Question

Polysomes lack
A. Proteins
B. mRNA
C. DNA
D. tRNA and rRNA

Answer 1

Hint: A polyribosome or polysome is a group of ribosomes bound to an mRNA molecule like “beads” on a “thread”. It consists of a complex of an mRNA molecule and two or more ribosomes that act to translate mRNA instructions into polypeptides.

Complete answer:
Polysomes are formed during the elongation phase when ribosomes and elongation factors synthesize the encoded polypeptide. Multiple ribosomes move along the coding region of mRNA, creating a polysome. The ability of multiple ribosomes to function on an mRNA molecule explains the limited abundance of mRNA in the cell. Polyribosome structure differs between prokaryotic polysomes, eukaryotic polysomes, and membrane-bound polysomes. Polysome activity can be used to measure the level of gene expression through a technique called polysomal profiling.

Eukaryotic polysomes exhibit linear configurations. Densely packed 3-D helices and planar double-row polysomes were found with variable packing including “top-to-top” contacts similar to prokaryotic polysomes. Eukaryotic 3-D polyribosomes are similar to prokaryotic 3-D polyribosomes in that they are “densely packed left-handed helices with four ribosomes per turn”. This dense packing can determine their function as regulators of translation, with 3-D polyribosomes being found in sarcoma cells. Polysomes lack the double helix DNA structure.

Hence, the correct option is C, i.e., DNA

Note: Polyribosomes bound to membranes are restricted by a 2-dimensional space given by the membrane surface. The restriction of inter-ribosomal contacts causes a round-shape configuration that arranges ribosomes along the mRNA so that the entry and exit sites form a smooth pathway. Each ribosome is turned relative to the previous one, resembling a planar spiral.