Question

7. Questions: When a new strand of DNA is synthesized, in which direction is the new strand made? What enzyme is inhibited by the fluoroquinolone antibiotic ciprofloxacin, and why does this inhibit bacterial growth? Is ciprofloxacin more likely to be bactericidal or bacteriostatic if it inhibits only the enzyme used during replication? The Meselson-Stahl experiment showed that DNA replication is semi-conservative: a. b. c. i. Describe the experiment that Meselson and Stahl did. i. Explain what semi-conservative DNA replication means

I would need help with question 7B and C please!

Thank You

Answer 1

B) this antibiotic inhibit DNA gyrase enzyme ,which is responsible for ATP dependent negative supercoiling of newly formed DNA.

• without supercoiling of DNA, it is not functional i.e. it can not be expressed, so proteins can not be made from transcription and translation process.
• so ciprofloxacin is more likely to be bacteriocidal.

C)

• (ii) semi-conservative DNA replication means that one strand of parental DNA is conserved each time, so one strand is newly formed and one is conserved from parents.
• (i) Meselson-stahl experiment

• they used E.coli for their experiment, to prove Semi-conservative mode of replication.

1. First they grow E.coli on media containing heavy Nitrogen (N¹⁴), so that all E.coli that grow on that media will have DNA containing heavy nitrogen.
2. then they grow that E.coli containing N¹⁴ DNA on the medium containing normal Nitrogen (N¹⁵).
3. they extracted DNA from E.coli from different generations (i.e. extract DNA from generation 1 and generation 2 in different tubes)
4. then they performed "cesium chloride gradient centrifugation" (this method saperate DNA based on how much heavy nitrogent it contains). for three samples, (1) DNA from parental E.coli (where all DNA was heavy) (2) DNA from generation 1 (3) DNA from generation 2
5. result of this centrifugation : (in 1^st) they found only one band at heavy region

(in 2^nd) they found 2 bands, 1 at heavy region and one at medium region

(in 3^rd) they found 3 bands, 1 at heavy, 1 at medium and 1 at light region

These bands are as given below :

• So they concluded their results as below :

they concluded that replication is semi-conservative process.