The nucleus is a spherical organelle found in eukaryotic cells that controls cell functions and contains DNA. It is surrounded by a double membrane and contains chromatin, nucleoplasm, and one or more nucleoli. The nucleus controls gene expression and protein synthesis, regulates cell growth and division, and maintains hereditary material. It allows for the exchange of molecules with the cytoplasm through nuclear pores in its membrane. The nucleus contains the cell's genetic material and plays a key role in controlling cell activities and functions.

1. Nucleus - Structure and Functions of Nucleus
The cell is the basic and structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life.
Cells are called the building blocks of life. The study of cells is called cell biology. The human body consists of trillions of cells, all
with their own particular function. Cells are the fundamental or key structure of all living organisms. The Cell provides a structure for
the body, takes in nutrients from food and carry out important functions.
There are two types of cells; one is prokaryotic (bacteria) and the other is eukaryotic (plant, animal, fungi). The prokaryotes have no
nucleolus- the DNA is in the cytoplasm, it can form small circular strands of DNA called plasmids. Likewise, Eukaryotic cells all have
their DNA enclosed in the nucleus. The nucleus is an organelle that consists of the genetic information for that organism. In an animal
cell, the nucleus is located in the central place of the cell. Likewise, in the plant cell, the nucleus is located more on the periphery due
to the large water-filled vacuole in the center of the cell.
What is the Nucleus?
The nucleus is a spherical-shaped organelle that is present in every eukaryotic cell. The nucleus is the control centre of eukaryotic
cells. It is also responsible for the coordination of genes and gene expression. The structure of the nucleus includes nuclear membrane,
chromosomes, nucleoplasm, and nucleolus. The nucleus is the most prominent organelle as compared to other cell organelles, which
account for about 10 percent of the volume of the cell. In general, a eukaryotic cell has the only nucleus. However, some eukaryotic
cells enucleate cells (without a nucleus) for example red blood cells. Some are multinucleate; it means it consists of two or more
nuclei, for example, slime mould.
The nucleus is detached from the rest of the cell or the cytoplasm by a nuclear membrane. The nucleus was the first organelle to be
discovered or detected. Antonie van Leeuwenhoek observes a ‘lumen’ , in the nucleus, in the red blood cells of salmon. Unlike
mammalian red blood cells, those of other vertebrates still contain nuclei.

2. Structure of the Nucleus
The cell nucleus consists of a nuclear membrane, called the nuclear envelope, nucleoplasm, nucleolus, and chromosomes.
Nucleoplasm, also called karyoplasm, is the matrix present inside the nucleus. The nuclear membrane separates the constituents of the
nucleus from the cytoplasm. 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 coordinating the flow of the molecules into and out of the nucleus
through nuclear pores. The nucleus of the cell contains DNA. The DNA controls the form, function, and growth of the cell. The
nucleus is similar to the brain in its functions of coordinating all the cell activities. The main components of the nuclear structure are
discussed below.
The Parts of the Nucleus are as Follows
 Nuclear membrane or envelope or karyotheca
 Chromatin threads or nuclear reticulum
 Nuclear sap or nucleoplasm or karyolymph
 Nucleolus.
Nuclear Membrane
The nuclear membrane is a double-layered system that encloses the elements of the nucleus. The outer layer of the membrane is
combined with the endoplasmic reticulum. The nuclear envelope is connected with the endoplasmic reticulum in such a way that the
internal compartment of the nuclear envelope continuous with the lumen of the endoplasmic reticulum. A liquid-filled space or
perinuclear space is present between the two layers of a nuclear membrane. The nucleus gets through the remaining of the cell or the
cytoplasm through several openings called nuclear pores. Such nuclear pores are the sites for the exchange of large molecules between
the nucleus and cytoplasm. The nuclear membrane is made up of lipoproteins, perinuclear space, pores, annuli material, an inner dense
lamella.
Chromosomes

