[Music] the central dogma of molecular biology is an explanation of the flow of genetic information within a cell and this can be divided into three stages replication where the DNA is duplicated transcription where the information from the DNA is transferred to RNA and then translation where this information that is being stored in RNA messenger RNA is used to build polypeptides that ultimately will make our proteins as a reminder DNA in protic cells is a single usually circular molecule the DNA has a right-handed Helix which we've introduced before and in its relaxed state it won't fit in the cell and there are approximately 10.5 bases per turn of that DNA and we mentioned this number just so it's easier to kind of understand what's going on now to fit this molecule into a procaryotic cell we have to undergo super coiling we can have negative super coiling and we can have positive super coiling negative super coiling happens when the DNA is Twisted about it's AIS in the opposite direction so it's against that of the right-handed double helix and these will unwind the DNA so instead of being 10.5 bases per turn we have less than that and that reliev stress that can be generated on the molecule and that is absolutely necessary during the process of replication which is where we're going to talk about this process the most but also in transcription this is also necessary so in replication and transcription that's where we're going to see negative super quiling and that's just because negative means it's easier for that DNA molecule to separate on the other hand some organisms or thermophilic archa for instance survive in high heat conditions and having this structure would mean it's a disaster for the cell because it would constantly underwinding positively wind their DNA so they will do that again in a different direction and they will overwind that DNA so they will go with that Helix and that means instead of being 10.5 bases per term we're now over that 10.5 bases per term and this helps them resist that denaturation that may come with those high heat conditions in prootic cells super coiling is introduced by enzyme called topoisomerases now there are several we're going to talk about the type two topoisomerases first and the one that you may recognize from other lectures is called DNA gyas DNA gyes is going to use ATP to break both strands of the DNA molecule so DNA gyes will bind to our DNA molecule as it binds to that molecule it's going to break both strands of the DNA so that's what we see here and what we see here that DNA gyres you can imagine like a big enzyme over here is holding on to these two strands it's actually coent attached to these two parts of that DNA molecule and it's holding them apart through this process using ATP what it will now do is it will take one of these ends and it will hold on to them but it will allow that intact strand that we're seeing here it will allow that intact Strand and it will pass that intact strand through the broken Strand and then it's going to stick them both together so what it's essentially doing is adding this extra coil so we can see that appearing in this structure so now this DNA molecule has one negative super coil on the other hand type one typ isomerases can relax positively or negatively super coil DNA so when it's positively or negatively super coiled and these ones are mostly ATP independent but they will only break one strand and because they only one break one strand they will allow these molecules to relax and then they will seal those back together that keeping that backbone intact is absolutely critical for the survival of the organisms