NUCLEUS(STRUCTURE)

NIYATI A. DIVANM.Sc. – 1(BIO-SCIENCE)

GENERAL INTRODUCTIONMost significant componentIt controls metabolic and hereditary

activities of the cell,so that it referred as “control center”

It contains DNA with complex of large variety of proteins-histones to form chromosomes

The genes within the chromosomes are the cell’s nuclear genome

It contains nucleic acid(DNA,RNA),nuclear protiens,nuclear enzymes(e.g. DNA polymarase,RNA polymarase, endonuclease, exonuclease,ligase etc.

HISTORY1831:Robert Brown [while he was studying

orchid’s outer layer,he observed an opaque area which he called Nucleus]

1838:Mittehias Schleiden [proposed that the nucleus

plays a role in generating cells,introduced name Cytoblast(cell builder)]

Occurrence and position Found in eukaryotic cell of the plants and

animals,absent in mature sieve tubes of higher plants, mammalian erythrocytes and bacteria

Located in the centre of the cell,but its position may change from time to time according to metabolic states of the cell

no. of the nucleus: -single nucleus(mononucleate) -two nuclei(binuleate) -more than two(polynucleate) Syncytial cells : in

animals(e.g.osteblast,straited muscle fibers) Coenocytic cells: in plants(e.g.siphonal

algae,certain fungai)

Shape and sizeIts shape is related with the shape of the cell-certain irregular-spheroid nuclei in isodiametrical cells-ellipsoid nuclei in cylindrical,prismatic or

fusiform cellsIts size is related with no. of the

chromosomes e.g.small in haploid cell and large in deploid cellThe size is also depends upon the volume of

the cell,amount of DNA,proteins and metabolic phase of the cell

Ultra structure Largest organelle of cell.Approximately 6µm in diameter.occupies about 10% of total volume of

cell.Different parts:1. Nuclear envelope2. Nuclear lamina3. Nucleoplasm4. Chromatin fibres5. nucleolus

I. Nuclear envelope Nuclear membrane ,outer most covering of nucleus,thus

separates the nucleoplasm from cytoplasm It act as a barrier to prevent macro molecules from diffusing

freely between nucleoplasm and cytoplasm It contains 2 cellular membranes:1} an inner membrane2} an outer membrane They are arranged parallel to one another These unit membranes are 80Å thick Lipoprotienous in nature; lipid -17 to 35% & protien-59 to 75% The space between 2 membranes –perinuclear space,150Å

broad The outer membrane is continuous with the membrane of

golgi body, mitochondria, ER, also studded with ribosomes

Nuclear pores:nuclear membrane is not continuous but broken at regular intervals,these broken areas are known as nuclear pores.

They are various in no. & depends on species & type of the cell,e.g. in mammalian cell-3000 to 4000 pores

Shape is octagonal due to the presenece of 8 granules arrenged symmetrically

Pore’s diameter is 600Å but molecules which are freely diffuse through gap are only about 9nm wide, due to the presence of regulatory system within the center of the pore

The pores are enclosed by a circular structure – annuli, pores & annuli are collectively called pore complex sometime pore complex remains covered by a thin membrane (septum) , which act as selective permeability.

2. Nuclear lamina Mostly composed of lamin proteins,which synthesized in the

cytoplasm & later transported into nucleus. The lamin monomer contains helical structure , 2 monomers

coiled around each other & form dimer-coiled coil. 2 of these dimer join side by side in an antiparallel

arrangment form tetramer-protofilament 8 of these filament arranged laterally & twist to form rope

like filament

3. Nucleoplasm The space between nuclear envelope & nucleolus is filled by

a transparent,semi solid,granular & acidophilic ground substence is known as nucleoplasm.

Nuclear components:-chromatin threads & nucleolus suspended in it.

It is composed of nucleoprotein & contains inorganic & organic substances like nucleic acid, proteins, enzymes & minerals

4. Nucleolus It is a large spherical & acidophilic dense granule also called suborganelle Discovered by Fontona in 1781 Size is depend on synthetic activities of the cell Main function is to synthesize r-RNA & produse

ribosomes

5.Chromatin fibres The nucleoplasm contains many thread like coiled &

much elongated structure –chromatin fibres Observe only in the interphase During cell division it become thick called-chromosome chromatin fibres are of 2 types:1. Heterochromatin: more compact DNA form2. Euchromatin: less compact DNA form