Genetics - Problem Drill 13: The Control of Gene Expression in Prokaryotes

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1 Genetics - Problem Drill 13: The Control of Gene Expression in Prokaryotes No. 1 of You have a cell where the lac repressor has a mutation that doesn t bind lactose. The cells are cultured in a low-glucose, high-lactose media. The expression of lac operon is. (A) Inducible by lactose (B) Constitutive (C) Inducible by high glucose (D) Induced by both lactose and high glucose (E) None of the above Lac repressor cannot bind to the operator; therefore, whether lactose binds the repressor or not, the operon is not inducible. B. Correct! Repressor binding, meaning the lac operon, will always be expressed. In other words, it is constitutive. Low glucose leads to synthesis of camp, which activates CAP and then activates lac operon; however, high glucose only inhibits such an effect. It cannot be induced by either. There is indeed one correct answer above. The key to answer this question is to understand how lac operon works: two regulation mechanisms: 1) regulated by repressor and lactose; 2) regulated by camp and CAP. (B)Constitutive

2 No. 2 of You have a cell where the expression of the lac operon is constitutive due to a mutation in the operator. If you add to that cell a completely wild-type version of the lac operon, you would expect. (A) The wild-type operon not to respond to lactose induction. (B) The endogenous original operon not to restore its regulation by lactose. (C) The endogenous operon not to respond to glucose or lactose. (D) The wild-type operon not to respond to lactose induction normally. (E) The endogenous operon to compete with the added wild-type operon for lac I binding. The added wild-type operon would function normally; therefore, it would respond to lactose induction. The lesion on the mutant is on the operator sequence, which cannot be corrected by adding a wild-type operon. The operator sequence is the site where the repressor binds; therefore, the mutant operon can still respond to glucose, which uses a CAP binding site. D. Correct! The wild type-operon would respond to lactose normally because all the components will be there. The endogenous operon has defects on its operator, meaning it cannot bind to the lac I; therefore, it will not compete with the wild-type operon. The operator sequence is for binding repressor. (D)The wild-type operon not to respond to lactose induction normally.

3 No. 3 of Assume that attenuation accounts for a tenfold regulation and that the trp repressor protein accounts for about a 70-fold regulation on the Trp operon. In a mutant cell line, the tryptophan repressor is mutant so that it cannot bind the trp operator. When tryptophan is removed from the media, the expressions of the tryptophan operon will. (A) Increase 10-fold (B) Increase 70-fold (C) Decrease 10-fold (D) Decrease 70-fold (E) Increase 700-fold A. Correct! When tryptophan is removed from media, the tryptophan operon will be induced by two different mechanisms: reduced attenuation and removing of the repressor from the operator sequence. Since repressor is non-functional, the tryptophan is only induced by 10-fold. The repressor is non-functional. When tryptophan is removed, the operon would start to make Trp; it is an induction, not inhibition. The effect will be an induction, not inhibition. Only the effect on attenuator is removed 10 fold. Key point: double mechanism to regulate the trp operon: attenuator and repressor. (A)Increase 10-fold

4 No. 4 of If you infect a bacterium simultaneously with a lambda phage carrying wild-type ci and a phage carrying mutant ci,. (A) The cell will undergo lysis. (B) The cell will undergo lysogeny. (C) The cell will not be infected. (D) The wild-type phage will infect the cells but not the mutant phage. (E) There is not enough information to determine the fate of the cell. If only in the mutant phage, it will undergo lytic phase. However, the co-infection leads to ci production and lysogeny establishment. B. Correct! The wild-type phage would generate enough ci for maintaining the lysogeny, even though the mutant wouldn t produce any ci. The ci mutant does not affect the infection ability of the phage. The ci mutant does not affect the infection ability of the phage. The phage cycle undergoes lysogeny first. Only when ci is mutated or suppressed by Cro, will it undergo lytic phase. Keep in mind: high ci lysogeny; high Cro lysis. (B)The cell will undergo lysogeny.

5 No. 5 of The camp-cap system provides positive control in the lac operon (super-imposed upon negative control by the laci gene). Based upon your understanding of these mechanisms, predict how the following mutations would affect expression and regulation of the lac operons: A mutation that inactivated the enzyme (adenylate cyclase) that synthesizes camp. (A) Cells cannot use glucose as the sole carbon source. (B) Cells grow poorly in glucose media. (C) Cells cannot use lactose as a carbon source. (D) No induction when lactose is added into the media. (E) No induction of lac operon when glucose is depleted. camp production is a second messenger that senses the low glucose condition; it does not affect the cell s ability to use glucose as a carbon source. See A. Cells can grow normally in glucose media. The lac operon is intact and all the necessary enzymes are present, so the cell can use lactose as a carbon source. Again, the lac operon is intact and it can be regulated through repressor-operator; there will be induction when lactose is added. E. Correct! When glucose is depleted, the camp would be generated to signal increase of lac operon activity. However, the enzyme needed for generating camp is mutated, so camp amount cannot be increased and lac operon cannot be induced in such a condition. The key to solving this problem is to understand the two different mechanisms for lac operon regulation. (E)No induction of lac operon when glucose is depleted.

