Dry Joining

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Amino acids, in common with other molecules, can be joined together by removing a molecule of water from between them. For example, here are two glycine molecules. A hydrogen atom (a) is removed from one molecule, a hydrogen (b) and oxygen atom (c) from the other. The molecules then join together. The removed atoms join together to make a water molecule.
On the young Earth this probably happened when a pool containing amino acids dried under the Sun. In life today there is a special process that controls which amino acids are joined together and in what order.