Which in vitro can divide forever?

• A. Primary cells
• B. In Vivo tests
• C. Immortalized cells
• D. Clinical trials

Answer: (C)

Immortalized cells can divide indefinitely in culture because they’ve acquired changes that bypass normal senescence and keep the cell cycle running. In standard lab cultures, most cells are primary cells with a finite replicative lifespan; they reach a limit (the Hayflick limit) as telomeres shorten and checkpoints trigger senescence or apoptosis. Immortalized cells often maintain telomere length or disable key regulatory pathways (like p53 or RB), allowing continual division. In vitro refers to lab culture, while in vivo tests and clinical trials refer to studies inside living organisms or humans, not endless cell division in culture. So the ability to divide forever in culture comes from immortalization.