DNA replication is a team effort it seems a little complicated at first to learn about all the different entities or players that have a role in replication but once you learn who they are what they do I promise it gets a little easier and so here are all the players in action and I just want to show you this sort of up front to give you an idea of the big picture of where all these different molecular entities sort of fit in the role of replication before going through each of them one by one and so I just want to point out two here first is this enzyme helicase depicted by this little blue triangle looking thing and it is responsible for actually unwinding or unzipping the DNA double helix and once that's done you end up with the leading strand down here and then the lagging strand right here and then DNA primase is an enzyme that comes in and it synthesizes a little RNA primer for DNA polymerase to use as sort of a starting point to start laying down the new nucleotides once that's done DNA ligase comes in ligates the little fragments on the lagging strand called Okazaki fragments and then there are a few other other players such as DNA gyrase single strand binding proteins and then another one called clamp proteins that that also play little sort of side roles in this whole process now DNA replication like a lot of other molecular processes proceeds in three enzymatically catalyzed and coordinated steps those being initiation elongation and termination and now comes the first major idea to really take note of and that's the fact that DNA is read in a 3 to 5 prime direction so DNA is read in a 3 to 5 prime direction and it is synthesized in the opposite direction from 5 prime to three prime so it's synthesized from five prime to three prime so let's see what this looks like in the leading strand here we have DNA polymerase that is adding each of the nucleotides one by one and here we have the template strand I'm trying to trace over that right here and you can see that the direction that DNA polymerase is moving is from the three prime end to the five prime end here so it's reading the template three to five and you can see that the new strand that is being synthesized is being synthesized in the opposite direction it is being synthesized from five prime to the three prime end and this often gets confused and so it's a favorite concept to be tested and the way I like to remember it is if you were to draw out some numbers say from one to five you would naturally read it in the direction of three to five so if you just remember that DNA is read 3 to five then you'll remember that the synthesis has to happen in the opposite direction from five to three so now that we've got that down we can move on and talk about initiation first so replication begins at specific locations in the genome and those are called origins of replication so this here would be say draw it out here an origin of replication and so back to this enzyme DNA helicase will helicase breaks the hydrogen bonds holding the two DNA strands together and unwinds this helix and in the human genome there are multiple origins of replication that go on in a single strand of DNA and these regions tend to be a tea rich or just rich in adenine and thymine bases and that's because if you remember just the basics of DNA structure the bonds between adenine and thymine there are just two hydrogen bonds rather than the three formed and say a cytosine to guanine pair and so it's easier for DNA helix to unzip just the two hydrogen bonds between adenine and thymine base pairs now the resulting structure at once helicase has done its unzipping has these two prongs which are made up of a single strand of DNA and the one of the strands is called the leading strand and the other one is called the lagging strand and each of these strands forms the template for DNA polymerase to synthesize the new strand of DNA and this whole complex here is known as the replication fork which kind of makes sense with the leading and lagging strand serving as the prongs if you will for this fork now next the enzyme DNA primase shown here this little blue doughnut looking thing synthesizes a short fragment of RNA called an RNA primer which is then paired with the template DNA before the main workhorse of replication DNA polymerase can synthesize a new strand of DNA by extending the three prime end of this existing nucleotide chain and it adds new nucleotides to a template strand one at a time via the creation of phosphodiester bonds now the energy for this process of DNA polymerization comes from hydrolysis of the high energy phosphate bonds between the three phosphates attached to each unincorporated base so let me show you what this looks like say we have a nucleotide and here you can see the triphosphate group the sugar and the base and when a nucleotide is being added to the growing DNA strand the formation of the phosphodiester bond between the proximal phosphate to the nucleotide so this phosphate right here to the growing chain is accompanied by the hydrolysis of this very high-energy phosphate bond here let me draw it with a color that's maybe easier to see and so once this bond is hydrolyzed you have the release of these to distil phosphate groups so we kind of go off on their own and known as a pyrophosphate group and then the enzymatic hydrolysis of this pyrophosphate group of this bond here consumes a second high energy phosphate bond and renders the reaction effectively irreversible now in general DNA polymerase is really highly accurate it has an intrinsic error rate of less than one mistake for every ten to the seventh nucleotides added and in addition some DNA polymerase is also have proofreading capabilities there are also post replication mismatch repair mechanisms that can fix the errors that DNA polymerase makes and together all of the steps enable replication fidelity of less than one mistake for every ten to the ninth nucleotides added now since the leading and lagging strand templates are oriented in opposite directions at this replication fork there is a major issue of how to achieve synthesis in the nascent or new lagging strand of the DNA whose direction of synthesis is the opposite direction of the growing replication fork so you can see that the replication fork is opening in this direction and yet the synthesis of this lagging strand is in the opposite direction so let's take a closer look at this lagging strand now because of the way the lagging strand is oriented to the rest of the DNA double helix which has yet to be unwound the lagging strand is synthesized in these short separate fragments as you can see here the primates reads the template DNA and initiate synthesis of the short complementary RNA primer then DNA Paul a which is different from Paul Delta in the leading strand comes in and extends this primed fragment which then forms these short fragments of newly synthesized DNA called Okazaki fragments which you only find in the lagging strand the RNA primers are then removed replaced with DNA and the fragments of DNA are joined together by this enzyme here DNA ligase now let's move on to the other are sort of special teams players in replication the first being DNA gyrase here which is a type of topoisomerase as helicase unwinds the DNA at the replication fork the DNA up ahead is forced to rotate and this process results in a buildup of twists in the DNA head the buildup forms this torsional resistance this twisting resistance that would eventually halt the progress of this replication fork and that's where DNA gyrase comes into play DNA gyrase comes into play this enzyme temporarily breaks the strands of DNA which relieves the tension in the super coils and it achieves this by adding negative super coils to the DNA helix now another set of special teams players are the single strand binding proteins once helicase has unwound the double helix bares single-stranded DNA tends to fold back on itself forming secondary structures and these structures can interfere with the movement of DNA polymerase to prevent this single strand binding proteins bind to the unwound DNA until the second strand is synthesized and then finally clamp proteins form a sliding clamp around DNA helping the DNA polymerase maintain contact with its template and thereby assisting with processivity or processing the DNA template to make new DNA and finally we can talk about termination now termination and bacteria because they have circular chromosomes and when the two replication forks meet each other on the opposite end of the parental chromosome and because eukaryotes have linear chromosomes DNA replications unable to reach the very end of the chromosome but instead ends at a telomere region of repetitive DNA close to the end and shortening of the telomeres occurs each time DNA is replicated which is a normal process that occurs in cells and is further discussed in another video