...require a living host cell to survive.
In animals and plants, there are two pairs of chromosomes - one set from a male and the other set from a female. But occasionally, the original fertilized cell doesn't quite divide correctly and more than two sets arises. The fertilized cell continues to divide with the result that all the cells have the extra set of chromosomes. This happens much oftener in plants and the plant will be sterile and can't form seeds.
Answer:
protein-coupled receptors.
