Semi conservative and conservative replication Watch

Weronika_m
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#1
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Does anyone know the predictions for semi conservative and conservative replication?
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username2488725
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What do you mean by predictions? Do you want to know how they discovered semi-conservative replication?
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Weronika_m
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(Original post by CrazyFool229)
What do you mean by predictions? Do you want to know how they discovered semi-conservative replication?
What did they predict was going to happen for semi conservative replication as well as for conservative replication before they conducted the experiment, so how did they think the DNA double helix would look like for each theory if that makes any sense?
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username2488725
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So when Watson and Crick determined the structure of DNA, they came up with the theory that DNA was replicated semi-conservatively. Then two scientists called Meselson and Stahl did an experiment with two isotopes of nitrogen, N14 N15 (so N15 has one more neutron in it making it denser) in order to see if Watson and Crick was correct.

Here's a breakdown of the experiment

1) They had two samples of bacteria, one broth that had the lighter isotope (N14) and also the heavier isotope, N15. Whilst the bacteria replicated, it took up some of this nitrogen into its DNA.
2) Some of this broth was taken and spun into a centrifuge to show that the bacteria had incorporated the nitrogen into its DNA, as the broth with the heavier nitrogen settled at the bottom whereas the lighter broth settled at the top.
3) Then the bacteria in the heavy nitrogen broth was taken out and put into the light bacteria broth for ONE replication cycle and then this was spun yet again
4) They discovered that after one replication, the DNA settled in the middle. This is because HALF of the old double helix was mixed with HALF of the new nucleotides in the lighter broth causing it to set in the middle due to the two densities. This meant that it is SEMI CONSERVATIVE because (semi = half, so half of the DNA was made up of the old nucleotides, so HALF of the nucleotides are conserved)

Hopefully that makes sense.
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ErraticPhysicist
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Hey,
The Meselson--Stahl experiment determined, conclusively, whether DNA replication was conservative or semi-conservative. If DNA replication was conservative, the results of the experiment would have been that each replicated strand would have consisted of a genetic marker (they used an isotope of nitrogen). However, the experiment illustrate that only one strand of the DNA from the two produced had the genetic marker on which supported the idea that one of the strands (per pair) was from the parental DNA strand and one was newly created through DNA replication (blah...blah...).
I would postulate that the only obvious difference would be the mechanism of how replication of DNA occurred, if it were conservative, no new DNA strands would have been generated in this process (I don't know where else they would have been, but the mechanism and process would have been different). The fact that the Meselson--Stahl experiment supported the semi-conservative replication idea, supports the concept that free-floating nucleotides pair up in a complementary fashion to both the template and coding strands of the parental DNA strands rather than by some other method.
I hope this cleared some things up for you.
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Weronika_m
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(Original post by ErraticPhysicist)
Hey,
The Meselson--Stahl experiment determined, conclusively, whether DNA replication was conservative or semi-conservative. If DNA replication was conservative, the results of the experiment would have been that each replicated strand would have consisted of a genetic marker (they used an isotope of nitrogen). However, the experiment illustrate that only one strand of the DNA from the two produced had the genetic marker on which supported the idea that one of the strands (per pair) was from the parental DNA strand and one was newly created through DNA replication (blah...blah...).
I would postulate that the only obvious difference would be the mechanism of how replication of DNA occurred, if it were conservative, no new DNA strands would have been generated in this process (I don't know where else they would have been, but the mechanism and process would have been different). The fact that the Meselson--Stahl experiment supported the semi-conservative replication idea, supports the concept that free-floating nucleotides pair up in a complementary fashion to both the template and coding strands of the parental DNA strands rather than by some other method.
I hope this cleared some things up for you.
Thank you!!!
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Weronika_m
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(Original post by CrazyFool229)
So when Watson and Crick determined the structure of DNA, they came up with the theory that DNA was replicated semi-conservatively. Then two scientists called Meselson and Stahl did an experiment with two isotopes of nitrogen, N14 N15 (so N15 has one more neutron in it making it denser) in order to see if Watson and Crick was correct.

Here's a breakdown of the experiment

1) They had two samples of bacteria, one broth that had the lighter isotope (N14) and also the heavier isotope, N15. Whilst the bacteria replicated, it took up some of this nitrogen into its DNA.
2) Some of this broth was taken and spun into a centrifuge to show that the bacteria had incorporated the nitrogen into its DNA, as the broth with the heavier nitrogen settled at the bottom whereas the lighter broth settled at the top.
3) Then the bacteria in the heavy nitrogen broth was taken out and put into the light bacteria broth for ONE replication cycle and then this was spun yet again
4) They discovered that after one replication, the DNA settled in the middle. This is because HALF of the old double helix was mixed with HALF of the new nucleotides in the lighter broth causing it to set in the middle due to the two densities. This meant that it is SEMI CONSERVATIVE because (semi = half, so half of the DNA was made up of the old nucleotides, so HALF of the nucleotides are conserved)

Hopefully that makes sense.
Thank you!!!
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