A point mutation is a type of genetic mutation in which a change occurs at a single nucleotide base pair in the DNA sequence. This change may involve substitution, insertion, or deletion of a single nucleotide. Even though the change is very small, it can sometimes lead to significant alterations in the structure and function of proteins produced by the gene.
One of the most well-known examples of a point mutation is Sickle Cell Anemia, a hereditary blood disorder. In this condition, a mutation occurs in the gene that codes for the beta chain of hemoglobin, the oxygen-carrying protein present in red blood cells. Normally, the sixth codon of the beta-globin gene codes for the amino acid glutamic acid.
In sickle cell anemia, a single base substitution occurs in the DNA sequence where the codon GAG is changed to GTG. Because of this mutation, the amino acid valine is produced instead of glutamic acid at the sixth position of the beta-globin chain. This small change alters the structure of hemoglobin and results in the formation of abnormal hemoglobin called hemoglobin S (HbS).
The abnormal hemoglobin causes red blood cells to become sickle-shaped instead of their normal round shape. These sickle-shaped cells are less efficient at carrying oxygen and may block blood vessels, leading to symptoms such as anemia, pain, and reduced oxygen supply to body tissues.
Thus, point mutation refers to a change in a single nucleotide of DNA, and sickle cell anemia is a classic example where a single base substitution in the beta-globin gene results in the production of abnormal hemoglobin and distorted red blood cells.