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A cell. (TV: The Lie of the Land)
Cells were a part of many living organisms' bodies.
The Doctor once explained, that amino acids would fuse to form cells, which would eventually evolve into plants and animals. (TV: City of Death)
A collection of cells were a cell culture. (COMIC: Culture Shock)
The Dalek Emperor claimed that only one human cell in a billion was fit to be nurtured to create Daleks. (TV: The Parting of the Ways)
As the Tenth Doctor explained to Rose Tyler, regeneration changed "every single cell" in his body. (TV: Born Again) The Ninth Doctor was forced to regenerate after absorbing the energy of the Time Vortex, which caused every cell in his body to die. (TV: The Parting of the Ways)
ARC, a companion of the Eleventh Doctor, did not have cells in his body. (COMIC: The Infinite Astronaut)
The process of progenation involved taking a sample of diploid cells, splitting them into haploids, recombining them in a different arrangement and growing the result into an offspring. (TV: The Doctor's Daughter)