Carbon is a unique element with the remarkable ability to form strong, stable chemical bonds with other atoms. Each carbon atom can form four bonds with other atoms. (Sometimes, two atoms will form more than one bond between themselves making a double bond or even a triple bond). This bonding ability allows carbon atoms to form chains of almost unlimited length. These chains can be closed on themselves to form rings or may branch wildly. This gives virtually infinite variety to the kinds of molecules that carbon can form. Carbon containing compounds are called organic compounds.

Atoms of hydrogen and oxygen and less frequently nitrogen, sulfur, or phosphorous bond to the carbon skeleton in a variety of ways to form small but complex molecules.

The four major types of macromolecules found in living cells—carbohydrates, lipids, proteins, and nucleic acids--are made of these smaller, repeating subunits called monomers. The monomers within one molecule are not always identical but they always have similar chemical structures. Monomers are joined together by a series of chemical reactions in a process called polymerization to form large, complex molecules called polymers. Poly means many. Another prefix you will run into later in your study of DNA testing is oligo. An oligomer is also a molecular made of repeating subunits but fewer in number than a polymer. For example, an oligonucleotide is a short chain of nucleotides whereas a polynucleotide may have many, many nucleotide subunits. You may also run across the terms dimer, trimer, and tetramer—oligomers composed of two, three, and four monomers respectively.