Question 1

Given the DNA sequence 5′-TAC AAA ATA CAG CGG-3′, which of these sequences represents a frameshift mutation?

Accepted Answer

A frameshift mutation occurs when one or more nucleotides are added or deleted, resulting in a shift in the reading frame. In this case, option B has an additional nucleotide (G) added after the fifth nucleotide, causing a frameshift mutation. Options A, C, D, and E are point mutations, where only one nucleotide is changed, but the reading frame is unaffected.

Question 2

Given the DNA sequence 5′-TAC AAA ATA CAG CGG-3′, which of these sequences represents a nonsense mutation?

Accepted Answer

A nonsense mutation is a type of mutation that results in a premature stop codon, leading to the production of a truncated protein. In this case, the change from TAC (Tyrosine) to TAG is a stop codon, resulting in a truncated protein. B, C, D, and E do not result in premature stop codons, so they are not nonsense mutations.

Question 3

Which of the following statements is true of non-homologous end joining (NHEJ)?

Accepted Answer

NHEJ is a double-strand break repair pathway (A) and is considered error-prone (B) because it can lead to insertions or deletions at the site of repair. It does not use a sister chromatid as a template, which is characteristic of homologous recombination.

Question 4

Gene conversion can be observed in fungi by identifying aberrant ratios in the ascus. Which of the following represent possible aberrant ratios caused when there is no mismatch repair of base-pair mistakes in the heteroduplex region?

Accepted Answer

Aberrant ratios in the ascus can result from gene conversion events in which one allele converts the other, leading to a heterozygous region. In the absence of mismatch repair, the heteroduplex region may contain base-pair mistakes, which can result in unequal crossing over or gene conversion. The aberrant ratios resulting from gene conversion without mismatch repair can result in imbalanced segregation of alleles, leading to non-parental ditype (NPD) tetrads with a 3:1:1:3 ratio. Thus, option B (3:1:1:3) is the best choice for aberrant ratios resulting from gene conversion without mismatch repair. The other options do not fit the expected ratios resulting from gene conversion without mismatch repair.

Question 5

Given the DNA sequence 5′-TAC AAA ATA CAG CGG-3′, which of these sequences represents a missense mutation?

Accepted Answer

The answer of Given the DNA sequence 5′-TAC AAA ATA...

Question 6

Which type of mutation is possible thanks to the redundant nature of the genetic code?

Accepted Answer

Silent mutations are possible thanks to the redundant nature of the genetic code. They occur when a change in the DNA sequence does not result in a change in the amino acid sequence of the protein because of the degeneracy of the genetic code, which allows multiple codons to code for the same amino acid.

Question 7

Which pathway is affected by an inherited mutation in the ATM gene?

Accepted Answer

The inherited mutation in the ATM (ataxia-telangiectasia mutated) gene affects the p53 repair pathway, which is responsible for detecting and repairing DNA damage. Individuals with mutations in the ATM gene are unable to activate the p53 pathway properly, which increases their risk of developing cancer. Therefore, the correct choice is C. The other options are not related to the ATM gene mutation.

Question 8

In eukaryotes, homologous recombination is initiated by __________.

Accepted Answer

Homologous recombination in eukaryotes is initiated by Spo11, which generates double-stranded DNA breaks. This break is then processed by a series of enzymes, including nucleases and recombinases, to facilitate the exchange of genetic material between homologous chromosomes.

Question 9

Which of these statements best describes gene expression in a damaged cell?

Accepted Answer

In a damaged cell, p53 is activated and leads to the transcriptional activation of BAX, which promotes apoptosis. Bcl-2 is then repressed, allowing apoptosis to occur.

Question 10

The compound 5-bromodeoxyuridine (BrdU)is a derivative of uracil, and if BrdU becomes incorporated during DNA replication, it pairs with adenine. This compound is best classified as which type of mutagen?

Accepted Answer

BrdU is a derivative of uracil, meaning it is structurally analogous to a normal nucleotide base. As such, it can be mistakenly incorporated into DNA during replication, leading to point mutations. This classifies BrdU as a base analog mutagen.

