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The nucleus was the first organelle to be discovered

Antonie van Leeuwenhoek (1632 1723). He observed a "Lumen", the

nucleus, in the red blood cells ofsalmon.[1]

The nucleus was also described by Franz Bauerin 1804[2] and in more detail

in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of

London

Brown was studying orchids under microscope when he observed an opaque

area, which he called the areola or nucleus, in the cells of the flower's outerlayer.[3] He did not suggest a potential function.

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In cell biology, the nucleus (pl. nuclei; from Latin nucleus ornuculeus,

meaning kernel) is a membrane-enclosed organelle found in eukaryotic cells.

It contains most of the cell's genetic material, organized as multiple long

linearDNA molecules in complex with a large variety ofproteins, such as

histones, to form chromosomes. The function of the nucleus is to maintain

the integrity of these genes and to control the activities of the cell byregulating gene expression the nucleus is, therefore, the control center of

the cell. The main structures making up the nucleus are the nuclear

envelope, a triple cell membrane and membrane that encloses the entire

organelle and unifies its contents from the cellularcytoplasm, and the

nucleoskeleton (which includes nuclear lamina), a meshwork within the

nucleus that adds mechanical support, much like the cytoskeleton, whichsupports the cell as a whole. Because the nuclear membrane is impermeable

to most molecules, nuclear pores are required to allow movement of

molecules across the envelope. These pores cross both of the membranes,

providing a channel that allows free movement of small molecules and ions.

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Nuclear transport is crucial to cell function, as movement through the pores isrequired for both gene expression and chromosomal maintenance. The

interior of the nucleus does not contain any membrane-bound

subcompartments, its contents are not uniform, and a number ofsubnuclear

bodies exist, made up of unique proteins, RNA molecules, and particular parts

of the mitochondria. The best-known of these is the nucleolus, which is mainly

involved in the assembly ofribosomes. After being produced in the nucleolus,ribosomes are exported to the cytoplasm where they translate mRNA.

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By the mid 1700s early microscope users had realized that living cells contain

a light gray sap with a darker, denser globule floating about in the sap. (This

structure is easily noted in the images of the human cheek cells forming the

borders of this page.) In 1831, Robert Brown used the word nucleus to describe

the dark, central globule.

The Cell Nucleus

(The word nucleus is Latin forlittle nut.)The nucleus of most cells averages about 5 m (.005 mm) in diameter. It is

surrounded by the nuclear envelope, a double membrane made of proteins

and lipids that separates it from the cytoplasm.

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Early on in the microscopic study of cells, it was found that adding stains or dyes to

thinly sliced tissue caused some structures inside cells to stand out. The material

in the nucleus absorbed stains so readily that it was named chromatin (from the

Greek chroma = color.) (Click on blue thumbnail to see this effect.)

To the early observers, the chromatin appeared to be tiny granules or delicately

intertwined threads scattered about inside the nucleus.

Today we know that chromatin is a complex of DNA and protein that forms

exceedingly long, thin, entangled threads called chromosomes (from the Greek

chroma = color + soma = body.) Only when the nucleus prepares to divide do

the chromosomes condense, becoming thick enough to be seen through a light

microscope as separate structures.

Each species has a characteristic number of chromosomes. A human body cell

(called a somatic cell) has 46 chromosomes in its nucleus.

A picture of the chromosomes of a cell taken during division can be cut out and

arranged to make a picture called a karyotype.

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y Brain of the cell

y Office of the factory

y Contains nearly all the cells DNA and with it the coded

instructions for making PROTEINS and other importantmolecules

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y Surrounds nucleus

y Made of 2 membranes

y Dotted with thousands of nuclear pores

y How do we get messages, instructions and blueprints out of theoffice?

y Allow material to move in and out of nucleus by using little

runners such as proteins, RNA and other molecules

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y Contain a granular material called

y CHROMATIN

y Chromatin= DNA + protein

y Usually spread out in nucleusy During cell division, chromatin clumps together or

condenseswe call this.

y CHROMOSOMES

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y Small dense region inside the nucleus

y Function: assembly of ribosomes begin

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The main function of the cell nucleus is to control gene expression and mediate

the replication of DNA during the cell cycle. The nucleus provides a site for

genetic transcription that is segregated from the location oftranslation in the

cytoplasm, allowing levels ofgene regulation that are not available to

prokaryotes.

Storage of hereditary material, the genes in the form of long and thin DNA(deoxyribonucleic acid) strands, referred to as chromatins.

Storage of proteins and RNA (ribonucleic acid) in the nucleolus.