6 No. 6 of Which of the following statements about the Lac Operon is correct? (A) The lac operon consists only of 3 structural genes, a promoter and an operator. (B) The lac operon includes a terminator. (C) There is a promoter for each of the three structural genes. (D) There is a promoter for each of the 6 structural genes. (E) There is one terminator for each structural gene. Along with 3 structural genes, a promoter and an operator, there is also a terminator. B. Correct! The lac operon does include a terminator. The three structural genes share one promoter. There are 3 structural genes in the lac promoter. There is one terminator for each lac operon. The lac operon is made up of three structural genes next to each other, a promoter, terminator and an operator. The lac operon was the first genetic regulatory mechanism completely described and is an important example in prokaryotic gene regulation. (B) The lac operon includes a terminator.

7 No. 7 of The Trp Operon. (A) Contains 5 structural genes. (B) Contains 6 structural genes and one promoter. (C) Functions by converting lactose into tryptophan. (D) The five structural genes code for 5 enzymes, which produce tryptophan. (E) Expression is regulated by the level of lactose. A. Correct! The Trp operon contains 5 structural genes. The Trp operon contains 5 structural genes. The trp operon converts chorismic acid into tryptophan. The five structural genes code for 3 enzymes involved in the production of tryptophan. The expression of the operon is regulated by an effector. The trp operon is an example of biosynthetic operon, and the expression of the operon is regulated by an effector. The operon is made up of 5 structural genes that code for three enzymes used to convert chorismic acid into tryptophan. The operon also has a gene that codes for a short oligopeptide, trpl and is used in attenuation. (A)Contains 5 structural genes.

8 No. 8 of Which of the following statements about Trp mrna is true? (A) The terminator stem-loop structure initiates transcription. (B) The tertiary structure of the terminator is a stem-loop. (C) The anti-terminator stops transcription. (D) The switch between the terminator and anti-terminator is dependent on the supply of lactose. (E) Trp mrna can form two secondary structures with opposite functions. The terminator stem-loop structure stops transcription. The stem-loop of the terminator is a secondary structure of the mrna. The anti-terminator allows transcription to continue. The switch between the terminator and anti-terminator is dependent on the supply of tryptophan. E. Correct! Trp mrna can form two secondary structures with opposite functions. TrpL mrna can form two secondary structures with opposite functions. Terminator stem-loop structure stops transcription. Anti-terminator structure allows transcription to continue. The switching between the terminator and anti-terminator structure depends on the supply of Trp. (E)Trp mrna can form two secondary structures with opposite functions.

9 No. 9 of What is the lytic cycle of a viral infection? (A) The lytic cycle of infection is when the virus infects a cell and then replicates itself, eventually killing the host cell. (B) The lytic cycle leads to permanent host cell production of the virus itself. (C) Unlike the lysogenic life cycle, the lytic cycle does not involve taking over the host cell DNA for viral production. (D) The lytic cycle is dependent on the initial step of the injection of viral DNA into the host cell, leading to viral replication. (E) Along with the lysogenic cycle, the lytic cycle kills the host cell to release the virus. A. Correct! The lytic cycle of infection is when the virus infects a cell and then replicates itself, eventually killing the host cell. While the virus is replicated during a lytic infection, the host cell eventually dies as a result of infection. For viral infection, including the lytic cycle, the host cell machinery must be used to replicate the virus. Adsorption onto the host cell is the first step of the lytic infection cycle. The lysogenic cycle results in viral production but not early host cell lysis. Viruses can follow two life paths. These two alternative paths are called lytic cycle or lysogenic cycle. In the lytic cycle, the virus infects the host and replicates. Upon completion of replication and new virion formation, the host cell is ruptured (lysed) and virion released. In the lysogenic cycle, the viral DNA incorporates into the host s DNA and is copied, along with the host DNA replication. (A)The lytic cycle of infection is when the virus infects a cell and then replicates itself, eventually killing the host cell.

10 No. 10 of Which of the following statements about the induction of the lytic cycle in Lambda Phage is correct? (A) The phage switch from lysogenic to the lytic cycle occurs when Cro proteins are dominant. (B) When cl proteins dominate, the phage switches to the lytic cycle. (C) Both Cro and cl have an operator for each gene. (D) When Cro protein concentrations are high, it stimulates its own production. (E) Decreased Cro and increased cl leads to the lytic cycle. A. Correct! The phage switch from lysogenic to the lytic cycle occurs when Cro proteins are dominant. When cl proteins dominate, the phase remains in the lysogenic cycle. Cor and cl genes share the same three operators. When Cro concentrations are high, it blocks its own expression. Increased Cro and decreased cl lead to lytic induction. The life cycle of lambda phages is controlled by ci and Cro proteins. When ci proteins dominate, phage remains in the lysogenic state. When Cro proteins are dominated, the phage enters the lytic state. ci encodes Lambda repressor. Cro encodes a protein that controls the repressor (and other genes). ci and Cro share the same three operators between the two genes but transcribes to different directions. (A)The phage switch from lysogenic to the lytic cycle occurs when Cro proteins are dominant.