Nucleus -
- Largest component of the cell.
- Nucleus is double membrane bound dense protoplasmic body that controls cellular metabolism, enclose all the genetic information, and is able to transmit the same to the next generation.
- Leeuwenheek first seen nucleus in fish R.B.C.
- Robert Brown coined the name nucleus. Robert Brown observed nucleus in pollen grain and stigma cells.
- Boveri indicated significance of nucleus in heredity.
- J. Hemmerling while working on unicellular alga Acetabularia proved that nucleus regulates the activity of cytoplasm and also heredity.
- Except mature mammalian R.B.C. and Sieve cell of phloem every living cell has a nucleus.
- Procaryotes have incipient nucleus devoid of nuclear membrane, nucleolus and histone. Such type of nucleus is called nucleoid.
- Details of nuclear structure, come to be known only after its study under electron microscope.
- Nucleus of undividing cell is called interphase nucleus.
NUMBER, SIZE, POSITION & SHAPE
- Normally a cell has single nucleus, this condition is called Uninucleate / Mononucleate / Monokaryotic.
- Sometimes a cell may have two nuclei called Binucleate / Dikaryotic cell e.g. Paramaecium caudatum has one macronucleus and one micronucleus, macronucleus controls metabolic activities and micronucleus controls genetic information. Paramaecium aurelia isTrinucleate with one macronucleus and two micronuclei.
- Multinucleate condition found in striated muscle cells, latex vessels, Opalina, Ascaris, polycaryoctes of bone marrow, fungi (Rhizopus) and algae (Vaucheria), which are formed by free nuclear divisions.
- Multinucleate condition in plants is called Coenocyte (coenocytic) whereas multinucleate condition in animals is called Syncytium.Multinucleate condition present in slime molds is called Plasmodium. Cell wall less fungi is called plasmodium. Coenocytic condition is formed by repeating karyokinesis and Syncytium condition is formed by fusion of cells.
- Nucleus usually occupies 20% of cells mass. Its size depend upon ploidy, cell maturity and cell function.
- Nucleus lies near the geometric centre of the cell, peripharal nucleus present in adipocytes & plant cells.
- Nucleus is rounded in young plants and polygonal in animal cells, oval or elliptical in mature plant cells.