EUKARYOTIC RIBOSOMES

 EUKARYOTIC RIBOSOMES

·   Ribosome, particle that is present in large numbers in all living cells and serves as the site of Protein synthesis.

·     The small particles that came to be known as Ribosomes were first described in 1955 by Romanian-born American Cell Biologist George E. Palade, who found them to be frequently associated and attached with the endoplasmic reticulum in eukaryotic cells.

·       A single actively replicating Eukaryotic cell may contain as many as 10 million Ribosomes.

·   The size of Ribosomes ranging from 3.5 MDa in lower Eukaryotes to 4.0 MDa in higher Eukaryotes.

·       Ribosomes are made up of Ribosomal proteins (50 %) and rRNA (50 %).

·       Ribosomes are usually made up of three or four rRNA molecules and anywhere from about 40 to 80 different Ribosomal proteins.

·       80 S type of Ribosomes are present in Eukaryotes. Each Ribosome is composed of two sub-units, a larger sub-unit 60 S and a smaller sub-unit 40 S. The sub-units typically are referred to in terms of their Sedimentation rate, which is measured in Svedberg units (S), in a centrifugal field.

·       Based on its location, Eukaryotic Ribosomes are classified into two types. They are

a)     Membrane bound Ribosomes

ü  Synthesize proteins for Membranes and Exocytosis.

ü  Attached to the Endoplasmic reticulum and other organelles.

b)     Free Ribosomes

ü  Found freely in cytoplasm

ü  Synthesize Proteins that function inside the Cytosol (Used inside the cell for Food metabolism)

·       Ribosomes are the major site of Protein synthesis.