Meselson and Stahl - Semiconservative DNA Replication

[MedPR]

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From mcat-review.org:

Meselson and Stahl proved this by experiment: Basically, they used heavy (15N) DNA as the old (pre-replication) DNA, and used light (14N) nucleotides for the synthesis of new DNA. They can tell the difference between heavy and light DNA by centrifugation. What they found was that when heavy DNA undergoes one round of replication in light nucleotides, the DNA made is of intermediate weight. After the second round of replication, the DNA is split between intermediate and light weight.
If DNA replication were completely conservative, only heavy and light DNA would be seen, and nothing in between. This was not the case.
If DNA replication were dispersive, everything would be of intermediate weight. Again, this was not the case because after the second round of replication, light DNA was seen.

I don't really get how this happens.

Say the original 15N nucleotides make up a DNA double helix that weighs 150g and the original 14N makes up a DNA helix that weighs 140g.

The first round of replication will yield DNA that weighs 145g.

Is the experiment saying that the second round yielded a mixture of 145gDNA and 140g DNA? Or is the "light weight" after the second round of replication something heavier than the original 14N DNA but lighter than the 145g DNA from the first round?

 

[MT Headed]

So you have this weird heavy strand that weighs 150g. We know it is really two 75g strands stuck together. And it is swimming in a soup of nucleotides that, if they were assembled into a double strand with the same information, the double strand would have weighed a more normal 140g. But it isn't assembled into a double or even a single strand. It's just a soup of spare parts.

Now imagine that weird heavy double strand of original dna undergoes a replication. What kinds of parts will it have on hand to create the complimentary strand? How much would the new single strand (glued to an old single strand) weigh?

What would happen if these two new double strands each underwent a replication, again swimming in a stew of normal weight nucleotide parts?

 

[MT Headed]

And I just have to plug audio osmosis here 🙂

Jon: ...and that's why it's called semi-conservative replication.
Jordan: 'Semi-conservative replication'? Jon, that sounds like moderate republicans having babies!

 

[chiddler]

And I just have to plug audio osmosis here 🙂

Jon: ...and that's why it's called semi-conservative replication.
Jordan: 'Semi-conservative replication'? Jon, that sounds like moderate republicans having babies!

lol

 

[MedPR]

So you have this weird heavy strand that weighs 150g. We know it is really two 75g strands stuck together. And it is swimming in a soup of nucleotides that, if they were assembled into a double strand with the same information, the double strand would have weighed a more normal 140g. But it isn't assembled into a double or even a single strand. It's just a soup of spare parts.

Now imagine that weird heavy double strand of original dna undergoes a replication. What kinds of parts will it have on hand to create the complimentary strand? How much would the new single strand (glued to an old single strand) weigh?

What would happen if these two new double strands each underwent a replication, again swimming in a stew of normal weight nucleotide parts?

I thought I got it, until I looked up a picture of a second replication cycle.

I completely understand the first replication, the two new strands each have 1 strand from the original. But what about the second replication? If answer to this next question is yes, then I understand. If not, then I'm still lost (obviously we are ignoring Conservative and Dispersive for now). In the semiconservative part of the image below, is the DNA being replicated with blue nucleotides only? As in, you put a red DNA into a pool of all blue DNA? So, in the second replication, the blue strands in the blue-red DNA are the new strands, and each of the blue-blue DNA contain 1 blue template strand and 1 blue new strand?

 

[MT Headed]

👍

Each half of any double stand serves as the 'old' template stand for duplication, and the new strand was assembled from the soup of available parts.

 

[MedPR]

👍

Each half of any double stand serves as the 'old' template stand for duplication, and the new strand was assembled from the soup of available parts.

Alright cool, thanks! I was confused about that picture because of the two blue-blue strands 🙂