A chemical reaction in which two or more relatively simple molecules combine chemically to form larger molecules containing repeating structural units of the original molecules. The small molecules are monomers (molecules that react to other molecules of the same or different compound to form a larger molecule such as polymer) and the larger ones are polymers (long chain or network molecules made up of repeated simple units or monomers). Natural monomers include hydrocarbons and amino acids. Natural polymers (or biopolymers) include proteins, polysaccharides and nucleic acids. The two molecules are linked or bonded together by polymerization that can be defined as the combination of many same or different molecules to form a product of higher molecular weight. When the combination results in the elimination of water, alcohol and the like, it is referred to as condensation polymerization. Without such elimination, it becomes addition polymerization. In the latter, monomers combine together with their structure remaining unchanged. In contrast, condensation polymerization results in a polymer that that is less massive than the two or more monomers that combined to form it. It is through this process that that amino acids link up to form proteins. There is another distinction that accounts for how a polymer is formed: chain growth polymerization (one monomer added at a time until a polymer is formed) and a more complicated process known as step-growth polymerization (at least two different polymers participate in the reaction between between dissimilar chemical compounds that are part of the monomer molecules). Naturally occurring polymeerization is a fundamental biological process, and without it brains, for example, could not be formed and develop.
See Amino acids, Carbohydrates, Filapodia, Growth cone, Myofibrils and myofilaments, Nucleic acid, Phosphorylation, Proteins