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.
In addition to heat, it's also common (although not necessary) for a combustion reaction to release light and produce a flame. In order for a combustion reaction to begin, the activation energy for the reaction must be overcome. Often, combustion reactions are started with a match or other flame, which provides heat to initiate the reaction.