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.
Reactions occur all the time. However, certain prequisites must be fulfiled before a reaction can occur. You cannot say, expect a reaction to occur between one substance and another if you keep both of them separated. Therefore, one theory that discusses how reactions occur is The collision theory. In the collision theory, for the reaction to occur reactants involved together must: Collide with the right orientation. Collide with sufficient energy.