2. Introduction
• Nucleus was discovered by Robert Brown in root of an orchid.
• It is a double membrane structure that encloses all the genetic information.
• Nucleus is present in all Eukaryotic cell except mature mammalian RBC.
• The nucleus is separated from the rest of the cell or the cytoplasm by a nuclear membrane.
• As the nucleus regulates the integrity of genes and gene expression, it is also referred to as the control
center of a cell.
• The shape of the nucleus is mostly round, it may be oval, disc shaped depending on the type of cell.
• Number- Mostly the cell are Monokaryonic but can be binucleate (example Paramecium) or multinucleate
(opalina , skeletal muscle cell, liver cell).
• Position- Nucleus is generally centric in position, but it is peripheral in adipocytes, Basal in columnar cell,
eccentric in Plant cell and subpended in center of vacuole by cytoplasmic stands in Spirogyra.
• Shape- shape is nucleus is variable, generally is spherical (example cuboidal cells), oval (example columnar
cell), discoidal (example flat cells of endothelium), kidney shaped (example meganucleus of Paramecium),
multilobed (neutrophils), bilobed (acidophils).
• Size- size of nucleus varies widely and its depend on- 1.nucleo- cytoplasmic index
2. Size of the cell
3. Function of the cells
3. Structure of nucleus
The structure of a nucleus encompasses the nuclear membrane,
nucleoplasm, chromosomes, and nucleolus.
1. Nuclear Membrane
• The nuclear membrane is a double-layered structure. The outer layer
of the membrane is connected to the endoplasmic reticulum.
• The nuclear envelope consists of phospholipids that form a lipid
bilayer.
• The envelope helps to maintain the shape of the nucleus and assists in
regulating the flow of molecules into and out of the nucleus
through nuclear pores. The nucleus communicates with the remaining
of the cell or the cytoplasm through several openings called nuclear
pores.
• Nuclear pores are the sites for the exchange of large molecules
(proteins and RNA) between the nucleus and cytoplasm.
• A fluid-filled space or perinuclear space is present between the two
layers of a nuclear membrane.
4. 2. Nucleoplasm
Nucleoplasm is the gelatinous substance within the nuclear
envelope.
Also called karyoplasm, this semi-aqueous material is
similar to the cytoplasm and is composed mainly of water
with dissolved salts, enzymes, and organic molecules
suspended within.
The nucleolus and chromosomes are surrounded by
nucleoplasm, which functions to cushion and protect the
contents of the nucleus.
Nucleoplasm also supports the nucleus by helping to
maintain its shape. Additionally, nucleoplasm provides a
medium by which materials, such as enzymes
and nucleotides (DNA and RNA subunits), can be
transported throughout the nucleus. Substances are
exchanged between the cytoplasm and nucleoplasm
through nuclear pores.
5. Chromosomes/ Chromatin - Chromosomes are present in the form of
strings of DNA and histones (protein molecules) called chromatin.
Chromatin: Chromosome is made up of chromatin.
Chromatin is made up of DNA, RNA and proteins.
At interphase, chromosomes are visible as thin chromatin fibres
present in the nucleoplasm. During cell division, the chromatin fibres
condense and chromosomes are visible with distinct features.
When a cell is “resting” i.e. not dividing, the chromosomes are organized
into long entangled structures called chromatin.
The chromatin is further classified into heterochromatin and euchromatin
based on the functions.
Heterochromatin- This is dark stained,thick and condensed part of
chromatin this part has more histone and less acidic protein.
Heterochromatin is genetically less active chromatin. Heterochromatin
occurs near nuclear membrane. Replicate in late s phase.
Euchromatin- This is lightly stained and diffused part of chromatin.
Which is transcriptionally or genetically more active. Generally
euchromatin lies at central part of nucleus. Replicate in early s phase.
6. Structural Organization of Chromatin
• Chromatin consists of DNA and associated proteins. DNA is packaged in a highly
organized manner in chromosomes.
• Nucleosomes are the basic unit of chromatin. It is 10 nm in the diameter.
• DNA packing is facilitated by proteins called histones. DNA is wound around
histone proteins to form a nucleosome.
• There are 5 types of histone proteins in the eukaryotic chromosomes, namely H1,
H2A, H2B, H3 and H4.
• Histones are positively charged due to the presence of amino acids with basic
side chains and it associates with negatively charged DNA due to the presence of
phosphate groups.
• Histone proteins play an important role in gene regulation.
• Nucleosomes are made up of 146 base pair of DNA coiled around a core of eight
histone molecules (2 molecules of 4 histone proteins).
• Linker DNA and the fifth histone (H1) pack adjacent nucleosomes to a 30 nm compact
chromatin fibers.
• These fibres form a large coiled loop held together by non-histone proteins (actin, 𝛂
and 𝛃 tubulin, myosin) called scaffolding proteins to form extended chromatin which
is 300 nm in diameter.
• Chromatin further condenses with the help of protein known as condensin, it
binds to DNA and wraps it into coiled loops and we get the compacted
7. 4. Nucleolus
• The nucleus is a dense, membrane-less structure
composed of RNA and proteins called the nucleolus.
• Some of the eukaryotic organisms have a nucleus that
contains up to four nucleoli.
• The nucleolus contains nucleolar organizers/NOR, which
are parts of chromosomes with the genes for ribosome
synthesis on them. The nucleolus helps to
synthesize ribosomes by transcribing and assembling
ribosomal RNA subunits. These subunits join together to
form a ribosome during protein synthesis.
• The nucleolus disappears during –prophase and reappears
in telophase.
8. Functions of Nucleus
• It controls the heredity characteristics of an organism.
• It main cellular metabolism through controlling synthesis of particular
enzymes.
• It is responsible for protein synthesis, cell division, growth and
differentiation.
• Stores heredity material in the form of deoxy-ribonucleic acid (DNA)
strands. Also stores proteins and ribonucleic acid (RNA) in the nucleolus.
• It is a site for transcription process in which messenger RNA (mRNA) are
produced for protein synthesis.
• It helps in exchange of DNA and RNA (heredity materials) between the
nucleus and the rest of the cell.
• Nucleolus produces ribosomes and are known as protein factories.
• It also regulates the integrity of genes and gene expression.
