Question

What do we understand by 70s and 80s ribosome

Answer 1

Ribosomes are ‘work benches’ for protein synthesis and are known as cell’s ‘protein factories’. Ribosomes are very small spherical (200-300 A0 in diameter), non-membranous structures composed of rRNA (55-65%) and proteins (35-45%) and are of two types:-

(i) 70S RIBOSOMES: Present in prokaryotes and in mitochondria and chloroplasts of eukaryotic cells. 80S RIBOSOMES ARE ABSENT IN PROKARYOTES.

(ii) 80S RIBOSOMES: Present in eukaryotes

Normally a typical cell contains 1000-5000 ribosomes. In cytoplasm, ribosomes are present either in free form or associated with RER, nuclear membrane or cytosolic surface of outer mitochondrial membrane.

In cytoplasm, ribosomes also exist in cluster or group. A group of ribosomes is known as ‘polyribosome’ or ‘polysome’. A polysome may have 2-70 ribosomes held together by a single strand of mRNA.

Each ribosome consists of two subunits— a small subunit and a large subunit held together by Mg2+ ions.

(i) 70S RIBOSOMES: Consist of a small subunit of 30S and a large subunit of 50S. (ii) 80S RIBOSOMES: Consist of a small subunit of 40S and a large subunit of 60S.