Question

A polysome is formed by
a. A cluster of ribosomes
b. Many ribosomes attached to mRNA
c. A cluster of ribosomal subunits
d. Many mRNA being attached to a ribosome
e. Clusters of cistrons

Answer 1

Ribosomes are granular structures discovered by George Palade in 1953, when he observed them under an electron microscope and described them as dense particles. They are membrane-less structures composed of proteins and ribonucleic acid (RNA).

Complete answer:
Ribosomes are present either in the cytoplasm floating freely or attached to the cell organelles. Ribosomes are useful for cells in performing protein synthesis. In a prokaryotic cell, ribosomes are found connected to the plasma membrane, with a size of 15nm by 20nm. These ribosomes are made up of two subunits, one of 50S and one of 30S, together forming a 70S ribosome. ‘S’ stands for Svedberg’s unit, which means the sedimentation coefficient. It describes the size and density of the ribosome. When several ribosomes are attached to a single mRNA, it forms a chain which is known as polyribosomes or polysome. These ribosomes present in the form of polysome translate the mRNA into proteins.

Hence, the correct answer is option (B).

Additional information:
In eukaryotic cells, ribosomes are made up of two subunits, together forming 80S ribosomes. Its main function is the same as that of prokaryotic ribosomes, which is to synthesise proteins. Proteins synthesised by ribosomes are required by the cell to perform various functions such as damage repair, or in chemical reactions.
Ribosomes are also found attached to the endoplasmic reticulum. The endoplasmic reticulum having ribosomes present on the outer surface are called rough endoplasmic reticulum due to the formation of a rough surface.

Note: Cistron is defined as a segment of molecules of DNA or RNA that helps in protein synthesis by coding for specific polypeptides. It is also a term used to define a gene that shows a definite behaviour in a genome.