Please select the best match for each term.
- A) Acidic
- B) Neutral, nonpolar
- C) Neutral, polar
- D) Atypical structure
- E) Basic
1) Glutamic acid
Skill: Factual recall
2) Glycine
Skill: Factual recall
3) Arginine
Skill: Factual recall
4) Serine
Skill: Factual recall
5) Proline
Skill: Factual recall
Answers: 1) A 2) B 3) E 4) C 5) D
MULTIPLE CHOICE
6) Which of the following is not a characteristic of proteins?
- A) High molecular weight
- B) Presence of nitrogen
- C) Presence of amino acids
- D) Presence of nucleic acids
- E) Structure built of one or more polypeptides
Answer: D
Skill: Factual recall
7) Where does translation take place?
- A) In the cytoplasm
- B) In the nucleus
- C) In the nucleolus
- D) Anywhere in the cell
- E) In the centriole
Answer: A
Skill: Factual recall
8) Every amino acid contains
- A) an amino group and a hydroxyl group.
- B) an amino group and an acetyl group.
- C) a hydroxyl group and a carboxyl group.
- D) a carboxyl group and an amino group.
- E) a carboxyl group and a hydroxyl group.
Answer: D
Skill: Factual recall
9) What is the function of a ribosome?
- A) To direct transcription
- B) To attach amino acids to the appropriate tRNAs
- C) To unwind the double-stranded DNA helix to enable translation
- D) To hold mRNA and tRNAs in the correct positions to enable translation
- E) To make more mRNA
Answer: D
Skill: Factual recall
10) What is the basic shape of a tRNA molecule?
- A) A three-dimensional cloverleaf
- B) A straight strand
- C) A right-handed helix
- D) A globular ball
- E) A β-pleated sheet
Answer: A
Skill: Factual recall
11) What is the role of tRNA in translation?
- A) It brings together two subunits of a ribosome.
- B) It couples an amino acid with aminoacyl-tRNA synthetase.
- C) It helps fold up the finished polypeptide chain.
- D) It catalyzes peptidyl transferase activity.
- E) It binds to an mRNA codon and carries the corresponding amino acid.
Answer: E
Skill: Factual recall
12) How can a carrot plant express a bacterial gene?
- A) Because the bacterial gene hijacks the carrot’s cellular machinery
- B) Because the genetic code is the same in both organisms
- C) Because of the wobble phenomenon
- D) Because of the degeneracy of the genetic code
- E) A carrot cannot express a bacterial gene.
Answer: B
Skill: Application of knowledge
13) In the genetic code, both AAU and AAC code for asparagine. For this reason, the code is said to be
- A) degenerate.
- B) nonspecific.
- C) universal.
- D) ambiguous.
- E) wobbly.
Answer: A
Skill: Factual recall
14) In a gene sequence, the DNA codon for tryptophan experiences a mutation at the first base position, changing it to T. What will the resulting amino acid be?
- A) Tryptophan (no change)
- B) Serine
- C) Arginine
- D) Threonine
- E) None (a stop codon will halt translation)
Answer: C
Skill: Problem-solving
15) A mutation causes a G to be inserted after the first base of the codon for tryptophan. How will this affect the growing polypeptide chain?
- A) It will not be affected.
- B) Elongation will stop prematurely.
- C) There will be a single amino acid substitution.
- D) The reading frame will be shifted to the left, and the wrong amino acids will be added from this point on.
- E) None of these
Answer: D
Skill: Problem-solving
16) A β-pleated sheet is a type of
- A) primary structure found in a protein.
- B) secondary structure found in a protein.
- C) tertiary structure found in a protein.
- D) quaternary structure found in a protein.
- E) heme structure found in a protein.
Answer: B
Skill: Factual recall
17) Which of the following events signals the termination of translation?
- A) The ribosome reaches the end of the mRNA.
- B) The ribosome reaches a start codon.
- C) The ribosome reaches a stop codon.
- D) The ribosome runs out of charged tRNAs.
