Ribosomes Ribosomes are ribonucleoprotein particles found in almost

Ribosomes: • Ribosomes are ribonucleo-protein particles found in almost all cells. • They are also known as “Protein factories” of the cell. • They are found in the cytoplasm or remain attached to E. R. & also bounded to m. RNA. • Free ribosomes are also present in the Mitochondria & Chloroplast. • Ribosomes were first observed by Claude in 1946 and named them as microsomes. • Palade in 1955 reported ribosomes for the first time from animal cells.

• Distribution of Ribosomes: • The ribosomes occur in cells both prokaryotic and eukaryotic cells. • In prokaryotic cells ribosomes often occur freely in the cytoplasm. • In eukaryotic cells the ribosomes often occur freely in cytoplasm or remain attached to the outer surface of the membranes of E. R. • The cells in which active protein synthesis takes place, the ribosomes remain attached with the membranes of E. R • Such cells are pancreatic cells, plasma cells, serous cells, osteoblasts, glandular stomach, thyroid cells, mammary gland cells.

• The cells which synthesize specific proteins for the intracellular utilization & storage, often contain large number of free ribosomes. • Such cells are erythroblasts, developing muscle cells, skin & hair. • Types of Ribosomes: • The ribosomes have 3 types depending on the size and sedimentation coefficient. • 1. 80 S Ribosomes • 2. 70 S Ribosomes • 3. 55 S Ribosomes

• The “S” referred as Svedberg unit or sedimentation coefficient. • Each ribosome consist of 2 subunits. • 1. A smaller subunit • 2. A Bigger subunit. Smaller Subunit Bigger Subunit RIBOSOME

• 80 S Ribosome: It is found in eukaryotes. It is larger in size and has a coefficient of 80 S. It has molecular wt of 5 x 106 daltons. It is composed of r- RNA and ribosomal proteins. The 80 S ribosome is made up of two subunits namely 60 S and 40 S. • 60 S--- 28 S r-RNA, 5. 8 S r-RNA & about 50 different proteins. • 40 S ---- 18 S r-RNA & 30 different proteins. • • •

• • 70 S Ribosome: It is found in prokaryotic cells. It is smaller than 80 S ribosomes. It has sedimentation coefficient of 70 S. It has molecular wt 3 x 106 daltons. Larger subunit is 50 S and smaller is 30 S. 50 S--- 23 S r-RNA, 5 S r-RNA and 30 proteins. 30 S---- 16 S r- RNA & about 20 different proteins.

• • • 55 S Ribosomes: It is found in Mitochondria & Chloroplast. The sedimentation coefficient is 55 S. It has 40 S (Larger) and 30 S (Smaller) subunit. 40 S contains 16 S RNA 30 S contains 12 S RNA Smaller Subunit Bigger Subunit RIBOSOME 70 S 30 S 50 S Dissociation of Ribosomes

• Ultrastructure of Ribosome • Lake’s model) • The fully asymmetrical model of ribosomes had been discovered by James Lake in 1981. • It is divided into 2 unequal parts the upper 1/3 is head and lower 2/3 is body. • From the body there originates the platform. • Between platform and head there is a cleft, having important functional regions. • It has a site for binding of IF-1, 2, 3 and also for codon-anticodon interaction. • The 50 S subunit has central protuberance and 2 lateral projections. • The platform of small subunit is aligned with the notch of the large subunit.

• Polyribosomes: • Protein synthesis does not take place on individual ribosome but on the groups or clusters of ribosomes. • These clusters are called as polyribosomes. • Polyribosomes may be called as polysomes or ergosomes. • The ribosomes of polyribosome may be arranged in a linear manner or rosette shaped cluster. • 2 70 S or 80 S ribosomes form dimmers of 100 S and 120 S respectively. • The 200 S polyribosome of Hb synthesis have 5 -6 ribosomes. • The number of ribosomes in a polyribosome depend on the length of m. RNA molecule that take part in protein synthesis.

• Functions of Ribosomes: • Ribosome is the site of protein synthesis. • It co-ordinates and brings all the components of protein synthesis together. • It carries the t. RNA fro nucleus to the site of protein synthesis. • t. RNA provides amino acid anticodon site. • Ribosomes also have protective function. • The m. RNA strand is protected from the action of the enzymes.