Question

Polysome is formed by
(a) several ribosomes attached to a single mRNA.
(b) many ribosomes attached to a strand of the endoplasmic reticulum.
(c) a ribosome with several subunits.
(d) ribosomes are attached in a linear arrangement.

Answer 1

Hint: These are the sites for protein synthesis found in the mitochondria and also on the surface of the endoplasmic reticulum known as the rough endoplasmic reticulum. In polysomes, many of these are attached to a single unit.

Correct step by step answer:
Polysome is formed by several ribosomes attached to a single messenger RNA. During the elongation phase, polysomes are formed when large clusters of 10 to 100 ribosomes and elongation factor synthesize the encoded polypeptide. Along the coding region of messenger RNA, many ribosomes move to create a polysome. The limited abundance of mRNA in the cell explains the ability of multiple ribosomes to function on an mRNA molecule.
So, the correct answer is, 'several ribosomes attached to a single mRNA'.

Additional Information:
- Each ribosome has two subunits and as soon as the translation is terminated, the two ribosomal subunits dissociate from each other and are re- associated during the next round of initiation of protein synthesis only.
- The structure of the polysome differs between prokaryotic polysomes, eukaryotic polysomes, and membrane- bound polysomes. Polysomal profiling is a technique used to measure the level of gene expression through polysome activity.
- Prokaryotic polysomes: These are found to form double- row structures. Here, the ribosomes are contacting each other through smaller subunits. They generally have a “sinusoidal” or zigzag or 3- D helical path.
- Eukaryotic polysomes: It has a linear configuration in cells. Similar to prokaryotic 3- D polyribosomes, the eukaryotic 3- D polyribosomes are “densely packed left- handed helices with four ribosomes per turn”.
- Membrane- bound Polyribosomes are restricted by a 2- dimensional space given by the membrane surface. The ribosomes are arranged along the mRNA in a round- shape configuration so that the entry and exit sites form a smooth pathway.

Note:
- Polysome-profiling is a technique commonly used to study translatomes and applies laborious extraction of efficiently translated mRNA from a large volume across many fractions.
- Polysomes consist of a complex molecule made up of one mRNA molecule with two or more ribosomes, this complex acts to translate mRNA's instruction into polypeptides.
- There are two types of contact between the small subunits- “top-to-top” or “top-to-bottom”, in the sinusoidal path.