Genetics Ch. 18

A new chemical pesticide developed for agricultural use was subjected to the Ames test. The rate of mutation seen in samples treated with the pesticide and liver extract was approximately half that of the samples treated with the liver extract alone; in samples treated with only pesticide, the mutation rate was about the same as that of untreated samples. Which of the following statements could you make about this pesticide?

- the pesticide is modified by liver enzyme -the pesticide is unlikely to be a mutagen -the pesticide may be protective against spontaneous mutation

Examples of environmental mutagens include:

-alkylating agents, which chemically modify bases -nucleotide base analogues -ionizing radiation

A framshift mutation could be caused by:

-an insertion or deletion of any number of base pairs that is NOT a multiple of 3, but not by base pair substitutions.

Mutations:

-are permanent changes in the DNA sequence or structure -produce allelic variation -are more likely to be harmful than beneficial

The mutation which causes sickle cell anemia in humans:

-is a base substitution -is a deleterious mutation -is a missense mutation

In Himalayan rabbits, the ears, feet, and muzzle are pigmented while the rest of the body is white. This is because they express an enzyme variant that is ineffective at body temperature but functions at the lower temperatures experienced by cells of the extremities. This is an example of:

A conditional mutation

Differentiate between deleterious and lethal mutations.

A deleterious mutation reduces chances of survival/reproduction of an organism. A lethal allele is an extreme example of a deleterious allele that leads to the death of the organism.

Define a mutation.

A heritable change in the genetic material.

Which of the following would be an example of a germ line mutation?

A man with normal chromosomes has a hip X-ray without a protective shield. Ten months later, his wife gives birth to a child with a chromosomal deletion.

In which situation would you expect the mutation frequency to increase over time?

A neutral allele is created at low mutation rate.

Describe a silent mutation.

A silent mutation is a change in the DNA sequence that does not alter the amino acid encoded. Like nonsense and missense mutations, silent mutations only occur in the protein coding region of a gene.

What might be a functional advantage to more efficient repair of actively transcribed genes?

Actively transcribed genes are presumably required for correct cellular function and thus mutations in them are more likely to have a deleterious effect. Also, these genes are loosely packaged, perhaps allowing more potential damage to bases. More efficient repair of these genes might offset an increased rate of damage.

Which of the following point mutations would be most likely to affect protein function?

CAA to TAA

Which of the following forms of DNA repair does NOT require DNA polymerase?

Direct DNA repair

Because they only affect a single amino acid, missense mutations do not have a significant effect on protein functions.

False- alteration of a single amino acid, especially to one with dissimilar properties, can have a dramatic effect on protein function.

Chromosomal rearrangements are unlikely to produce an obvious phenotype, since they do not remove any DNA.

False- rearrangements can disrupt genes, interfere with chromatin structure, and have position effects on gene expression.

Provided the number of base pairs is a multiple of three, insertion of additional DNA in the coding region of a protein is unlikely to have a dramatic effect on protein function.

False- while insertions in multiples of three will not cause a frameshift, addition of a single amino acid may be enough to disrupt protein function. Further, trinucelotide repeat expansions are essentially additions of base pairs in multiples of three, and they can cause major disruptions in protein structure.

A bacterial cell experiences a mutation as a result of exposure to nitrogen mustard and then divides several times to produce a total of eight cells. How many of the resulting cells would you expect to contain the mutation?

Four (nitrogen mustard is an alkylating agent and so changes one member of a base pair)

Individuals with Cockayne syndrome are more likely to:

Have an increased rate of somatic mutation.

Explain how xeroderma pigmentosum can be caused by mutations in so many different genes.

In all cases of xeroderma pigmentosum, the nucleotide excision repair pathway is disrupted. Since many different proteins are required for this pathway to function correctly, mutations in any one protein of the pathway might be expected to lead to a similar phenotype.

Considering spontaneous mutations caused by deprivation, deamination, and tautomeric shifts, explain why transitions are more common than transversions.

In depurination, only purines are affected but there is a 50:50 chance of insertion of a purine vs. a pyrimidine at the apurinic site after DNA replication. However, this would be expected to lead to equal numbers of transitions and transversions. Deamination only causes transitions, as it creates thymine or uracil from cytosine (indirectly, in the case of thymine). Tautomeric shifts also only cause transitions, as they still only allow base pairing between one purine and one pyrimidine but the specific pairs are changed. Therefore, two of these three causes of spontaneous mutations are only capable of creating transitions while the third should produce equal numbers of transitions and transversions.

