Nucleus, the commanding centre of the cell

Nucleus is the site where DNA the hereditary material is found which controls the activities of the cell. This is the site where ultimate dictatorship is the rule and efficiency of this master molecule is amazing and not even understandable or imaginable.

The Nucleus (L., nux=nut) is the heart of the cell. Synonym: Karyon (Gk)

Nucleus

History

• Named and discovered by Robert brown 1833 in plant cells sooner he identified it as a common feature in both plant and animal cells.
• W. Flemming (1879), used the term chromatin for chromosomal network found in the nucleus.
• The presence of nuclear membrane was first reported by O. Hertwig (1893)
• Strasburger (1882) introduced the term cytoplasm and nucleoplasm. Barbara McClintock in 1934 identified nucleolar organisers in the chromosomes.
• The term nucleolus was coined by Bowman, but first reported by Fontana (1781)
• Later Hammerlings grafting experiment with Acetabularia clearly demonstrated the role of nucleus in heredity.

Hammerling's Acetabularia grafting experiment that established nucleus as the store house of hereditary material.

After the discovery of cell and its universal presence in living things, the next question was which is the CPU of the cell? Many tried to solve this problem using uni-celled organisms which has regeneration capacity.

The first insight into this problem was given by Balbiani. He dissected protozoan’s into two halves and found out that nucleated halves regenerated, grew and survived. But enucleated halves degenerated and died.

Later an experiment was conducted where, nuclei was incorporated into enucleated amoeba. Soon the amoeba retained its growth, feeding and pseudopodial activity (for movement) suggesting the role of nucleus in manifesting all these functions.

Are there any cells in eukaryotes without nucleus?

Nucleus is present in all eukaryotic cells. But the notable exceptions are sieve tubes in plants; sieve tubes are phloem elements that are meant for transport of food from photosynthetic sites to different tissues of the plant. Sieve tube lacks nucleus. But associated with sieve tube, a cell with conspicuous nucleus are present and is rightly called as companion cells. Companion cells controls and co-ordinate the activity of sieve tubes.

In animals, red blood cells, the oxygen carriers lack nucleus. These cells are called red blood corpuscles (RBC,s) rather than cells (corpus means body or corpse).

How many nuclei are there in a cell?

Normally, a cell contain single nucleus. But rarely may be bi-nucleate (aramaceum, liver cells) or poly nucleate. In animals, poly nucleate cells are referred are syncytial cells (osteoblast (~ 100 nuclei /cell). Poly nucleate plant cells are called coenocytes. Many fungal hyphae are coenocytic. Vaucheria, the siphonal algae has 100s of nuclei in a common cytoplasm. The presence of many nuclei in a common cytoplasm is called as coenocytic condition.

Nucleus Ultra Structure

What is the size of nucleus compared to total cell volume of the cell?

Nucleus occupies nearly 10% of cells total volume.

Size ranges from 3-25um in diameter.

Which are the factors that determine the size of the nucleus?

The size of the nucleus depends on the no. of chromosomes or ploidy, total volume of the cell and metabolic phase of the cell. The size of the nucleus of the haploid cell is comparatively smaller than diploid cell.

How to calculate the size of the nucleus?

R. Hertwig proposed a formula for deducting the size of the nucleus of a particular cell

NP=Vn/Vc-Vn

NP=nucleoprotein index, Vc= volume of the cell and Vn=volume of the nucleus.
Functions of Nucleus