DNA Gyrase

[IL Pre Med]

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Why is this wrong?

Fluoroquinolones are a class of antibiotics that inhibit DNA gyrase. Which of the following would be inhibited?

A. Condensing of DNA
B. Unwinding of DNA

I answered B but the answer is A

Doesn't Gyrase unwind DNA that is supercoiled?

 

[Temperature101]

Why is this wrong?

Fluoroquinolones are a class of antibiotics that inhibit DNA gyrase. Which of the following would be inhibited?

A. Condensing of DNA
B. Unwinding of DNA

I answered B but the answer is A

Doesn't Gyrase unwind DNA that is supercoiled?

Helicase unwind DNA but gyrase prevent supercoiling ( ie condensing).

 

[IL Pre Med]

Helicase unwind DNA but gyrase prevent supercoiling ( ie condensing).

Ah okay, I guess I was just picturing it wrong in my head. Thanks!

 

[navho77]

Helicase unwind DNA but gyrase prevent supercoiling ( ie condensing).

No, gyrase does not prevent supercoiling, it is what does the supercoiling (negative supercoiling). The supercoiling is in turn what causes bacterial DNA to condense since there are no histone proteins to wrap around.

You might have been thinking about topoisomerase (of which gyrase technically is one, but it's kind of the "opposite" one of the one involved in unwinding, at least for MCAT purposes). Topoisomerase is what relaxes the tension upstream of the replication fork when the DNA is being unwound by helicase, so if topoisomerase were inhibited, the unwinding process would go haywire (although, I guess it would still be unwound, albeit in a faulty, useless way).

 

[Temperature101]

No, gyrase does not prevent supercoiling, it is what does the supercoiling (negative supercoiling). The supercoiling is in turn what causes bacterial DNA to condense since there are no histone proteins to wrap around.

You might have been thinking about topoisomerase (of which gyrase technically is one, but it's kind of the "opposite" one of the one involved in unwinding, at least for MCAT purposes). Topoisomerase is what relaxes the tension upstream of the replication fork when the DNA is being unwound by helicase, so if topoisomerase were inhibited, the unwinding process would go haywire (although, I guess it would still be unwound, albeit in a faulty, useless way).

I should have been more precise by saying gyrase relaxes positive supercoiling by introducing negative supercoiling... I was looking at it from a standpoint that positive supercoiling condenses DNA and by preventing ( or introducing negative) supercoiling, DNA become uncondense.

 

[navho77]

I should have been more precise by saying gyrase relaxes positive supercoiling by introducing negative supercoiling... I was looking at it from a standpoint that positive supercoiling condenses DNA and by preventing ( or introducing negative) supercoiling, DNA become uncondense.

Right, what you're saying is true, but only if the DNA is positive supercoiled to begin with. In that case, introducing negative supercoils would uncondense the DNA. In bacteria, however, the circular DNA starts from a "0" point, so introducing negative supercoils has a net affect of condensing the DNA. The key is that supercoiling itself is inherently going to condense the DNA if starting from a point of no supercoiling to begin with, no matter whether it is positive or negative, and that is what I believe the OP's question was going for.