- Certain chemical mutagens, known as intercalating agents, insert themselves between the stacked bases of the DNA double helix. How does this mechanism typically lead to mutations?
Answer: By distorting the helix, often causing DNA polymerase to slip and introduce insertions or deletions.
Intercalating agents fit between the DNA bases, distorting the helical structure. This distortion can confuse DNA polymerase during replication, causing it to either insert an extra base or skip a base, which leads to frameshift mutations (insertions or deletions). A describes the action of base analogs or alkylating agents. B is more characteristic of high-energy radiation. D is incorrect; these agents cause mutations, they do not repair them.
- A mutation in a DNA template strand changes the triplet from 3'-GGC-5' to 3'-GGT-5'. The corresponding mRNA codons are 5'-CCG-3' and 5'-CCA-3' respectively. Both of these codons specify the amino acid Proline. Which statement best explains this phenomenon?
Answer: The genetic code is degenerate, meaning multiple codons can specify the same amino acid.
This is a silent mutation. The correct explanation is that the genetic code is degenerate, which means that most amino acids are coded for by more than one codon. In this case, both CCG and CCA code for Proline, so the mutation has no effect on the amino acid sequence. A is incorrect as transcription faithfully copies the DNA template. C is incorrect as ribosomes do not have such a proofreading function for the mRNA sequence. D uses the term 'neutral mutation' incorrectly in this context; while this silent mutation is indeed neutral in effect, the underlying reason it produces the same protein is degeneracy, not the properties of the amino acid.
- A point mutation changes an mRNA codon from 5'-AAG-3' (lysine) to 5'-AGG-3' (arginine). Both lysine and arginine are basic, positively charged amino acids. What is the most likely consequence of this mutation on the protein's function?
Answer: A minimal or no effect on function, as the substituted amino acid has similar chemical properties.
This is an example of a missense mutation where the substituted amino acid is chemically very similar to the original. This is sometimes called a neutral mutation. Because lysine and arginine are both basic and positively charged, they can often substitute for one another without significantly altering the protein's structure and function, especially if they are not in a highly specific active site. A and C are too extreme for a conservative substitution. B is incorrect because the amino acid has changed, so it is not a silent mutation by definition.
- The mutation responsible for sickle-cell anemia is a missense mutation where the DNA codon GAG is changed to GTG. This results in the amino acid glutamic acid being replaced by valine. How does this single amino acid change cause the sickling of red blood cells?
Answer: It replaces a hydrophilic amino acid with a hydrophobic one, causing hemoglobin molecules to polymerize under low oxygen conditions.
Glutamic acid is negatively charged (hydrophilic), while valine is nonpolar (hydrophobic). This substitution creates a hydrophobic patch on the surface of the hemoglobin molecule. Under low oxygen conditions, this patch causes hemoglobin molecules to stick together and form long, rigid polymers, distorting the red blood cell into a sickle shape. A describes a nonsense mutation. C is incorrect; the oxygen-binding site itself is not directly altered. D is incorrect; the protein is folded and functional under normal oxygen conditions, the problem arises from polymerization under low oxygen stress.
- A scientist develops skin cancer due to a mutation in a skin cell caused by excessive UV radiation exposure. The scientist later has a child. What is the likelihood of this specific UV-induced mutation being inherited by the child?
Answer: 0%, because the mutation is in a somatic cell and not in the germline cells (gametes).
Only mutations that occur in germline cells (sperm or egg cells) can be inherited by offspring. Mutations that occur in somatic cells (all other body cells, like skin cells) are not passed down. Therefore, the mutation that caused the scientist's skin cancer will not be in the child. The concepts of Mendelian inheritance (B and D) do not apply to the heritability of somatic mutations. The assumption in C that UV radiation affects gametes in the same way it affects skin cells is incorrect.
- Which missense mutation would most likely cause a complete loss of the enzyme's function?
Answer: A mutation that replaces valine (nonpolar) inside the hydrophobic pocket with aspartic acid (acidic/polar).
Replacing a nonpolar amino acid (valine) with a charged, polar amino acid (aspartic acid) within a critical hydrophobic pocket would disrupt the chemical environment required for substrate binding. This change in properties at a crucial location is very likely to abolish the enzyme's function. A is a substitution of one nonpolar amino acid for another, which may have a minor or no effect (a neutral mutation). B describes a mutation in a non-critical region, which is less likely to cause a complete loss of function. D describes a mutation in a non-coding region (intron) that would be spliced out and therefore have no effect on the final protein.
- Mutations are the ultimate source of genetic variation, which is essential for evolution. Which statement correctly links mutation to natural selection?
Answer: Mutations arise randomly, and natural selection acts upon the resulting phenotypic variation.
The central principle is that mutations (changes in DNA) occur randomly, without regard for their potential benefit or harm. This random variation is the raw material for natural selection. The environment then 'selects' for individuals whose traits (resulting from their genetic makeup) are best suited for survival and reproduction. A suggests a directed, Lamarckian view of mutation, which is incorrect. B is incorrect as most mutations are neutral or deleterious. D is incorrect as natural selection acts on variation; eliminating all mutations would halt evolution.
- A single nucleotide insertion occurs in the second to last codon of the coding sequence of a gene. How would the effect of this mutation likely compare to a single nucleotide insertion near the beginning of the gene?
Answer: It would be less severe, because the frameshift affects only a very small portion of the amino acid sequence at the C-terminus.
