- 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?
Answer: The enzyme will likely lose its function due to steric hindrance from the larger amino acid disrupting the active site's shape.
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.
- 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?
Answer: A short DNA oligonucleotide that contains the correct, wild-type sequence.
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.
- Which statement accurately describes a key advantage of the CRISPR-Cas9 system over the use of traditional restriction enzymes for targeted gene modification?
Answer: Restriction enzymes cut at fixed, short recognition sites, while CRISPR-Cas9 can be programmed to cut almost any specific, longer DNA sequence.
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.
- A frameshift mutation is generally more disruptive to a protein's function than a base substitution mutation. What is the underlying reason for this?
Answer: Base substitutions only affect a single codon, while a frameshift alters all downstream codons from the mutation point.
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.
- 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?
Answer: The C-terminal end, from the third amino acid onwards, will be completely different and likely truncated.
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.
- A base substitution mutation occurs in a gene. In which location is this mutation least likely to alter the phenotype of the organism?
Answer: In the middle of a large intron, distant from any exon-intron splice junctions.
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.
- 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?
Answer: The mutation created a new codon that specified the same amino acid as the original codon.
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.
- 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?
Answer: All cells derived from the mutated stem cell, including various blood cells, will be toxin-resistant, but the trait is not heritable.
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.
- 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?
Answer: It generates highly reactive free radicals that can cause chemical changes to bases or breaks in the DNA backbone.
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.
- 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?
Answer: A silent substitution.
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.
- 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?
Answer: A small insertion or deletion of bases, leading to a frameshift and a premature stop codon.
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.
- 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?
Answer: Translation will be terminated immediately, resulting in no protein synthesis.
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.
- 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?
Answer: The guide RNA directs the Cas9 enzyme to cut at DNA sequences that are similar, but not perfectly identical, to the intended target.
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.
- 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?
Answer: Germline modifications are heritable and alter the gene pool of subsequent generations.
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.
- 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?
Answer: It changes a hydrophilic amino acid to a hydrophobic one, creating a 'sticky' patch on the protein's surface under low oxygen conditions.
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.
- 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?
Answer: The virus delivers a functional gene copy that integrates into the host's genome, allowing for transcription of the correct protein.
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.
- 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?
Answer: A sequence of five amino acids will be incorrect, but the reading frame for the rest of the polypeptide will be restored.
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.
