Final answer:
A change in amino acids in the hemoglobin protein causes red blood cells to change shape in sickle cell anemia and block blood flow. The altered shape is caused by a substitution of valine for glutamic acid in the hemoglobin B chain.
Step-by-step explanation:
In sickle cell anemia, a change of one amino acid in the hemoglobin B chain causes a change in the structure and function of the protein. Specifically, the amino acid glutamic acid is replaced by valine in the sixth position of the two beta-chains.
This change leads to the formation of long fibers in hemoglobin molecules, which distort the shape of the red blood cells and cause them to assume a crescent or "sickle" shape. This altered shape can result in the blockage of blood vessels, leading to various health problems.