Viruses are described as obligate intracellular parasites because they
Correct answer: D. lack the ribosomes and enzymes needed to make their own proteins and ATP
- A. are smaller than any known bacterium
- B. carry both DNA and RNA in the same particle
- C. have a cell wall made of peptidoglycan
- D. lack the ribosomes and enzymes needed to make their own proteins and ATP
Explanation
A virus particle has nucleic acid and protein but no ribosomes, no respiratory enzymes and no ATP generating system, so it can only replicate by taking over a living host cell's machinery. Small size alone does not force parasitism, since many free living bacteria are smaller than the largest viruses. A virus also never carries both DNA and RNA as its genome, and it has no cell wall of any kind.
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About Viruses
Viruses are acellular infectious particles made of genetic material enclosed by a protein capsid, sometimes with a lipid envelope. Their replication depends on a host cell and may follow lytic or lysogenic cycles, so viral structure and reproduction differ fundamentally from those of cellular organisms such as bacteria.
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