Nucleus

Nucleus:

Nucleus is a membrane enclosed organelle that contains the chromosome. It is present inside the cell.

• The study of nuclei of cells is known as Karyology.
• Nucleus is present in all cells except mature RBC's and sieve cells of xylem.
• The shape of nucleus varies. It can be oval, disc shaped, lobular, irregular etc
• In prokaryotes the nucleus is not covered by well defined membranes. In prokaryotes it is called as prokaryon/genophore/nucleoid.
• Normally a cell contains one nucleus but their number varies in certain cells e.g. binucleate liver cells and cartilage, polynucleate in osteoblast and skeletal muscles etc.

Nucleus is made up of four parts:

1. Nuclear Membrane
2. Nucleoplasm/Karyoplasm
3. Chromatin Network
4. Nucleolus

Different parts of Nucleus

1) Nuclear Membrane:

It is a perforated double membrane. The pores have pore complex to regulate the size of pore. It seems to separate the contents of nucleus from cytoplasm. The nuclear membrane is made up of phospholipid bilayer.

2) Nucleoplasm:

It is an alkaline semifluid in which chromatin material and nucleoli are embedded. It contain DNA Polymerases, RNA Polymerases, minerals, nucleoproteins, nucleotides and water. It helps to maintain the shape and structure of nucleus.

3) Chromatin Network:

Chromatin Network is made up of

• Heterochromatin
• Euchromatin

Heterochromatin:

It is condensed chromatin, darkly stained and tightly coiled. The genes are inactive.

Euchromatin:

It is diffused chromatin, lightly stained and loosely coiled. The genes are active.

4) Nucleolus:

It is darkly stained organelle, dense and eccentric in position. The number of nucleoli varies in different species. The nucleoli disappear during cell division and reformed on the nucleolar chromosome at telophase.