A monomer (from Greek mono "one" and meros "part") is an atom or a small molecule that may bind chemically to other monomers to form a polymer.; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex. The most common natural monomer is glucose, which is linked by glycosidic bonds into polymers such as cellulose and starch, and is over 76% of the weight of all plant matter. Most often the term monomer refers to the organic molecules which form synthetic polymers, such as, for example, vinyl chloride, which is used to produce the polymer polyvinyl chloride (PVC).
Amino acids are natural monomers that polymerize at ribosomes to form proteins. Nucleotides, monomers found in the cell nucleus, polymerize to form nucleic acids – DNA and RNA. Glucose monomers can polymerize to form starches, glycogen or cellulose; xylose monomers can polymerise to form xylan. In all these cases, a hydrogen atom and a hydroxyl (-OH) group are lost to form H2O, and an oxygen atom links each monomer unit. Due to the formation of water as one of the products, these reactions are known as dehydration or condensation reactions.
Isoprene is a natural monomer and polymerizes to form natural rubber, most often cis-1,4-polyisoprene, but also trans-1,4-polyisoprene..
The lower molecular weight compounds built from monomers are also referred to as dimers, trimers, tetramers, pentamers, octamers, 20-mers, etc. if they have 2, 3, 4, 5, 8, or 20 monomer units, respectively.  Any number of these monomer units may be indicated by the appropriate Greek prefix; e.g. a decamer is formed from 10 monomers. Larger numbers are often stated in English or numbers instead of Greek. Molecules made of a small number of monomer units, up to a few dozen, are called oligomers.
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