A cell may be uni or multinucleated. If a multinucleated condition arises due to fusion of cells, it is called syncytium e.g., plasmodium, body of slime moulds, young xylem vessels and if due to repeated nuclear divisions without cytokinesis, it is called coenocytic e.g. Vaucheria, Rhizopus. Nucleus was reported by Robert Brown (1831) in orchid cells. Strasburger (1882) proved that nucleus arises from pre existing nucleus by division. Hertwig and van Beneden showed the role of nucleus in fertilization. Hammerling (1953) by his grafting experiments on Acetabularia (largest unicellular green, marine alga) proved the role of nucleus in heredity, growth and morphology. 1/10th of volume of cell is occupied by nucleus. In a cell, there is a definite nucleo cytoplasmic ratio. Nucleocytoplasmic index is Volume of nucleus/ (Volume of cell – Volume of nucleus). About 10% of nuclear membrane bears simple of compound pores. Nucleus has 80% proteins (65% non histone, Mol. Wt. high, rich in tyrosine and tryptophan, acidic and forms enzymes and helps in RNA transaction. 15% proteins are basic, histone proteins, Mol. Wt. low, rich in lysine and arginine. DNA : histone ratio 1 : 1. Nucleosomes are structural units of chromatin. Term was given by Outdet. A nucleosome is an octamer of histone proteins and has a core of 8 molecules of histone proteins (two each of H2A, H2B, H3, H4) bounded by 13/4 turns of DNA having about 166 base pairs. H1 histone does not form nucleosome. Size of a chromosome varies from 0.5 to 32 μ. Minimum number of chromosomes n = 2 e.g., Haplopappus. Maximum number is 2n = 1262 in Adder’s fern (Ophioglossuin). In animals, minimum number is 2n = 2 in Ascaris sp. And maximum number is 2n = 1600 in Aulacantha and Radiolarians.
