Ribosomes

Prokaryotic ribosome

Ribosomes are small but complex molecular machine that contains both protein and ribonucleic acid (RNA). They are generally found in the cytoplasmic matrix of a bacterial cell; and some of them are bound to the plasma membrane. They are the site of protein synthesis i.e. translation (links amino acids together in the sequence specified by mRNA to form protein). Those proteins that remain within the cell are synthesized by matrix ribosomes, whereas plasma membrane ribosomes synthesize those proteins that are transported outside.

Prokaryotes have 70S ribosome, consisting of a large (50S) and a small (30S) subunit. They are about 20 nm in diameter and molecular weight is about 2.7 million. The S in 70S stands for Svedberg unit. This unit is a measurement of sedimentation in centrifugation i.e. greater the Svedberg value, faster the particle travel during centrifugation. Also, heavier and compact particles have higher Svedberg number. The 50S subunit consists of 5S rRNA (ribosomal RNA), 23S rRNA and 31 proteins whereas 30S subunit contains 16S rRNA and 21 proteins.

Eukaryotic ribosome

Eukaryotic ribosomes are larger than prokaryotic ribosomes; they are 80S ribosomes and are about 22nm in diameter with 60S and 40S subunit. Their molecular weight is about 4 million. The 40S subunit contains 18S rRNA and 60S subunit contains 28S rRNA, 5.8S rRNA and a short 5S rRNA. They can be free in the cytoplasmic matrix or can be associated with endoplasmic reticulum. Endoplasmic reticulum bound to ribosomes are called rough endoplasmic reticulum (RER) and those without ribosomes are called smooth endoplasmic reticulum (SER). In RER, ribosome is attached through its 60S subunit.

Proteins are synthesized by both free and endoplasmic bound ribosomes. Nonsecretory and nonmembranous proteins are synthesized by free ribosomes. Ribosomes bound to ER synthesize protein that either inserts into ER as integral membrane protein or enters into lumen for transport and secretion. These synthesized proteins are mostly functionally inactive and they require proper folding. Molecular chaperone aids this process and make them functionally active and also prevents them from aggregating into nonfunctional structures. Chaperones also help in the transport of protein into eukaryotic organelles like mitochondria. Generally during translation, multiple ribosomes are linked to a single mRNA at different locations and polypeptide synthesis occurs. This complex of mRNA and multiple ribosomes is known as polysome or polyribosome.