Viruses

Viruses are described in NCERT Class 11, Chapter 2 (Biological Classification) as obligate parasites whose genetic material may be either DNA or RNA, but never both, and the type present characterises different groups of viruses. In plant-infecting viruses, including the tobacco mosaic virus, the genetic material is usually single-stranded RNA, whereas many animal viruses carry single- or double-stranded RNA or double-stranded DNA, and most bacteriophages possess double-stranded DNA. Hence, for TMV the correct genetic material is single-stranded RNA. Double-stranded DNA is incorrect because it typifies bacteriophages and several animal viruses rather than TMV. Double-stranded RNA occurs in some viruses such as certain animal viruses, but not in the tobacco mosaic virus. Single-stranded DNA is found in a few viruses but again not in TMV. Surrounding this nucleic acid is a protein coat, the capsid, built from subunits called capsomeres arranged in helical symmetry in TMV. A consistency check confirms the answer: TMV is the classic example used historically to demonstrate that RNA can serve as hereditary material, so its genome being single-stranded RNA is fully in line with established virology and the conclusions of Fraenkel-Conrat's reconstitution experiments.