Deck 15: Genetic Analysis of Cellular Processes

Question 1

Fill in the blank. Another name for a genetic screen is a ______ screen.

Answer 1

mutant

Question 2

Tryptophan is an essential amino acid that Neurospora crassa needs in order to be able to grow. The tryptophan biosynthetic pathway has three main steps by which the amino acid tryptophan is produced from chorismate. In an experiment, three N. crassa strains that were suspected of having defects in tryptophan synthesis were cultured on minimal medium (without tryptophan) and on minimum medium (plus tryptophan), with the following results. Which strain is defective in the tryptophan biosynthetic pathway?
$$\begin{array} { | l | c | c | } \hline & \text { Minimal medium } & \begin{array} { c } \text { Minimal medium } \\+ \text { tryptophan }\end{array} \\\hline \text { Strain 1 } & \text { no growth } & \text { growth } \\\hline \text { Strain 2 } & \text { no growth } & \text { no growth } \\\hline \text { Strain 3 } & \text { growth } & \text { growth } \\\hline\end{array}$$

A) Strain 1
B) Strain 2
C) Strain 3

Answer 2

Strain 1

Question 3

Arginine is synthesized in a pathway involving several enzymes and biosynthetic intermediates. Five Neurospora crassa strains were cultured on minimal media containing various additives, and the results are shown. Match each N. crassa strain with its description.


$$\begin{array} { | c | c | c | c | c | } \hline & \text { glutamate } & \text { ornithine } & \text { citrulline } & \text { arginine } \\\hline \text { Strain A } & \text { growth } & \text { growth } & \text { growth } & \text { growth } \\\hline \text { Strain B } & \text { no growth } & \text { no growth } & \text { growth } & \text { growth } \\\hline \text { Strain C } & \text { no growth } & \text { no growth } & \text { no growth } & \text { no growth } \\\hline \text { Strain D } & \text { no growth } & \text { no growth } & \text { no growth } & \text { growth } \\\hline \text { Strain E } & \text { no growth } & \text { growth } & \text { growth } & \text { growth } \\\hline\end{array}$$

Answer 3

Answer: Answers are shown in the following table. Strain C is defective in a different biosynthetic pathway, because supplementation with arginine does not rescue the growth phenotype. Note that strain A is also technically not defective in the arginine pathway, but shows normal growth so this is wild-type with respect to the conditions under consideration here. The remaining strains can grow if they are provided with a supplement that occurs after the enzyme defect in the pathway. Thus, growth in strain E with ornithine, citrulline, and arginine but not with glutamate indicates that the defect is in enzyme 1.
$$\begin{array} { | c | c | c | c | c | c | } \hline & \text { glutamate } & \text { ornithine } & \text { citrulline } & \text { arginine } & \\\hline \text { Strain A } & \text { growth } & \text { growth } & \text { growth } & \text { growth } & \text { Wild-type } \\\hline \text { Strain B } & \text { no growth } & \text { no growth } & \text { growth } & \text { growth } & \text { defective in enzyme 2 } \\\hline \text { Strain C } & \text { no growth } & \text { no growth } & \text { no growth } & \text { no growth } & \begin{array} { c } \text { not defective in the } \\\text { arginine biosynthesis } \\\text { pathway }\end{array} \\\hline \text { Strain D } & \text { no growth } & \text { no growth } & \text { no growth } & \text { growth } & \text { delective in enzyme 3 } \\\hline \text { Strain E } & \text { no growth } & \text { growth } & \text { growth } & \text { growth } & \text { defective in enzyme 1 } \\\hline\end{array}$$

Question 4

Forward genetics involves disruption of a known gene to see what phenotypes will be produced. Reverse genetics involves introducing random mutations and examining the resultant phenotypes to discover the causative gene.

Question 5

Which of the following features of genetic screens was not key to the success of Beadle and Tatum's early Neurospora crassa experiments with nutritional auxotrophs?

A) Targeted insertional mutagenesis
B) Easily-screened phenotype
C) Efficient screening procedure
D) Use of an appropriate model organism

Question 6

Components and analogs of the lac operon are used by researchers as experimental tools. Match the molecule with its activity.

-IPTG

A) Binds to lac repressor
B) derepresses lac operon when lactose is present
C) turns blue when lac operon is turned on
D) breaks down lactose

Question 7

Components and analogs of the lac operon are used by researchers as experimental tools. Match the molecule with its activity.

-allolactose

A) Binds to lac repressor
B) derepresses lac operon when lactose is present
C) turns blue when lac operon is turned on
D) breaks down lactose

Question 8

Components and analogs of the lac operon are used by researchers as experimental tools. Match the molecule with its activity.

-X-gal

A) Binds to lac repressor
B) derepresses lac operon when lactose is present
C) turns blue when lac operon is turned on
D) breaks down lactose

Question 9

Components and analogs of the lac operon are used by researchers as experimental tools. Match the molecule with its activity.

-β-galactosidase

A) Binds to lac repressor
B) derepresses lac operon when lactose is present
C) turns blue when lac operon is turned on
D) breaks down lactose

Question 10

Fill in the blank. In the lac operon, cAMP-CAP is produced and binds to the CAP binding site when ____ is low

Question 11

Which of the following is true of dominant loss-of-function mutations?

A) The mutant phenotype is seen even if an additional functional copy of a gene is present
B) The mutation is present in trans but not in cis
C) The mutant phenotype is seen only in haploids
D) The mutant phenotype is seen only when two defective copies of a gene are present

Question 12

During investigation of the lac operon, Jacob and Monod found that mutations in the same gene did not always lead to the same phenotypic outcome. For example, some mutations in the lacI gene led to constitutive lactose breakdown, whereas other mutations led to an inability to ferment lactose under any conditions. Which of the following statements are true about lacI^s mutations? Please select all that apply.

