IB Biology Quiz: Understand Mutations And Gene Editing
17 questions · exam conditions
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Understand Mutations And Gene EditingQuestion 1 of 17

Within the coding sequence of a single exon, an insertion of one nucleotide is followed 15 bases downstream by a deletion of one nucleotide. What is the most likely effect on the resulting polypeptide?

The entire polypeptide sequence will be altered from the point of the insertion onwards.
The polypeptide will be unchanged because the two mutations cancel each other out perfectly.
A sequence of five amino acids will be incorrect, but the reading frame for the rest of the polypeptide will be restored.
Only the two amino acids at the sites of mutation will be affected, with the rest of the polypeptide being normal.
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IB Biology Quiz

IB Biology Quiz: Understand Mutations And Gene Editing

Practice Understand Mutations And Gene Editing in IB Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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This quiz focuses on Understand Mutations And Gene Editing, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Biology.

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Question 1

Within the coding sequence of a single exon, an insertion of one nucleotide is followed 15 bases downstream by a deletion of one nucleotide. What is the most likely effect on the resulting polypeptide?

  1. The entire polypeptide sequence will be altered from the point of the insertion onwards.
  2. The polypeptide will be unchanged because the two mutations cancel each other out perfectly.
  3. A sequence of five amino acids will be incorrect, but the reading frame for the rest of the polypeptide will be restored. (correct answer)
  4. Only the two amino acids at the sites of mutation will be affected, with the rest of the polypeptide being normal.
Explanation: The initial insertion causes a frameshift. This frameshift continues for 15 bases. Since each codon is 3 bases long, 15 bases correspond to 5 codons (15 / 3 = 5). The subsequent deletion restores the original reading frame. Therefore, the five amino acids coded by the affected 15-base region will be incorrect, but the remainder of the polypeptide sequence will be translated in the correct frame.

Question 2

A researcher uses the CRISPR-Cas9 system to create a gene knockout in a cell line. No donor DNA template is supplied with the Cas9 and gRNA. The cell repairs the induced double-strand break via the non-homologous end joining (NHEJ) pathway. What is the most likely molecular outcome at the target site?

  1. Perfect, error-free repair of the DNA break, restoring the original wild-type sequence.
  2. Insertion of a large segment of DNA from another chromosome at the break site.
  3. A small insertion or deletion of bases, leading to a frameshift and a premature stop codon. (correct answer)
  4. Correction of a pre-existing mutation by using the homologous chromosome as a repair template.
Explanation: Non-homologous end joining (NHEJ) is an error-prone repair mechanism. It often results in the insertion or deletion of a few nucleotides (indels) at the site of the double-strand break. If the number of inserted or deleted bases is not a multiple of three, this causes a frameshift mutation, which usually leads to a premature stop codon and a non-functional protein, thus 'knocking out' the gene.

Question 3

Scientists aim to correct a disease-causing point mutation in a patient's cells using CRISPR-Cas9. To achieve precise correction of the sequence rather than just gene disruption, what must be delivered to the cells along with the Cas9 protein and guide RNA?

  1. A short DNA oligonucleotide that contains the correct, wild-type sequence. (correct answer)
  2. An inhibitor of the non-homologous end joining (NHEJ) pathway.
  3. A second, different guide RNA that targets the wild-type allele for protection.
  4. A reverse transcriptase enzyme to create a permanent copy of the guide RNA.
Explanation: For precise gene correction, the cell's homology-directed repair (HDR) pathway must be utilized. After CRISPR-Cas9 cuts the DNA, HDR can use a template to repair the break. By providing a DNA template containing the correct sequence, scientists can guide the cell to repair the cut and simultaneously replace the mutated sequence with the wild-type version.

Question 4

A significant concern in the therapeutic application of CRISPR-Cas9 is the occurrence of off-target effects. What is the direct molecular basis of these unintended mutations?

