Biology Today — May 2017

•    Each ribosome consists of one larger and one smaller subunit.

•    The smaller subunit fits over larger one and requires Mg2+ for binding.

•    They are found in all living cells except RBCs.

•    They occur in cytoplasm of the cell and in organelles like mitochondria

and chloroplast. The cytoplasmic ribosomes may remain free in

cytoplasm or attached to ER.

•    They are the sites of protein synthesis and are known as protein

factories.

•    Chemically ribosome is made of two parts: proteins and rRNA.

•    It has 4 sites for specific attachments: mRNA binding site, A or aminoacyl

binding site, P or peptidyl site and E or exit site.

• The size of ribosome is measured by sedimentation coefficient in the centrifuge and is measured as Svedberg unit called S.
Two subunits of 80S ribosome are 60S and 40S while 70S ribosome has 50S and 30S subunits.

Subunit

30S

Subunit

40S 21 proteins

33 proteins

40 proteins 34 proteins

Fig.: Components of 80S and 70S ribosomes.

18S RNA 16S RNA

28S RNA 23S RNA

5S RNA 5S RNA

5.8S RNA

80S 70S

Subunit

50S

Subunit

60S

300 – 340A° 200 – 290A°

Functions

•    Protein factories: Ribosomes are sites for polypeptide or protein synthesis and therefore act as protein factories.

•    Free ribosomes synthesise structural and enzymatic proteins for use inside the cell. The attached ribosomes synthesise

proteins for transport.

•    Ribosomes provide enzymes (e.g., peptidyl transferase) and factors for condensation of amino acids to form polypeptide.

•    Ribosome contains rRNAs for providing attaching points to mRNA and tRNAs. Ribosome also has a tunnel for mRNA so

that it can be translated properly.

•    It also provides protection to newly synthesised polypeptide by enclosing it in groove of larger subunit of ribosome.