A) lacI^s mutants produce a lac repressor protein that cannot bind to DNA
B) lacI^s mutants cannot be complemented in trans
C) The operator binding sequence is changed in lacI^s mutants
D) lacI^s mutants are unable to utilize lactose
E) Introduction of a functional operon on a plasmid rescues the lacI^s phenotype

Question 13

Nüsslein-Volhard and Wieschaus used Drosophila melanogaster as a model organism in genetic screens to examine developmental biology. Why was Drosophila a good model organism for this study?
Please select all that apply.

A) Drosophila embryos can be studied easily
B) Haploids can grow so no consideration of dominant and recessive alleles is needed
C) Drosophila has a short generation time
D) Eye color can be easily followed in Drosophila
E) Balancer chromosomes are available in Drosophila

Question 14

A diagram of the mutagenesis and crossing strategy used by Nüsslein-Volhard and Wieschaus to generate and maintain segmentation mutants is shown. Male flies were exposed to EMS (a mutagen) and mated to a non-mutagenized female. How did the researchers ensure that the offspring of Cross B (i.e., the offspring seen in generation C in the diagram) were heterozygotes that carried a mutagenized chromosome? Please select all that apply.

A) CyO homozygotes die
B) Flies homozygous for segmentation mutations usually die
C) Only male flies were mutagenized
D) The female parent in Cross B carries a temperature-sensitive lethal allele

Question 15

Match the missing words in the following phrase. In the Nüsslein-Volhard and Wieschaus experiment, embryos lacking in a contiguous region across segments had mutations in [A] genes. Mutants lacking every other segment had defects in [B] genes. Mutants that had alterations in specific regions of the embryo were defective in [C] genes. Finally, mutants that exchanged the identity of a region for that of another region were defective in [D] genes.

Question 16

Fill in the blank. In genetic screens, the chance of discovering a novel gene decreases as the screen proceeds. When the chance of finding a new gene is very low and the effort expended to discover a new gene is not justifiable, the screen has been _________.

Question 17

Which of the following is an advantage of a genome-wide mutagenesis screen compared to a random mutagenesis screen?

A) Genome-wide mutagenesis is unbiased
B) Each gene can be targeted in a genome-wide screen
C) Phenotypic analysis is not needed to determine gene function in a genome-wide screen
D) Random mutagenesis identifies important regulatory regions

Question 18

Hartwell and colleagues used the yeast Saccharomyces cerevisiae to examine the cell cycle. However, most cell cycle mutants are lethal. How did the researchers get around this problem in their experiments?

A) The researchers grew S. cerevisiae cells carrying mutations alongside wild-type cells to counteract the effects of the mutation
B) The researchers grew S. cerevisiae as a haploid and used sub-lethal mutants for the genes of interest
C) The researchers grew S. cerevisiae as a diploid and maintained the mutations in a heterozygote
D) The researchers used conditional mutants

Question 19

Fill in the blank. Temperature-sensitive mutations exhibit the mutation when grown at the restrictive temperature. They exhibit a wild-type phenotype when grown at a _____ temperature.

Question 20

Match the missing words in the following phrase. Experiments showed that cell cycle mutants stalled at specific points in the cycle. Mutants could be classified in relation to recognizable steps called [A] events. Cells with a particular mutation stalled at [B] point in the cell cycle, despite the individual cells starting the experiment at [C] point in the cycle. This indicated that the gene product was critical at a specific point in the cycle but was not always needed throughout the cycle. These critical points were termed the [D] point for each gene.

Question 21

The ras gene is a major component of which sort of pathway?

A) Cell transport
B) Cell cycle
C) Biochemical synthesis
D) Signal transduction

Question 22

Fill in the blank. Receptor tyrosine kinases add a _______ to specific tyrosine residues in target proteins

Question 23

Which characteristic of Drosophila melanogaster was exploited in the screen that led to the discovery of the ras pathway?

A) Ability to fly
B) Sex linkage
C) Chemotaxis
D) Phototaxis

Question 24

Many receptor tyrosine kinase genes are essential, making it difficult to assess the function of such genes. How did researchers get around this problem when examining the roles of the sev gene?

A) The researchers looked at mutations in related genes instead of mutations in the sev gene
B) The researchers used sub-lethal mutations in the sev gene
C) Haploid flies were used for experiments
D) The researchers used conditional mutants

Question 25

To identify genes in the sev pathway, sev mutants were further mutagenized and the phenotype examined. Mutations that alleviate the sev phenotype are called suppressor mutations and mutations that exacerbate the sev phenotype are called synthetic enhancer mutations.

Question 26

When Nüsslein-Volhard and Wieschaus conducted their genetic screen in Drosophila, they continued to find new genes of interest with increasing numbers of experiments, but the number of new mutations they discovered tailed off after about 2000 chromosomes had been tested.

Question 27

Which of the following methods have been used to generate libraries of disrupted genes for an organism? Please select all that apply.

A) RNAi
B) Promoter trapping
C) Targeted insertion
D) Cytogenetics

Question 28

Which of the following are advantages of random mutagenesis compared to targeted mutagenesis? Please select all that apply.

A) Random mutagenesis is unbiased
B) Targeted mutagenesis is limited to a single mutation type
C) Targeted mutation only reduces gene expression rather than knocking-out the genes
D) Follow-up experiments are more complex with targeted mutagenesis