A frameshift mutation alters the reading frame from the point of the mutation onwards. If the insertion occurs near the very end of the coding sequence, only the last one or two amino acids will be changed, and the stop codon might be shifted slightly. The vast majority of the protein will be normal. In contrast, a frameshift near the beginning of the gene would alter almost the entire amino acid sequence, making it far more severe. Therefore, the mutation at the end is less severe. B is an overgeneralization; not all frameshifts lead to a completely non-functional protein, especially if they are near the end.
- Which event is an example of a spontaneous mutation, as opposed to an induced mutation?
Answer: An incorrect nucleotide being incorporated by DNA polymerase during replication that is not corrected by proofreading.
Spontaneous mutations arise from natural processes within the cell, such as errors during DNA replication. An uncorrected mistake by DNA polymerase is the classic example. Induced mutations are caused by external agents called mutagens. X-rays (A), UV light (B), and chemical agents like ethidium bromide (C) are all examples of mutagens that induce mutations.
- A genetic disorder is characterized by the production of a short, non-functional protein that is missing a significant portion of its C-terminal end. Which type of gene mutation is the most likely cause?
Answer: A nonsense mutation that changes an amino acid codon into a stop codon.
A nonsense mutation introduces a premature stop codon into the mRNA sequence. This causes the ribosome to terminate translation early, resulting in a truncated (shortened) protein that is almost always non-functional, matching the description. A silent mutation would have no effect on the protein. A missense mutation (B) would change only one amino acid, altering the protein but not truncating it. An in-frame deletion (D) would remove a single amino acid, which would shorten the protein by one residue but not cause the significant truncation described.
- A deletion of exactly three consecutive base pairs occurs in the middle of a gene's coding sequence. What is the most probable effect on the resulting polypeptide?
Answer: It will result in a polypeptide that is shorter by one amino acid but has the correct sequence otherwise.
The genetic code is read in triplets (codons). Deleting exactly three base pairs removes one full codon. This results in the loss of one amino acid from the polypeptide chain, but it does not shift the reading frame for the rest of the gene. Therefore, the subsequent amino acid sequence will be correct. A is incorrect because the reading frame is maintained. B is incorrect as an entire amino acid is lost, which is not a silent mutation. D is unlikely unless the three deleted bases coincidentally create a stop codon from the surrounding sequence.
- A gene mutation causes a single nucleotide deletion at the 15th codon of a gene's coding sequence. A different mutation causes a single nucleotide substitution at the same position. Why is the deletion likely to have a more severe effect on the final polypeptide's function?
Answer: The deletion alters the reading frame from that point onward, changing all subsequent amino acids and likely introducing a premature stop codon.
A single nucleotide deletion causes a frameshift mutation. This changes the way all subsequent codons are read by the ribosome, leading to a completely different amino acid sequence downstream of the mutation and often results in a premature stop codon. A substitution, in contrast, only affects a single codon and thus a single amino acid, which may or may not have a significant effect on the protein's function. B is incorrect because both mutations occur in the DNA and their repairability doesn't determine the severity of their effect on the polypeptide if they persist. C incorrectly describes the effect of a single nucleotide deletion; a three-nucleotide deletion would remove one amino acid. D incorrectly describes how ribosomes handle frameshifts; they continue translating, albeit with an incorrect frame, until a stop codon is reached.
- The DNA base triplet 3'-ATG-5' on the template strand is mutated to 3'-ATC-5'. What is the most likely classification of this mutation's effect?
Answer: Nonsense
First, transcribe the original and mutated DNA triplets to mRNA codons. The original DNA template 3'-ATG-5' is transcribed into the mRNA codon 5'-UAC-3', which codes for Tyrosine. The mutated DNA template 3'-ATC-5' is transcribed into the mRNA codon 5'-UAG-3'. UAG is one of the three standard stop codons. A mutation that changes an amino acid-coding codon into a stop codon is a nonsense mutation.
- Exposure to UV radiation can cause the formation of thymine dimers, where adjacent thymine bases on the same DNA strand become covalently linked. If not repaired, what is the most likely consequence during DNA replication?
Answer: The dimer physically blocks DNA polymerase, which can lead to errors or stalling of replication.
Thymine dimers create a structural distortion or lesion in the DNA double helix. This distorted structure cannot be read correctly by the standard DNA polymerase, often causing it to stall or to insert incorrect bases opposite the dimer, leading to mutations. The key issue is the physical blockage and misreading. A is what should happen, but the dimer prevents it. B is a more drastic outcome than what typically happens. D is biochemically incorrect.
- [HL] A researcher is investigating a genetic disorder caused by a single nucleotide deletion. They plan to use CRISPR-Cas9 to correct the mutation in affected cells in vitro. What essential components must be introduced into the cells to achieve this correction via homology-directed repair (HDR)?
Answer: A DNA template containing the correct sequence, the Cas9 enzyme, and a corresponding guide RNA (gRNA).
For precise gene correction using CRISPR, three components are needed: 1) the Cas9 enzyme to create a double-strand break, 2) a guide RNA (gRNA) to direct Cas9 to the specific target location in the genome, and 3) a DNA repair template containing the desired (correct) sequence. The cell's homology-directed repair (HDR) pathway uses this template to fix the break, thereby incorporating the correction. A is incomplete as it would likely lead to gene knockout via NHEJ, not correction. C and D omit the critical Cas9/gRNA system needed to create the targeted break that initiates the repair process.
