Molecular Cell Biology - Problem Drill 09: Gene Expression in Prokaryotes

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1 Molecular Cell Biology - Problem Drill 09: Gene Expression in Prokaryotes Question No. 1 of Which of the following statements about gene expression in prokaryotes is correct? Question #1 (A) In prokaryotes, the control of gene expression has two levels: mrna level and operon level. (B) In prokaryotes, the control of gene expression has two levels: mrna level and protein level. (C) mrna levels are only transcriptionally controlled through operons. (D) Control at protein level mainly involves mrna degradation. In prokaryotes, the control of gene expression has two levels: mrna level and protein level. B. Correct! In prokaryotes, the control of gene expression has two levels: mrna level and protein level. mrna levels are mainly transcriptionally controlled through operons, but there is also anti-sense regulation of mrna stability. Control at protein level mainly involves protein degradation. In prokaryotes, the control of gene expression has two levels: mrna level and protein level. mrna level is mainly transcriptionally controlled through operons, but there is also anti-sense regulation of mrna stability. The operon transcriptional control is the main control level in prokaryotes. Control at protein level mainly involves protein degradation.

2 Question No. 2 of Which of the following statements about transcription in bacteria is correct? Question #2 (A) Portions of DNA upstream of the promoter region contain specific sequences for transcription factor (TF) binding. (B) Portions of RNA upstream of the promoter region contain specific sequences for transcription factor (TF) binding. (C) The binding of a transcription factor to the promoter controls the RNA polymerase activity and therefore expression of a gene. (D) The binding of a transcription factor to the coding sequence controls the RNA polymerase activity and therefore expression of a gene. Portions of DNA upstream of the coding region contain specific sequences for transcription factor (TF) binding. Portions of DNA upstream of the coding region contain specific sequences for transcription factor (TF) binding. C. Correct! The binding of a transcription factor to the promoter controls the RNA polymerase activity and therefore expression of a gene. The binding of a transcription factor to the promoter controls the RNA polymerase activity and therefore expression of a gene. Transcriptional regulation in bacteria mainly involves the regulation on a promoter. Promoter is portions of DNA upstream of the coding region and contains specific sequences for transcription factor (TF) binding. The binding of TFs to these sites controls the RNA polymerase activity and therefore expression of a gene. Prokaryotes promoter contains a -35 consensus sequence and a -10 consensus sequence called TATA Box, also known as Pribnow box. Transcription starts from certain start site and then followed by coding sequence.

3 Question No. 3 of Which of the following statements about operons is correct? Question #3 (A) There are two kinds of genes in an operon: Structure genes and glucose genes. (B) The genes in an operon are transcribed individually as separate mrnas that are cleaved before translation. (C) The genes in an operon are transcribed together, sometimes as a single mrna that is cleaved before translation. (D) In eukaryotes (but not prokaryotes), genes are organized into functional groupings called operons. There are two kinds of genes in an operon: Structure genes and Regulator genes. The genes in an operon are transcribed together - Sometimes as a single mrna that is cleaved before translation. C. Correct! The genes in an operon are transcribed together Sometimes as a single mrna that is cleaved before translation. In prokaryotes (but not eukaryotes), genes are organized into functional groupings called operons. In prokaryotes (but not eukaryotes), genes are organized into functional groupings called operons. Operons consist of two or more adjacent coding regions that are controlled by the same transcription factors. The genes in an operon are transcribed together, sometimes as a single mrna that is cleaved before translation. This is called polycistronic.

4 Question No. 4 of Which of the following statements about trans- and cis-acting elements is correct? Question #4 (A) Cis-acting Elements are DNA sequences in the vicinity of the structural portion of a gene that are required for gene expression i.e., Operator and promoter. (B) Cis-acting Elements are DNA sequences that bind to the cis-acting sequences to control gene expression. (C) Trans-acting Elements are factors that bind to the trans-acting sequences to control gene expression. (D) Trans-acting Elements are DNA sequences that bind to the cis-acting sequences to control gene expression. A. Correct! Cis-acting Element are DNA sequences in the vicinity of the structural portion of a gene that are required for gene expression i.e., Operator and promoter. Cis-acting Element are DNA sequences in the vicinity of the structural portion of a gene that are required for gene expression i.e., Operator and promoter. Trans-acting Elements are factors that bind to the cis-acting sequences to control gene expression. Trans-acting Elements are factors that bind to the cis-acting sequences to control gene expression. For transcription, there are two classes of elements that may regulate the promoter activity: Cis-acting Element, DNA sequences in the vicinity of the structural portion of a gene that are required for gene expression, for example Operator sequences and promoter sequences. Trans-acting Elements are factors that bind to the cisacting sequences to control gene expression, for example RNA polymerase, transcription factors.