Question 11

Studies of gene mutation frequencies have shown that __________.

Accepted Answer

Mutations are generally rare, indicating genome stability, and mutation frequencies can vary among organisms and genes, suggesting some genes are more prone to mutations.

Question 12

What phenotype would you expect to see in a strain of E. coli with a mutation in the phr gene?

Accepted Answer

The phr gene encodes for a photolyase enzyme that repairs UV-induced DNA damage, specifically thymine dimers. Therefore, a mutation in the phr gene would result in a decreased ability of E. coli to repair UV-induced damage, leading to an increase in UV-induced mutations.

Question 13

What type of mutation is seen here? Wild type: 5′-TAC AAA ATA CAG CGG-3′
Mutation: 5′-TAC AAG ATA CAG CGG-3′

Accepted Answer

The mutation involves a change from an adenine (A) to a guanine (G), which is a purine to purine substitution, characteristic of a transition mutation.

Question 14

A strain of E. coli is unable to use the UV repair pathway because of an apparent absence of DNA helicase activity. Which UV repair gene is likely mutated in this strain?

Accepted Answer

The uvr-D gene encodes a DNA helicase that is involved in the UV repair pathway, specifically in the nucleotide excision repair process. A mutation in uvr-D would result in the absence of DNA helicase activity, impairing the repair of UV-induced DNA damage.

Question 15

An intercalating agent such as aflatoxin is more likely to cause which type of mutation?

Accepted Answer

Intercalating agents, such as aflatoxin, insert themselves between the base pairs of DNA, causing frameshift mutations, which result in the deletion or insertion of nucleotides.

Question 16

You have conducted an Ames test on a given compound. Which of the following would be classified as a positive result on the Ames test?

Accepted Answer

A positive result in the Ames test is indicated by the growth of a his^- strain on a his^- plate, suggesting that the compound has caused mutations that revert the his^- strain to his⁺, allowing it to grow without histidine.

Question 17

Mutations can also occur within the regulatory regions of genes. Fragile X is an example of a disorder caused by one such mutation that increases the length of the long arm of the X chromosome and ultimately decreases mRNA stability. Which type of regulatory mutation causes Fragile X?

Accepted Answer

Fragile X is caused by a triplet-repeat expansion mutation in the FMR1 gene, located on the X chromosome. This mutation leads to an abnormal increase in the number of CGG repeats in the 5' untranslated region of the FMR1 mRNA, resulting in decreased mRNA stability and a functional loss of the FMR1 protein. This type of regulatory mutation affects gene expression by altering the stability and translation of the mRNA rather than the structure of the resulting protein.

Question 18

You would like to induce a transversion mutation into a sequence of DNA. Which type of chemical mutagen would give you the best chance of inducing the correct mutation without causing transition mutations as well?

Accepted Answer

Oxidative agents are known to cause transversion mutations by altering the structure of DNA bases, leading to incorrect base pairing, whereas other agents like base analogs and deaminating agents are more likely to cause transition mutations.

Question 19

Which type of mutation converts a nucleotide to an alternative structure with the same composition but a slightly different placement of rare, less stable hydrogen bonds that cause base-pair mismatch?

Accepted Answer

A tautomeric shift occurs when a nucleotide temporarily switches between its keto and enol forms, which alters its hydrogen bonding patterns and can lead to base-pair mismatch. This type of mutation is a common cause of point mutations. Depurination, deamination, transition, and transversion all involve more significant changes to the nucleotide structure, such as the loss or substitution of a base, and do not necessarily involve altered hydrogen bonding patterns.

Question 20

If one Holliday junction region is resolved by an NS cut and the other by an EW cut, the resulting chromosomes __________.

Accepted Answer

When one Holliday junction is resolved by an NS (north-south) cut and the other by an EW (east-west) cut, the resulting chromosomes are recombinant, leading to new combinations such as A₁ and B₂ or A₂ and B₁.

Quiz 12: Gene Mutation, Dna Repair, and Homologous Recombination