Ribosomes are minute spherical membrane less cytoplasmic organelles present in both prokaryotic and eukaryotic cells consisting of RNA and some special proteins and perform the key role in protein synthesis. Ribosomes are usually designated according to their rate of sedimentation. Structure and function of ribosomes is described bellow.
Ribosomes contain 80-85% of the cell’s ribonucleic acids (RNA). Ribosomes are made up of 40-50% of ribosomal RNA or rRNA and 50-60% of protein and metallic ion like magnesium ion.
LOCATION: Ribosomes are found in the cytoplasm of both prokaryotic and eukaryotic cells. The ribosomes may be present in the cytosol as free or unattached (found in both eukaryotic cytoplasm and prokaryotic cytoplasm) or in bound form like attached to the membrane of endoplasmic reticulum and nucleus membrane (mainly found in eukaryotic cells). Some ribosomes are also present in mitochondria, plastid and in nucleus.
TYPES: Depending on the rate of sedimentation ribosomes are two type – a. 70S ribosome present in the prokaryotic cells and b. 80S ribosomes present in the eukaryotic cells.
STRUCTURE OF RIBOSOMES: The general structure of eukaryotic and prokaryotic ribosome is similar. These are spherical in shape and do not contain membrane. These are differentiated into two subunits:
- A Smaller Subunit, which is 30S type in prokaryotic cells and 40S type in eukaryotic cells. And it is consisting four parts head, base, platform and cleft.
- A Larger Subunit, which is 50S type in prokaryotic and 60S type in eukaryotic cells. The smaller subunit is associated with the larger subunit like a cap.
The subunits of ribosomes also contain three functional site which helps to bind and decode the mRNA codons and the tRNA anticodons and play as the main or initiative site for protein synthesis. These are I. Amino acyl site or acceptor site (A-site) for the amino acids containing tRNA, II. Peptide site or P-site where peptide chain starts to grow, III. Exit site or E-site for the decoded tRNA.
FUNCTION OF RIBOSOMES: Ribosomes are the main site of protein synthesis and plays many other roles in cell maintenance. The function of ribosome is described below-
PROTEIN SYNTHESIS: Ribosomes synthesize all transmembrane proteins. It helps to translate or decode the codes of mRNAs and associated the amino acids and form the peptide chains and synthesize proteins. For this special function of protein synthesis ribosomes are known as the Protein Factory. Both subunits of ribosomes participate in protein translation.
- Ribosomes also help to carry the newly synthesized proteins from the cytoplasm into the endoplasmic reticulum where the ultimate processing and sorting of proteins is done.
- Sometimes ribosomes also can help to protect the newly formed polypeptide chains from the degradative reactions of the degradative enzymes present in the cytoplasm.
Ribosomes also play some other important function in cell maintenance.