Regarding Chlamydiae, which one of the following is MOST accurate?

  •   Their life cycle consists of a metabolically inactive particle in the extracellular phase.
  •   They can replicate only within cells because they lack the ability to produce certain essential mRNAs.
  •  They are gram-positive rods that do not form spores.
  •  They exhibit swarming motility on a blood agar plate.
  •   They replicate in the nucleus of infected cells, where they form inclusions that are useful diagnostically.