Regeneration

The Fifth Doctor regenerates in The Caves of Androzani.

Regeneration is the process by which the Time Lords maintain their centuries-long lifespans. It is generally described as a "genetic reshuffling" which a Time Lord may go through if his or her body is worn out by age or has suffered some life-threatening injury or infection. Time Lords may even regenerate at will.

Control over Regeneration

While the Doctor apparently has never had much choice in what the finished product looks like, others, like Romana, seem adept enough at the process to custom design their new appearances, such as when she went through several bodies before finally deciding on a copy of Princess Astra. (DW:Destiny of the Daleks)

This is most likely because all the Doctor's regenerations have to date been involuntary, the result of mortal injuries or, in one case, having a regeneration forced upon him by the Time Lords prior to his exile on Earth.

Personality

After each regeneration there is a marked change in a Time Lord's personality. This is caused by the fact that regeneration is an all or nothing cellular regeneration. Simply put, a Time Lord can only regenerate every cell in their body at the same time (though they can regrow body-parts within 15 hours of the regeneration) (DW: The Christmas Invasion)

The process isn't perfect, however, and the genetic equivalent of 'bit errors' appear in the DNA of the regenerated cells. This is what causes the appearance of the Time Lord to change. However, because even the cells of the brain regenerate as well, their brain chemistry and organization will change. Although the aspects of their personality caused by "nurture" will not change, the "nature" contribution to their personality will change.

So a regenerated Time Lord will share the same core moral values as they did pre-regeneration but may be, for example, more likely to lose their temper (or vise versa), or more depressive (or vise versa) or more energetic (or vise versa) depending on the new balance of hormones and neural arrangements in their brain.

Regenerations can also cause psychological "break" points where the regenerated Time Lord assimilates their past experiences and evolves from it in a sudden jump rather than gradually.

For example, the Second Doctor's primary attribute is excitability and youthful enthusiasm -- possibly a reaction to being trapped in a fading and sore body for the previous couple of centuries. The Fourth Doctor seems to dismiss vanity and arrogance from his life -- possibly because those were the character flaws that ultimately killed his previous incarnation. The Sixth Doctor seems much more controlling and resentful of his companion -- possibly because he, at some level, sees his previous "death" as her fault.

The Doctor's Regenerations

Following is a list of how each Doctor has regenerated, thus far.

1. First Doctor: apparently succumbs to old age. (DW: The Tenth Planet)
2. Second Doctor: a forced regeneration and exile to Earth by the Time Lords. (DW: The War Games)
3. Third Doctor: radiation poisoning from the Great One's cave of crystals on Metebelis III. (DW: Planet of the Spiders)
4. Fourth Doctor: falls from the Pharos Project radio telescope. (DW: Logopolis)
5. Fifth Doctor: spectrox toxaemia. (DW: The Caves of Androzani)
6. Sixth Doctor: injured as The Rani attacks the TARDIS (DW: Time and the Rani)
7. Seventh Doctor: dies on the operating table while undergoing surgery for gunshot wounds. (DW: Doctor Who: The TV Movie)
8. Eighth Doctor: unknown as of yet, but presumably as a result of the Time War.
9. Ninth Doctor: cellular degeneration caused by absorbing the energies of the time vortex from Rose, which she in turn had absorbed from the heart of the TARDIS. (DW: The Parting of the Ways)

The Eighth Doctor unofficially regenerated into the Alternative Ninth Doctor, also presumably due to injuries caused by the Time War.

Limitations

It was established that a Time Lord can regenerate twelve times before permanently dying, though as with most such "rules" there were occasionally exceptions. (DW:The Deadly Assassin) It is possible that the twelve-regeneration limit is maintained by convention, but can be extended in exceptional circumstances. The Master is offered "Regeneration: a whole new life cycle" as an incentive to rescue the four Doctors from the Death Zone. (DW: The Five Doctors)

In some circumstances, a Time Lord may be unable to regenerate, despite having an ample number of lives still available. The Elixir of Life, distilled by the Sisterhood of Karn from their Sacred Flame, can help in these situations. (DW: The Brain of Morbius)

The Doctor's arch-enemy, the Master, found his own remedy for the end of his regenerative cycle. He first attempted and failed to use the Eye of Harmony on Gallifrey to renew himself, though he was able to absorb enough energy from it to extend his life for a time. (DW: The Deadly Assassin) He was finally able to gain access to the Source on Traken, and used some its powers to steal the body of Nyssa's father Tremas. (DW: The Keeper of Traken) When this body was finally destroyed by the Daleks on Skaro, the Master's essence entered that of a human on Earth, and in this body the renegade Time Lord tried to use the powers of the Eye of Harmony's dimensional link on the Doctor's TARDIS to steal the Doctor's remaining lives. (DW: Doctor Who: The TV Movie)

Rassilon is said to have achieved a cycle of perpetual regeneration, becoming immortal. It was for this secret that Lord President Borusa sent four of the Doctor's incarnations and their companions into the Death Zone, where Rassilon lay in eternal sleep in the Dark Tower. (DW: The Five Doctors) It may also have been this particular method of perpetual regeneration which Mawdryn and his team of scientists attempted to steal from the Time Lords, with the result that they became undying creatures who continually aged and regenerated until the Doctor was able to finally help them die. (DW: Mawdryn Undead) Legends suggest that Rassilon became immortal when he was turned into a vampire by the Great Vampire himself. (DW: Goth Opera)

Science

The exact mechanism that makes regeneration possible has not been stated, though many theories have been made. These varying explanations may or may not be compatible with each other.

• One explanation was Cardinal Rassilon had been investigating a method of regenerating decaying and diseased tissue via a series of permanently carried self-replicating biogenic molecules. The cells of a Gallifreyan body can be repaired, restored and reorganised. This will result in a wholly new physical form. The brain cells would similarly be rearranged, though to a lesser degree, thus ensuring the new incarnation will replicate the memories and personality of the former incarnation. Cardinal Rassilon intended for this mechanism only to be used upon the Gallifreyan elite. He also inputed a parameter of 12 regenerative cycles to avoid decaying biogenic molecules. (DW: Zagreus)

• Another theory attributes regeneration to a "nanomolecular virus" that rebuilds the body much like the "self-replicating biogenic molecules". (DW: The Gallifrey Chronicles)

• A third theory is that Time Lords have triple-helix DNA: the third strand was added by Rassilon to make regeneration possible. (DW: The Crystal Bucephalus)

• Another explanation is that regeneration may be linked to what is known as the "Rassilon Imprimatur", the symbiotic nuclei of a Time Lord that bonds him or her to a TARDIS, and allows his or her body to withstand the molecular stresses of time travel.

Development of the Concept

Regeneration was first introduced when the First Doctor (William Hartnell) changed into the Second Doctor (Patrick Troughton) in the closing moments of "The Tenth Planet." Tradition has it that Hartnell himself thought up the idea of the Doctor being able to change into a new body as a means of keeping the series going after his departure. In "Power of the Daleks" (the first story featuring the Second Doctor), the Doctor described himself as having been "renewed", and also said that the change was "part of the TARDIS."

The process was not actually referred to as "regeneration" until the end of "Planet of the Spiders," when the Third Doctor (Jon Pertwee) changed into the Fourth (Tom Baker). Prior to this, the Doctor was simply described as having "changed his appearance."