Q1. Huntington’s disease is caused by a short tandem CAG repeat in the HTT gene. Individuals with fewer than 35 CAG repeats in the HTT gene do not develop Huntington’s disease. Individuals with 40 or more CAG repeats will develop Huntington’s disease.
A transgenic mouse has the HTT gene. A scientist wants to investigate whether inserting a stop codon into the HTT gene might prevent the mouse from developing the disease. Which of the following tools would be most useful in inserting a stop codon into the HTT gene?
Correct Answer: (a)
CRISPR-Cas9 is used for gene editing, and with the correct donor DNA, it could be used to insert a stop codon into a gene. Gel electrophoresis separates DNA fragments by size, so choice (B) is incorrect. Choice (C) is incorrect because PCR amplifies the number of copies of a specific DNA sequence. Restriction enzymes cut DNA at specific sequences, so choice (D) is incorrect.
Q2. A forensic scientist cuts DNA from a crime scene and DNA from a suspect with the same enzyme. Which tool should the scientist use to separate the DNA fragments according to their size?
Correct Answer: (c)
Gel electrophoresis separates fragments of DNA according to their size. Bacterial transformation involves the uptake and expression of foreign DNA by bacteria, so choice (A) is incorrect. CRISPR-Cas9 is used for gene editing, so choice (B) is incorrect. PCR amplifies specific sequences of DNA, so choice (D) is also incorrect.
Q3. Why is a heat-stable DNA polymerase required for the polymerase chain reaction (PCR)?
Correct Answer: (b)
The denaturation stage that occurs during each cycle of PCR requires high temperatures that would denature most enzymes, so a heat-stable DNA polymerase is required for PCR. Heat-stable DNA polymerases do not add DNA nucleotides at a faster rate than other DNA polymerases, so choice (A) is incorrect. PCR primers anneal to the target DNA sequence, not to the DNA polymerase, so choice (C) is incorrect. Choice (D) is incorrect because heat-stable DNA polymerases are not more accurate than other DNA polymerases.
Q4. A scientist at a pharmaceutical company wants to create bacteria that will synthesize human growth hormone.
What should the scientist use to add the DNA code for human growth hormone to a plasmid?
Correct Answer: (b)
DNA ligase attaches DNA fragments together, joining them with phosphodiester bonds. Choice (A) is incorrect because bacterial transformation is used to insert naked, foreign DNA into a cell. Gel electrophoresis separates DNA fragments by size, so choice (C) is incorrect. Polymerase chain reaction is used to amplify the number of copies of a specific sequence of DNA, so choice (D) is incorrect.
Q5. You set up four RT-qPCR reactions all containing the same amount of total RNA. When the reactions are complete, you see that you have 10,000 units of fluorescence after 20 cycles for sample A, after 25 cycles for sample B, after 18 cycles for sample C, and after 22 cycles for sample D. Which of the samples had the most copies of target mRNA at the start of the RT-qPCR?
Correct Answer: (c)
In quantitative PCR, the 'cycle threshold' (Ct or Cq) is the cycle number where the fluorescence signal crosses a specific level. A lower Ct value indicates that the target sequence reached the threshold sooner, meaning there was a higher starting concentration of the target mRNA. Sample C reached the threshold in only 18 cycles, the fewest among the group.
Q6. The Ti plasmid of Agrobacterium usually induces tumors called galls on infected plants. When Agrobacterium is used to make transgenic plants, why does no gall form?
Correct Answer: (b)
To use Agrobacterium as a vector, the Ti (tumor-inducing) plasmid is 'disarmed.' This involves genetically engineering the plasmid to remove the T-DNA genes that cause tumor formation (galls) and replacing that space with the specific transgene the scientist wishes to introduce into the plant.
Q7. Huntington’s disease is caused by a short tandem CAG repeat in the HTT gene. Individuals with fewer than 35 CAG repeats in the HTT gene do not develop Huntington’s disease. Individuals with 40 or more CAG repeats will develop Huntington’s disease.
Which tool would be most useful for estimating the size of the HTT gene isolated?
Correct Answer: (b)
Gel electrophoresis separates DNA fragments by size and would be most useful for estimating the size of the HTT gene isolated. DNA ligase attaches DNA fragments together, so choice (A) is incorrect. Polymerase chain reaction amplifies specific sequences of DNA, so choice (C) is incorrect. Restriction enzymes cut DNA at specific base pair sequences, so choice (D) is also incorrect.
