NUCLEUS Nucleus is major cell organelle It houses

NUCLEUS § Nucleus is major cell organelle. § It houses all the cell’s DNA. § It was discovered by Robert brown in 1833. § Nuclear material bounded by memrane---- Eukaryotic. § Not bounded---- Prokaryotic. § The chromatin material suspended in the cytoplasm is called as nucleoid. Robert Brown

Occurrence: § Found in nearly all eukaryotic cells of plants & animals. § Mammalian RBcs, Sieve tubes of higher plants do not possesses nucleus. § In such cells it is present in immature condition. Position: §. Usually it is present in the center of the cell. § Position changes as per metabolic state of the cell. § Glandular cells has basal position nucleus.

Number: § § § § Generally a cell contains single nucleus. Number of nucleus may vary cell to cell. Cells are differentiated in to … 1. Anucleate: without nucleus. 2. Mononucleate: with single nucleus. 3. Binucleate: having two nuclei. 4. Polynucleate: cells with many nuclei. e. g. polynucleate cells animal of animal is called as syncytium (epidermis of Ascaris) § Polynuceate cells of plants (ceonocytic) e. g. Vaucheria.

Shape: § Shape of the nucleus is different in different cells. § It is Spherical § Ovoid § Rounded § Bilobed § Multilobed § Cylindrical § Moniform (like string of beads) § Horse-shoe shaped § Branched.

Size: § Size of the nucleus also variable in many cells. § It is about 3µm in diameter. § Actually size depends on no of chromosomes present. § Haploid cells contain small sizes than the diploid & polyploid cells. § Thus, the size of nucleus depends on volume of cytoplasm. § Generally it occupies 10% of the total cell volume.

Ultrastructure of Nucleus § The interphase nucleus composed of following four principal components: Ø 1. Nucear envelope Ø 2. Nuclear Sap Ø 3. Chromatin material Ø 4. Nucleolus

Nuclear envelope § A membrane bounded the nucleoplasm. § It is also called as karyotheca. § It is composed of outer & inner unit membrane. § The outer membrane is rough due to the presence of Ribosomes. § It is directly associated with E. R. & Ribosomes. § Proteins synthesized on ribosomes. § Both unit membranes seperated by perinuclear space which is about 80 -100 A 0.

§ Lipid droplets, antibodies and crystalline deposits are found in perinuclear space. § The inner membrane is associated with proteinaceous mass called as fibrous lamina. § Small openings are present at different intervals to nuclear membrane is called as nuclear pores. § These pores shows the presence of ring (circular) like structure called as annulus. § The annulus extends in to cytoplasm & nucleoplasm. § Annulus transports the material from nucleoplasm to cytoplasm & vice-versa.

Chemical composition of Nuclear Envelope § Nuclear membrane composed of chemically… § Proteins (59 -75% ) in different cells. § Lipids (17 -35 %) § Carbohydrates in low concentrations. § 20 different types of proteins. § Many enzymes. § DNA & RNA § NE shows ATPase activity. § ATPase concentration is found to be high at Nucear pores.

Functions of Nuclear Envelope § Protective (genetic material is well protected). § Transport of materials. (active & passive). § Exchahnge through E. R. § Pinocytosis. § Macromolecules pass through Nuclear pores. § Provides attachment for structural elements of Cytoplasm & Nucleoplasm.

Nuclear Sap § The space between nucleoplasm and nuclear membrane is filled with a transparent semisolid, granular and acidophilic substance is known as nuclear sap or nucleoplasm. § It contains nucleolus and chromatin fibres. § It contains… § a). Nucleic acids- DNA & RNA. § b). Proteins- basic proteins & acidic proteins. § c). Enzymes- DNA polymerase & RNA polymerase. § d). Lipids- in small amount. § e). Minerals- Phosphorus, Potassium, Sodium, Calcium and Magnesium are present.

Chromatin Material § The nuceoplasm contains coiled, thread-like elongated structure, stained by basic dyes. § These threads are k/as chromatin fibres and are observed only in the interphase nucleus. § During cell division, they become short & thick and are k/as chromosomes. § Two types of chromatin materials have been recognized… § (a). HETEROCHROMATIN: § The darkly stained condensed region of the chromatin is heterochromatin. § The condensed portion of nucleus is k/as chromocenters or karyosomes. § The heterochromatin occurs around the nucleolus and at the periphery. § It contains small amount of DNA and large amount of RNA. § It is supposed to be metabolically and genetically inert.

§ (b). EUCHROMATIN: § The lightly stained and diffuse region of chromatin is k/as euchromatin. § It contains large amount of DNA. § It is metabolically active i. e. it is a site of RNA synthesis.

NUCLEOLUS § The nucleus contains a large, spherical and acidophilic dense granular structure k/as nucleolus. § It was first discovered by Fontana in 1781. § Nucleolus is absent from lower organism like bacteria, yeast, some algae, mammalian RBC, spermatozoa. In all other cells nucleolus is present. § Size of the nucleolus depends on the synthetic activity of cells. § Nucleoli are small in cells showing little protein synthesis. § Large cells where protein synthesis is going on actively as oocytes, neurons, secretory cells etc. § The number of nucleoli depends on no of sets of chromosomes. § A diploid cell contains two nucleoli and haploid with single nucleolus.

Ultrastructure of Nucleolus § The nucleolus is surrounded by thick covering called perinucleolar chromatin. § The perinucleolar chromatin projects into the nucleolus to form the intranucleolar chromatin. § The peri & intranucleolar chromatin are rich in DNA. § The DNA serves as a template for the synthesis of RNA. § The interior of the nucleolus is filled with a proteinous ground substance called matrix. § The matrix contains a number of fibrils and granules. § The nucleolus is composed of RNA, DNA, proteins and enzymes. § The enzymes are acid phosphatases, nuclease, phosphorylase.

Types of Nucleolus § Based on the distribution of granules nucleoli are classified in to… § 1). Homogenous nucleolus: § The granules are uniformly distributed throughout the nucleolus. § 2). Heterogenous nucleolus: § The granules are occurring in groups. § 3). Ring nucleolus: § The granules are arranged along the periphery of nucleolus in the form of ring.

Nucleolar organizer § It is constricted area of the chromosome where nucleolus is formed. § This area is also called as secondary constriction. § The chromosome containing the nucleolar organizer is called the nucleolar chromosome. Functions of Nucleolus 1. RNA Synthesis: Nucleolus is the active site of RNA Synthesis 2. Ribosomal subunit formation: 40 s & 60 s ribosomal subunits.

Functions of Nucleus Ø It is a store house of heredity material. Ø It controls heredity characteristics of an organism. Ø Protein Synthesis. Ø Cell division, growth & differentiation. Ø Site for transcription process for m. RNA production. Ø Exchange of DNA & RNA. Ø Nucleolus produces Ribosomes. Ø It regulates the integrity of gene and gene expression.