5 Question No. 5 of Which of the following statements about transcription factor function is correct? Question #5 (A) Transcription factors including repressors and activators, work independently of RNA polymerase to modulate (B) Transcription factors including repressors and activators work with RNA Helicase to modulate its activity. (C) Repressor negatively regulates RNA polymerase activity, i.e. decreases (D) Repressor positively regulates RNA polymerase activity, i.e. increases Transcription factors include repressors and activators work with RNA polymerase to modulate its activity. Transcription factors include repressors and activators work with RNA polymerase to modulate its activity. C. Correct! Repressor negatively regulates RNA polymerase activity, i.e. decreases Repressor negatively regulates RNA polymerase activity, i.e. decreases Transcription factors include repressors which negatively regulate RNA polymerase activity; and activators which positively regulate RNA polymerase activity. Repressor slows down transcription and activators enhance

6 Question No. 6 of Which of the following statements about the lac operon is correct? Question #6 (A) The lac operon is an operon required for glucose utilization by bacteria. (B) Lac operon contains one regulatory gene, the i gene which codes for the activator of the lac operon (lac I), also the lac operon includes three structural genes (z, y, and a). (C) Lac operon contains one regulatory gene, the i gene which codes for the repressor of the lac operon (lac I), also the lac operon includes three structural genes (z, y, and a). (D) The y gene codes for transacetylase. The lac operon is an operon required for lactose utilization by bacteria. Lac operon contains one regulatory gene, The i gene which codes for the repressor of the lac operon (lac I), also the lac operon includes three structural genes (z, y, and a). C. Correct! Lac operon contains one regulatory gene, the i gene which codes for the repressor of the lac operon (lac I), also the lac operon includes three structural genes (z, y, and a). The y gene codes for permease, which increases permeability of the cell to b- galactosides. Lac operon contains one regulatory gene, the i gene which codes for the repressor of the lac operon (lac I). It has its own promoter; and three structural genes (z, y, and a). The z gene: b-galactosidase (b-gal), responsible for the hydrolysis of the disaccharide lactose into its monomeric units, galactose and glucose. The y gene codes for permease, which increases permeability of the cell to b-galactosides. The a gene encodes a transacetylase. z, y and a share one promoter.

7 Question No. 7 of Which of the following statements about glucose catabolic repression of the lac operon is correct? Question #7 (A) When glucose is depleted in the media, camp level decrease, and this leads to enhanced (B) When glucose is depleted in the media, camp level increase, and this leads to enhanced (C) When activated CAP binds to the operator sequence of the trp operon it enhances (D) When activated CAP binds to the operator sequence of the lac operon it down regulates When glucose is depleted in the media, camp level increase, and this leads to enhanced B. Correct! When glucose is depleted in the media, camp level increase, and this leads to enhanced When activated CAP binds to the operator sequence of the lac operon it enhances When activated CAP binds to the operator sequence of the lac operon it enhances Glucose inhibits Lac operon via camp and CAP. CAP is short for catabolite activator protein; it binds to CAP-binding sequence and enhances RNA polymerase activity. When cells are cultured in glucose, the camp level is low; hence No CAP binding to Lac Operon takes place and the Lac operon is off. When glucose is depleted in the media, camp level increases, camp then binds to CAP and activates CAP, the activated CAP binds to operator sequence of lac operon and enhance

8 Question No. 8 of Which of the following statements about the tryptophan operon is correct? Question #8 (A) TrpR itself does not bind to the operator sequence. (B) TrpR itself binds to the operator sequence. (C) Terminator stem-loop structure permits transcription and the Antiterminator structure stops (D) Terminator stem-loop structure permits transcription and the Antiterminator structure stops transcription of the lac operon. A. Correct! TrpR itself does not bind to operator sequence. TrpR itself does not bind to operator sequence. Terminator stem-loop structure stops transcription and the Anti-terminator structure permits transcription to continue. Terminator stem-loop structure stops transcription and the Anti-terminator structure permits transcription to continue. Trp operator is also regulated by attenuator, which is located at the 5 end of operon, codes for a short 14 aa oligopeptide. It contains two consecutive trp codons, serves as a measure of the tryptophan supply in the cell. TrpL mrna can form two secondary structures with opposite functions. Terminator stem-loop structure: stop Anti-terminator structure: allow transcription to continue.

9 Question No. 9 of Which of the following statements about phage infection of bacteria is correct? Question #9 (A) When a cell is infected, the cell can undergo either the lytic cycle or the lysogeny cycle. (B) When a cell is infected, the cell undergoes the lytic cycle. (C) When ci proteins predominate the phage enters the lytic state. (D) Latent Infection results in cell lysis and release of the virus. A. Correct! When a cell is infected, the cell can undergo either the lytic cycle or the lysogeny cycle. When a cell is infected, the cell can undergo either the lytic cycle or the lysogeny cycle. When ci proteins predominate the phage remains in the lysogenic state. Latent Infection results in integration of viral DNA into host DNA (lysogenesis). When a cell is infected, the cell can undergo either the lytic cycle or the lysogeny cycle. The phage switch is dependent on two proteins: Cro and ci. ci encodes Lambda repressor. Cro encodes a protein that controls the repressor (and other Genes). The life cycle of lambda phages is controlled by ci and Cro proteins: when ci proteins predominate: phage remains in the lysogenic state; When Cro proteins predominate: phage enters the lytic state. ci and Cro share same three operator between the two genes, but transcribes to different directions, they repress each other s expression.

10 Question No. 10 of Which of the following statements about the induction of the lytic cycle is correct? Question #10 (A) When exposed to stress, a protein called RecA is de-activated. (B) When exposed to stress, a protein called RecA is activated. (C) Increased Cro and increased ci leads to lytic cycle. (D) Increased Cro and decreased ci prevents the lytic cycle. When exposed to stress, a protein called RecA is activated. B. Correct! When exposed to stress, a protein called RecA is activated. Increased Cro and decreased ci leads to lytic cycle. Increased Cro and decreased ci leads to lytic cycle. When exposed to stress, a protein called RecA is activated. RecA can cleave ci and the Cleaved ci lose their ability to bind to DNA. The transcription of repressor itself is no longer activated, so less ci is produced, Increased Cro and decreased ci leads to lytic cycle.