This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
In sickle cell disease: September 2007 
Select and explain. 
A. Glutamtic acid, at position No.5 of beta-globin chain of haemoglobin is replaced by valine, B. Glutamtic acid, at position No.6 of beta-globin chain of haemoglobin is replaced by valine, C. Valine at position No.6 of beta-globin chain of haemoglobin is replaced by glutamtic acid, D. Valine at position No.5 of beta-globin chain of haemoglobin is replaced by glutamtic acid
B: Glutamtic acid, at position No.6 of beta-globin chain of haemoglobin is replaced by valine
Answer: B: Glutamtic acid, at position No.6 of beta-globin chain of haemoglobin is replaced by valine 
Explanation: 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