Sickle-cell anemia is an inherited blood disorder in which one of the hemoglobin subunits is replaced with a different form of hemoglobin. Partial DNA sequences of the HBB gene for normal hemoglobin and for sickle-cell hemoglobin are shown in Figure 1.

Partial sequence for normal hemoglobin:

Partial sequence for sickle-cell hemoglobin:

Figure 1. Comparison of partial DNA sequences for normal hemoglobin and hemoglobin with a sickle-cell mutation

Which of the following best describes the type of mutation shown in Figure 1 that leads to sickle-cell anemia?

Question

Sickle-cell anemia is an inherited blood disorder in which one of the hemoglobin subunits is replaced with a different form of hemoglobin. Partial DNA sequences of the HBB gene for normal hemoglobin and for sickle-cell hemoglobin are shown in Figure 1.

Partial sequence for normal hemoglobin:

Partial sequence for sickle-cell hemoglobin:

Figure 1. Comparison of partial DNA sequences for normal hemoglobin and hemoglobin with a sickle-cell mutation

Which of the following best describes the type of mutation shown in Figure 1 that leads to sickle-cell anemia?

BlackTom AI · Accepted Answer

To begin, let's lay out the question in our own words and review the answer choices.
Option 1: Insertion. This would add one or more nucleotides into the DNA sequence, which often shifts the reading frame or adds extra amino acids. In the context of sickle-cell disease, the key change is not an insertion event but a specific base change, so this option does not describe the mutation responsible here.
Option 2: Deletion. A deletion removes one or more nucleotides from the sequence. While deletions can cause frameshifts or loss of function, the sickle-cell mutation is not described as a deletion; it is a base substitution that alters one codon, not a missing segment.
Option 3: Substitution. A substitution replaces one nucleotide with a different nucleotide. In sickle-cell anemia, the replacement of adenine with thymine (in the DNA sense) changes one codon in the beta-globin gene, resulting in a different amino acid in the hemoglobin protein. This type of change is characteristic of a single-base substitution, which can alter a codon and, consequently, the protein sequence.
Option 4: Frameshift. Frameshift mutations occur when nucleotides are inserted or deleted in numbers not divisible by three, shifting the reading frame of the codons downstream and usually producing a drastically different protein. The sickle-cell mutation does not involve a reading-frame shift; it changes a single amino acid without altering the downstream frame.

With those distinctions in mind, consider which option best matches the described mutation in the figure. The different hemoglobin variants arise from a specific single-base change in the HBB gene, altering one codon and hence one amino acid, rather than adding or removing nucleotides or shifting the entire reading frame. This aligns with a base-substitution event, not an insertion, deletion, or frameshift.

Therefore, the option describing a substitution best captures the type of mutation leading to the sickle-cell change in the sequence. The other options would produce different alterations in the gene product that are not consistent with the classic sickle-cell mutation mechanism.

APBIO142-030 with labs S19 - CollegeBoard* AP Exam Prep: Multiple Choice

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