Smallest DNA virus is –
**Core Concept**
The question is testing knowledge of viral classification, specifically the smallest DNA virus. DNA viruses are classified based on their size, shape, and genetic material. The smallest DNA viruses are typically those that infect bacteria, known as bacteriophages.
**Why the Correct Answer is Right**
The correct answer is a bacteriophage, specifically the PhiX174 virus, which is a small, single-stranded DNA virus that infects *E. coli*. It has a genome of approximately 5,386 base pairs and a diameter of about 27 nanometers. The PhiX174 virus is a model organism for studying viral replication, assembly, and gene expression. Its small size and simple genome make it an ideal system for understanding the fundamental principles of viral biology.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular virus, making it too vague to be a correct answer.
**Option B:** This option is incorrect because it does not refer to a specific virus or bacteriophage, and most viruses that infect eukaryotic cells are larger than the PhiX174 virus.
**Option C:** This option is incorrect because it is a type of virus that infects plants, and while some plant viruses are small, they are not the smallest DNA viruses.
**Clinical Pearl / High-Yield Fact**
The PhiX174 virus is often used as a model system in molecular biology and virology, and its small size and simple genome make it an ideal system for understanding the fundamental principles of viral biology.
**Correct Answer:** C. PhiX174 virus