According to Aristotle, the two most important elements of the tragedy and of its plot are peripeteia and anagnorisis. Does the play Doctor Faustus contain these elements?
Peripeteia could be described as a "reversal of fortunes" or a "turning point in a story"; it is some undoing of the truth as it is known. Anagnorisis is best described as "discovery" or "revelation." Thus, a combination of the two might include such tragic events as betrayal.
Doctor Faustus depicts the classic "deal with the devil" storyline, with a well-learned scholar exchanging his soul for a demonic servant and access to magical conjurations. What separates Faustus from other tragic heros is that he makes this choice not out of necessity, or trickery, but with full cognition. Faustus is perfectly aware that he is trading his soul for a few fleeting years of earthly power, and will be eternally damned afterward. Faustus is also repeatedly warned against this bargain, encouraged to repent, or to call on God; yet he never does so, always arguing that "what's done is done" and there's no point in trying.
While this may be a matter of opinion, I think that peripeteia and anagnorisis appear in Act V. Faustus is in his final hours on earth, and it is only now, at the last minute, that the full magnitude of his choices begin to weigh upon his conscience. Suddenly he realizes how long "eternity" really is. Suddenly his demonic servants taunt and torment him and promise him endless pain. Suddenly he sees the face of an angry God. The discovery is that Faustus finally knows what is in store for him, and what it truly means, and how poor of a deal he has struck with Lucifer. The great reversal or turning point is that he really does care more about his soul and salvation than earthly pleasures, but it is only now, at the last minute, that it truly is too late.