Nucleus

   ( Latin nuculeus – kernel or nut)

Nucleus is a membrane bound organelle containing chromosomes and nucleolus. The nucleus is the brain of eukaryotic cells.

Robert Brown in 1833 discovered a prominent body within the cell and termed it nucleus. Van Hammerting’s experiment (1934) showed the role of the nucleus in controlling the shape and features of the cell.

Usually, the nucleus is round and is the largest organelle in the cell. The nucleus varies in diameter from 11 to 22 micrometers. The nucleus typically occupies about 10% of a eukaryotic cell’s volume, making it one of the cell’s most prominent features.

Generally, there is only one nucleus per cell, but nucleus is absent in RBC’s of Human beings and sieve cells of plants.

Some organisms like paramecium  and tapetal cells has two nucleus and this condition is called as binucleate. Some organisms like slime moulds (Mucor), siphonales (Vaucheria) have more than two nucleus, this condition is called multinucleate. The multinucleate animal cells are called syncytial cells (e.g., epidermal cells of Ascaris) and the multinucleate plant cells are called coenocytic.

The shape of the nucleus varies considerably. In most of the cells, it is spherical in shape. It may be cylindrical or elliptical.

Structure:

The nucleus is comprised of the following structures:

1. The nuclear membrane or karyotheca              2. The nuclear sap or nucleoplasm, 

3. Chromatin fibres   and                                     4. The nucleolus

Nuclear Envelope

The nucleus is enclosed by a double membrane called the nuclear envelope or nuclear membrane or karyotheca. The nuclear envelope seperates the nuceloplasm from the cytoplasm. Each membrane is about 75 to 90 Ao thick and lipoproteinous in nature. The two membranes remain separated by a space of 100 - 150 Ao.  The space between two membranes is called as perinuclear space.

The outer membrane is continuous with rough endoplasmic reticulum and is studded with ribosomes. The inner membrane is linked by firbrous filament protein called lamins, which forms a scaffold of filaments called the nuclear lamina.

Nuclear Pore

The nuclear envelope is perforated with thousands of pores called the nuclear pore. The nuclear pore were first demonstrated by Callan and Tomlin (1950) in amphibian oocytes.

The number of pores per unit area of nuclear envelope varies with the cell type and with the physiological state of the cell. The nuclear pores are circular in surface view and have an diameter between 10 – 100 nm.

The nuclear pore has a complex organisation. So the entire structure of nuclear pore is called nuclear pore complex. The pore complex consists of two rings or annuli with an inside diameter of 80nm

. A large particle is present in the nuclear pore forming the central plug.

Eight radial subunits or spokes extend from the inner and outer membrane where they join. Actually, they form a ring of subunits 15-20 nm in diameter. Each sub-units project a spoke-like unit into the centre, so that the pore looks like a wheel with 8 spokes from the top.

The nuclear pore forms an aqueous channel connecting the cystol with the interior of the nucleus. When materials are to be transported through the pore, it opens up to form a channel some 25nm wide, that is large enough to get such large assemblies as ribosomal subunits through. The nuclear pore complexes are active; that its, it requires energy.

All proteins synthesized in cytoplasm and those needed by the nucleus must be imported into it through the NCPs, these includes histones, ribosomal proteins, transcription factors, splicing factors etc.

Molecules and macromolecular assemblies like ribosomal subunits, mRNA, tRNA are exported from the nucleus through the NCPs.

Function of  Nuclear Envelope

The Nuclear envelope regulates and facilitates transport between the nucleus and the cytoplasm.

It serves to separate the genetic component (chromosomes) from the protein synthesising machinery (ribosomal and ER). It provides protection to DNA against mutagenic effects of cytoplasmic enzymes.

The nuclear envelope provides a surface for the attachment of structural elements of the cytoplasm i.e., microtubules and microfilaments.

The attachment of chromosomes to nuclear envelope assists in their condensation and separation during cell division.

Envelope is involved in the formation of ER, Golgi complex.

Nucleoplasm:

The nucleus is filled with a homogenous, transparent, acidophilic fluid called the nucleoplasm or nuclear sap or karyolymph.

The nuclear components such as chromatin threads and the nucleolus remain suspended in the nucleoplasm.

The nucleoplasm is composed of nucleic acids, proteins, enzymes and minerals. The most important nuclear enzymes are the DNA polymerase, RNA polymerase, NAD synthetase, alkaline phosphatise.

In nucleoplasm events like – i. Replication of DNA, ii. Transcription of RNA, iii. Transport of materials etc occurs.

Nucleolus:

The nucleolus (- a small nucleus) is a large, spherical, ball like body found in nucleus. It was discovered by Fontana. In cell biology, nucleolus is a ‘suborganelle’of the cell nucleus, which is an organelle.

The size of the nucleolus is found to be related with the synthetic activity of the cell. Therefore, the cells which synthesize the proteins contain comparatively large-sized nucleoli, eg., Oocytes, neurons and secretory cells.

No membrane seperates the nucleolus from the nucleoplasm. Calcium ions are supposed to maintain its intact organisation. Nucleoli are made of protein and ribosomal DNA (rDNA) sequences of chromosomes. The rDNA is a fundamental component since it serves as the template for transcription of the ribosomal RNA (rRNA) for inclusion into new ribosomes.

Nucleoli disappear during metaphase of cell division. After daughter cells complete separation, nucleoli reform around the nucleolus organizer regions (NORs) of the chromosomes.

During the period between cell division, when the chromosomes are in their extended state, 1 or more chromosomes have loops extending into a spherical mass called the nucleolus. The nucleolus is organized from the ‘nucleolar organising regions’on different chromosomes. Nucleoli are formed around the DNA loop that extends from the nucleolar organizer.  A number of chromosomes get together and transcribe ribosomal RNA at this site. Here three kinds of ribosomal RNA molecules (28S, 18S, 5.8S) used in the assembly of the large and small subunits of ribosomes are synthesized. 

28S, 18S and 5.8S ribosomal RNA is transcribed by RNA polymerase I from hundred of tandemly-arranged rDNA genes distributed on different chromosomes. The rDNA-containing regions of these chromosomes cluster together in the nucleolus.

The nucleolus that is active in the synthesis of ribosomes typically exhibits three regions under electron microscope.

1. Fibrillar Centre (FC):

This pale-staining part represents the innermost region of the nucleolus. It is made up by a network of fine (4-5 nm thick) fibrils. These fibrils are formed of rDNA, rRNA and ribonucleoprotein particles(RNP).

2. Granular component (GC):

This is the outermost region of the nucleolus where processing and maturation of pre-ribosomal particles occur.

. 3. Amorphous matrix: this is the proteinous ground substance in which granules and fibrils remain suspended.

Chromatin fibres

the nucleoplasm  contains  thread like, coiled and much elongated structures called the chromatin fibres. Chromatin fibres are observed only in the interphase nucleus. During cell division chromatin fibres become thick and ribbon-like structures known as the chromosomes.

 

Chemically chromatin is a nucleoprotein. It is composed of nucleic acid and protein.

Function of Nucleus

Nucleus is the dynamic centre which controls and regulates various metabolic activities like growth, protein synthesis, metabolism, reproduction.

Vehicle for transmission of hereditary characters through chromosomes during cell division.

The synthesis of ribosomal RNA takes place inside the nucleolus