What is 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.
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.
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 xylum.The prokaryotes have an incipient nucleus,called nucleoid or prokaryon or genophore or false nuclues.
Usually there is a single nucleus per cell i.e.,mononucleate condition,e.g,Acetabularia.
It is without of nucleus.RBCs of mammals,phloem sieve tube,trachids and vessels of xylam.
Ciliate ,Protozoans like Paramecium.
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 of nucleus:
It varies widely,generally spherical e.g.,cuboidal germ cell,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.
The size of nucleus is variable i.e.,5-30micron.In metabolically active cells size of the nucleus is larger than metabolically inactive cells.
Enzymes like polymerases are abundantly present and help in synthesis of DNA and RNA.
The nucleus is composed of following structure.
Structure of nucleus:
It is a bilayered envelope.Each membrane is about 60-90 Angstrom 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 angstrom).
Nuclear membrane is porous and has 1,000-10,000 octagonal nuclear pores.Each nuclear pore is about 400-1,000 angstrom in diameter.Callan and Tamlin(1950) first to observe nuclear pore in nuclear membrane.The nuclear proes are enclosed by circular structure are called annuli.The pore and annuli together are called pore complex or pore basket.
It is formed by the fusion of ER elements during the telophase of cell division.
- It regulates the nucleo-cytoplasmic interactions.
- It allows the passage of inorganic ions,small organic molecules ,ribosomal subunits,RNAs and proteins through nuclear pores.
- It maintains the shape of the nucleus.