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.
In biology, particularly molecular and cellular biology, dimers are most often observed in signaling. They are crucial to understanding chemical reactions 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 monomeric proteins. These monomers will dimerize, or join together, usually upon the binding of a signal to the receptor of each monomer. These signals can be a growth factor, a phosphate group from Adenosine triphosphate (usually through a kinase protein), or a ligand.
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).