from: http://sickle.bwh.harvard.edu/scd_background.html
The top diagram is for normal hemoglobin; the bottom diagram is for the sickle cell mutant-type molecules. This simple change alters the behavior of hemoglobin molecules when under low oxygen conditions. The Hs S form polymer-like molecules.
A single base change in the DNA results in one altered amino acid in one of the types of hemoglobin subunits.
A graphics model of the abnormal polymer that forms.
The top graphic is for normal hemoglobin; the bottom is for sickle cell disease. It is fascinating that a single change in one amino acid can cause a change in hemoglobin functioning and also affect cell shape.