  1. The Cas9 enzyme mutates and begins to cut DNA at random, non-specific sequences.
  2. The guide RNA directs the Cas9 enzyme to cut at DNA sequences that are similar, but not perfectly identical, to the intended target. (correct answer)
  3. The cell's DNA repair machinery becomes overactive after the initial cut and introduces errors throughout the genome.
  4. The Cas9 protein itself acts as a chemical mutagen, directly altering the structure of DNA bases away from the target site.
Explanation: Off-target effects occur because the guide RNA can have partial complementarity to other sequences in the genome. If the similarity is high enough, the gRNA can bind to these unintended sites and direct the Cas9 enzyme to create a double-strand break there, leading to mutations at locations other than the intended target.

Question 5

High-energy radiation, such as X-rays, is a potent mutagen. What is a primary mechanism by which this type of radiation induces mutations in DNA?

  1. It causes the insertion of random nucleotides, leading to frameshift mutations.
  2. It is incorporated into the DNA strand in place of a normal base during replication.
  3. It generates highly reactive free radicals that can cause chemical changes to bases or breaks in the DNA backbone. (correct answer)
  4. It specifically methylates cytosine bases, silencing gene expression without changing the DNA sequence.
Explanation: High-energy ionizing radiation, like X-rays, can strip electrons from molecules, particularly water, creating highly reactive free radicals. These radicals can attack the DNA molecule, causing a variety of damage, including chemical modification of bases (leading to mispairing) and single- or double-strand breaks in the sugar-phosphate backbone.

Question 6

A point mutation changes the mRNA codon for an amino acid from CUU to CUC. Both of these codons specify leucine. Which terms best describe this mutation?

  1. A missense substitution.
  2. A nonsense substitution.
  3. A frameshift deletion.
  4. A silent substitution. (correct answer)
Explanation: The mutation is a substitution because one base (U) was replaced by another (C). It is described as 'silent' because, due to the degeneracy of the genetic code, the new codon (CUC) codes for the exact same amino acid (leucine) as the original codon (CUU). Therefore, the primary structure of the resulting protein is unchanged.

Question 7

The ethical debate surrounding human gene editing often distinguishes between somatic and germline modifications. Why is germline gene therapy significantly more controversial than somatic gene therapy?

  1. Somatic therapy is less effective at treating genetic disorders than germline therapy.
  2. Somatic therapy can only be performed on adult patients, limiting its application.
  3. The technology for germline editing is inherently more prone to dangerous off-target effects.
  4. Germline modifications are heritable and alter the gene pool of subsequent generations. (correct answer)
Explanation: The core ethical issue is heritability. Somatic gene therapy affects only the individual being treated. Germline therapy, performed on reproductive cells or embryos, results in changes that are passed down to all future descendants. This raises profound ethical questions about altering the human gene pool, consent, and unforeseen long-term consequences.

Question 8

The mutation causing sickle-cell anemia changes the mRNA codon from GAG to GUG, substituting valine for glutamic acid. How does this single amino acid change lead to the polymerization of hemoglobin molecules?

  1. It introduces a premature stop codon, leading to a truncated and unstable hemoglobin protein.
  2. It changes a hydrophilic amino acid to a hydrophobic one, creating a 'sticky' patch on the protein's surface under low oxygen conditions. (correct answer)
  3. It shifts the reading frame during translation, resulting in a completely different protein that aggregates easily.
  4. It prevents the correct folding of the polypeptide chain into its secondary structure, exposing the hydrophobic core.
Explanation: Glutamic acid is hydrophilic and negatively charged, while valine is hydrophobic. This substitution occurs on the surface of the hemoglobin protein. Under low oxygen conditions, this exposed hydrophobic patch on one hemoglobin molecule sticks to a complementary patch on another, leading to the formation of long, rigid fibers (polymerization) that distort the red blood cell.

Question 9

A missense mutation in an enzyme replaces alanine (a small, nonpolar amino acid) with tryptophan (a large, nonpolar amino acid) in a tightly packed region of the enzyme's active site. What is the most probable effect on enzyme function?

