Behind the helicase molecules that are moving down the DNA, we have two DNA polymerase 3s and two DNAG 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 5' to 3' direction. So we see 5' here to 3' here. And this is because of this group, this functional group here. This is a hydroxyl group or an OH group, and that is necessary for the enzyme to catalyze the new bond that is being formed in this backbone.
So DNA polymerase 3, important, but it can't start this process because it can add to something that's not there. It hasn't got a 3' OH. So this is where DNA G primase comes in. DNA G primase is going to provide the 3' OH 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 uridine instead of thymine. So that's going to pair with adenine 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 nucleotides for guanine. And that would be indicative that those are ribonucleotides, i.e. they're found in RNA versus GTP, GDTP and GMP that would be found in DNA because they have this deoxy group associated with that nucleotide. So the deoxy ribose 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, DNAG primase, is it lays down this primer complementary to the DNA and it provides a 3'OH. It doesn't need a primer itself to get going.
So it provides this primer. And once it provides this 3'OH, then DNA polymerase 3 can now interact with that 3'OH 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.