3. Chromosomes are present in the form of strings of DNA and protein molecules called chromatin. The chromatin is classified further
into heterochromatin and euchromatin based on the functions. The heterochromatin is a highly condensed, transcriptionally inactive
form; mostly present adjoining to the nuclear membrane. On the other hand, euchromatin is a mild, less condensed organization of
chromatin, which is found amply in a transcribing cell. Chromatin threads are associated with one another and form a network called
chromatin reticulum. At the time of cell division, the chromatin threads isolated from one another become thicker or massive and
smaller and are now termed as chromosomes. It is primarily nucleoprotein, made up of nucleic acid and basic protein histone. Nucleic
acid contains sugar, nitrogenous bases, phosphate, and is a very complex organic acid.
Nucleic Acids are of Two Types
DNA (Deoxyribonucleic acid) especially found in the cytoplasm in soluble form and is called soluble RNA. It is also present in some
amounts in the ribosomes of nucleus, chromatin, and nucleolus. It is synthesized from DNA and is piled up in the nucleolus. It travels
to the cytoplasm and gets attached to the ribosome.
Chromatin is basophilic in type and most of the chromatin material is transferred into the specific number of chromosomes during cell
division. The chromatin material may be heterochromatin, sex chromatin, and euchromatin.
Nuclear Sap
The nuclear membrane encloses the clear, homogeneous, transparent, colloidal liquid of variable consistency. It is chiefly organized of
nucleoproteins, a small amount of inorganic and organic substances like nucleic acids, proteins dissolved phosphorus, ribose sugars,
minerals, enzymes, and nucleotides.
Nucleolus
The nucleolus is a solid, spherical-shaped structure found inside the nucleus. Some of the eukaryotic organisms have a nucleus that
consists of up to four nucleoli. The nucleolus plays an implied/indirect role in protein synthesis by producing ribosomes.

4. These ribosomes are cell organelles made up of RNA and proteins; they are transported to the cytoplasm, which is then attached to the
endoplasmic reticulum. The ribosome is the protein-producing organelles of a cell. The nucleolus disappears when a cell undergoes
division and reforms after the completion of the cell division.
Characteristics of Nucleolus
Some of the important features are summarised below.
 One or more nucleoli may be present within a nucleus. Four nucleoli are found in each nucleus in an onion.
 The nucleolus disappears in the late prophase stage.
 Reappears in the telophase stage
 It is a storehouse of RNA.
Functions of the Cell Nucleus
The cell nucleus controls the hereditary characteristics of an organism. This organelle is also responsible for protein synthesis, growth,
cell division, and differentiation. The important function is carried out by a cell nucleus. The following are the important functions of
the nucleus summarised below.
 Chromatin is referred to as the storage of hereditary material, the genes in the form of long and thin DNA strands.
 The Nucleolus is referred to as the storage of proteins and RNA in the nucleolus.
 The Nucleus is a site for transcription in which messenger RNA is produced for protein synthesis.
 The nucleus functions as the exchange of hereditary molecules that is RNA and DNA between the nucleus and the rest of the
cell.
 During cell division, chromatins are arranged into chromosomes in the nucleus.
 It functions in the production of ribosomes in the nucleolus.
 The nucleus functions the selective transportation of regulatory factors and energy molecules through nuclear pores.

5. The nucleus is the control centre of an organism as it regulates the integrity of genes and gene expression. The nucleus contains all the
genetic material of an organism like DNA, genes, chromosomes etc.
Distribution of Nucleus
Different cell types are categorized based on the presence or absence of the cell. The different types are mentioned below
 Uninucleate cell:
It is also referred to as monokaryotic cells, mostly plant cells which contain a single nucleus.
 Bi-nucleate cell:
It is also called a dikaryotic cell. It contains two nuclei at a time. The examples are one paramecium (have mega and
micronucleus), balantidium, and liver cells and cartilage cells.
 Multinucleate cells:
It is also known as the polynucleated cell which contains more than 2 nuclei at a time. For example, plants latex cells and latex
vessels. In animals, striated muscle cells and bone marrow cells.
 Enucleate cells:
Cells without a nucleus are called enucleate cells. However, some living cells like mature sieve tubes of phloem and RBC’s of
mature mammals lack nuclei.
In conclusion of the article we have learnt about the nuclear structure and functions. We have also learnt about different types
of cells based on the presence and absence of the nucleus.
Nuclear pore Definition:
A nuclear pore is a part of a large complex of proteins, known as a nuclear pore complex that spans the nuclear envelope, which is the
double membrane surrounding the eukaryotic cell nucleus. There are approximately 1,000 nuclear pore complexes (NPCs) in the
nuclear envelope of a vertebrate cell, but it varies depending on cell type and the stage in the life cycle
Structure:

