Act 1, Scene 3: Antonio signs the bond with Shylock for 3000 ducats, simultaneously signing his fate; as the collateral for not paying is a pound of Antonio's flesh.
Act 2, Scene 6: Jessica elopes with Lorenzo, the Christian, betraying her own blood and nation because of Shylock.
Act 3, Scene 1: Shylock gives his revenge speech to the Christians, illustrating how Jews and Christians aren't all that different.
Act 4, Scene 1: Shylock takes Antonio to court, because he didn't pay him back in 3 months. Portia also gives the mercy speech to convince Shylock to not take the flesh.