A wild type allele:

Is the allele that is most common in the population

Based on the physiological adaptation theory, bacterial populations that had not been exposed to T1 infection:

None of the above

Which of these is NOT a mechanism by which a chromosome rearrangement could affect gene function?

None- all of these may be caused by rearrangements and affect gene function).

What type of mutations might you expect to alter much or all of the primary structure of a protein?

Nonsense or frameshift

Many eukaryotic genes have two stop codons located close to each other at the end of the gene. How might this be relevant for cells that carry a nonsense suppressor mutation?

Nonsense suppressors could allow translational read through of normal stop codons as well as the nonsense mutation. Presence of a second stop codon (especially if it is a different codon) would allow normal termination even if the nonsense suppressor allowed read through of the first stop codon.

As a result of the Chernobyl nuclear disaster, mutation rates in both humans and plants have been demonstrated to be greatly increased. What type of mutations would you expect to find in this population?

Small deletions or chromosomal rearrangements (result of cross linking or chromosomal breaks) would be more common than base substitutions.

Differentiate between spontaneous and induced mutations.

Spontaneous mutations result from abnormalities in normal biological processes such as DNA replication; induced mutations are caused by exposures to environmental mutagens such as UV radiation.

The main difference between the physiological adaptation theory and the random mutation theory is:

The cause of the mutation (both theories rely on spontaneous, not induced, mutations).

The gene that is mutated in cases of cystic fibrosis encodes an ion transporter protein normally expressed in the respiratory and digestive tracts along with other locations. Individuals homozygous for the most common mutation in this gene (ΔF508) died at very early ages prior to the advent of advanced medical intervention. Despite this, the mutation exists at relatively high frequency in populations of European descent. One potential explanation is that heterozygotes for such mutations may be relatively immune to the lethal effects of diseases such as typhoid fever. Which statement below is most accurate?

The deltaF508 cystic fibrosis mutation is a lethal mutation.

For a gene with a mutation rate of 1 x 10-5 per generation, what additional information would you need to calculate the mutation frequency?

The mutation rate does not tell you anything about the current mutation frequency. To calculate mutation frequency, you need to know the population size (total number of alleles) and the number of mutant alleles.

How is mismatch repair able to determine the strand with the correct DNA sequence?

The parental strand is methylated and the newly made strand if not. The methylated strand is used as a template to replace mismatches in the newly made strand.

Why is alkyltransferase only able to be used once?

The protein is chemically changed by the addition of a methyl group.

A woman with mild symptoms of myotonic muscular dystrophy has a daughter with a very severe form of the disease. Which of the following statements is most correct?

This is an example of anticipation.

A splice site mutation leading to loss of an exon can affect production of regions of the protein encoded in downstream eons even if they are spliced correctly.

True

DNA repair systems typically work in three major steps: detection of the error, removal of the abnormality, and replacement of the removed DNA.

True

Increasing environmental levels of mutagen is likely to lead to increasing frequency of deleterious alleles.

True

The Luria-Delbruck fluctuation test was designed to differentiate between appearance of a trait due to random, spontaneous mutations vs. appearance of the trait due to adaptation to environmental conditions.

True

In the Ames test:

mutagens will cause an increase in the number of revertants

In a haploid organism, lethal alleles would be expected to:

none of the above (a high mutation frequency is very unlikely since lethal alleles are not heritable in this case. they could only arise by new mutation).

Mitochondrial DNA polymerase does not have an error checking mechanism such as that of nuclear DNA polymerase. This would be expected to lead a higher rate of which type of mutation in mitochondrial DNA?

spontaneous mutations

In Drosphila, the Torso protein is required for activation of a signaling pathway leading to formation of the head and tail of embryos. If Torse is missing...

suppressor, intergenic suppressor, up promoter (since D-raf requires activation by Torso, this is unlikely in this scenario, however), missense. The mutation could NOT be a down promoter, intragenic suppressor, silent, nonsense, or neutral mutation. In the absence of additional information, you cannot determine if this mutation is deleterious, beneficial, or even lethal on its own.