Sickle cell anaemia is

Options

A. Caused by substitution of valine by glutamic acid in the beta globin chain of haemoglobin
B. Caused by a change in a single base pair of DNA
C. Characterized by elongated sickle like RBcs with a nucleus
D. An autosomal linked dominant trait

Answer

### Step-by-Step Solution:

1. **Understanding Sickle Cell Anemia**: Sickle cell anemia is a genetic disorder that affects the shape and function of red blood cells (RBCs). In this condition, the RBCs become rigid and take on a sickle or crescent shape.

2. **Genetic Basis**: The disorder is caused by a mutation in the beta-globin gene, which is located on chromosome 11. This gene is responsible for producing a component of hemoglobin, the protein in red blood cells that carries oxygen.

3. **Type of Mutation**: The mutation that leads to sickle cell anemia is a single point mutation. This means that a single nucleotide in the DNA sequence is changed. Specifically, the codon GAG, which normally codes for the amino acid glutamic acid, is mutated to GUG, which codes for valine.

4. **Effect on Hemoglobin**: The substitution of valine for glutamic acid alters the properties of hemoglobin. Normal hemoglobin (HbA) is soluble and allows red blood cells to maintain a round shape. However, the mutated hemoglobin (HbS) tends to polymerize under low oxygen conditions, leading to the sickle shape of the red blood cells.

5. **Clinical Implications**: The sickle-shaped cells can block blood flow in small vessels, leading to pain and potential organ damage. This condition is inherited in an autosomal recessive pattern, meaning that an individual must inherit two copies of the mutated gene (one from each parent) to exhibit the disease.

### Summary:
Sickle cell anemia is caused by a single point mutation in the beta-globin gene on chromosome 11, leading to the substitution of valine for glutamic acid in hemoglobin, resulting in sickle-shaped red blood cells.

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