hi everyone and welcome to learn a double biology for free aware miss astrick today's session is the second part of the DNA replication videos if you haven't seen the first one then just click the card at the top so you can see how DNA replicates yes today's session is going to be on mrs. Anand Starr's experiment which is the evidence for semiconservative replication so if you are new around it then click subscribe give it a thumbs up if you enjoy today's video we will be doing at the end a summary mind map of the investigation and flow diagram of each step so grab some paper and a pen so you can make notes if you wish so the first thing is the evidence are there collecting and what it's to try and prove Watson and Crick discovered the structure of DNA in 1953 with significant help from Rosalind Franklin's research on x-ray diffraction and once they discovered the structure of DNA they then started to hypothesize how DNA replicates and they came up with two key ideas they suggested that DNA must replicate either following conservative replication or semiconservative replication so let's go through what those two hypotheses are say the first one semiconservative replication this is where they suggested that from the original DNA when it replicates to make two lots of new DNA each time you'll conserve one of the original strands and the other strand will be made up of newly synthesized DNA so you are semi conserving because each time you are conserving one entire original strand which is then also combined with one of the newly synthesized strands hypothesis two is conservative replication and in this model they suggested that the original DNA after application would remain intact and the newly synthesized DNA would combine together so each time you are fully conserving the original DNA so Mickelson and stars experiment then is to prove which type of replication occurs and their experiment did prove it was semiconservative replication so let's go through their experiment first of all though we're going to go through some background information some basic facts which will help you to understand the experiment when we get to it so facts number one DNA bases are nitrogenous what we mean by that is they contain nitrogen and if you look over at the nucleotide the DNA nucleotide we've got the phosphate group the deoxyribose pentose sugar and here is the nitrogenous base which would be either adenine thymine cytosine or guan in the nitrogenous bases and we can see that it does contain nitrogen fact number two this experiment is going to be using two isotopes of nitrogen we're going to have the 14 N and 15 n isotope and the 14 n is slightly lighter 15 n is heavier now just a recap from GTC chemistry if you can't remember one isotope is what that means is it's the different forms of an element in which two proton and the electoral number are the same but what differs is the number of neutrons so last key point is bacteria will when you grow them you have to provide a growth medium which will have nutrients such as glucose water and they'll also need a source of nitrogen and in their experiment the bacteria are provided with one of the two isotopes and the bacteria will incorporate these isotopes to make the DNA nucleotides so we're going to focus on that third point a bit more the fact that I said when bacteria grow they take in the nitrogen isotopes in their growth medium to make the new DNA nucleotides what that means then is the bacteria that are grown in the medium containing only 14 n and so the isotope of nitrogen which is 14 n all of their DNA will only contain the 14 n I so tight so that DNA will actually be lighter in trust the bacteria which are only growing in the growth medium with 15n all of their DNA the nitrogenous bases will contain the 15 and isotope and they'll be slightly heavier and the way we can demonstrate this is by putting the DNA samples in a centrifuge and this is what we'd expect to see so if you were to spin the DNA samples from the bacteria that grown in the 15n isotope all of the DNA would settle at the bottom because it's more dense it's a heavier isotope in contrast the bacteria that were grade in the 14n if we were to separate out the DNA and then spin it in a centrifuge we'd then see that the DNA settles at a much higher position because it's less dense if you do want to have a recap on centrifugation and how to separate DNA I've put a link here at the top so you can see my video on self fractionation and ultracentrifugation which explains this whole process in a bit more detail so now we've got those basic facts we're going to go through their actual experiment recording their results and then seeing how this proves semiconservative replication so it's at this point you wanna pause the video get yourself some paper to write these notes as we go and if you're going to do that you will need a whole a full sheet of paper turn it landscape put these headings on and when you're ready press play again say the three columns I have generation which I'm summarizing this gene this is referring to how many grounds are bacterial replication have occurred and therefore DNA replication so generation zero we're not allow there's no replication that's occurred at this stage that's our start point then we've got generation 1 2 & 3 which are the later generations so each of these represents how many rounds and position have occurred in the middle I'm going to be showing the composition of the DNA so whether the double helix will contain a 15 n heavy or 14 n light strand of DNA and then finally if we were to put that DNA sample in the centrifuge where the DNA would settle so the first part of their experiment they grew eco light in a growth medium that only contained 15m so when the DNA