What causes sickle cell anemia, and how does a base substitution mutation affect mRNA and the polypeptide sequence in hemoglobin?

SouravNovember 8, 2024

Answer

Sickle cell anemia is a genetic disorder caused by a specific mutation in the gene that encodes for the beta chain of hemoglobin, the protein responsible for transporting oxygen in red blood cells. This condition arises from a base substitution mutation, where a single nucleotide change alters the DNA sequence.

Causes of Sickle Cell Anemia

Genetic Mutation

• The mutation responsible for sickle cell anemia involves a change from adenine (A) to thymine (T) in the DNA sequence of the beta-globin gene, specifically changing the codon from GAG to GTG. This alteration occurs on the non-template strand, while the corresponding change on the template strand is from CTC to CAC.
• This single nucleotide change results in a different mRNA codon during transcription, leading to a significant alteration in the amino acid sequence of hemoglobin.

Inheritance Pattern

• Sickle cell anemia follows an autosomal recessive inheritance pattern, meaning that an individual must inherit two copies of the mutated gene (one from each parent) to develop the disease. If only one copy is inherited, the individual will have sickle cell trait but typically will not exhibit symptoms.

Effects of Base Substitution Mutation on mRNA and Polypeptide Sequence

Impact on mRNA

• The base substitution mutation changes the DNA sequence, which subsequently alters the mRNA transcript. Specifically:
  • The original DNA sequence (GAG) transcribes to mRNA as CUG.
  • After mutation, the new DNA sequence (GTG) transcribes to mRNA as CUG, which codes for valine instead of glutamic acid.

Impact on Polypeptide Sequence

• The amino acid change occurs at position six of the beta-globin polypeptide chain:
  • Normal hemoglobin has glutamic acid (Glu) at this position.
  • In sickle cell hemoglobin, this is replaced by valine (Val) due to the mutation.
• This substitution significantly affects the structure and properties of hemoglobin:
  • The presence of valine, a hydrophobic amino acid, causes hemoglobin molecules to aggregate under low oxygen conditions, forming long, rigid fibers that distort red blood cells into a sickle shape.
  • These sickle-shaped cells are less flexible and can obstruct blood flow in small vessels, leading to various complications such as pain crises, increased risk of infections, and chronic anemia due to rapid cell destruction