In the recessive condition in humans known as sickle-cell anemia, the beta-globin polypeptide of hemoglobin is found to be abnormal. The only difference beteween it and the normal beta-globin is that the sixth amino acid from the N-terminal end is valine, whereas the normal beta-globin has glutamic acid at this position. Please explain how this occurred in terms of differences in the DNA and the mRNA.