Sickle Cell Anemia

Sickle cell anemia is a disease that contains abnormal hemoglobin which means sickle hemoglobin. It affects red blood cells (RBC) as it causes the cells in a sickle shape.

Red blood cells are also called as erythrocytes that carry oxygen from lungs to rest of the body. Hemoglobin is the protein made by red blood cells. This pigment is red in colour that gives blood its red colour.

Hemoglobin carries oxygen from the lungs to rest of the tissues and returns carbon dioxide from the body tissues to the lungs again.

Sickle cell anemia affects hemoglobin and thus do not deliver oxygen to the tissues efficiently. Sickled red blood cells have difficulty binding to oxygen. Sickle shape red blood cells block capillaries their sickle or collapsed shape is not efficient.

Hb A/Hb A is tge genotype of normal person. Hb A/Hb S is the genotype of mild sickle cell anemia. Hb S/Hb S is the genotype of person with sickle cell anemia.

Sickle cell anemia is caused by single nucleotide mutation in which thymine is in place of an adenine. SNP is called as single nucleotide polymorphism. Hemoglobin contains heme group, two alpha chains (chromosome 16, q arm) and two beta chains (chromosome 11, q arm). SNP for sickle cell anemia occurs on the 17th nucleotide of the beta chain. SNP codes for valine instead of glutamic acid.

Sequence for Hb A is palindrome which is CCTGAGG. Sequence for Hb S is not a palindrome CCTGTGG. Hb A codes for glutamic Acid and HbS codes for valine. Mst II is the restriction enzyme which is used to detect Hb S. Mst II cuts at CCT(N)AGG–>A
where, N can be either be A or T that depends on the genotype.

Plasmodium causes malaria and it spends most of its life cycle in red blood cells. But person with sickle cell anemia can either have mild or no malaria because plasmodium will not able to reproduce in sickle-shaped red blood cells.

Sickle cell less severe in people of Indian descent compared to those of African descent. The people of Indian descent contain 20- 30% fetal hemoglobin which resists sickling. This is because Fetal hemoglobin (HbF) change the phenotype of sickle cell anemia by inhibiting the deoxy sickle hemoglobin polymerization. Therefore, Fetal hemoglobin is not sickle shaped and able to transport oxygen efficiently.

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