00:02
So suppose mesocynostar labeled bacteria dna with heavy nitrogen and 15 stable isotope and with radioactive carbon and then allowed the bacteria to grow two generations in the presence of light and nitrogen and non -radioactive carbon.
00:20
If they extract dna from the second generation of bacteria and centrifuge it, a, how many bands of dna would be in a centrifuge tube? so in this case, we're talking about the semi -conservative dna replication model.
00:43
Let's say the first generation of dna, we use black.
00:53
So this black is labeled with n15 and also with radioactive.
01:03
Now it was transferred into light isotope, which is n14.
01:10
After the first round of dna replication, the two strands of the same double -stranded dna separate.
01:17
So this is why we call it semi -conservative.
01:19
You can see that for the new dna, half of them come from old dna.
01:24
Then the new strand is made of n14.
01:28
Okay.
01:29
And then, so now you have two double -stranded dna that both are hybrid of n15 and n14.
01:45
Now the second round, again, the black and the red, they will separate from each other and each will be used as a template to make new.
01:54
Again, you can see that second round, you have four double -stranded dna.
02:02
Two of them are hybrid.
02:04
Two of them are totally red.
02:07
So now, how many bands will there be in a centrifuge tube from the second generation? so we can see that if we have the centrifuge tube, you'll have one band that is a hybrid of n15 and n14 because it has a medium density...