Nucleus is a site fortranscription in which messenger RNA (mRNA) are

produced for the protein synthesis.

Exchange of hereditary molecules (DNA and RNA) between the nucleus and rest

of the cell.During the cell division, chromatins are arranged into chromosomes in the

nucleus.

Production ofribosomes (protein factories) in the nucleolus.

Selective transportation of regulatory factors and energy molecules through

nuclear pores.

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CELL COMPARTMENTALIZATION

The nuclear envelope allows the nucleus to control its contents, andseparate them from the rest of the cytoplasm where necessary. This is

important for controlling processes on either side of the nuclear

membrane. In some cases where a cytoplasmic process needs to be

restricted, a key participant is removed to the nucleus, where it interacts

with transcription factors to down regulate the production of certain

enzymes in the pathway. This regulatory mechanism occurs in the case ofglycolysis, a cellular pathway for breaking down glucose to produce

energy. Hexokinase is an enzyme responsible for the first the step of

glycolysis, forming glucose-6-phosphate from glucose. At high

concentrations offructose-6-phosphate, a molecule made later from

glucose-6-phosphate, a regulator protein removes hexokinase to the

nucleus,[44]

where it forms a transcriptional repressor complex with nuclearproteins to reduce the expression of genes involved in glycolysis.[45]

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In order to control which genes are being transcribed, the cell separates some

transcription factorproteins responsible for regulating gene expression fromphysical access to the DNA until they are activated by other signaling pathways.

This prevents even low levels of inappropriate gene expression. For example, in

the case ofNF-B-controlled genes, which are involved in most inflammatory

responses, transcription is induced in response to a signal pathway such as that

initiated by the signaling molecule TNF-, binds to a cell membrane receptor,

resulting in the recruitment of signalling proteins, and eventually activating thetranscription factor NF-B. A nuclear localisation signal on the NF-B protein

allows it to be transported through the nuclear pore and into the nucleus, where it

stimulates the transcription of the target genes.[6]

The compartmentalization allows the cell to prevent translation of unspliced

mRNA.[46] Eukaryotic mRNA contains introns that must be removed before

being translated to produce functional proteins. The splicing is done inside thenucleus before the mRNA can be accessed by ribosomes for translation.

Without the nucleus, ribosomes would translate newly transcribed

(unprocessed) mRNA, resulting in misformed and nonfunctional proteins.

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The nucleus regulates all cell activity. It does this by controlling the enzymespresent. The chromatin is composed of DNA. DNA contains the

information for the production of proteins. This information is encoded in the 4DNA bases. Adenine, thymine, cytosine, and guanine. The specific

sequence of these bases tells the cell what order to put the amino acids.There are three processes that enable the cell to manufacture protein:Replication allows the nucleus to make exact copies of its DNATranscription allows the cell to make RNA working copies of its DNAIn translation the Messenger RNA is used to line up amino acids into a

protein molecule

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Cell Nucleus: Structure

The structure of a cell nucleus consists of a nuclear membrane

(nuclear envelope), nucleoplasm, nucleolus, and chromosomes.

Nucleoplasm, also known as karyoplasm, is the matrix present inside

the nucleus. Let's discuss in brief about the several parts of a cell

nucleus.

Nuclear Membrane

The nuclear membrane is a double-layered structure that encloses the

contents of the nucleus. The outer layer of the membrane is connected to

the endoplasmic reticulum. A fluid-filled space or perinuclear space is

present between the two layers of a nuclear membrane. The nucleus

communicates with the remaining of the cell or the cytoplasm through

several openings called nuclear pores. Such nuclear pores are the sites

for exchange of large molecules (proteins and RNA) between the nucleus

and cytoplasm.

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Chromosomes

Chromosomes are present in the form of strings ofDNA and histones (protein

molecules) called chromatin. The chromatin is further classified into

heterochromatin and euchromatin based on the functions. The former type is a

highly condensed, transcriptionally inactive form, mostly present in adjacent to

the nuclear membrane. On the other hand, euchromatin is a delicate, less

condensed organization of chromatin, which is found abundantly in atranscribing cell.

Nucleolus

The nucleolus (plural nucleoli) is a dense, spherical-shaped structure present

inside the nucleus. Some of the eukaryotic organisms have nucleus that

contains up to four nucleoli. The nucleolus plays an indirect role in protein

synthesis by producing ribosomes. These ribosomes are cell organelles made

up of RNA and proteins; they are transported to the cytoplasm, which are then

attached to the endoplasmic reticulum. Ribosomes are the protein-producing

organelles of a cell. Nucleolus disappears when a cell undergoes division and

is reformed after the completion of cell-division.

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