Can you explain to me how mutation take place in sickle-cell anemia?

Important gene...: Sickle cell anemia is caused by a mutation in the gene that tells your body to make hemoglobin. Hemoglobin allows red blood cells to carry oxygen from your lungs to all parts of your body. In sickle cell anemia, the abnormal hemoglobin causes red blood cells to become rigid, sticky and misshapen. These abnormal red blood cells lead to the various symptoms of the disease.