How does solid phase peptide synthesis work

Solid phase peptide synthesis is performed from the C-terminus to the N-terminus of the peptide (opposite to direction in nature).

The general peptide synthesis reaction cycle is comprised of:

1. Incorporation of the first amino acid to the polystyrene solid support also known as resin. This amino acids is protected at the N-terminus to avoid unwanted reaction, usually with an Fmoc group.
2. Removal of N-terminal protection group of this amino acid.
3. Activation of C-terminal group of the next amino acid with protected N-terminus group.
4. Coupling reaction of these two amino acids.
5. Re-start of 2nd cycle to add another amino acid or cleavage of full-length peptide from the resin.