A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes may both occur.
The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions, and the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which graphically present the starting materials, end products, and sometimes intermediate products and reaction conditions.
All living organisms are dependent on three types of very large molecules for essentially all of their biological functions. These molecules are DNA, RNA and proteins, and are classified as biological macromolecules. Without DNA, RNA and proteins, all known forms of life could not exist. This is because each molecule plays an indispensable role in biology. DNA is an informational macromolecule that encodes the complete set of instructions (the genome) that are required to assemble, maintain, and reproduce every living organism. Proteins are responsible for catalyzing the myriad biochemical reactions that are required to provide food and energy for every organism, and to carry out all of the additional functions of any given organism, for example movement, neural function, vision, or photosynthesis. RNA is multifunctional, but its primary responsibility is to make proteins, according to the instructions encoded within a cell’s DNA. The simple summary is that DNA makes RNA, and then RNA makes proteins.
While many typical cellular molecules (for example sugars and fats) contain tens, or rarely hundreds, DNA, RNA and proteins are typically composed of thousands of atoms (millions for most DNA molecules).