In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids. It is a quaternary structure of a protein.
A homo-dimer would be formed by two identical molecules (process called homodimerization). A hetero-dimer would be formed by two different macromolecules (process called heterodimerization).
Most dimers in biochemistry are not connected by covalent bonds with the exception of disulfide bridges. An example of this would be the enzyme reverse transcriptase, which is made of two different amino acid chains.[1]
Some proteins contain specialized domains to ensure dimerization (dimerization domains).