The Meselson-Stahl experiment provided the necessary...

The Meselson-Stahl experiment provided the necessary evidence to discover the mechanism by which DNA replicates. They accomplished this discovery by first culturing DNA with the heavy 15N nitrogen isotope. They then allowed the "heavy" DNA to replicate with DNA grown in normal 14N nitrogen. The density of each generation of replicated DNA was recorded by marking its position in a test tube after centrifugation. The position of each generation was then compared to the positions of pure 15N DNA and pure 14N DNA. Suppose that the first generation after replication revealed two bands after being centrifuged: one at the pure 14N mark and one at the pure 15N mark. Which of the following methods of replication would this observation support? Semiconservative replication dispersive replication transcrptive replication Rolling circle replication Conservative replication

Transcript

00:01 The meselson -starr experiment provided the necessary evidence to discover the mechanism by which dna replicates.

00:08 So, they accomplished this discovery by first coltran dna was heavy n15 nitrogen.

00:15 Then they allowed the heavy dna to replicate with dna grown in normal n14.

00:21 The density of each generation replicated dna was recorded by marking its position in a test tube with centrifugation.

00:28 So basically it's a density centrifugation.

00:31 The position of each generation was then compared to the position of pure n15 dna in the pure n14 dna.

00:39 Suppose that the first generation after the replication revealed two bands after being centrifuged, one at pure 14n mark and one at pure 15n mark.

00:49 Which of the following method of replication would the observation support? so let me talk about the real dna replication model we call it a semi -conservative and then we can go back to this hypothetical model.

01:02 So let's say this is the real semi -conservative model.

01:09 So what happened is you have first of all double -stranded dna.

01:14 Both strands has n15 because the bacteria has grown in n15 media for a long time.

01:21 So all the dna molecules are made up in n15.

01:24 Then you switch this n -e.

01:28 Coli bacteria to a regular media that has n14 only.

01:33 So after one round of replication, you'll actually see that due to a semi -conservative model, the two parental strands separate and then the new strand is being made by using the new n14.

01:54 So you can see now you go from one double -stranded dna to two double -stranded dna.

02:00 This is one round.

02:05 And then you end up having two and then each is a hybrid.

02:08 So if we do a density centrifugation before the dna replication, in the tube you'll see a heavy, pure n15 band.

02:22 But due to the semi -conservative model, then if you do another density gradient centrifugation, you actually will see a hybrid which is in between the n14 because the n14 should be somewhere here.

02:48 But due to the semi -conservative model, this both dna, double -stranded dna is a hybrid...

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