Question

The most striking example of point mutation is found in a disease called
A. Night blindness
B. Thalassemia
C. Down syndrome
E. Sickle cell anaemia

Answer 1

Hint: Point mutation is a type of mutation that occurs at molecular level. Abnormal changes in genetic material are known as mutation. In point mutation changes occur in a single base pair of DNA.

Step by step answer:Sickle cell anaemia is the best example for point mutation (insertion or deletion occur in a single base pair. In this disease in the globin protein of our respiratory pigment ‘haemoglobin’ a defect occurs. The gene that is responsible for the synthesis of the beta-globin chain of haemoglobin is denoted as HbA. In the DNA of this gene, nucleated base thymine is substituted by adenine. DNA synthesizes mRNA and this mRNA has the information to synthesis peptides.
The sequence of amino acids in the peptide chain is determined by the sequence of bases of codons of mRNA. Due to this substitution in DNA, the mRNA synthesized has an abnormal codon as GUG instead of GAG. So the protein synthesized from this defected mRNA has valine amino acid at 6th position instead of glutamic acid. This affects the quality of haemoglobin and during less oxygen supply in the blood, haemoglobin is not able to carry oxygen. RBC of blood becomes sickle-shaped; this condition is called Sickle cell anaemia.
Hence option D is correct.

Note: The defective gene is denoted by HB^s . This is a recessive gene able to cause this disease expressed in homozygous condition. Gene for haemoglobin synthesis is present on an autosome. So this disease is also known as Autosomal recessive disease.