BACKGROUND INFO: For purposes of genetic counseling, you wish to determine the h
ID: 18121 • Letter: B
Question
BACKGROUND INFO:For purposes of genetic counseling, you wish to determine the hemoglobin genotypes for a couple who both have family histories of sickle cell disease.
Since hemoglobin S - Hb S - and hemoglobin A - Hb A - have different net charges, if each is placed in an electrical field, the two types of hemoglobin proteins will migrate differently. (Note: Separation of molecules via this type of electrophoresis - unlike the gel electrophoresis considered in Unit 16 - depends on the different net electrical charge (not the size – which is essentially the same for both hemoglobin proteins.)
QUESTION:
Will Hb A or Hb S move further toward the positive pole upon electrophoresis? Hint: Recall the difference in charge due to the point mutation substituting valine for glutamate in Hb S.
Explanation / Answer
Hemoglobin S now carries valine instead of glutamic acid. The valine has more of a NEUTRAL charge in comparison with the glutamic acid which has a NEGATIVE charge. Electrophoresis separates molecules on the basis of their size and charge. Negatively charged molecules move faster as they start on the negative side and move towards the positive (they are attracted to the positive charge). As these molecules are both hemoglobin - roughly the same size - it comes down to the charge. Hence. The normal Hemoglobin - - - - HgA will move faster towards the positive electrode due to the glutamic acid. Remember - the purpose of all this electrophoresis is simply to GET them to have some kind of separation - this test can help one identify a person with normal hemoglobin, a sickle cell carrier, or a person that has sickle cell anemia (each electrophoretic profile will be different).
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