1. In which of this condition is Ara operon functional?
a) When repressor binds to O2 and I1
b) When repressor binds to O1 and I2
c) When repressor binds to I2 and I1
d) When repressor binds to O2 and Pc
Explanation: When repressor binds to I1 and I2 then RNA polymerase faces no hindrance in binding to the promoter and the genes are turned on. On the other hand when induces the repressor bind to O2 and I1 which leads to the looping out and turning off.
2. Which of the Ara promoter regulation is controlled by binding of repressors on the O1 regulating unit?
a) PA
b) PB
c) PC
d) PD
Explanation: When Ara C is produced in excess it bind to O1 and represses the promoter PC. This is true as o1 lies closer to PC. However the other promoter PBAD has a regulating unit O2 that acts from a distance by looping out.
3. Which of these operons is anabolic?
a) Lac
b) Ara
c) Trp
d) Phe
Explanation: Trp operon is anabolic as it codes for enzymes that help in the synthesis of the amino acid tryptophan. On the other hand Ara and Lac operons are catabolic as they help in breaking down the respective sugar.
4. Which of these systems give the best mode for turning trp operon off?
a) Repressor
b) Attenuator
c) Repressor with a downstream poly A tail
d) Repressor with an attenuator
Explanation: A repressor provides 70 fold lowering of expression while an attenuator provided an additional 10 fold lowering of expression. Together they have a net effect of 700 fold repression over the trp operon.
5. A Lac repressor is a tetramer repressed when bound to the inducer. The trp repressor is a ____________________
a) Dimer inactivated when bound to the inducer
b) Dimer activated on inducer binding
c) Tetramer inactivated on inducer binding
d) Tetramer activated on inducer binding
Explanation: A trp repressor is a dimer which can’t bind to the cis element unless it is bound to the inducer tryptophan. Only then it undergoes conformational change such that it then binds to the operator.
6. Which of these properties do not agree with trp operon attenuator?
a) It brings about repression of trp operon
b) It consists of one stem loop system
c) It has two codons for tryptophan in sequence
d) Ribosome stalls at the attenuator
Explanation: Attenuator does’t just contain one loop but it can form two in presence of tryptophan and an alternate one in absence of the same for B. subtilis and one terminator and another anti-terminator loops for E. Coli. The other facts about trp operon are absolutely correct.
7. Which segments of the attenuator together form the repression loop?
a) Segment 1-2
b) Segment 2-3
c) Segment 3-4
d) Segment 1-4
Explanation: In the absence of tryptophan when the RNA polymerase stalls at segment 1 of the attenuator it hinders the formation of loop 1-2. The alternate loop so formed is loop 2-3 which is the attenuation loop
8. We know that lactose has a positive impact on the activity of the lac operon. Tryptophan’s presence has a ________________
a) Positive feedback
b) Negative feedback
c) No difference
d) Highly positive impact
Explanation: Trp binding to the TRAP makes a trp-TRAP complex which binds to the RNA and stabilizes the terminator loop. Hence, Tryptophan’s presence has a negative feedback effect.
9. If in an experiment you mutate the two trp codons in the attenuator to ala codon what will be the impact on repression?
a) Repression will be normal
b) Reduced repression
c) No repression so the gene would be constitutively turned on
d) Activation will take place
Explanation: The basis of attenuation at the attenuator is the presence of two trp operon which stalls the RNA polymerase when trp is deficient. Mutating the trp to ala eliminates the attenuator’s mode of repression. However, there is still an intrinsic mode of repression by binding of a repressor, which is still active.
10. Which of these items have a positive impact on PEDCBA transcription?
a) Trp-TRAP
b) Trp-ANTI TRAP
c) Anti-Trp with TRAP
d) Anti-Trp with Trp-TRAP
Explanation: Anti- trp is an auto-regulating molecule that is sensitive to different levels of tryptophan. When tryptophan conc. is high enough to form trp-TRAP complex yet lower than necessary it binds to the Trp-TRAP complex thus preventing repression.