- E) The polypeptide chain folds into a protein.
Answer: C
Skill: Factual recall
18) A peptide bond forms between the ________ of one amino acid and the ________ of another.
- A) amino group, carboxyl group
- B) amino group, R group
- C) carboxyl group, sulfide group
- D) amino group, phosphate group
- E) R group, R group
Answer: A
Skill: Factual recall
19) The majority of naturally occurring amino acids are
- A) acidic and polar.
- B) basic and nonpolar.
- C) acidic and nonpolar.
- D) neutral and nonpolar.
- E) neutral and polar.
Answer: D
Skill: Factual recall
20) Molecular chaperones
- A) help guide proteins from the nucleus to the cytoplasm.
- B) help guide proteins from the cytoplasm to the endoplasmic reticulum.
- C) help fold proteins properly.
- D) are proteins with structural function.
- E) are proteins with enzymatic function.
Answer: C
Skill: Factual recall
21) The concept that a set of three nucleotides specifies a particular amino acid provides the basis for
- A) the one-gene-one-enzyme hypothesis.
- B) the one-gene-one-polypeptide hypothesis.
- C) the genetic code.
- D) biochemical reactions among nucleic acids.
- E) All of these
Answer: C
Skill: Factual recall
22) In eukaryotes, the AUG initiation codon is located in the ________ sequence.
- A) Shine—Dalgarno
- B) Kozak
- C) Khorana
- D) Goldberg—Hogness
- E) Pribnow
Answer: B
Skill: Factual recall
23) How many naturally occurring amino acids are used by ribosomes to construct proteins?
- A) 46
- B) 64
- C) 3
- D) 20
- E) 10
Answer: D
Skill: Factual recall
24) Which of the following statements is not true of proteins destined for the ER?
- A) They have a signal sequence.
- B) The signal sequence for the ER consists of 15-30 amino acids.
- C) The proteins are modified with carbohydrates in the ER.
- D) The signal sequence is subsequently removed from the mature protein.
- E) The SRP binds to the protein and prevents entry into the ER until translation is completed.
Answer: E
Skill: Factual recall
25) Which of the following statements is correct?
- A) For every codon, there is one amino acid.
- B) For every amino acid, there is one codon.
- C) A codon may signify more than one amino acid.
- D) Three codons make up an amino acid.
- E) Three amino acids make up a codon.
Answer: A
Skill: Factual recall
TRUE/FALSE
26) During translation, mRNA codons bind to complementary tRNA anticodons.
Answer: TRUE
Skill: Factual recall
27) Molecules of mRNA are translated from 3′ to 5′, which is the opposite direction from which they were made.
Answer: FALSE
Explanation: They are translated from 5′ to 3′.
Skill: Factual recall
28) The Shine—Dalgarno sequence is a purine-rich region just upstream of a start codon and is necessary for the initiation of protein synthesis in prokaryotes.
Answer: TRUE
Skill: Factual recall
29) With absolutely no exception, all organisms use the same genetic code for the production of proteins.
Answer: FALSE
Explanation: Some organisms have minor changes in the code, such as in the nuclear genome of Tetrahymena and the mammalian mitochondria.
Skill: Factual recall
30) There are 64 sense codons in the genetic code and 61 different types of tRNA molecules.
Answer: FALSE
Explanation: There are 64 codons but only 61 sense codons.
Skill: Factual recall
31) Protein synthesis begins when ribosomes bind to the UGA codon.
Answer: FALSE
Explanation: UGA is a stop codon. Protein synthesis begins when ribosomes bind to the AUG (start) codon.
Skill: Factual recall
32) The genetic code is comma-free, nonoverlapping, and almost universal.
Answer: TRUE
Skill: Factual recall
33) In the genetic code, when the first two nucleotides of a triplet are identical and the third letter is U or C, the codon always codes for the same amino acid.
Answer: TRUE
Skill: Factual recall
34) A new initiation event cannot occur on an mRNA molecule until the first ribosome falls away.
