Download A change that makes a polypeptide defective has been discovered

3.24 A: Genetic Variation at the Molecular Level Quiz
1.
PROCTOR VERSION
A change that makes a polypeptide defective has been discovered in its amino acid sequence.
The normal and defective amino acid sequences are shown below.
Researchers are attempting to reproduce the conditions and events that resulted in this defective
amino acid sequence. Which statement is the best prediction of the conditions and events that
the researchers will most likely find produced the defective polypeptide?
(A) During DNA replication, there was a deletion of a single nucleotide that codes for lysine
(Lys), resulting in translation of a polypeptide without the Lys amino acid.
Distractor Rationale:
This answer suggests the student may understand that deletion of a single nucleotide during
replication would most likely produce changes in the amino acid sequence and the resulting
protein, but does not understand that it would cause a change in all of the amino acids that
were transcribed after the point of deletion, because the deletion of a single base is a
mutation that affects all of the codons “downstream” from the affected base.
(B) During mRNA transcription, the codon that codes for Lys was removed from the transcript,
resulting in translation of a polypeptide without the Lys amino acid.
Rationale:
This answer suggests the student understands that if the codon, a sequence of three
nucleotides, that codes for lysine is removed during transcription, the resulting polypeptide
would not contain that specific lysine amino acid, and the structure or function of the
resulting protein could be affected.
(C) During translation, the transfer RNA matched an incorrect amino acid with the mRNA strand,
resulting in a defective polypeptide.
Distractor Rationale:
This answer suggests the student may understand that if the tRNA places an incorrect
amino acid in the polypeptide chain in place of lysine, the polypeptide would be defective,
but does not understand that this would not simply result in the deletion of the lysine amino
acid, but instead would substitute the incorrect amino acid in place of lysine.
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3.24 A: Genetic Variation at the Molecular Level Quiz
PROCTOR VERSION
(D) During protein folding, one of the amino acids was mistakenly removed during the final stage
of protein assembly, resulting in a defective polypeptide.
Distractor Rationale:
This answer suggests the student may understand that certain helper proteins can alter
other proteins that have been denatured, but does not understand that amino acids are not
removed from the polypeptide by these helper proteins because their role is to refold
denatured proteins, not to change protein sequences.
Aligned to: LO 3.24 CA 3.24: Predict Impact of Change in Genotype
2.
A transfer RNA (tRNA) molecule for phenylalanine is shown in the diagram below, and an mRNA
codon chart is provided as a reference.
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3.24 A: Genetic Variation at the Molecular Level Quiz
PROCTOR VERSION
A yeast cell has a defective enzyme that attaches the lysine amino acid (Lys), instead of a
phenylalanine amino acid (Phe), to tRNAs with the anticodon AAG. Which statement describes
the most likely result in the cell?
(A) None of the proteins in the cell will contain phenylalanine, because the defective tRNA
molecule will not transcribe the appropriate DNA sequence into a phenylalanine amino acid.
Distractor Rationale:
This answer suggests the student may understand that this particular tRNA molecule would
not be able to translate UUC into the proper amino acid, but confuses translation and
transcription because he or she does not understand that during transcription, a DNA
sequence is transcribed into mRNA, and during translation, tRNA translates the sequence
from the mRNA into an amino acid sequence. Further, phenylalanine would still be present
in the protein because there are other tRNAs that also code for phenylalanine.
(B) Proteins in the cell will include lysine instead of phenylalanine at amino acid positions
specified by the codon UUC, because of the defective enzyme.
Rationale:
This answer suggests the student understands that due to the defective enzyme, the given
tRNA molecule will place lysine, instead of phenylalanine, where there is a UUC codon on
the mRNA transcript because the given tRNA anticodon will incorrectly pair UUC with lysine.
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3.24 A: Genetic Variation at the Molecular Level Quiz
PROCTOR VERSION
(C) The cell will correct the defect, because other enzymes will attach phenylalanine to tRNAs
with lysine-specifying anticodons to compensate for the defective enzyme.
Distractor Rationale:
This answer suggests the student may understand that enzymes could modify the activity of
other tRNA molecules, but does not understand that this would produce more mutant
proteins because all locations where lysine should be on the protein would contain
phenylalanine instead.
(D) The cell will adapt and change any mRNA sequences that include the codon UUC to the
codon UUU, because the codon UUU also codes for phenylalanine.
Distractor Rationale:
This answer suggests the student may understand that the mRNA codon UUU also codes
for phenylalanine, but does not understand that the cell will not be able to reconfigure the
original mRNA sequence to accommodate for the defective enzyme since mRNA is
transcribed from DNA and genetic information always flows from DNA to mRNA to tRNA.
Aligned to: LO 3.24 CA 3.24: Predict Impact of Change in Genotype
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3.24 A: Genetic Variation at the Molecular Level Quiz
3.
