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Regeneration is a biological ability exhibited by the Gallifreyan Time Lords, a race of humanoids originating on the planet Gallifrey. This process allows a Time Lord who has become too old or mortally wounded to undergo a transformation into a new physical form and a somewhat different personality.

Genetic background
Gallifreyan's have a Triple Helix DNA, comprising of 69 chromosomes divided into 23 homogeneous triads. Humans, by comparison, only have 46 chromosomes divided among 22 pairs of autosomes and one pair of sex chromosomes.

The extra third strand of Gallifreyan DNA was added by Rassilon to make Regeneration possible, and was created to extend the already amazing Gallifreyan lifespan. This strand stores the regenerative information within a Gallifreyan's Bioplasmic Field that surrounds their bodies. During regeneration this information is conveyed to permanently carried self-replicated biogenic molecules, which permeate a Time Lord body, via a bio-language called Nucleolingua Symbiotica.

Mechanism
Time Lord bodies are filled with self-replicating biogenic molecules. These biogenic molecules work ceaselessly to repair and prune damaged and malformed cells, thus giving a Time Lord amazing recuperative powers. Because of this it takes hundreds of years for a Time Lord to show signs of aging.

Biogenic molecules
The biogenic molecules are similar to symbiotic nuclei and gives Time Lords a symbiotic self-renewing cell structure. Their bodies will dispose of foreign objects and implants fairly quickly. By "retreating" into a healing trance most injuries (such as burns and diseases) can be healed in a day, although repairing broken bones or psychic trauma often takes considerably longer. While in the healing trance, the Gallifreyan will appear dead to most humanoids, but Gallifreyans can tell they're alive. Their body temperature will drop so low that frost sometimes even forms on the body. While extreme pain will automatically induce a healing trance some training is required to enter one voluntarily. They can also selectively regenerate specific organs while in the trance.

Process
Regeneration itself is "triggered" by the release of the hormone Lindos from a Time Lords Limbic Gland (automatically in response to a near-death crisis or when the body wears out), which instructs the biogenic molecules to rebuild the body and restructure the biodata of the subject cell by cell at the molecular level, using the information contained in the Bioplasmic Field. Through Regeneration, the cells of a Time Lord's body are repaired, restored and reorganized. This "genetic reshuffling" will result in a wholly new physical appearance.

To a lesser degree, even the brain cells will similarly be rearranged, causing the brain chemistry and synaptic organization to change. Because of the new balance of hormones, neurotransmitters and neural arrangements in their brain, there is a marked alteration in the Time Lord's collection of emotional, thought and behavioral patterns. So while a regenerated Time Lord will still have the same memories and core moral values as they did pre-Regeneration, there is a moderate to significant change in personality along with the physical.

At the first Regeneration the biogenic molecules are programmed to build into the new body a number of improvements and backup systems including a second heart and other ancillary systems.

Limits
It is very dangerous for Gallifreyans who haven't passed First Maturation (i.e. teenagers) to regenerate. For this reason it is usually forbidden for Gallifreyans to regenerate until they become a Time Lord or are 500 years old.

After 12 regenerations the biogenic molecules begin to decay naturally. Gallifreyans are limited to 13 bodies because their minds and bodies can only withstand the strain of twelve regenerations.