Question

Polysome is formed by
A. A ribosome with several subunits.
B. Ribosomes attached to each other in a linear arrangement.
C. Several ribosomes attached to a single mRNA.
D. Many ribosomes attached to a strand of the endoplasmic reticulum.

Answer 1

Hint: Ribosomes are found in the cells floating in the cytoplasm or may be attached to the endoplasmic reticulum making it the rough endoplasmic reticulum. They are used to synthesize proteins in the cells. Polysomes as such are made with ribosomes.

Complete answer: Polysomes are large polymers which are formed by joining several ribosomes attached to a single messenger RNA, i.e. mRNA. These large clusters having about 10-100 ribosomes which are actively translating protein synthesis constitute the polysomes. Along with the coding region of mRNA, many of the ribosomes move along it to create a polysome and thereby elongate the encoded protein. Each ribosomal unit has two subunits and thereby when the translation is terminated these subunits dissociate from each other and they get joined again for the other round of protein synthesis.
There are structural differences between the structure of prokaryotic, eukaryotic and membrane-bound polysomes. A technique named polysome profiling is commonly used to measure the level of gene expression through the activity of polysomes. Prokaryotic polysomes have a double row of structures. In these, ribosomes are found to contact each other through smaller subunits. They are found to have a sinusoidal or zigzag or 3-D helical path. Eukaryotic polysomes seem to have a linear configuration. The eukaryotic polysomes seem to have 3-D configuration which are densely packed left-handed helices having four ribosomes per turn. In membrane-bound polysomes, they are restricted to a 2-D space bounded by a membrane surface. These ribosomes are found to be arranged along the mRNA in round shape configuration. It is arranged in such a way that the entry and exit both sites form a smooth pathway.
So, the correct answer is option A.

Note: There is a technique named polysome profiling which is commonly used to study translates and this applies a very laborious extraction of mRNA which is efficiently translated from a very large volume across many fractions. It is an important technique.