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Chemistry
In chemistry, a dimer refers to a molecule composed of two similar subunits or monomers linked together. It is a special case of a polymer. It can refer to halide chemistry, involving halogen bonding. Its more common usage refers to dimers as 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.
Biology
In biology, particularly molecular and cellular biology, dimers are most often observed in signaling. They are crucial to understandings in biochemistry as well. In this case, a dimer is a protein complex made up of two subunits that are not necessarily covalently linked. In fact, they may initially be monomer proteins. These monomers will dimerize, or join together, upon the binding of a factor to the receptor of each monomer.
An example of this dimerizing activity involves the RAS-independent receptor tyrosine kinase that activates Phospholipase C-gamma. When a growth factor binds to two monomeric Epithelial Growth Factor (EGF) receptor (or Platelet-Derived Growth Factor (PDGF) receptor), the receptors will dimerize and phosphorylate each other at the SH2 binding domains on the cytoplasmic portion of the receptor. The Phospholipase C-gamma isoform has SH2 domains that bind to the newly phosphorylated SH2 binding domain of the dimerized growth factor receptors. Upon binding, the Phospholipase C-gamma will be activated, and will be close to the membrane phospholipid that it is designed to cleave.
In a homodimer the two subunits are identical, and in a heterodimer they differ (though they are often still very similar in structure).
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