The nucleotide triplet CTC in the sixth position of the l3-chain in DNA forms the complementary nucleotide on (mRNA) that codes for glutamic acid. A point mutation on the l3-chain resulting in the nucleotide triplet CAC forms a com¬plementary nucleotide on mRNA that codes for valine. In sickle cell anemia, you would expect the comple¬mentary nucleotide triplet on mRNA from 5' to 3' to read
Note that by changing the CTC triplet to a CAC, the messenger RNA (mRNA) changes from GAG, which normally codes for glutamic acid in the sixth position of the -chain of hemoglobin, to GUG, which now codes for valine. This point mutation of a single base pair is responsible for sickle cell anemia.
GAG
CTC
GTG
GUG