Final answer:
The fundamental difference between a virus and other mobile genetic elements is that viruses lack cellular components, are obligate intracellular parasites, and require a host cell for replication.
Step-by-step explanation:
The fundamental difference between a virus and other types of mobile genetic elements is that viruses are noncellular and lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane.
Instead, a virion (an individual virus) consists of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope made of protein and phospholipid membranes derived from the host cell.
Additionally, viruses are obligate intracellular parasites, meaning they must infect a living cell and utilize the host's cellular machinery to replicate and produce more virions. Other mobile genetic elements, such as transposons, can move within and between genomes but do not require a host cell for reproduction.