Questions: Which one of the following statements is a reason for NOT classifying viruses in one of the three domains of life (Bacteria, Archaea, Eukarya) rather than in a fourth separate domain? Some viruses can incorporate their genome into a host's genome. Viruses are not composed of cells. Viruses are obligate parasites. Viruses direct anabolic pathways of host cells.

The answer is the second one: Viruses are not composed of cells.

Explanation for each option:

1. Some viruses can incorporate their genome into a host's genome.

   • This is a characteristic of some viruses, but it is not a reason for not classifying them in one of the three domains. Other organisms can also integrate genetic material, such as through horizontal gene transfer.

2. Viruses are not composed of cells.

   • This is the correct reason. The three domains of life (Bacteria, Archaea, Eukarya) consist of cellular organisms. Viruses lack cellular structure, which is a fundamental criterion for classification within these domains.

3. Viruses are obligate parasites.

   • While viruses require a host to replicate, this characteristic alone does not exclude them from being classified within the domains, as some cellular organisms are also obligate parasites.

4. Viruses direct anabolic pathways of host cells.

   • This describes how viruses manipulate host cells for replication, but it is not a reason for their exclusion from the three domains.

In summary, the primary reason viruses are not classified within the three domains of life is that they are not composed of cells.