11th Class Biology Cell - The Unit Of Life Nucleus

The nucleus also called director of the cell. It is the most important part of the cell which directs and controls all the cellular function.

Discovery : The nucleus was first observed by Robert Brown (1831), in orchid root cells. Nucleus plays determinative (in heredity) role in cell and organism, that was experimentally demonstrated by Hammerling (1934) in surgical experiments with green marine unicelled algae Acetabularia.

Occurence : A true nucleus with definite nuclear membrane and linear chromosome, is present in all the eukaryotes except mature mammalian RBCs, sieve tube cell of phloem, tracheids and vessels of xylem. The prokaryotes have an incipient nucleus, called nucleoid or prokaryon or genophore or false nucleus.

Number : Usually there is a single nucleus per cell i.e., mononucleate condition, e.g., Acetabularia.

(1) Anucleate (without nucleus) : RBCs of mammals, phloem sieve tube, trachids and vessels of xylam.

(2) Binucleate : e.g., Ciliate, Protozoans like Paramecium.

(3) Polynucleate : e.g., fungal hyphae of Rhizopus, Vaucheria. Polynucleate condition may be because of fusion of a number of cells. i.e., syncytium, coconut endosperm or by free nuclear divisions without cytokinesis i.e., coenocyte.

Shape : It varies widely, generally spherical e.g., cuboidal germ cells, oval e.g., columnar cells of intestine, bean shaped  in paramecium, horse-shoe shaped in Vorticella, bilobed, e.g., WBCs (acidophils), 3 lobed e.g., basophil, multilobed e.g., neutrophils, long and beaded form (moniliform) e.g., stentor and branched in silk spinning cells of platy phalyx insect larva.

Size : The size of nucleus is variable i.e., \[530\mu .\] In metabolically active cells size of the nucleus is larger than metabolically inactive cells.

Chemical composition

Proteins \[=80%,\text{ }DNA=12%,\text{ }RNA=5%,\text{ }Lipids=3%\]

Enzymes like polymerases are abundantly present and help in synthesis of DNA and RNA.

Ultrastructure : The nucleus is composed of following structure.

(1) Nuclear membrane : It is also called nuclear envelope or nucleolemma or karyotheca, was first discovered by Erclab (1845).

Structure : It is a bilayered envelope. Each membrane is about 60-90Å thick lipoproteinous and trilaminar. Outer membrane, called ectokaryotheca (with ribosome) and inner membrane is called endokaryotheca (without ribosome). Two membranes are separated by a fluid-filled intermembranous perinuclear space (about \[100-300{AA}\]).

Nuclear membrane is porous and has \[1,000-10,000\] octagonal nuclear pores. Each nuclear pore is about \[400-1,000\text{ }{AA}\] in diameter (average size is \[800\text{ }{AA}\]). Callan and Tamlin (1950) first to observe nuclear pore in nuclear membrane. The nuclear pares are enclosed by circular structure are called annuli. The pore and annuli together are called pore complex or pore basket.

Origin : It is formed by the fusion of ER elements during the telophase of cell division.

Functions

(i) It regulates the nucleo-cytoplasmic interactions.

(ii) It allows the passage of inorganic ions, small organic molecules, ribosomal subunits, RNAs and proteins through nuclear pores.

(iii) It maintains the shape of the nucleus.

(2) The nucleolus (Little nucleus plasmosome) : It was first observed by Fontana (1781) in the skin cells of an eel. Bowman (1840) coined term 'nucleolus'. Wagner (1840) gave its light microscopic structure.

Position : It is generally associated with nucleolar organizer region (NOR) of the nucleolar chromosomes. It is absent in muscle fibres, RBC, yeast, sperm and prokaryotes.

Number : Generally, a diploid cell is with two nucleoli but there are five nucleoli in somatic cell of man and about 1000 nucleoli in the oocytes of Xenopus.

Structure : (De Robertis et.al 1971). A nucleolus is distinguishable into following regions :-

(1) Chromatin : The nucleolus is surrounded by perinucleolar chromatin.

(2) Pars fibrosa : Fibrils of \[80100\text{ }{AA}\] size form a part of the nucleolus.

(3) Pars granulosa : Granules of \[150200\text{ }{AA}\] diameter constitute the granular part of the nucleolus. Ribosome formation takes place in this part so it is called assembly line of ribosome.

(4) Pars amorpha : The granules and the fibrils lie dispered in an amorphous proteinaceous matrix. Nucleolus is stained by “pyronine”. It is not bounded by any limiting membrane.

Chemical composition : Nucleolus is mainly formed of RNA and non histone acidic proteins. It is a store house of rRNA.

Origin : A nucleolus is formed at specific sites, called the nucleolar organizers, present on certain chromosomes region (NOR).

Functions

(i) It is seat of biogenesis of rRNA and also stores rRNA.

(ii) It plays important role in spindle formation during cell division.

(iii) It receives the ribosomal proteins from the cytoplasm, combines the rRNAs and ribosomal proteins to form ribosomal subunits. So it is also called ribosome producing machine or factory.

(3) Nucleoplasm : It is also called karyolymph. It is transparent, homogenous, semifluid, colloidal, ground substance present inside the nuclear membrane. It contains Nucleic acid (DNA and RNA), Proteins (Basic proteins and acidic protein), Enzyme (DNA and RNA polymerase, NAD synthetase etc.), Minerals (K, Na, Ca, Mg etc.) and Ribonucleoproteins.

The nucleoplasm helps in maintaining the shape of nucleus formation of spindle protein of NAD, ATP, DNA, RNAs and ribosomal subunits. Plasmosome and karyosome combindly called “amphinucleoli”.

(4) Chromatin fibres /Nuclear chromatin : The nucleoplasm contains many thread like, coiled and much elongated structures which take readily the basic stains such as “basic fuchsine”. These thread like structures are known as chromatin fibre. They are uniformly distributed in the nucleoplasm. They are observed only in the “interphase stage”.

(5) Nuclear matrix : It is network of proteinaceous fibrils. It is outer thicker part is called fibrous lamina (Haris and James, 1952).