6. a) The human nuclear pore complex (hNPC) is a 110 megadalton (MDa) structure. b) The proteins that make up the nuclear pore
complex are known as nucleoporins; each NPC contains at least 456 individual protein molecules and is composed of 34 distinct
nucleoporin proteins. c) About half of the nucleoporins typically contain solenoid protein domains—either an alpha solenoid or a beta-
propeller fold, or in some cases both as separate structural domains. d) The other half show structural characteristics typical of
"natively unfolded" or intrinsically disordered proteins, i.e. they are highly flexible proteins that lack ordered tertiary structure. These
disordered proteins are the FG nucleoporins, so called because their amino-acid sequence contains many phenylalanine—glycine
repeats. Significance: Nuclear pore complexes allow the transport of molecules across the nuclear envelope. This transport includes
RNA and ribosomal proteins moving from nucleus to the cytoplasm and proteins (such as DNA polymerase and lamins),
carbohydrates, signaling molecules and lipids moving into the nucleus. It is notable that the nuclear pore complex (NPC) can actively
conduct 1000 translocations per complex per second.
Nuclear transport Introduction:
Nucleoporin-mediated transport is not directly energy requiring, but depends on concentration gradients associated with the RAN
cycle. Ran (RAs-related Nuclear protein) is a 25 kDa small G protein that is essential for the translocation of RNA and proteins
through the nuclear pore complex. The RAN Cycle: a) Ran exists in the cell in two nucleotide-bound forms: GDP-bound and GTP-
bound. b) RanGDP is converted into RanGTP through the action of RCC1, the nucleotide exchange factor for Ran. RCC1 is also
known as RanGEF (Ran Guanine nucleotide Exchange Factor). c) Ran's intrinsic GTPase-activity is activated through interaction with
Ran GTPase activating protein (RanGAP), facilitated by complex formation with Ran-binding protein (RanBP).d) GTPase-activation
leads to the conversion of RanGTP to RanGDP, thus closing the Ran cycle.
Import of Proteins
a) Any cargo with a nuclear localization signal (NLS) exposed will be destined for quick and efficient transport through the pore.
Several NLS sequences are known, generally containing a conserved sequence with basic residues such as PKKKRKV. Any material
with an NLS will be taken up by importins to the nucleus. b) The classical scheme of NLS-protein importation begins with Importin-α

7. first binding to the NLS sequence, which then acts as a bridge for Importin-β to attach. c) The importinβ—importinα—cargo complex
is then directed towards the nuclear pore and diffuses through it. d) Once the complex is in the nucleus, RanGTP binds to Importin-β
and displaces it from the complex. e) Then the cellular apoptosis susceptibility protein (CAS), an exportin which in the nucleus is
bound to RanGTP, displaces Importin-α from the cargo. The NLS-protein is thus free in the nucleoplasm. f) The Importinβ-RanGTP
and Importinα-CAS-RanGTP complex diffuses back to the cytoplasm where GTPs are hydrolyzed to GDP leading to the release of
Importinβ and Importinα which become available for a new NLS-protein import round.
Export of Proteins
a) Some molecules or macromolecular complexes need to be exported from the nucleus to the cytoplasm, as do ribosome subunits and
messenger RNAs. Thus there is an export mechanism similar to the import mechanism. b) In the classical export scheme, proteins
with a nuclear export sequence (NES) can bind in the nucleus to form a heterotrimeric complex with an exportin and RanGTP (for
example the exportin CRM1). c) The complex can then diffuse to the cytoplasm where GTP is hydrolysed and the NES-protein is
released. d) CRM1-RanGDP diffuses back to the nucleus where GDP is exchanged to GTP by RanGEFs. e) This process is also
energy dependent as it consumes one GTP. Export with the exportin CRM1 can be inhibited by Leptomycin B.
Chromatin Definition:
Chromatin is a complex of DNA and protein found in eukaryotic cells. Its primary function is packaging long DNA molecules into
more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in reinforcing the
DNA during cell division, preventing DNA damage, and regulating gene expression and DNA replication. During mitosis and
meiosis, chromatin facilitates proper segregation of the chromosomes in anaphase; the characteristic shapes of chromosomes visible
during this stage are the result of DNA being coiled into highly condensed chromatin.