Discussion
If the replication theory suggested by Watson and Crickâs model was correct, then the density of the double stranded DNA after 2 rounds of replication in 14N medium would be:
*This question is included in 05. DNA Structure and Function, question #5
| (A) | half heavy, half light |
| (B) | ... |
| (C) | ... |
| (D) | ... |
| (E) | ... |
| (F) | ... |
The solution is
Posted: 05/16/2013 07:11
I'm totally lost with this one. Anyone wang to chime in with this one?
Posted: 05/16/2013 10:39
Kiki, I agree with Arcadia Admin, be specific about which part you got lost in.
That said, let's give you some help.
Please keep in mind that the replications happen in a medium of light isotope (n14) Nitrogens, so all new strands will be N14 (light).
Since the passage states that the semi-conservative theory is confirmed, a new DNA double helix always contains one old copy, and another brand new one.
Let's trace step by step, assuming 1 bacterium to start with.
[Step 0]. Start with heavy isotope bacterium (from N15 or heavymedium). Place in N14 or light medium.
[Step 1]. Replication happens. DNA splits into two strands. Each strand is an N15 or heavy strand. Each strand gets coupled with a new strand / N14. So the result is two DNAs consisting of N15-N14 (intermediate).
[Step 2]. Replication happens again. N15-N14 DNA splits into N15 and N14 strands, and serve as templates for new strands (all N14), resulting in N15-N14 (intermediate) and N14-N14 (light) DNAs.
The same happens for the other DNA. Hence we have 4 DNAs: 2 intermediates, 2 lights.
I hope this helps.
That said, let's give you some help.
Please keep in mind that the replications happen in a medium of light isotope (n14) Nitrogens, so all new strands will be N14 (light).
Since the passage states that the semi-conservative theory is confirmed, a new DNA double helix always contains one old copy, and another brand new one.
Let's trace step by step, assuming 1 bacterium to start with.
[Step 0]. Start with heavy isotope bacterium (from N15 or heavymedium). Place in N14 or light medium.
[Step 1]. Replication happens. DNA splits into two strands. Each strand is an N15 or heavy strand. Each strand gets coupled with a new strand / N14. So the result is two DNAs consisting of N15-N14 (intermediate).
[Step 2]. Replication happens again. N15-N14 DNA splits into N15 and N14 strands, and serve as templates for new strands (all N14), resulting in N15-N14 (intermediate) and N14-N14 (light) DNAs.
The same happens for the other DNA. Hence we have 4 DNAs: 2 intermediates, 2 lights.
I hope this helps.
Posted: 05/16/2013 10:00
Kiki B, a suggestion. After you have read the explanation, be specific about the parts where you got lost. This will help those who want to help you.
Posted: 05/16/2013 12:25
Could you supply a visual explanation. I just don't get the concept
Posted: 05/16/2013 21:24
See if this helps.
Posted: 06/17/2013 21:18
It is basically math. When you have a group of N15 E. Coli bacteria dividing in N14 then first round of division would leave you with all intermediate. Next round being divided again in N14 leaving now 50% intermediate and 50% light. The concept being before division you have a double helix of both strands being heavy. Dividing in N14 would form all double helixes with one strand heavy and one strand light making it intermediate. Next division would give make the heavy strand bond to a light strand and the light strand to bond with another light strand. You now have one group that has 1 heavy and 1 light, and another group that has 2 lights. Thus making it half intermediate and half light.