Question

What is polysome?

Answer 1

Hint: A polysome (polyribosome) is a structure that bears at least two or more ribosomes and an mRNA sequence.

Complete answer:
Polysome is a group of ribosomes involved in mRNA translation. A polyribosome (polysome or ergasome) is bound to an mRNA molecule like “beads” on a “thread”. So, polysomes consist of a complex of an mRNA molecule and two or more ribosomes involving mRNA translations into polypeptides. Earlier polysomes were named "ergosomes". It was characterized by Jonathan Warner, P.M. Knopf and Alex Rich. Polysomes are formed during the elongation phase of translation. Structurally prokaryotic polysomes are different from eukaryotic polysomes, and membrane-bound polysomes. Polysome activity gives a measure of the level of gene expression. This technique is called polysomal profiling.

Note: Polysome can be viewed under an electron microscopy technique by methods such as staining, metal shadowing, and ultra-thin cell sections. The cryo-electron microscopy techniques give a high-resolution image. It is a more precise method to determine the structure. Different cells produce different structures of polysomes. Prokaryotic polysomes form double-row structures that give “sinusoidal” (zigzag) or 3-D helical path. Eukaryotic polysomes exhibit linear configurations, 3D helices and planar double-row polysomes are found. Membrane-bound polyribosomes are restricted by the membrane surface. This produces a round-shape configuration and arranges ribosomes along the mRNA. Polysomal Profiling is a technique in which cycloheximide is used to arrest translation and a sucrose gradient separates the resulting cell extract by centrifugation. In cultured cells and tissues, the technique helps to track the translational status of an identified mRNA and measure ribosome density. The technique is utilized to compare the translational status of mRNAs in different cell types.