**Core Concept**
In sickle cell anemia, a specific mutation in the beta-globin gene leads to the substitution of glutamic acid (glutamine's neutral form) with valine, causing hemoglobin S (HbS) to polymerize under deoxygenated conditions, resulting in the characteristic sickling of red blood cells.

**Why the Correct Answer is Right**
This substitution occurs due to a point mutation in the beta-globin gene, where glutamic acid's codon (GAG) is replaced by valine's codon (GTG) in the hemoglobin beta chain. This missense mutation leads to the incorporation of valine instead of glutamic acid, changing the protein's structure and function. As a result, the hemoglobin molecule becomes more prone to polymerization under conditions of low oxygen tension, leading to the characteristic sickling of red blood cells in sickle cell anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** A nonsense mutation would result in a premature stop codon, leading to a truncated protein product, which is not the case in sickle cell anemia.
**Option C:** The beta chain is not degraded in sickle cell anemia; rather, the mutation leads to an abnormal protein that causes the disease.
**Option D:** There is no deletion of the beta chain in sickle cell anemia; the mutation is a point mutation, not a deletion.

**Clinical Pearl / High-Yield Fact**
Sickle cell anemia is a classic example of a genetic disorder caused by a single point mutation, highlighting the importance of precise DNA sequencing in diagnosing and understanding genetic diseases.

**✓ Correct Answer: B. Missense mutation of beta chain**