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MCAT
Course: MCAT > Unit 5
Lesson 4: DNA- DNA questions
- Eukaryotic gene transcription: Going from DNA to mRNA
- DNA
- Molecular structure of DNA
- Antiparallel structure of DNA strands
- Telomeres and single copy DNA vs repetitive DNA
- Leading and lagging strands in DNA replication
- Transcription and mRNA processing
- Speed and precision of DNA replication
- Translation (mRNA to protein)
- Differences in translation between prokaryotes and eukaryotes
- DNA repair 1
- DNA repair 2
- Semi conservative replication
- Protein modifications
- Jacob Monod lac operon
- DNA structure and function
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Semi conservative replication
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IST‑1 (EU)
, IST‑1.M (LO)
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DNA replication has three possible methods: conservative, dispersive, and semi-conservative. The Meselson-Stahl experiment proved that DNA replicates in a semi-conservative way, meaning each new DNA pair has one old strand and one new strand. Created by Efrat Bruck.
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I feel as if this video is confusing... are dispersive and conservative replication actual types of replication or just theories? I feel like this should be addressed at the beginning before talking about the different types(38 votes)
After Watson and Crick proposed the double helix model of DNA, three models for DNA replication were proposed: conservative, semiconservative and dispersive. But Meselson and Stahl proved semiconservative is actual the mode of DNA replication . So, dispersive and conservative replication are just hypothesis which were proved wrong.(77 votes)
How did they prove it?(8 votes)
To summarize their experiment, they used "heavy" or N15 nitrogen to label the DNA for many generations. They had this thing called an ultracentrifuge which was able to make bands based on size. All the heavy nitrogen made a band that could be called the N15 band because both strands had heavy nitrogen. They were using E. coli I believe and transferred it from the heavy nitrogen containing medium to one containing normal N14 nitrogen. They examined the DNA of the first generation from this using the ultra centrifuge and found one band which was different from the N15 band. This was the N15/N14 band(because one strand was N15, the old, and the other N14, the new). The next generation had 2 bands, the N15/N14 band and one that was lighter, an N14/N14 band. This proved it was semiconservative. If it had been conservative, the first generation would have shown 2 bands (N15 and N14, not the middle band seen). Also, dispersive would have just made a mess.(42 votes)
- This video seems incomplete to me.
How did they prove semi-conservative? What was the basic experimental design? What data did they observe that supported the hypothesis of semi-conservative replication? What WOULD they have needed to observe to support one of the other theories?
I know the answers are given in a comment below, but this video should be reworked to include these parameters.(9 votes)
Oh I agree and I addressed that in my above comment. if you are going to make a video about replication, you should DEFINITELY include the M&S experiment and explain a bit more on the other THEORIES of replication.(2 votes)
- I understand the difference between the three different replications but how do you figure out the percentage of 14N and 15N contained in the DNA for each?(1 vote)
15N is heavier than 14N. Meselson and Stahl used that fact to tell which kind of replication is correct by putting the DNA into liquid in test tubes as it reproduced, generation after generation.
Step 1. grow bacteria (they used E.Coli) with only 15N present. Extract the DNA and put it in a test tube
Step 2. grow bacteria with only 14N present (second generation) and extract the DNA again
Step 3. grow bacteria with only 14N present (third generation) and extract the DNA again.
In the test tubes, the first step resulted in most of the DNA near the bottom of the tube, because 15N is heavy.
In step 2, because DNA replication is semi-conservative, the DNA rested at a half-way point between where 14N and 15N DNA would be (because each daughter DNA would contain half 15N and half 14N ).
The third step is where things get complicated:
4 daughter DNA molecules are produced in the 3rd generation (two from each parent). Two daughters with the 15N parent are now half 14N-half 15N. The other two daughters have the 14N parent, so they have purely 14N nitrogen in their DNA. This would result in a test tube with one half-half layer, and another 14N layer of DNA floating above it.
Here's a link to a diagram that can explain the experiment better than i can:
http://www.nature.com/scitable/content/18551/sadava_11_11_large_2.gif(3 votes)
What organisms use conservative replication and which ones use dispersive replication?(1 vote)
None. DNA replicates in accordance with the semi-conservative model. Conservative and dispersive replication models are incorrect.(1 vote)
- Can we program XNA( Xeno nucleic acid) to do Conservative replication or dispersive replication?(1 vote)
Video transcript
- Let's take a piece of DNA. And let's just unwind it into its two strands. And just to help us to remember that DNA is a very, very long molecule, I'm gonna put arrows here on our two strands of DNA. And the question I want to ask you is, "If we were to replicate this DNA, "what would the end result look like?" So, I'm kind of skipping
over the entire process of how the DNA is replicated and focusing just on the product. And so we have three choices. The first is conservative replication. And in conservative replication we have our old pair of DNA and then we synthesize a
completely new pair of DNA. So you can see the old pair, that looks just the same as
what we had before, in yellow. And then we have a completely new pair which is represented in blue. Our next choice is dispersive replication. And in dispersive replication, we're gonna end up with two pairs of DNA. And in each one of those pairs we have some old DNA and new DNA dispersed within that double strand of DNA. So you can see there's yellow and blue mixed up together. And it wouldn't necessarily
have to be in the ratio that I drew it in. I drew it in this kind of neat ratio where the yellow and
blues are the same size. But perhaps the yellows
would be a little bit bigger and maybe some of the blue parts smaller or vice versa. And the third option we have is semi-conservative replication. And in semi-conservative replication, each pair has one old strand, that you see in yellow, of course, and one new strand, that's in blue. And this question was
answered by two scientists. One by the name of Meselson and one by the name of Stahl. And they conducted a famous experiment which was named after them. So the Meselson Stahl experiment. And in this experiment they proved that DNA replication is semi-conservative. So, this is how DNA is replicated.