Non-cellular life

Biological entities such as viruses and prions that lack cellular organization. Whether they are alive is disputed because they depend on cells for reproduction or other life processes.

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Cell theory: The principle that cells are the basic units of life and that cells arise from existing cells. Its definition of life’s basic unit makes acellular entities a direct challenge.
Viral replication: The process by which a virus uses host-cell machinery to make viral genomes and components. It explains how viruses reproduce without carrying out independent cellular division.
Cellular life: Organisms organized into cells, the smallest units capable of independent life processes. It is the cellular counterpart against which acellular entities are defined.
Virus evolution: The study of how viruses originated and diversified through evolutionary change. Its competing origin hypotheses bear directly on whether viruses arose from cellular life.
Virus: An infectious agent whose genome replicates inside host cells and is packaged in a protein coat. Viruses are the best-known entities whose cellular status and living status are disputed.
Bacteriophage: A virus that infects bacteria and reproduces using bacterial cells. Phages make the dependence of viral reproduction on living cells especially clear.
Life: The condition associated with organized systems that maintain themselves, reproduce, and evolve. The disputed boundary of life determines whether acellular entities belong within it.
Escape hypothesis: A hypothesis that viruses evolved from genetic elements that escaped from cells. It proposes a cellular source for at least some viral lineages.
Prion: An infectious protein that propagates by inducing a misfolded shape in corresponding proteins. Prions transmit biological information without cells or nucleic acids.
Prion replication: The propagation of abnormal protein shapes as misfolded proteins convert matching proteins. This mechanism reproduces an infectious state without genomes or cells.