  1. The enzyme's function will be unaffected because both amino acids are nonpolar.
  2. The enzyme will become more active due to the increased size of the amino acid in the active site.
  3. The enzyme will likely lose its function due to steric hindrance from the larger amino acid disrupting the active site's shape. (correct answer)
  4. The primary structure of the enzyme will be unchanged, but its quaternary structure will be disrupted.
Explanation: Even though both amino acids are nonpolar, tryptophan is significantly larger than alanine. Placing a bulky amino acid in a space evolved for a small one can cause steric hindrance, physically disrupting the precise three-dimensional geometry of the active site. This change in shape is likely to prevent the substrate from binding correctly, leading to a loss of or reduction in enzyme function.

Question 10

A mutation alters the DNA sequence in a gene's coding region, but analysis shows that the primary structure of the translated polypeptide is completely unchanged. Which statement provides the most accurate explanation?

  1. The mutation was repaired by cellular enzymes before transcription occurred.
  2. The mutation created a new codon that specified the same amino acid as the original codon. (correct answer)
  3. The mutation occurred in the third position of a codon, which never affects the amino acid sequence.
  4. The mutation was a frameshift that was quickly restored by a second, compensatory mutation.
Explanation: This describes a silent mutation. The genetic code is degenerate, meaning that most amino acids are coded for by more than one codon. It is possible for a base substitution to change a codon into another codon that specifies the exact same amino acid (e.g., changing CCU to CCC, both of which code for proline). A is incorrect because if it were repaired, there would be no mutation. C is an overgeneralization; while the third position is often redundant ('wobble'), changes there can still sometimes alter the amino acid. D describes a more complex scenario that would still result in some altered amino acids.

Question 11

A mutation occurs in a gene within a hematopoietic stem cell in the bone marrow of an adult mouse. This mutation confers resistance to a specific toxin. What are the long-term implications of this single somatic mutation for the mouse?

  1. The mouse will pass the toxin resistance trait on to approximately half of its offspring.
  2. All cells derived from the mutated stem cell, including various blood cells, will be toxin-resistant, but the trait is not heritable. (correct answer)
  3. Only the original hematopoietic stem cell will be resistant to the toxin, with no effect on its differentiated daughter cells.
  4. The mouse's gametes will now carry the mutation, making the toxin resistance a heritable trait for all future generations.
Explanation: Somatic mutations occur in non-gametic cells and are not passed to offspring. Since the mutation is in a stem cell, all cells that differentiate from it will carry the mutation. Therefore, the mouse will develop a population of toxin-resistant blood cells, but this trait will not be inherited by its progeny. A and D are incorrect because somatic mutations are not heritable. C is incorrect because the mutation is passed to all daughter cells during mitosis.

Question 12

A base substitution mutation occurs in a gene. In which location is this mutation least likely to alter the phenotype of the organism?

  1. In an exon, changing a codon from UGG (Tryptophan) to UGA (Stop).
  2. In the promoter region, modifying a key recognition site for RNA polymerase.
  3. In the middle of a large intron, distant from any exon-intron splice junctions. (correct answer)
  4. In an exon, changing a codon for a nonpolar amino acid to one for a polar amino acid.
Explanation: Introns are non-coding sequences that are removed from the pre-mRNA during splicing. A mutation deep within an intron, far from the critical splice sites at its boundaries, is unlikely to affect the final mRNA sequence or the protein it codes for, and thus is unlikely to alter the phenotype. Mutations in exons (A, D) or regulatory regions like promoters (B) have a much higher probability of impacting protein function or gene expression.

Question 13

A mutation in the DNA template strand changes the sequence from 3'-TAC-5' to 3'-ATC-5' at the beginning of a gene. What is the most likely outcome during protein synthesis?

