Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologyhardmcq1 pt

In sickle cell disease, valine replaces glutamic acid at position 6 of the beta-globin chain. Which outcome follows most directly from this substitution?

A.Transcription stops because the altered codon cannot be read by RNA polymerase.
B.A hydrophobic patch appears on the protein surface, so hemoglobin molecules stack into fibers that distort red blood cells.
C.Peptide bonds fail to form without glutamate, so the four subunits drift apart.
D.Oxygen binding becomes irreversible, so hemoglobin never releases oxygen to tissues.

Explanation

Core Concept

Glutamic acid normally contributes a charged, hydrophilic side chain at position 6 of the beta chain, while valine contributes a nonpolar hydrophobic one. Placing valine on the outer surface creates a sticky patch that fits into a pocket on another hemoglobin molecule. Repeated docking drives polymerization into stiff fibers under low-oxygen conditions, bending red blood cells into the sickle shapes that lodge in capillaries. One amino acid change in primary structure therefore propagates upward through higher-order structure to an organism-level phenotype.

Correct Answer

BA hydrophobic patch appears on the protein surface, so hemoglobin molecules stack into fibers that distort red blood cells.

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