pradeep's Biology Text Book .
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What is the unit of Living organism ?
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