Resemble small bacteria, but like viruses they can only multiply by invading the cells of another life form.

Rickettsia

Explanation:

Rickettsia are usually considered as a separate group of bacteria. These microorganisms which multiply by binary fission inside their host cells are a group of obligately intracellular gram-negative mircoorganisms, having cell walls that look exactly as that of a typical bacteria cell wall. They possess no flagella. They are carried as parasites such as lice and tcks, and also cause diseases such as typhus.

it has one origin of replication, but it proceeds in two directions - d)

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