Answer: FALSE
Explanation: Many ribosomes may simultaneously be translating each mRNA, forming a structure called a polysome.
Skill: Factual recall
35) In eukaryotes, the translation initiation complex forms in association with the 40S ribosomal subunit.
Answer: TRUE
Skill: Factual recall
SHORT ANSWER
36) A DNA codon reads GCC. Give the corresponding mRNA codon, tRNA anticodon, and amino acid.
Answer: mRNA codon: CGG; tRNA anticodon: GCC; amino acid: alanine.
Skill: Problem-solving
37) If a tRNA anticodon reads GAI, to which mRNA codon(s) will it bind? What amino acid will the tRNA carry?
Answer: It will bind to CUU, CUC, or CUA, which all code for leucine.
Skill: Problem-solving
38) Part of a DNA gene sequence reads CAT. If a mutation occurs that changes the T to A, will the final protein be affected?
Answer: Both CAT and CAA encode the amino acid valine, so the final protein will not be affected.
Skill: Problem-solving
39) If a poly(C) is used as an mRNA and translated, a poly-glycine polypeptide is formed. If on the other hand a poly(G) mRNA is used, a poly-proline polypeptide is made. However, if both poly(C) and poly(G) are used in the same reaction, no polypeptide is made. Explain why.
Answer: Since mRNA is single stranded, and guanines pair with cytosines, the poly(C) would pair with the poly(G) to give double stranded RNA that could not be used as a template for translation.
Skill: Conceptual understanding
40) Why is methionine the first amino acid to be added to every polypeptide chain?
Answer: Because the start codon, AUG, specifies methionine. Since the start codon signals the beginning of translation, methionine is always the first amino acid added to a new polypeptide chain.
Skill: Conceptual understanding
41) Describe and differentiate among the primary, secondary, and tertiary structures of a protein. To which kinds of interactions can each of these stages be ascribed?
Answer: Primary protein structure refers to the sequence of amino acids on the polypeptide chain, which is determined by the gene nucleotide sequence. Secondary structure is the regular folding and twisting of a single polypeptide chain into a variety of shapes. This can be ascribed to weak bonds (electrostatic or hydrogen bonds) between NH and CO groups that are near each other on the chain. Tertiary structure refers to the folding and twisting of the amino acid chain into a three-dimensional conformation. This can be ascribed to interactions among R-groups of individual amino acids.
Skill: Conceptual understanding
42) When does quaternary structure occur?
Answer: Quaternary structure occurs only in proteins that consist of more than one polypeptide subunit. The folded subunits come together to form the quaternary structure of the protein.
Skill: Conceptual understanding
43) In the experiments performed on bacteriophage T4 by Francis Crick and others, in which it was shown that the genetic code was triplet, what were the researchers trying to identify?
Answer: They were trying to identify revertants from mutant rII to wild-type r+ phage that would have resulted from frameshift mutations due to the addition or deletion of a single base pair.
Skill: Factual recall
44) What are the two sulfur-containing amino acids that contribute to a protein’s tertiary structure?
Answer: Cysteine and tryptophan.
Skill: Factual recall
45) How did von Ehrenstein et al. demonstrate that the specificity of codon recognition lies in the tRNA molecule and not in the amino acid it carries?
Answer: They chemically converted the cysteine on tRNA.Cys molecules to alanine in vitro. This Ala-tRNA.Cys was used to synthesize hemoglobin, which normally contains a cysteine on each α and ß chain. The resulting hemoglobin molecules contained alanine at the position normally occupied by cysteine, which indicated that the tRNA had read the cysteine codon and inserted the alanine it carried.
Skill: Conceptual understanding
46) Of the codons AAU, AUG, UAA, and UAU, which is considered a nonsense codon? What does this mean?
Answer: UAA is a nonsense, or stop, codon. It does not code for any amino acid and signals the end of translation.
Skill: Factual recall
47) What is the wobble hypothesis, and what implications does it have for base-pairing rules and selective pressure on codons?
