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.
Quick Answer. Chemical reactions occur because different types of molecules interact with each other to create new substances. Many of these interactions happen naturally and are not man-made. When one molecule interacts with another one, a chemical reaction occurs due to the breaking of old bonds and the forming of new ones.