monomer
Meaning of monomer: The single unit a chain is built from, whether that chain is a plastic, a starch, a protein or a strand of DNA.
Definition of monomer
A monomer is a small molecule able to bond with others of its kind to build a long chain called a polymer. The units need not be identical: twenty different amino acids go into proteins and four different building blocks into DNA, while an industrial plastic may repeat one unit millions of times. Living cells usually join them by a reaction that releases a molecule of water.
How do monomers become a chain?
By reacting end to end, over and over. Inside cells the join is usually made by stripping a hydrogen from one unit and a matching group from the next, which releases a molecule of water and leaves a bond behind. Amino acids link this way into a protein, simple sugars link into starch and glycogen, and nucleotides link into DNA. Breaking a chain runs the same reaction backward by adding water, which is exactly what digestive enzymes do to food.
Are the units always the same?
No, and this is where short definitions mislead. A polymer built from one repeating unit certainly exists, and most industrial plastics are like that. Biological chains are not. Twenty amino acids are available for a protein and four bases for a strand of DNA, and the order they are placed in is the information the molecule carries. Change a single unit in a protein of hundreds and it can fold wrongly or stop working, which is how one altered letter in a gene produces disease. A chain of identical units carries no message at all.
Which monomers matter in the body?
| Monomer | Chain it builds | Role of the chain |
|---|---|---|
| Amino acid | Protein | Enzymes, structure, signaling |
| Nucleotide | DNA and RNA | Storing and reading information |
| Glucose | Glycogen and starch | Stored fuel |
| Vinyl chloride | PVC plastic | Tubing, bags and other equipment |
A second and looser use of the word turns up in protein science. Many proteins work only as pairs or clusters, and a single copy standing alone is described as the monomer, two joined copies as a dimer. Nothing has been polymerized there; the word simply means one unit. Context separates the two senses.
Used in a sentence
The chain is assembled one monomer at a time as the ribosome reads the message.
Good to know
The units in a biological polymer are not identical, contrary to a common shorthand, and it is the order in which different monomers are strung together that carries the information in DNA and sets the shape of a protein.
Word origin
Where “monomer” comes from
Greek monos, meaning single, joined to meros, meaning part, so the word names one part of a larger chain.