How is the base sequence changed in sickle cell anemia?

The correct option highlights a specific mutation in the base sequence related to sickle cell anemia, which is caused by a single nucleotide change in the beta-globin gene of hemoglobin. In this case, the change from GAG to GTG results in the substitution of the amino acid glutamic acid (Glu) with valine (Val) at the sixth position of the beta-globin chain.

This single point mutation alters the properties of the hemoglobin molecule, leading to a tendency for the hemoglobin to polymerize under low oxygen conditions, thus causing the characteristic sickling of red blood cells. This modification directly impacts the efficiency of oxygen transport and the overall health of individuals with this condition.

Understanding that sickle cell anemia is primarily caused by a specific nucleotide substitution in the genetic code—where the original sequence (GAG) is changed to (GTG)—is crucial for grasping how genetic mutations can lead to significant physiological changes and diseases.