[Music] behind the helicase molecules that are moving down the DNA we have two DNA polymerase 3s and two DNA G primases of these two enzymes it's usually argued that DNA polymerase 3 is the most important because it will catalyze the addition of new nucleotides so it's what's adding in to this growing strand here these new nucleotides however this enzyme is only able to synthesize in the five Prime to three prime Direction so we see five Prime here to three prime here and this is because of this group this functional group here this is a hydroxy group or an O group and that is necessary for the enzyme to catalyze the new bond that is being formed in this backbone so DNA polymerize 3 important but it can't start this process because it can add to something that's not there it hasn't got a three prime o so this is where DNA G primase comes in DNA G primase is going to provide the three prime o for DNA polymerase 3 it lays down an RNA primer of about 10 to 12 nucleotides so count these out there's about 10 to 12 in here these are going to be complementary to the DNA now remember that in RNA we see urine instead of thyine so that's going to pair with Adine here a and another thing that you may come across is you may come across for example GTP or GDP or even GMP these are all modified um nucleotides for guanine and that would be indicative that those are ribonucleotides I.E they're found in RNA versus GTP G dtp and GMP that would be found in DNA because they have this deoxy group associated with that nucleotide so the deoxy ribos in these particular nucleotides so that's just a little bit of a side and we'll probably come back to that during lecture period as well so what this enzyme does DNA G prise is it lays down this primer complimentary to the DNA and it provides a three prime o it doesn't need a primer itself to get going so it provides this primer and once it provides this three Primo then DNA polymerase 3 can now interact with that three prime o and start this addition of the nucleotides for the DNA strand now as a reminder as we go into this next process we have both a leading DNA strand which is continuous replication which is what we're seeing here but we also have lagging strands where the replication is discontinuous and we'll come back to that here as well in a moment