Ribosomes
Chemical composition:

• ribosomes are ribonucleo-proteins.
• ribosomes are consists of RNA and  Proteins in almost equal proportion.

Assembly of ribosomes:

• ribosomes are assembled in the nucleolus.
• from nucleolus they are transported to the cytoplasm through the nuclear pores.

Form and physical structure:

• they exist in two forms; either dispersd in the cytoplasm or attached with rough endoplasmic reticulum as a tiny granules.
• ribosomes consists of two subunits.
• larger subunit 60 S
• smaller subunit 40 S
• attachment of both ribosomes results in 80 S.
• attachment of both ribosomes is controlled by Mg ions.
• ribosomes are attached to 5 end of mRNA through the smaller subunits.

Funtions:

• ribosomes are factory for the protein synthesis.
• A group of ribosomes attached to mRNA is known as polysome or polyribosome.