This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on “Operons : Lac and Ara”.
1. You want your bacterial culture to grow well so you made an enriched media with all forms of carbohydrates. Which of this carbohydrate should you restore first if you want the culture to keep growing at the same rate?
Explanation: Prokaryotes first use all the glucose in the media to derive energy. Only when they sense a low glucose level they search alternative energy sources like lactose, fructose or galactose.
2. In a mero-diploid you have one I- and one I+ Repressor gene in the two lac operons. I+ is wild type while I- is a mutant which produces a repressor that doesn’t bind. What would be the functionality of the diploid?
a) Function normally
b) Will be non-functional
c) May or may not function
d) Will have impaired functioning
Explanation: The set of gene product produced from the wild type I+ loci will be sufficient to induce repression in both the operons. Thus, the system will not be impaired or non-functional.
3. Which of these Lac operon mero- diploid will be constitutively turned on? Considering the terms has their usual meaning.
a) I+ + P + O+ + Z+ + Y+ + A+ / I+ + P+ O+ + Z+ + Y+ + A+
b) I+ + P + OC + Z+ + Y+ + A+ / I+ + P+ O+ + Z+ + Y+ + A+
c) Is + P + O+ + Z+ + Y+ + A+ / I+ + P+ O+ + Z+ + Y+ + A+
d) I+ + P + O+ + Z+ + Y+ + A+ / I– + P+ O+ + Z+ + Y+ + A+
Explanation: OC is an operon that can’t bind to the repressor. Consecutively it is always turned on. IS produces a repressor which binds tightly to operon and never lets it turn on- This is the case of constitutively turning off.
4. Which of these combinations will result is am operon that can never produce beta galactosidse?
a) I– + P + O+ + Z+ + Y+ + A+ / I+ + P+ O+ + Z+ + Y+ + A+
b) I+ + P + OC + Z– + Y+ + A+ / I+ + P+ O+ + Z– + Y+ + A+
c) Is + P + OC + Z+ + Y+ + A+ / I+ + P+ OC + Z+ + Y+ + A+
d) I-d + P + O+ + Z+ + Y+ + A+ / I– + P+ O+ + Z+ + Y+ + A+
Explanation: This is because although this operon is constitutively turned on, yet it has a mutated lac Z gene which produces beta galactosidase. This mutation exists in both of the loci so it can’t produce this enzyme even if the operon is on.
5. Which of the Ars operon regulation site is actually preventing RNA polymerase from binding and producing the Ara genes BAD?
Explanation: Although O2 is much farther from the ara promoter P yet it can regulate P as it loops out and reaches the site. On the other hand O1 however being closer doesn’t control the promoter of BAD genes.
6. Which of these Ara genes is a mode of feedback autoregulation?
a) Ara A
b) Ara B
c) Ara C
d) Ara D
Explanation: As ara C level rises it itself binds beside its promoter preventing the RNA polymerase to bind to the same there by having a negative feedback control. Ara BAD promoter lacks this type of control and is regulate din different way.
7. 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.
8. Which of the Ara promoter regulation is controlled by binding of repressors on the O1 regulating unit?
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.
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