STD-11 UNIT-3 CHA-8
CELL : STRUCTURE AND FUNCTIONS
Nucleus Introduction
- Nucleus (L. nucleus - kernel) is a specialized double membrane bound protoplasmic body which contains all the genetic information for controlling cellular metabolism and transmission to the posterity.
- A nucleus in the non- dividing or metabolic phase is called interphase nucleus.
- Like other cellular structures, living unstained nucleus does not show much internal differentiation.
- For detailed study of nucleus, the cells must be properly killed , fixed and stained.
- Nucleus is the largest cell organelle.
- Though first observed by Leeuwenhoek in red blood cor puscles of fish, nucleus was first studied in orchid root cells by Robert Brown in 183.
- A nucleus is present in all living eukaryotic cells with the excep tion of mature sieve cells of vascular plants and red blood corpuscles of mammals.
- Even here a nucleus is present during the early stages of their development.
- Presence of hereditary information in the nucleus was proved by the work of Joachim Hammerling (1953) on single celled alga Acetabu laria.
- Commonly cells are uninucleate, that is, they possess a single nucleu.
- The protistan Paramecium caudatum has two nuclei (bi nucleate), macronucleus for controlling metabolic activities of the organism and micronucleus pos sessing hereditary information.
- Multinucleate or polynucleate condition is found in some cells of bone marrow, striated muscles, latex vessels, sev eral fungi and algae.
- Multinucleate animal or protistan cells are called syncytial cells (e.g. ,epidermis of Ascaris) while in plants and fungi they are called coenocytic cells (e.g. , Rhizopus , Vaucheria ).
- Acellular slime moulds have a multinucleate proto plasmic body called plasmodium.
- Nucleus is usually found in the re gion of maximum metabolic activity in the cytoplasm.
- Commonly it is situated in the geometric centre of the cell.
- In plant cells it is pushed to peripheral position on one side due to the develop ment of a large central vacuole.
- Nucleus is peripheral in fat - storing cells or adipocytes, and basal in glandular cells.
- It is suspended in central vacuole by cytoplasmic strands in Spirogyra.
Shape
- The nuclei are generally rounded in outline.
- They appear oval or elliptical in plant cells having large central vacuoles.
- Disc - shaped nuclei occur in the cells of squamous epithelium, lobed in white blood corpuscles and irregularly branched in silk spinning cells of insects.
- DNA- 9-12 %. RNA- 5 %. Lipids- 3 %. Basic Proteins- 15 %. Acid proteins, neutral proteins and enzymes- 65 %. Traces of minerals like Calcium. Magnesium). Potassium and Sodium ( Phosphorus is a constituent of DNA , RNA and acid proteins
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