A pressure-induced laminar separation bubble developing on a flat plate in incompressible flow is studied by means of direct numerical simulation. These simulations shall lead to a better understanding of bursting of this bubble. Transition from laminar to turbulent flow in the shear layer occurs either unforced or is triggered by small disturbance input with a fixed frequency and fixed spanwise wave number. Starting from a fully attached boundary layer with impulsively imposed pressure gradient, the flow separates and eventually transition to turbulence occurs within the detached shear layer. The resulting short bubble is shown to be converged in time to a statistically steady state, while possessing essential features of a short laminar separation bubble. Disturbance input is required to maintain this short bubble. Switching-off disturbance input yields a continuously growing separation bubble that develops towards a long-bubble state. Two different forms of bubble bursting are observed, which are both simulated as a dynamic process. The first form can be observed during start-up and it is characterized by the change between a bubble with laminar and turbulent reattachment. The second form occurs after switching off disturbance input and it is characterized by the inability of the transition process to reattach the flow.