PROCTOR VERSION
The diagram below shows a segment of an mRNA molecule and the polypeptide sequence that it
produces during translation. The table below the diagram shows how each codon is translated
during protein synthesis in the ribosome.
A mutation in the original DNA sequence occurs that results in the deletion of a uracil nucleotide
from the mRNA molecule, as shown in the diagram below.
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3.24 A: Genetic Variation at the Molecular Level Quiz
PROCTOR VERSION
Which statement best predicts the resulting change in the polypeptide?
(A) The third amino acid will be leucine instead of phenylalanine, because the missing
nucleotide only affects the third codon.
Distractor Rationale:
This answer suggests the student may understand that a base deletion will change a codon
so that it is translated into a different amino acid, but does not understand that this
frameshift will also change all of the codons appearing after the deletion because each RNA
triplet has been shifted and no longer carries the correct code after the point in the sequence
where the deletion occurred.
(B) The third and fourth amino acids will be leucine and alanine instead of phenylalanine and
glycine, because the missing nucleotide only affects the third and fourth codons.
Distractor Rationale:
This answer suggests the student may understand that a base deletion will change a codon
as well as the codon following it due to a shift in each triplet, but does not understand that
this will also change all codons appearing after the deletion, including the stop codon,
because each RNA triplet “downstream” from the deletion no longer carries the correct code.
Since none of the stop codons begins with the nucleotides “AA,” the fifth codon can no
longer be a stop codon and the mRNA sequence will continue to be translated.
(C) The third, fourth, and fifth amino acids will be changed and the mRNA will continue to be
translated, because the stop codon will be changed to a coding triplet.
Rationale:
This answer suggests the student understands that a base deletion will change the codon
with the deletion and all codons after the deletion, including the stop codon, because each
RNA triplet “downstream” from the deletion is affected.
(D) The third amino acid will remain the same despite the deletion, because the first two
nucleotides in the codon ultimately determine which amino acid the codon is translated into.
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3.24 A: Genetic Variation at the Molecular Level Quiz
PROCTOR VERSION
Distractor Rationale:
This answer suggests the student may understand that changing the third base in a codon
may not always change the amino acid that is produced due to the redundancy of the
genetic code, but does not understand that, in this case, the amino acid is changed, as are
all amino acids translated after the deletion, because of the frameshift of the codons that
follow the deletion.
Aligned to: LO 3.24 CA 3.24: Predict Impact of Change in Genotype
4.
The diagrams below show transcription and translation in a prokaryotic cell and in a eukaryotic
cell.
A eukaryotic gene is inserted into the genome of a prokaryotic cell. Which statement best
predicts how transcription and translation of the genetic information would be affected?
(A) Transcription and translation of the eukaryotic gene would produce a smaller protein in the
prokaryotic cell, because prokaryotes utilize fewer amino acids than eukaryotic cells do.
Distractor Rationale:
This answer suggests the student may understand that prokaryotic cells are not as complex
as eukaryotic cells, but does not understand that eukaryotic genes cannot be directly
inserted into prokaryotic cells because eukaryotic genes contain many non-coding regions
that would not be removed since mRNA processing does not occur in prokaryotes. These
differences between eukaryotic and prokaryotic cells during the transcription and translation
processes would result in extra genetic material in the prokaryotic cell.
(B) Many more copies of the eukaryotic protein would be produced, because transcription and
translation occur at the same time, and are therefore much faster in prokaryotic cells.
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3.24 A: Genetic Variation at the Molecular Level Quiz
PROCTOR VERSION
Distractor Rationale:
This answer suggests the student may understand that transcription and translation occur at
the same time in prokaryotic cells and that this increases the rate of protein synthesis, but
does not understand that this will not result in extra copies of the protein being produced
because these processes would be regulated by other factors (such as regulatory genes).
(C) Transcription and translation of the eukaryotic gene would produce the same protein in the
prokaryotic cell, because the process of protein synthesis is the same in all organisms.
Distractor Rationale:
This answer suggests the student may understand that the processes of transcription and
translation are very similar in prokaryotic and eukaryotic cells, but does not understand
that the proteins produced in the prokaryotic cell would not be the same because, unlike
eukaryotic cells, the non-coding sequences (introns) of the mRNA in the prokaryotic cell
have not been removed.
(D) The eukaryotic protein produced in the prokaryotic cell would have many additional amino
acids, because non-coding regions of nucleic acids are not removed in prokaryotic cells.
Rationale:
This answer suggests the student understands that in eukaryotic cells, the non-coding
sequences of the mRNA are removed, so the eukaryotic gene would produce additional and
unnecessary amino acids in the protein, because the same mRNA processing does not
occur in prokaryotes, and thus the mRNA transcript would contain many non-coding regions
(introns) that would be translated into additional amino acids.
Aligned to: LO 3.24 CA 3.24: Predict Impact of Change in Genotype
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