Answer: The wobble hypothesis refers to the degeneracy at the third nucleotide position of a codon, in which the base at the 5′ of the tRNA anticodon can pair with more than one type of base at the 3′ end of the codon. For example, the same leucine tRNA molecule with anticodon GAG can read two different leucine codons (CUC and CUU). This allows for neutral mutations to occur in genes at the third position, but not the first or second, of a codon. Therefore, there is said to be higher selective pressure on the first and second positions in a codon, causing mutations at those positions to be much more rare.
Skill: Analytical reasoning
48) How are proteins sorted into their appropriate cell compartments in eukaryotes?
Answer: Proteins with specific “signal” sequences (a hydrophobic amino terminal extension) are directed to the ER during translation, while proteins without signal sequences remain in the cytoplasm. Inside the space of the ER, the signal sequence is removed from the polypeptide. The protein is modified further by addition of specific carbohydrate groups that direct them to their final destination.
Skill: Factual recall
49) Would a two-letter code with four different nucleotides be sufficient to encode the 20 amino acids found in cells? Why or why not? What does the three-letter code imply about the nature of the genetic code?
Answer: A two-letter code would only encode 16 (42) amino acids, so it would not be sufficient. A three-letter code generates 64 (43) possible codes, which is more than enough for the 20 amino acids found in cells. This implies that some amino acids may be specified by more than one codon.
Skill: Problem-solving
50) Describe a cell-free protein synthesizing system with which you could determine unambiguously which codons specify which amino acids.
Answer: You could use a system with synthetic mRNAs that contain only one type of base and then observe which type of polypeptide is synthesized. The polypeptide should contain only one type of amino acid. For example, a poly(U) mRNA would contain only UUU triplets and would produce a polypeptide chain entirely of phenylalanine.
Skill: Analytical reasoning
iGenetics: A Molecular Approach, 3e (Russell/Bose)
Chapter 7 DNA Mutation, DNA Repair, and Transposable Elements
MATCHING
Please select the best match for each term.
- A) A mutation that changes a codon from one that represents an amino acid to one that signals a chain termination
- B) A mutation that causes a change in a single base pair
- C) A mutation in a gene that causes no detectable change in the protein product
- D) A mutation that changes a codon from one amino acid to another
- E) A mutation that causes an addition or deletion of one or two base pairs in a gene
1) Nonsense mutation
Skill: Factual recall
2) Missense mutation
Skill: Factual recall
3) Frameshift mutation
Skill: Factual recall
4) Point mutation
Skill: Factual recall
5) Neutral mutation
Skill: Factual recall
Answers: 1) A 2) D 3) E 4) B 5) C
MULTIPLE CHOICE
6) Which of the following nucleotide changes leads to a transition mutation?
- A) Adenine to guanine
- B) Adenine to cytosine
- C) Guanine to cytosine
- D) Thymine to guanine
- E) Guanine to thymine
Answer: A
Skill: Application of knowledge
7) Which of the following is not mutagenic?
- A) 5BU
- B) AZT
- C) Nitrous acid
- D) Hydroxylamine
- E) Acridine
Answer: B
Skill: Factual recall
8) Base analogs may cause mutations because
- A) they modify the chemical structure and properties of the normal base.
- B) they insert themselves between adjacent bases on the DNA strand and cause an extra base to be inserted during replication.
- C) they remove amino groups from bases, causing them to pair with the wrong base during replication.
- D) they may exist in alternate chemical states that pair with different DNA bases than the normal state during replication.
- E) Both A and C
Answer: D
Skill: Conceptual understanding
9) The deamination of cytosine creates
- A) 5-methyl cytosine.
- B) uracil.
- C) 2-aminopurine.
- D) 5-bromouracil.
- E) thymine.
Answer: B
Skill: Factual recall
10) Thymine dimers are commonly caused by
- A) ultraviolet radiation.
- B) ionizing radiation such as X-rays.
- C) tautomers.
- D) alkylating agents.
