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.
No, many endothermic reactions occur only when heat is constantly added, like when you fry an egg. Heat must be applied throughout the entire process in order for the reactions that cook the eggs to continue.