This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on “Operons : Trp”.
1. Which of these operons is anabolic?
a) Lac
b) Ara
c) Trp
d) Phe
View Answer
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.
2. 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
View Answer
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.
3. 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
View Answer
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.
4. 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
View Answer
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.
5. 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
View Answer
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.
6. 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
View Answer
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.
7. 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
View Answer
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.
8. 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
View Answer
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.
9. When uncharged tRNA concentration is low what will you expect as the activity of tryptophan operon?
a) Low
b) Medium
c) High
d) Very high
View Answer
Explanation: When uncharged tRNA concentration is low this is because of lack of trp. This inactivates anti-trap gene and thus there is no attenuation at all in trp gene. Thus activity is high.
10. How many stem loop structures play important role in complete system of tryptophan gene regulation?
a) 3
b) 4
c) 6
d) 8
View Answer
Explanation: 3 stem loop for the attenuation and termination in trp gene itself and another 3 for attenuation and termination of anti-trp gene. Overall there are 6 stem loop structures which regulate the entire functioning.
11. Which of this mode of trp regulation is not possible in eukaryotes?
a) Anti-TRAP Trp-TRAP
b) Anti-Terminator stabilization
c) Attenuation of trp
d) Trp-TRAP binding
View Answer
Explanation: Attenuation of trp is based on ribosomal stalling at two trp codons which leads to the formation of attenuation loop. This is only possible for co-transcriptional translation which is absent in eukaryotes.
12. In which microorganism will you find attenuation by alternate loop formation due to ribosomal stalling?
a) S. aureus
b) E. coli
c) S. typhimurium
d) B. subtlis
View Answer
Explanation: This is a mechanism found in Bacillus subtilis; where other microorganisms like E. coli have a different mechanism. E. coli has the anti–TRAP trp-TRAP mechanism.
13. Which operon behaves similar to Bacillus subtilis trp operon in terms of attenuation?
a) Lac operon
b) His operon
c) Phe operon
d) Ara operon
View Answer
Explanation: His operon, similar to B. subtilis trp operon, has a streach of 6 His codons in its attenuator which stalls the ribosome and helps in attenuation.
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