In sickle cell disease: September 2007

Ans. B: Glutamtic acid, at position No.6 of beta-globin chain of haemoglobin is replaced by valine Sickle haemoglobin (HbS) is a structural variant of haemoglobin in which glutamic acid, an amino acid, at position No.6 of beta-globin chain of haemoglobin is replaced by valine. This happens due to change of a nucleotide, adenine to thymine (GAGgGTG) of codon 6 of beta-globin gene. This substitution of amino acid changes the net charge of haemoglobin, oxygen affinity and three-dimensional structure thus rendering it as unstable haemoglobin. Sickle haemoglobin gets polymerized at low oxygen tension and deforms the red blood cell from discoid shape to sickle like (crescent) form