thus far in our discussion on DNA replication we discussed the process by which our double stren DNA molecule must unwind itself before replication actually takes place and we said that a special type of protein known as DNA helicase must bind to the origin of replication on the double trand the DNA and this helps break the hydrogen bonds that exist between our adjacent nitrogenous base is on our adjacent nucleotides and this therefore unwinds and unzips our double stranded DNA and exposes the single stranded DNA molecule so basically our DNA helicase binds to the origin of replication and as it moves it breaks the hyren bonds and it unwinds our singl stranded DNA molecules and after we expose the section s another type of enzyme known as singl stranded DNA proteins or simply SSB proteins bind to those exposed regions and they allow they keep the two single strands from reassociating and reforming the hydrogen bonds so these singl stranded binding proteins are shown in Green in this diagram so once that takes place another enzyme known as DNA gyas binds onto our double stranded DNA and it basically creates or introduces negative super coils and this decreases the stress that is involved with the process of unwinding now once we actually unwind our doubl stranded DNA molecule and we expose these single stranded DNA regions another type of molecule known as primase which is basically an RNA polymerase creates primers or RNA primers remember an RNA primer is basically a sequence of nucleotides that are needed for a DNA polymerase to bind and to begin the synthesis of our daughter strand now how exactly does DNA polymerase actually synthesize our da strand well basically DNA polymerase Act as a catalyst it catalyzes the formation of our phosphodiester bonds so it takes the nucleotides that are found in the environment so these are free nucleotides and attaches these nucleotides together via phospho diaster bonds or phospho diaster linkages in the process every time we form a phosphodiester linkage by using the DNA polymerase we release a pyro phosphate into the environment and this basically drives the process of DNA replication now DNA polymerase can only read the parent strength in the 3 to five Direction and this implies that it can only synthesize The New Daughter DNA molecule in the five to three directions so to see what we mean let's take a look at the following Di diagram so we have the DNA helicase that binds at the origin of replication eventually it moves a certain distance and it ends up at the location known as the fork of replication the fork of replication is basically the location of our DNA helicase so we can imagine that DNA helicase moves in the left directions towards the left along the xaxis we have the DNA gay that basically induces those negative super coils that decreases the stress involved with unwinding and we have the SSB proteins the single stranded binding proteins that basically act to make sure that those two single strands do not reassociate with one another and once we unwind what happens is the DNA polymerase shown in red basically binds onto our single stranded DNA molecules and it creates those phospho diester bonds by combining by attaching our nucleotides together so first we form the RNA primer by using our primase enzyme and this is shown in purple and then we basically create those other nucleotides by using our RNA polymerase or DNA polymerase and notice that for the case of the parent strand that runs from the from the three to the five Direction the DNA polymerase basically creates the nucleotides beginning with a five and ending with the three end so we see that DNA polymerase has no problem synthesizing our daughter strand on the parent molecule that runs 3 to 5 however what happens in this case for the the parent strand that begins with the five and ends with the three if our DNA polymerase attaches to this side it must synthesize in the three to five Direction and that is not allowed remember the DNA polymerase can only synthesize in the five to three Direction it cannot synthesize from the three to five Direction so we see that this strand that is formed by using the three to five pair in strand is known as the leading Strand and it's known as the leading strand because it is synthesized continuously without much problem however how exactly does the DNA polymerase actually synthesize the other parent strand that runs from the five to the three Direction so let's take a look at the following diagram and let's determine how this actually takes place so let's begin with our parent strand that runs from the three to the five directions so we can imagine that DNA helicase attaches to the origin of replication it moves a certain distance and it unwinds our double stranded DNA so first we have the primase that lays down the RNA primer so let's suppose we have our RNA primer as shown and then what happens our DNA polymerase basically continuously forms our nucleotide piece by piece so we form the following continuous Strand and notice the formation actually takes place in the same exact direction as the movement of our helicase so the helicase this enzyme moves to the left along our xaxis and the replication also takes takes place in the same directions toward towards the left along our x axis so because this is the three end this end of The New Daughter new uh strand is our five end and this is the three end and this makes sense because our DNA polymerase can only synthesize in the five to three Direction but what about this case what happens here notice the same thing cannot happen here because if that happened because this is the five end the first nucleotide must be the three end and our DNA polymerase cannot synthes synthesize in the 3 to 5n so instead what happens is we use our primase and the primase instead of forming only one RNA primer we form many RNA primers so we form as many RNA primers as we can so once we form the RNA primer as our DNA polymerase moves this way we see that our nucleotides are basically formed in the backward Direction going this way so as our DNA polymerase moves this way along our parent strand that runs from the three to five the one that runs from the 5 to three it takes place in the opposite direction going this way so so we form backwards with respect to the movement of our helic so we basically form going this way this goes here this goes here this goes here and this goes here so this is known as the lagging strand because it lags behind the leading strand so the leading strand is formed continuously and our DNA polymerase moves in the same direction as the motion of the fork as the motion of our helicase but for the lagging strand our direction of the DNA polymerase is backwards it's reversed and this is important because it basically ensures that the DNA polymerase forms in the five to three directions so for the LA uh for the lagging strand so let's designated using our red color this end is the five end and this end is the three end so we see going this way we form the five to three end and this we form the five to three and and that makes makes sense because DNA polymerase can only form the new strands in the five to three Direction and each one of these individual fragments individual pieces are known as the okas Sagi fragment so once again for the leading strand for this strand here the DNA polymerase uses the primer to initiate replication so this purple section is the primer our DNA polymerase binds onto our primer and it moves along this fashion along this direction in a continuous fashion adding the nucleotides continuously piece by piece in the forward Direction in the same direction as the movement of this DNA helicase but to synthesize the other strand known as the Ling strand primase creates many RNA primers as far as possible down our parent strand that runs in the five to three Direction and then DNA polymerase Works backwards with respect to direction of the movement of this replication fork so the replication fork moves this way this is synthesized this way but this is synthesized in the opposite direction and we see that the DNA helicase basically reads the parent strand in the allowed 3 to five Direction and it forms the lagging strand in the five to3 Direction so therefore the polymerase forms the laging Strand in a piecewise fashion so piece by piece in a discontinuous manner and each one one of these pieces is known as the okas Sagi fragments now this process is important because it basically ensures two important things firstly as our helicase moves this way we we see that the DNA polymerase is able to actually form the leading Strand and our lagging strand at about the same exact time and this type of process also ensures that the DNA polymerase creates both of these strands in the five to3 Direction so going this way we synthesize in a 5 to3 fashion and going backwards we also synthesize in the 5 to three fashion which is the only way by which DNA polymerase actually synthesizes our new daughter DNA strands now once we actually form all these okas doy fragments once we form the lagging strand we have to connect those lagging strands and the way we connect those strands is we remove these purple primers and we replace them with the proper nucleotides and we form we connect the nucleotides by forming the phosphodiester bonds and the enzyme that basically does this is known as DNA ligase so once the okasi fragments are formed an enzyme called DNA liase connects the okas Sagi fragments by creating phosphodiester linkages between our adjacent okas Sagi fragment so this is the process by which DNA is replicated so