Question

Largest known RNA virus is
A. TMV
B. T1 phage
C. Citrus tristeza
D. $\phi \times 174$

Answer 1

Hint: CTV is the most prevailing virus infecting citrus plants. It has a large genome size of 19.3 kb. It consists of a positive-sense single-stranded RNA as its genetic material. It is the largest known plant infecting virus.

Complete answer: CTV or Citrus tristeza virus is a positive sense ssRNA virus. It infects an economically important large number of citrus plants. It belongs to the flexuous rod virus family. It is 2000 nm long. It has a diameter of around 12 nm and a 19.3 kb long genome. These large dimensions as compared to other RNA viruses make it the largest known RNA virus.
Option one, TMV is the Tobacco Mosaic virus. It is a positive sense ssRNA (single-stranded RNA) virus belonging to the Tobamovirus genus. It affects many plants, especially the tobacco plant. Its body is 300 nm in length and 18 nm in diameter. Its genome size is 6.3-6.5 kb.
Option two is the T1 phage. It is a Bacteriophage that attacks bacteria. Its genome is a dsDNA molecule, that is of 50,000 bp. Bacteriophages are very minute virions as compared to other viruses.
Option third is the Citrus tristeza virus. It is the largest known RNA virus. It has the largest genome of 19.3 kb length. It is much larger as compared to other plant infecting viruses.
The last option is $\phi \times 174$ or Bacteriophage $\phi \times 174$. It is also a bacteria-killing virus and is much smaller in size than CTV. It has the smallest genome size of 5,386 nucleotides only. It is a positive sense single-stranded DNA virus.
Thus, from the given options, the largest genome size and dimensions are of CTV or Citrus tristeza.
So, the correct option is option C

Note: We should know that although all the viruses are much smaller particles as compared to bacteria. But they can be categorized as the largest or smallest based on their genome sizes. Interestingly, all the viral species have very different genome types and sizes. The type of the genome they carry gives them their characteristic viral infection properties. CTV is an invasive citrus plant virus and its genome is much larger to study and thus it is not feasible yet to control it efficiently.