To clarify, english isn’t my first language; the title might sound like I assume that it should be dispersive, which would make this a question about an opinion and therefore against the rules, but I know and understand that it’s semi conservative, I just don’t understand how its proven. thanks in advance for replies 🙂
In: Biology
I’m surprised you didn’t get any replies yet. Okay well I’ll explain the experiment.
You have two DNA strands. When the cell replicates, you could either end up with the strands separating and acting as template for the new strands, where you have two sets of strands, each containing an original parental one. This is called semi conservative, the correct model.
Or, you have the double stranded DNA just photocopies itself as is, and you end with two sets of strands, one is the original pair and one is a totally new pair. That’s the conservative model.
Or you could have the double stranded DNA just chop itself up in pieces and the pieces are then duplicated. Then these pieces, old and new, are randomly dispersed to each pair of DNA strands. You end up with a cocktail of DNA strands containing segments from parent and new. This is the dispersive model.
Now to know which is true, Meselson and Stahl did this beautiful experiment. They got ecoli and incubated them with heavy nitrogen, it contains an extra neutron. Functionally this is the same as normal nitrogen, but you can tell them apart when you centrifuge on a density gradient. So anyway, when the ecoli were incubated with N15 for several generations, virtually all DNA in these bacteria was built with N15.
Then they got these ecoli and let them divide once in N14 media (20 min), then again (20 min) and so on. They isolated the DNA, and centrifuged it on the gradient to see what you got. Now let’s look at how the results would look like if each model was true: assume a single N14 Strand is like 100 AU (arbitrary mass or density or whatever units) and N15 is 200 AU.
Semi conservative: the end result should be that all the progeny of the bacteria have DNA whose half is N14 and other half is N15. So on the gradient you should get only one band, and its density should be 300 AU. As the more and more bacteria divide, you should get a second band that keeps getting more intense, one at 100 AU. You have to draw this to understand.
Conservative: here you should get two bands for the first cycle, equal bands, one at 200+200 AU (400) and one at 100+100 (200 AU). Next cycles you’ll just keep getting a more and more intense 200 AU band.
Dispersive: here you should get theoretically one band in the first cycle, where half each Strand or so is N15 and the other N14. So should be 300 AU. And as you go to higher cycles, you should be approaching the pure N14 Band gradually, which is at 200 AU. So you’ll have many intermediate bands along the way.
If you draw this it’ll get very clear. Anyway, they did this and found the results predicted by the semi conservative.
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