was analyzed from these eco light all of the DNA was made up of two strands of 15n and the way that they knew that's what the DNA contained was from the results after centrifugation so we had all of the DNA settling at the bottom and what this Shadis was that all of the DNA must contain two strands of the 15n isotope DNA what they then did was they took all of the e.coli out of that growth medium and then they put it into a growth medium which only contained the 14 ni sa' tape and they allowed the bacteria to divide once only what would then be happening is in that one round of division after you've seen the DNA replication video you'll see that the double helix splits into two and each those strands acts as a template for DNA replication so we're conserving that original DNA one strand each and all of the new DNA will be formed from this 14 n isotope because that is all that is available to them in this growth medium so the new strand this time will be blue which is representing my light 14 n isotope so all of their DNA nucleotides only contain 14 n in their bases so now when we put this DNA sample into the centrifuge we still only get one band but it's shifted to a higher position and that's because this time order the DNA contains one strand of the heavier 15 n and one strand of the lighter 14 n so it's a middling density so that's why it's moved up slightly higher position in the test tube so now if the bacteria are still left in the 14 n growth medium and they're allowed to replicate for a second time so this time same thing again we get the two strands of each of the DNA molecules separates and acts as a template so we have the too heavy and we have also two light chains this time which are acting as a template but all of the new DNA is still only containing the 14n isotope so the second strand which is made up of the newly synthesized DNA will be 14 n all of them so this time out of our four double helixes we have two which are one strand 15 n one strand 14 n but now we have to double helixes which are two strands of 14 n and that's why for the first time we now get to bands in our centrifuge the middle band is made up of one strand of 15 n one strand 14 n and the second band which is the one that settled much much higher up is because it's less dense it's lighter and that's because it's composed of two strands of 14 n so we'll do one more round of division to the third round of division but the bacteria is still only being grown in the 14 n growth medium so same again all of these double helixes will now be split so each of the strands acts as a template so let's just split them into the templates and all of the new DNA strands will be made up of 14 n so here's our second strand for all of them so we've now got eight double helixes two out of eight or one-quarter are a combination of one strand 15 n one strand 14 n and all of the rest so six out of eight or three quarters are 14 n 14 n so in the centrifuge this time we still have two bands but the proportions are different 25% will be 15 n 14 n and 75% will be 14 and 14 n and the way you can visualize those proportions our percent figure Sheen is the thickness of the bands so that was Mickelson and stars experiment they were growing the bacteria in different growth mediums according to the isotope of nitrogen they then spun the DNA samples in the centrifuge and they looked at the position that the DNA settled in and from that they then did the section in the middle they worked out the proportion of 14 and 15 n in each DNA sample and that's how they realized it must be semiconservative replication because each time you can see the DNA splits into its two templates and one of the strands is conserved one is nearly synthesized so it's the last little bit on how this proves it was semi conservative and not conservative I've got on the top that first part of the semiconservative replication that we just looked at underneath is what we would have expected if replication was conservative so after the first round of division conservative replication states that's the original DNA will remain intact and the newly synthesized DNA will combine together as well so about G one generation one we would have expected after centrifugation to have two bands one band would be at the top because it would contain two strands of 14 N one band would be at the bottom because it would contain two strands of 15 n however that's not what meselson and stahl found they found 100% of the DNA was in the middle and that is how they disproved that it could not be conservative replication so that's it for Mickelson and stars experiment just a quick recap so Mickelson and stars experiment was to prove Watson and Crick's hypothesis and that was the hypothesis that DNA replicates semiconservative li the experiment involves growing bacteria in mediums containing two different isotopes of nitrogen the 14n isotope which is lighter and the 15 isotope which is heavier the nitrogen isotopes are incorporated into the DNA nucleotides and that is to make the nitrogenous bases and lastly DNA can be separated according to its density using a centrifuge and that is it for DNA replication semiconservative replication and missiles in style there are lots of questions available for this or miss Esther accom so if you click to practice questions and have a go at the topic 1 assessment booklets they'll be within there or if you go to the topic 1 questions page there'll be a booklet called DNA replication which tests the semiconservative replication process as well as a lot of questions on mucin and stars experiment so if you have found this helpful to date then please give it a thumbs up and make sure to subscribe so you can keep up to date with all of the latest videos [Music] [Music]