The origin of replication (also called the replication origin) is a particular sequence in a genome at which replication is initiated.
It is the interaction of two or more substances, resulting in chemical changes in them. Chemical reactions take place when some kind of energy is supplied to them. The supplied energy may be in the form of heat, light or mechanical shaking for proper contact between the reactant molecules or electricity.
If the activation energy of a reaction is high, then it requires a large amount of energy to initiate the reaction. The situation arising when a spontaneous reaction has a large activation energy is similar to rolling a ball over a hill.