Nucleus is known as the master cell organelle as it regulates various metabolic activities through synthesis of various proteins and enzymes.
The nucleus in eukaryotic cell is made up of nuclear envelope, nucleoplasm, nucleolus and chromatin network.
1. Nuclear envelope:
a. Nuclear envelope is a double layered delimiting membrane of nucleus.
b. Two membranes are separated from each other by perinuclear space (10 to 50nm).
c. Outer membrane is connected with endoplasmic reticulum at places and harbours ribosomes on it.
d. The inner membrane is lined by nuclear lamina- a network of protein fibres that helps in maintaining shape of the nucleus.
e. The two membranes along with perinuclear space help in separating nucleoplasm from cytoplasm. However, nuclear membrane is not continuous.
f. There are small openings called nucleopores on the nuclear membrane.
g. The nucleopores are guarded by pore complexes which regulate flow of substances from nucleus to cytoplasm and in reverse direction.
2. Nucleoplasm or karyolymph:
a. The nucleoplasm or karyolymph contains various substances like nucleic acids, protein molecules, minerals and salts.
b. It contains chromatin network and nucleolus.
3. Nucleolus:
a. Nucleolus is made up of rRNA and ribosomal proteins and it is known as the site of ribosome biogenesis.
b. The rRNA and ribosomal proteins are transported to cytoplasm and are assembled together to form ribosomes.
c. Depending on synthetic activity of a cell, there are one or more nucleoli present in the nucleoplasm. For e.g. cells of oocyte contain large nucleolus whereas sperm cells contain small inconspicuous one.
d. Nucleolus appear as dense spherical body present near chromatin network.