  1. Translation will begin, but the first amino acid will be isoleucine instead of methionine.
  2. Translation will be terminated immediately, resulting in no protein synthesis. (correct answer)
  3. The protein will be synthesized normally, as this represents a silent mutation.
  4. The ribosomal subunit will not initiate translation because the corresponding mRNA codon is not a start codon.
Explanation: The original DNA template sequence 3'-TAC-5' is transcribed into the mRNA start codon 5'-AUG-3'. The mutated sequence 3'-ATC-5' is transcribed into the mRNA codon 5'-UAG-3'. UAG is a stop codon. Therefore, when the ribosome scans the mRNA, it will encounter a stop codon instead of a start codon, and translation will be terminated before it can even begin, leading to no protein production.

Question 14

Which statement accurately describes a key advantage of the CRISPR-Cas9 system over the use of traditional restriction enzymes for targeted gene modification?

  1. Restriction enzymes cut at fixed, short recognition sites, while CRISPR-Cas9 can be programmed to cut almost any specific, longer DNA sequence. (correct answer)
  2. CRISPR-Cas9 can only function in bacterial cells, whereas restriction enzymes function in all cell types.
  3. CRISPR-Cas9 cuts DNA to produce only blunt ends, which are more useful for gene editing than the sticky ends from restriction enzymes.
  4. Restriction enzymes are naturally occurring proteins, while the entire CRISPR-Cas9 system is artificially synthesized and more stable.
Explanation: The primary advantage of CRISPR-Cas9 is its programmability. A restriction enzyme recognizes a single, short (typically 4-8 bp), and often palindromic DNA sequence. In contrast, the CRISPR-Cas9 system's target specificity is determined by a ~20 nucleotide guide RNA, which can be easily synthesized to match almost any desired sequence in the genome, offering far greater flexibility and precision.

Question 15

A frameshift mutation is generally more disruptive to a protein's function than a base substitution mutation. What is the underlying reason for this?

  1. Base substitutions only affect a single codon, while a frameshift alters all downstream codons from the mutation point. (correct answer)
  2. Frameshift mutations are more difficult for DNA repair enzymes to detect and correct.
  3. Frameshift mutations always result in a premature stop codon, while base substitutions never do.
  4. Base substitutions can only occur in introns, whereas frameshift mutations can only occur in exons.
Explanation: A base substitution changes only one codon, affecting at most a single amino acid (and may even be silent). A frameshift mutation, caused by an insertion or deletion of a number of nucleotides not divisible by three, changes the triplet reading frame. This means every codon from the point of the mutation onwards is read incorrectly, leading to a completely different and usually non-functional amino acid sequence, and often a premature stop codon.

Question 16

Gene therapy to treat recessive genetic disorders often involves using a modified virus to deliver a functional copy of a gene. How does this approach work at the cellular level?

  1. The virus delivers a functional gene copy that integrates into the host's genome, allowing for transcription of the correct protein. (correct answer)
  2. The virus uses its machinery to find and precisely correct the mutated allele in the host cell's DNA.
  3. The virus introduces a drug that binds to and inactivates the faulty protein produced by the mutated gene.
  4. The virus delivers RNA interference molecules that degrade the mRNA transcribed from the mutated gene.
Explanation: This describes gene augmentation therapy. For a recessive disorder, the problem is the lack of a functional protein. The viral vector acts as a delivery vehicle to introduce a correct, functional copy of the gene into the patient's cells. This new copy can then be transcribed and translated to produce the missing protein, compensating for the defective alleles.

Question 17

An insertion of a single nucleotide occurs in the third codon of a gene's coding sequence. Which part of the resulting polypeptide will be most affected compared to the wild-type?

  1. Only the third amino acid will be different.
  2. The N-terminal end, including the first two amino acids, will be incorrect.
  3. The C-terminal end, from the third amino acid onwards, will be completely different and likely truncated. (correct answer)
  4. The polypeptide will be identical, as the ribosome can skip over the inserted nucleotide.
Explanation: A single nucleotide insertion causes a frameshift mutation. The reading frame is shifted from the point of insertion (the third codon) onwards. This means that not only will the third amino acid likely be incorrect, but every subsequent codon will be misread, leading to a completely different amino acid sequence at the C-terminal end. This frameshift also often creates a premature stop codon, truncating the protein.