Amino Acids – Biological Molecules Ep 3

Amino acids are the building blocks of proteins. In this series we have already seen how nucleotides are the monomers joined together to make polynucleotide chains in DNA and RNA. Similarly, in proteins, amino acids are the monomers joined together to make polypeptide chains.


Firstly, let’s look at the structure of an amino acid. There are actually 20 different amino acids (see list at the bottom of the page), but they all have the same basic structure. On one side there is an amine group (also called an amino group) – a nitrogen atom covalently bonded to two hydrogen atoms. On the other side there is a carboxyl group (also called a carboxylic acid group) – a carbon atom with a double covalent bond to an oxygen atom and a single covalent bond to a hydroxyl group (OH).

An amino acid

Off the center carbon there is an R group. The R symbolises that the group attached here is variable. This is what gives the 20 amino acids unique structures. In the simplest amino acid (glycine) the R group is just one hydrogen atom. But in methionine for example, the R group contains three carbon atoms, seven hydrogen atoms and one sulfur atom.

Joining amino acids

To form a polypeptide chain, amino acids must be joined to each other with peptide bonds. This happens during translation at the ribosomes. Joining amino acids is a condensation reaction; for every peptide bond formed, a molecule of water is produced. (To break a peptide bond, a molecule of water is added – this is a hydrolysis reaction.) The diagram below shows how two amino acids are joined together to form a dipeptide (di = two), but the chain can be made as long as necessary. The two different functional groups go on reacting with each other to form a naturally occurring condensation polymer. With 20 amino acids to choose from, you can see how almost infinite combinations of different lengths are possible.

Two amino acids join to form a dipeptide

The amino acid sequence is the primary structure of a protein. Polypeptide chains are folded and processed before becoming the final active protein. We will take a closer look at protein structure later on in the series.


  • Amino acids are the monomers of polypeptide chains (proteins).
  • There are 20 amino acids (listed below).
  • They have an amino group, a carboxyl group, and a variable R group.
  • They can join together with peptide bonds – this involves a condensation reaction which produces water.

Full list of the 20 common amino acids: alanine, isoleucine, leucine, methionine, valine, phenylalanine, tryptophan, tyrosine, asparagine, cysteine, glutamine, serine, threonine, aspartic acid, glutamic acid, arginine, histidine, lysine, glycine, proline.

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