Ribosomes are small, electron microscopic, dense, ribonucleoprotein particles which occur in cytoplasm either freely or attached to ER. They are also known as microsomes or palade particles or commonly known as protein factories. They occur both in prokaryotic and eukaryotic cells except RBC, sperm cells. The number of ribosomes in cells depends on upon their RNA content of the cell. They occur in large number in active protein synthesizing cells. eg; plasma cells, endocrine cells, meristematic cells(dividing cells).
On the basis of sedimentation coefficient(S), is measured in Svedberg unit. Two types of ribosomes are categorized as;
|The 70s type||80s type|
|It mainly occurs in prokaryotic cells as well as inside eukaryotic cell organelles like mitochondria, chloroplast.||It occurs in eukaryotic cells where the cytoplasm is free and in the bound forms.|
|It consists of subunit 30s and 50s.||It consists of two sub units40s and 60s.|
|It contains RNA: protein ratio 60: 40.||It contains RNA: protein ratio 40:60.|
Ribosomelacks any covering membrane and consist of two unequal subunits.
The smallest subunit fits like the cap over the larger subunit called base unit. These subunits associate only at the time of protein synthesis and at other times they remain in dissociated form. The association and dissociation of ribosomal subunits depend upon the concentration of magnesium ion. It is chemically composed of rRNA and proteins i.e. r is ribosomal.
Polysome is a group of ribosomes which remain attached to the same mRNA during protein synthesis. Polysomes are formed when multiple copies of the same protein are required inside the cell.
rRNA (ribosomal RNA)
It is the constituent of ribosome.
It is the messenger RNA which carries the message for protein synthesis from DNA.
It is transfer RNA which transfers amino acid to the site of protein synthesis.