Most viruses that infect plants possess
- A.single-stranded DNA
- B.single-stranded RNA
- C.double-stranded DNA and RNA
- D.double-stranded RNA only
Correct answer
Explanation
The correct answer is B, single-stranded RNA. The great majority of plant viruses carry their genes as single-stranded RNA.
Viruses contain either DNA or RNA, never both, wrapped in a protein coat. Most viruses that infect plants, such as tobacco mosaic virus (TMV), potato virus Y and cucumber mosaic virus, have single-stranded RNA genomes. Bacteriophages, the viruses that infect bacteria, usually have double-stranded DNA, and animal viruses come in both kinds. TMV was the first virus to be discovered: Dmitri Ivanovsky showed in 1892 that the agent of tobacco mosaic disease passed through filters that held back bacteria, and M. W. Beijerinck later called it 'contagium vivum fluidum'.
A is wrong: single-stranded DNA is found only in a few plant viruses, such as the geminiviruses.
C is wrong: no virus carries both DNA and RNA as its genome.
D is wrong: double-stranded RNA plant viruses exist but are uncommon.
Exam tip: plant viruses = mostly ssRNA (TMV); bacteriophages = mostly dsDNA; a virus has DNA or RNA, never both.
Practice Questions
View allConsider the following animals :
1. Hedgehog
2. Marmot
3. Pangolin
To reduce the chance of being captured by predators, which of the above organisms rolls up/roll up and protects/protect its/their vulnerable parts?
- A.1 and 2
- B.2 only
- C.3 only
- D.1 and 3
Show answer
Hedgehogs and pangolins curl into a tight ball when threatened, covering the soft belly and face with spines or scales; marmots do not.
Read the full explanation →Which one of the following is used in preparing a natural mosquito repellent?
- A.Congress grass
- B.Elephant grass
- C.Lemongrass
- D.Nut grass
Show answer
Lemongrass and its close relative citronella grass yield aromatic oils that keep mosquitoes away, so they are the base of many natural repellents.
Read the full explanation →Which one of the following devices is non-ohmic ?
- A.Conducting copper coil
- B.Electric heating coil
- C.Semi conductor diode
- D.Rheostat
Show answer
A diode does not obey Ohm's law: the current through it is not proportional to the voltage across it. An ohmic conductor gives a straight-line graph of current against voltage, so its resistance stays constant at a fixed temperature.
Read the full explanation →What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)
- A.286 u
- B.106 u
- C.83 u
- D.53 u
Show answer
Anhydrous sodium carbonate is Na₂CO₃, and 2 × 23 + 12 + 3 × 16 = 106 u. Formula mass is the sum of the atomic masses of all the atoms in one formula unit of an ionic compound.
Read the full explanation →Which one of the following minerals is used as a fuel in nuclear power stations ?
- A.Bauxite
- B.Quartz
- C.Feldspar
- D.Pitchblende
Show answer
Pitchblende, or uraninite, is the chief ore of uranium, the fuel used in most nuclear power stations. Pitchblende is a heavy, black mineral made mostly of uranium oxide.
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