Nucleus

By

Muhammad Uzair Khan

Discovery

It was discovered by A.V Leeuwenhoek for the first time.

Franz Bauer also described it in 1804. Scottish Botanist Robert Brown

explained it in more detail in 1831

Definition of Nucleus

In cell biology, the nucleus (pl. nuclei; from Latin nucleus or nuculeus, meaning kernel) is a membrane-enclosed organelle found in eukaryotic cells.

It is the control center of the cell.

Functions

It stores the cell hereditary material, or DNA.

Site of DNA replication Site of DNA transcription to mRNA Ribosomal formation It coordinates the cell’s activities, which

include growth, intermediary metabolism, protein synthesis and reproduction(cell division) by regulating gene expression.

Size & Shape

It is a spherical organelle. It is approximately 3-10 μm in diameter Usually only one nucleus is present in

each cell but certain cells maybe binucleate (e.g., some liver cells) or multinucleate (e.g., skeletal muscle cells)

Liver Cells(Binucleate)

Muscle cells(Multinucleate)

Structure of Nucleus

Structure of Nucleus is made up by the following

Nuclear Envelope Nuclear Pores Chromatin Nucleoplasm Nucleolus

Nuclear pores

Nucleus

Nuclear Envelope

The nuclear envelope, otherwise known as nuclear membrane, consists of two cellular membranes, an inner and an outer membrane, arranged parallel to one another.

The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum (RER), and is similarly studded with ribosomes.

The space between the membranes is called the perinuclear space.

The inner surface of the NE is bound to a thin filamentous network (lamins proteins) called the nuclear lamina. It provides mechanical support to the NE and serves as sites for attachment for chromatin fibers.

The nuclear lamina is composed mostly of lamin proteins

Mutations in lamin genes leading to defects in filament assembly are known as laminopathies (e.g. a rare form of muscle dystrophy)

Nuclear Pores

The nuclear pores are the gateways across which movement of RNAs and proteins takes place between the nucleus and cytoplasm in both direction.

Proteins synthesized in the cytoplasm cross the nuclear envelop to initiate replication and transcription of genetic material. Similarly, mRNA, tRNA and ribosomal subunits built in the nucleus cross through the nuclear pores to the cytoplasm.

They have approximately 100 nm diameter and consists of around 100 proteins which allows the free passage of small water-soluble molecules while preventing larger molecules.

The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope.

Each pore contains a donut-shaped, eightfold-symmetric ring-shaped structure at a position where the inner and outer membranes fuse.

Attached to the ring is a structure called the nuclear basket that extends into the nucleoplasm, and a series of filamentous extensions that reach into the cytoplasm.

Both structures serve to mediate binding to nuclear transport proteins.

Nucleoplasm

The nucleoplasm is a highly viscous liquid that surrounds the chromosomes and nucleoli.

Similar to the cytoplasm of a cell. it is enveloped by the nuclear envelope. Many substances such as nucleotides

(necessary for purposes such as the replication of DNA) and enzymes (which direct activities that take place in the nucleus) are dissolved in the nucleoplasm.

Chromatin

It appears in the form of fine to coarse granules distributed irregularly within the nucleus.

The chromatin has been found to consist of DNA and associated basic proteins. The DNA exists in the form of flexible rod-like structures known as chromosomes.

Chromatin is further divided into two more types1.) Heterochromatin- highly condensed2.) Euchromatin- less condensed

chromatin

chromosomes

Chromatin Function

Packages DNA into a smaller volume to fit in the cell.

Strengthen the DNA to allow mitosis and meiosis

Serve as a mechanism to control expression. Histones are the proteins closely associated

with DNA molecules. They are responsible for the structure of chromatin and play important roles in the regulation of gene expression.

Nucleolus

The nucleolus is a discrete densely stained structure found in the nucleus.

A nucleolus is composed of Ribonucleic acid (RNA) and associated proteins.

Under Electron Microscope, it exhibits a granular structure.

The nucleolus produces ribosomes, which move out of the nucleus and take positions on the rough endoplasmic reticulum where they are critical in protein synthesis.