Nucleus:

• Reported by Robert Brown in orchid cells.
• Hammerling by his grafting experiments on Acetabularia (largest unicellular green, marine alga) proved the role of nucleus in heredity, growth and morphology.
• Found in all eukayotic cells, except mature phloem sieve tube elements and nature red blood cells of mammals.
• A cell may be uni-or multinucleate. If a multinucleated condition arises due to fusion of cells, it is called syncytium.
• Nucleus has 80% proteins [65% non histone acidic rich in tyrosine and tryptophan) 15% proteins are basic, histone proteins, Rich in lysine and arginine].

Structure:

1. Nuclear Envelope:

• Composed of two membrane. Te outer membrane is continuous with the endoplasmic reticulum (ER) and it may be covered with ribosomes carrying out protein synthesis. It has nuclear pores/(80 nm) extending upto inner membrane.
• Perforated by nuclear pores which allow exchange of substance between the nucleus and the cytoplasm.
• The inner membrane of the nuclear membrane is attached to a layer of fibrous proteins 50 to 80 nm thick that is known as the nuclear lamina. Terminal ends of chromatin fibres or telomeres are embedded in nuclear or fibrous lamina.

2. Nucleoplasm/Nuclear sap/Karyolymph – Strasburger: Proteinaceous, jelly like with pH 7.4 ± 0.2. It has nucleosides, enzymes, (DNA and RNa polymerase) ribosomes, Mg, Fe, Mn, Ca and nucleoproteins.
3. Nucleolus: First discovered by Fontana (1781), Ribosomal factory.

• Rounded colloidal body without surrounding membrane. Ca⁺⁺ ions keep it intact.
• Mainly consists of r-RNA and proteins.
• Nucleolus is attached to the chromatin or special type of chromosome called nucleolar organizing chromosome at a specific place called nucleolar organizing region (NOR) or secondary constriction. It consists of following four parts.

1. Granular region: Outermost (cortical) region where processing and maturation of pre-ribosomal particles occurs.
2. Fibrillar region: formed of a large number of 50-80 Å long fibrils called nucleonema. The fibrils are made up of both protein and RNA and are believed to be precursors of granules.
3. Amorphous matrix: homogenous proteinaceous ground substance of the nucleolus called as ‘para amorpha’
4. Fibrillar centre or nucleolar chromatin: The innermost region called pars chromosoma. Ribosomal RNA (rRNA) and genes of nucleolar organizing regions are located in this region (containing nuclear genes for rRNA synthesis).

Function:

Principal site for development of ribosomal RNAs. Nuclear proteins such as histone proteins are synthesized. Essential for spindle formation during cell division.