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In chemistry, a dimer is a molecule composed of two similar subunits or monomers linked together. It is a special case of a polymer. Among the most common dimers are certain types of sugar; sucrose, for example, is a dimer of a glucose molecule and a fructose molecule.
A physical dimer is a term that designates the case where intermolecular interaction brings two identical molecules closer together than other molecules. There are no covalent bonds between the physical dimer molecules. Acetic acid is such a case where hydrogen bonds provide the interaction.
In biology, a dimer is a protein complex made up of two subunits. In a homodimer the two subunits are identical, and in a heterodimer they differ (though they are often still very similar in structure). The subunits do not need to be covalently linked, and usually aren't.