Final answer:
Normal and sickle-cell polypeptides migrate differently in electrophoresis because of a point mutation causing a single amino acid change; this affects the protein's charge and conformation, altering its movement through the gel.
Step-by-step explanation:
Normal betaA polypeptide and sickle-cell betaS polypeptide products migrate differently in gel electrophoresis because they have different amino acid compositions due to a genetic mutation. In sickle cell anemia, the normal glutamic acid at the sixth position of the beta-globin chain is replaced by valine caused by a point mutation in the DNA. This alteration in primary structure leads to a change in the charge and shape of the hemoglobin protein, which affects its movement through a gel during electrophoresis.
A normal beta-globin chain will have a different electrical charge and conformation compared to the mutated sickle-cell form due to the different amino acid properties. Because the electric charge of the polypeptide chain influences its behavior in an electric field, the abnormal hemoglobin with the valine substitution moves differently when an electric current is applied during electrophoresis. This difference in migration patterns allows for the distinction between normal and sickle hemoglobin, which is crucial for diagnosing sickle cell anemia.