RIBOSOMES

RIBOSOMES
The ribosome is the main site for protein synthesis. It is spheroidal particle consisting of a large and a small subunit. In prokaryotes (E. coli), the intact particle has a sedimentation coefficient of 70S (S = Svedberg unit) where the small subunit is 30S and the large subunit is 50S. Each 30S subunit contains one 16S ribosomal RNA (rRNA) molecule and 21 different proteins. Each 50S subunit contains one 5S rRNA molecule, one 23S rRNA molecule and 32 different proteins. In eukaryotes, in intact particle is 80S and dissociates to give 40S small subunit and 60S large subunit. Each 40S subunit contains one 18S rRNA and 30 proteins. Each 60S subunit contains 28S rRNA, 5.8S rRNA and 40 proteins. Each subunit contains two transfer-RNA-binding sites—A-site (aminoacyl site) and P-site (peptidyl site). During proteins synthesis more ribosomes attach to a single messenger RNA molecule and form a polysome. It also protects the mRNA during protein synthesis.