- E) intercalating agents.
Answer: A
Skill: Factual recall
11) Xeroderma pigmentosum is a human genetic disease caused by
- A) elevated levels of cholesterol in the blood.
- B) failure to produce pigment that protects the skin cells from UV light exposure.
- C) defective DNA excision-repair mechanisms.
- D) mutations that inactivate tumor suppressor genes.
- E) loss of genes controlling the SOS
Answer: C
Skill: Factual recall
12) Mutation frequency is the
- A) number of mutations per gene per generation.
- B) number of mutations per nucleotide per generation.
- C) number of mutations per cell per generation.
- D) number of a specific mutation in a defined population.
- E) total number of mutations in a defined population.
Answer: D
Skill: Factual recall
13) The codon 5′-AAA-3′ codes for the amino acid lysine. Which of the following mutations in this codon is a neutral mutation?
- A) 5′-ATA-3′ to isoleucine
- B) 5′-AGA-3′ to arginine
- C) 5′-AAG-3′ to lysine
- D) 5′-CAA-3′ to glutamine
- E) 5′-AAC-3′ to asparagine
Answer: B
Skill: Application of knowledge
14) Spontaneous mutation rates are greatly reduced by
- A) exposure to ionizing radiation.
- B) reverse mutations.
- C) base-modifying agents.
- D) DNA repair mechanisms.
- E) performing the Ames test.
Answer: D
Skill: Factual recall
15) A mutation during DNA replication causes a G to be inserted after the first base of the codon for tryptophan. How will this affect the growing polypeptide chain?
- A) It will not be affected.
- B) Elongation will stop prematurely.
- C) There will be a single amino acid substitution.
- D) The reading frame will be shifted to the left, and the wrong amino acids will be added from this point on.
- E) An extra amino acid will be added, but the reading frame will not be affected.
Answer: D
Skill: Reasoning and logic
16) In Drosophila, the wild-type eye color is red. A mutation, vermilion, causes vermilion-colored eyes, unless there is a mutation in another gene, suv, which, when homozygous or hemizygous, results in eyes that are the wild-type red even in the presence of the vermilion mutation. This is an example of
- A) forward mutation.
- B) reverse mutation.
- C) intragenic suppression.
- D) intergenic suppression.
- E) back mutation.
Answer: D
Skill: Application of knowledge
17) A transposon is
- A) a DNA segment that can insert itself at one or more sites in a genome.
- B) a “jumping gene.”
- C) a DNA segment that may cause mutations in genes or chromosomal rearrangements.
- D) a mobile genetic element that may or may not leave a copy of itself in its original site when it moves to a new site.
- E) All of these
Answer: E
Skill: Factual recall
18) Nonsense suppressors are usually mutations in genes coding for
- A) proteins.
- B) enzymes.
- C) mRNA.
- D) tRNA.
- E) rRNA.
Answer: D
Skill: Factual recall
19) Which of the following transposable elements are found in eukaryotes but not in prokaryotes?
- A) IS elements
- B) Families of autonomous and nonautonomous elements
- C) Retrotransposons
- D) Ty elements
- E) B, C, and D only
Answer: E
Skill: Factual recall
20) In order for the dissociation element (Ds) mutations in corn to be stable,
- A) an Ac element must be present.
- B) an Ac element must not be present.
- C) the Ds must contain the gene for transposition.
- D) the DNA must not be replicated.
- E) None of these
Answer: B
Skill: Factual recall
21) An IS (insertion sequence) element contains
- A) a transposase gene only.
- B) a transposase gene and additional genes.
- C) a transposase gene and inverted repeats at the ends.
- D) a transposase gene, additional genes, and inverted repeats at the ends.
- E) genes but not a transposase gene.
Answer: C
Skill: Factual recall
22) What is conservative transposition?
- A) No net increase in the number of transposable elements in the genome.
- B) A net increase in the number of transposable elements in the genome.
- C) Movement of transposable elements with replication of the element.
- D) Movement of transposable elements without replication of the element.
