Which statement best describes a basic characteristic of viruses?

• A. They have both DNA and RNA genomes simultaneously in each virion.
• B. They are composed of cells with ribosomes and cytoplasm.
• C. They reproduce independently by binary fission in nutrient broth.
• D. They are obligate intracellular parasites that require a host cell to replicate.

Answer: (D)

Verified step by step guidance

1. Understand that viruses are unique infectious agents that differ fundamentally from cellular life forms such as bacteria and eukaryotic cells.
2. Recall that viruses do not have cellular structures like ribosomes or cytoplasm, which are essential for independent metabolism and reproduction.
3. Recognize that viruses contain either DNA or RNA as their genetic material, but never both simultaneously within the same virion.
4. Know that viruses cannot reproduce by binary fission or any independent means; instead, they must infect a host cell and use the host's machinery to replicate.
5. Conclude that the defining characteristic of viruses is that they are obligate intracellular parasites, meaning they require a host cell to carry out replication.