Q8. What is the basis of separation of different DNA fragments by gel electrophoresis?
Correct Answer: (b)
DNA molecules have a uniform negative charge-to-mass ratio, so they all migrate toward the positive electrode. The agarose gel acts as a molecular sieve; smaller DNA fragments move through the pores more easily and quickly than larger fragments, resulting in separation based strictly on length (size).
Q9. What property of DNA causes it to move toward the positive electrode in gel electrophoresis?
Correct Answer: (d)
The phosphates in the backbone of the DNA double helix have a slight negative charge and are attracted to the positive electrode. The nitrogen atoms in the nitrogenous bases are not charged, so choice (A) is incorrect. Hydrogen bonds between the base pairs do not result in a net negative charge, so choice (B) is incorrect. The oxygen atoms in the deoxyribose sugars are not charged, and thus choice (C) is incorrect.
Q10. Huntington’s disease is caused by a short tandem CAG repeat in the HTT gene. Individuals with fewer than 35 CAG repeats in the HTT gene do not develop Huntington’s disease. Individuals with 40 or more CAG repeats will develop Huntington’s disease.
Which of the following tools would be most useful in amplifying the number of copies of the HTT gene so that more DNA would be available for analysis?
Correct Answer: (c)
Polymerase chain reaction makes multiple copies of a specific DNA sequence and would be the best choice for amplifying the number of copies of the HTT gene. Choice (A) is incorrect because CRISPR-Cas9 is used for gene editing. Gel electrophoresis separates fragments of DNA by size, so choice (B) is incorrect. Restriction enzymes cut DNA at specific sequences, so choice (D) is incorrect.
Q11. If a PCR is started using 10 pieces of template DNA, how many pieces of DNA would there be after 10 cycles?
Correct Answer: (c)
PCR amplification follows an exponential growth pattern defined by the formula N = N0 × 2n, where N0 is the starting number and n is the number of cycles. In this case, 10 × 210 = 10 × 1024 = 10,240, which is approximately 10,000.
Q12. Which of the following statements is accurate for DNA replication in your cells, but not PCR?
Correct Answer: (c)
During in vivo DNA replication, the lagging strand is synthesized in short Okazaki fragments that must be joined together by the enzyme DNA ligase. In PCR, the target sequence is typically small enough that the polymerase completes the entire strand from a single primer, and there are no fragments to seal. Furthermore, PCR uses heat-stable Taq polymerase, whereas human cells use heat-labile polymerases.
Q13. A forensic scientist recovers a very small amount of evidence at a crime scene. The scientist would like to amplify the number of copies of DNA in the evidence sample. Which of the following techniques should the scientist use?
Correct Answer: (d)
Polymerase chain reaction is used to make multiple copies of (amplify) specific DNA sequences. Choice (A) is incorrect because bacterial transformation is the process in which bacteria absorb and express foreign DNA. CRISPR-Cas9 is used in gene editing, so choice (B) is incorrect. Gel electrophoresis is used to separate fragments of DNA by size, so choice (C) is incorrect.
Q14. Which of the following is used to cut DNA at specific base pair sequences?
Correct Answer: (d)
Restriction enzymes (also known as restriction endonucleases) cut DNA at specific base pair sequences. DNA ligase connects pieces of DNA, so choice (A) is incorrect. Choice (B) is incorrect because gel electrophoresis is used to separate DNA fragments according to their size. Polymerase chain reaction (PCR) is used to amplify specific sequences of DNA, so choice (C) is incorrect.
Q15. A scientist at a pharmaceutical company wants to create bacteria that will synthesize human growth hormone.
The scientist has successfully created a plasmid that contains the DNA code for the human growth hormone gene. Which technique should the scientist use to insert the plasmid into a cell?
Correct Answer: (a)
Bacterial transformation inserts naked, foreign DNA into a bacterial cell. CRISPR-Cas9 is used for gene editing, not inserting plasmids into a cell, so choice (B) is incorrect. Choice (C) is incorrect because gel electrophoresis is used to separate DNA fragments according to their size. Polymerase chain reaction is used to amplify the number of copies of a specific DNA sequence, so choice (D) is incorrect.
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