- E) Transposition without disruption of normal gene product activity.
Answer: D
Skill: Factual recall
23) Dissociation elements (Ds) in plants are examples of
- A) activator transposons that can direct their own transposition.
- B) nonautonomous elements that require activation by an autonomous element.
- C) mutator genes that increase the spontaneous mutation frequencies of other genes.
- D) repeated sequences that are targeted by a transposase.
- E) Both B and C
Answer: E
Skill: Factual recall
24) Which procedure would you use to detect a nutritional mutation in microorganisms?
- A) Visible inspection
- B) Replica plating
- C) Controlled crosses
- D) Isolation at high temperature
- E) Plating on antibiotic-containing medium
Answer: B
Skill: Conceptual understanding
25) Genetic manipulation in Drosophila may be assisted by the use of
- A) P
- B) Ty
- C) factor.
- D) bacteriophage Mu.
- E) Tn10.
Answer: A
Skill: Factual recall
TRUE/FALSE
26) LINEs and SINEs are repetitive sequences in humans that can also, as retrotransposons, insert into genes and cause disease.
Answer: TRUE
Skill: Factual recall
27) Changes in heritable traits result from adaptation to environmental influences.
Answer: FALSE
Explanation: Changes in heritable traits result from random mutations.
Skill: Conceptual understanding
28) Somatic mutations may be inherited by the next generation.
Answer: FALSE
Explanation: Only germ-line mutations may be heritable; somatic mutations only affect the individual in which the mutation occurs.
Skill: Conceptual understanding
29) 5-bromouracil (5BU) is a mutagen because it is an analog of the base thymine and may pair with guanine instead of adenine if it is incorporated into a DNA strand.
Answer: TRUE
Skill: Factual recall
30) A silent mutation is a change in the DNA sequence that alters the amino acid sequence of the encoded protein but does not change its function.
Answer: FALSE
Explanation: A silent mutation is a change in the DNA sequence that does not change the amino acid sequence of the protein due to the redundancy of the genetic code.
Skill: Factual recall
31) A tautomer is an uncommon form of DNA base that naturally exists along with the common form.
Answer: TRUE
Skill: Factual recall
32) The DNA polymerase proofreading mechanism maintains a low mutation rate in eukaryotic genes.
Answer: FALSE
Explanation: The DNA polymerase proofreading mechanism is found only in bacteria, where it detects the insertion of incorrect bases following DNA replication.
Skill: Conceptual understanding
33) A transposon may carry genes for proteins that enable their transposition as well as genes for other functions such as drug resistance.
Answer: TRUE
Skill: Factual recall
34) Composite transposons contain a central region with genes and repeated sequences at their ends but do not terminate with IS elements.
Answer: FALSE
Explanation: Composite transposons contain a central region with genes flanked at both sides by IS elements as well as terminal inverted repeats.
Skill: Factual recall
35) A cointegrate is characteristic of replicative transposition.
Answer: TRUE
Skill: Factual recall
SHORT ANSWER
36) An mRNA codon reads GUA. If a transition mutation occurs at the third base pair, will the final protein be changed?
Answer: Yes. Valine will be inserted in place of an asparagine.
Skill: Application of knowledge
37) A point mutation changes a codon from UCG to UAG. What will happen to the resulting polypeptide?
Answer: UAG is a stop codon, so the polypeptide will terminate prematurely.
Skill: Application of knowledge
38) What are two ways that a reverse mutation can occur?
Answer: A mutation in the same gene can restore the original phenotype, or a suppressor mutation at a second site in the genome can suppress the forward mutation.
Skill: Factual recall
39) How does an intercalating agent such as ethidium bromide cause mutations?
Answer: The substance fits between adjacent bases on one strand of a DNA sequence, causing the complementary strand to be miscopied during replication. This usually results in a frameshift mutation.
Skill: Factual recall
40) How can PCR be used to induce site-specific mutations in DNA?
