Question

Ribosomes attach to the endoplasmic reticulum by their
A. 50'S' sub-unit
B. 60'S' sub-unit
C. 40'S' sub-unit
D. More than one option is correct

Answer 1

Hint: The major function of the ribosomes in both prokaryotes and eukaryotes is to aid in the translation of proteins. It is done at the ER where the large subunit of the ribosome binds to the ER membrane and then translates proteins. Both 70s in prokaryotes and 80s in eukaryotes perform the same function.

Complete answer: Ribosomes are protein molecules found within the cell and majorly on the endoplasmic reticulum. The ribosomes attach themselves to the surface of the ER via various membrane receptors. This attachment of ribosomes gives the characteristic rough visual of the ER and thus is known as the Rough ER. Ribosomes in eukaryotes are 80s ribosomes and in prokaryotes are 70s. The ‘s’ stands for Svedberg constant which is a unit denoting the sedimentation rate of the protein. The 70s ribosome in prokaryotes is made up of two subunits, namely 50s and 30s subunits. In the case of the 80s ribosome in eukaryotes, it is made of two subunits namely 60s and 40s. The 60s and 50s subunits are called the large subunits and 40s and 30s are called the small subunit. The large subunit only binds to the ER membrane and houses the t-RNA for translation of proteins, whereas the small subunits act as the binding site for mRNA. The small subunit is bound to the large subunit by several protein-protein interactions and the gap in between them acts as the conveyor belt through which the mRNA is read and translated into polypeptide chain via the t-RNA.
Thus, the correct answer is option D.

Note: Only the large subunit of the ribosome binds to the ER membrane. The small subunit has a binding site for m-RNA and the large subunit has binding sites for tRNA. Once the entire mRNA is read, the polypeptide, m-RNA, and the t-RNA dissociate from the ribosome, and the polypeptide is sent to the Golgi apparatus.