Synonyms:
Extra cytoplasmic organelle, apparatus of heredity, director of cell, control room, or ‘heart of cell’ or cell brain.
History:
A V. Leeuwenhoek (1672):
First observed nucleus in the RBC of Salmon fish.
Robert Brown (1831):
He first described and named the term nucleus in orchid root cells.
Strasburger (1882):
He proved that nucleus arise from pre-existing nucleus by division Joachim Hammerling (1934): A Danish biologist who perform grafting experiments on Acetabularia.(a unicellular marine green alga) and first demonstrated that nucleus is the store house of hereditary information or controller of cell or cell brain.
Definition:
Nucleus is a specialized double-membrane bounded organelle which contains the genetic information for controlling metabolism, growth and differentiation of the cell. Nucleus is the largest of all cell organelles and its study is called karyology.
Occurrence:
Nucleus present in all eukaryotic cells. But in mature sieve tube elements of phloem and mature RBCs of mammals’ nuclei lost secondarily at maturity. Prokaryotic cells do not possess organized nucleus. They have a nucleoid, a highly folded circular DNA molecule without a membrane.
Position:
In young and normal cell, it is usually located in the centre of the cell. However, in mature plant cells it is mostly pushed to peripheral region due to development of a large central vacuole. Sometimes nucleus occupies unusual position, such as seen in Spirogyra, where it is seen suspended in the central vacuole by cytoplasmic strands.
Number:
Most of the eukaryotic cells are generally uninucleate or eukaryen (one nucleus); but it may vary in different cells. Paramecium, liver cells, cartilage cells are binucleate or dikaryon (two nucleic). Multinucleate or polykaryon condition is also found in some cells of bone marrow, striated muscles, latex vessels, several fungi and algae.
In animals, the multinucleate cells are called syncytial cells, whereas in plants and fungi they are known as coenocytic cells (e.g., alga Vaucheria, fungus Rhizopus). Acellular slime moulds have a multinucleate protoplasmic body called Plasmodium. A cell is called cybrid or heterokaryon when it contains nuclei of two types, produced by cell fusion.
Shape and Size:
In majority of the cells nuclei appear spherical. However, in mature plant cells they appear oval or elliptical. The nucleus is mostly spherical in shape. However, it may become somewhat oval or elliptical in plant cells having large central vacuole. A typical interphase nucleus is usually 5-25 µm in diameter and occupies about 10% of the cell volume.
Functions of Nucleus:
1. It controls the ongoing activities of the cell by determining which protein molecules are produced by the cell and when they are produced. It is done through the formation of RNAs (mRNA, rRNA, and tRNA) which control synthesis of different enzymes.
2. It stores most of the cell’s genetic information, passing it on to the daughter cells in the course of cell division. Chromatin part of nucleus possesses all the genetic information that is required for growth and development of the organism, its reproduction, metabolism and behavior.
3. Ribosomes, essential for protein synthesis in a cell, are formed in the nucleolus of the nucleus.
4. It initiates and controls cell division in the organism.
5. It induces genetic variation that contributes to evolution.
6. It brings about cell differentiation by keeping only certain genes operational in particular cells.