Answer: Two nested pairs of primers are used, the internal pair having a targeted mismatch that is copied into the newly synthesized DNA fragments. The external primers are used to replicate the mutated fragments.
Skill: Application of knowledge
41) For a particular gene, if one gene in a million experiences a mutation each generation, what is its mutation frequency?
Answer: 10-6
Skill: Application of knowledge
42) A tumor suppressor gene is cloned and mutagenized in vitro, then injected into mouse embryos to create knockout mice. How could you identify the heterozygous knockout mice and create mice susceptible to tumors?
Answer: Reared heterozygous mice can be screened molecularly to identify the ones with the mutation. Interbreeding these mice will result in some homozygous knockout progeny, which in turn will show rapid development of tumors.
Skill: Analytical reasoning
43) Describe how the Ames test is used to determine whether a particular chemical is mutagenic.
Answer: Strains of the bacteria Salmonella typhimurium that are auxotrophic for histidine are plated on a histidine-lacking nutrient agar with rat liver cells. The unknown chemical is added to the agar plate, and the plate is observed for bacterial growth that would indicate reverse mutations caused by the chemical resulting in histidine prototrophy.
Skill: Conceptual understanding
44) What kind of mutation-detection procedure can be used to detect the white-eye mutation in Drosophila?
Answer: Screening for this mutation can be done by inspection since it is a visible mutation.
Skill: Conceptual understanding
45) How does the nucleotide excision repair (NER) system in E. coli work, and what kinds of DNA damage does it repair?
Answer: The NER system repairs thymine dimers and other serious distortions of the DNA helix. It is a system of four proteins (UvrA, UvrB, UvrC, and UvrD) that work in a complex to recognize a distortion in the DNA, make cuts on either side of the lesion, release the short single-stranded cut segment, and allow DNA polymerase and ligase to fill in the gap with new bases.
Skill: Factual recall
46) Explain how IS elements produce target-site duplications when they move.
Answer: The enzyme transposase makes a staggered cut in the target sequence of the recipient DNA. The IS element is inserted, becoming joined to the single-stranded ends. The gaps are then filled in by DNA replication, resulting in a copy of both the IS element and the target sequence.
Skill: Conceptual understanding
47) Barbara McClintock discovered that a controlling element was responsible for the mutant-spotted phenotype (purple spots on white) in corn kernels. Explain how this works.
Answer: A normal C gene produces purple pigment in the corn kernel cells. If this gene is disrupted by the transposon Ds (a “mobile controlling element”), no pigment is produced, resulting in a white kernel. Reversions of this mutation can occur if Ds transposes out of the gene during development, resulting in spots of purple pigment on the kernel. The transposition of Ds is controlled by an autonomous “activator” element, Ac.
Skill: Conceptual understanding
48) What is a retrotransposon, and how does it differ from typical transposons?
Answer: Most transposons move by a DNA-to-DNA mechanism, but retrotransposons (such as yeast Ty elements, Drosophila copia elements, and human LINEs and SINEs) transpose via an RNA intermediate using a transposon-encoded reverse transcriptase.
Skill: Factual recall
49) How was the adaptive method for acquiring mutations disproved?
Answer: Luria and Delbruck argued that if mutations occurred due to exposure to a particular environment, then the number of cells that had the mutation would be similar in different identical populations. However, if the mutations occurred randomly, then the number of cells with that particular mutation would differ or fluctuate in identical populations exposed to the same environmental factor. They proved the second argument to be true by infecting identical bacterial cultures with the bacteriophage T1 and counting the number of resistant colonies. The number of resistant colonies fluctuated widely in the different cultures.
Skill: Factual recall
50) Explain what a mutator gene is and give an example.
Answer: A mutator gene is a gene in which a mutation results in a much higher-than-normal mutation frequency for all other genes. For example, mutD mutations in E. coli affect the function of DNA polymerase III. This causes defective 3′-to-5′ proofreading repair activity and results in many incorrectly inserted nucleotides being left unrepaired.
Skill: Conceptual understanding
