The nucleus isa membrane bounded organelle which is found on Eukaryotic cells. It is also thelargest cellular organelle in animal cells and is 5µm in diameter. Mostly thecells are uni-nucleate, and some cells may have more than one nuclei. But thereare few types of cells which lack a nucleus. Ex: mammalian red blood cells.
Thenucleus carries the hereditary information or the genetic material of the cellwhich are arranged within the chromosomes. These chromosomes are formed by thelong linear DNA molecules and different types of proteins including Histone.The DNA and protein complex which makes the chromosomes is called chromatins.
Each eukaryotic cell has a specific number of chromosomes. Ex: typical humancell has 46 chromosomes.The double membranes around the nucleus is called thenuclear envelope. The nuclear envelope consists of phospholipids which isimpermeable to larger molecules. The two membranes are separated by a space of20-40nm.
The function of this envelope is to maintain the shape of the nucleusand to regulate the molecules which flows in and out of the nucleus. The outermembrane of the nuclear envelope is attached with the Rough EndoplasmicReticulum. There are pores along the nuclear envelope which provides a channelto transport molecules across the nuclear envelope. They are known as nuclearpores and are about 100nm in diameter. This helps in the transportation of moleculessuch as RNA, different proteins, carbohydrates, and some DNA. Within thenucleus there is a viscous liquid called nucleoplasm which is also calledKaryoplasm.
It is mainly composed of water, different enzymes, dissolved saltsand some suspended organic molecules. The nucleolus and the chromosomes arekept intact within the nucleus by the nucleoplasm. In animal cells a meshstructure is present which is called the nuclear lamina that provides thesupport to the envelope.