Which of the following do viruses typically require in order to infect and replicate within a host?

• A. A living host cell
• B. Free ribosomes in the environment
• C. Autonomous metabolic enzymes
• D. Photosynthetic pigments

Answer: (A)

Verified step by step guidance

1. Understand that viruses are obligate intracellular parasites, meaning they cannot carry out metabolic processes or reproduce on their own outside a host cell.
2. Recognize that viruses lack the cellular machinery necessary for independent replication, such as ribosomes or metabolic enzymes.
3. Recall that viruses rely on the host cell's machinery, including ribosomes and enzymes, to synthesize viral components and assemble new virus particles.
4. Evaluate each option: free ribosomes in the environment cannot support viral replication because viruses need the organized environment of a living cell; autonomous metabolic enzymes and photosynthetic pigments are not features viruses possess or require.
5. Conclude that the essential requirement for viral infection and replication is a living host cell, which provides the necessary machinery and environment for the virus to reproduce.