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ETD 45 A. What is the role of each ofhte 7 major enzymes involved in DNA Replication? B. Why does there have to be a leading and lagging strand?
January 24, 2014 at 8:08am


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Kevin Turek
January 25, 2014 at 10:43am

Parent: My son explained that there are many enzymes involved involved with DNA replication, but there are only seven that they went over in class. 1. There is helicase, which separates the two DNA strands. 2. Single stranded binding proteins stop the strands from coming back together. 3. Topoisomerase keeps ahead of helicase making sure no kinks appear, or stabilize this. Also it zips it back up in the end. Primase will then lay down the RNA primers. 5. DNA polymerase 3 will lay down DNA blocks. 6. DNA polymerase then removes the primers and fills in the gaps with DNA. 7. Then ligase links together the Okazaki fragments. Then Kevin told me that the names leading and lagging strands are a misnomer because they happen at the same time but the lagging strand has to do more steps then the leading strand. And because of this they have to  keep up with the leading strand.

Student: Again this was another concept that was so new to my mom. She knew about DNA and RNA but only knew about enzymes that broke down food and other substrates. These 7 were completely new. I just opened my ETD and read off the definitions and showed her the diagrams from the book. She was still confused. Guess I better study this more.


Sophie Wulfing
January 26, 2014 at 9:39pm

Helicase rips the helix.  SSBPS neutralizes charges to keep the split strands from reattaching.  Topoisomorase works in front of the helicase ans stablizes the nucleotides before they're ripped.  Primase lays down RNA primer "starting blocks".  DNA pol-3 lays down DNA starting at the RNA primers.  DNA Pol-1 replaces RNA primers with DNA.  Last one is DNA Ligase, links all the fragments together.

When laying down DNA, it has to be done from 3-prime to 5-prime.Leading strand can start stright from the 3 on the DNA and work its way to the 5.  But the lagging strand is opposite (5 to 3) so needs to jump forward and work its way back.  It's called the lagging strand because it has more steps in the process.

Sophie- I don't really understand topoisomerase and how it stabilizes the nucleotides so the helicase can run smoothly. It's also hard to remember the directionality that the enzymes have to run. Also, the reason why the RNA is laid down is because it can run from 3' to 5', and works as a starting block for the okasaki fragments.

Caroline Pedro
January 26, 2014 at 9:48pm


Helicase - unwinds the double helix.  Single strands binding proteins latch onto the DNA to block DNA reattaching to itself.  Topoisomerase - in front of the Helicase relieves the stress so the process is fluid.  Primase - enzyme that lays down the starting blocks (made from RNA).  DNA Pol-3 - lays down DNA from 5 prime to 3 prime.  DNA Pol-1 removes the Primase RNA primers and replaces with DNA.  DNA Ligase links all the fragments.  

As DNA unwraps 3 to 5 is the leading strand, but the 5 to 3 strand needs to build backwards (from 3 to 5) so it must do it in more steps.  


I learned that the primers were made out of RNA. I thought that RNA came after the primers, and were removed. But it turns out that the Primers are the RNA that are getting removed by DNA Pol I.

Riley Hajek
April 02, 2014 at 10:06pm

I learned that DNA replication is a very involved process because many enzymes are needed.  Each one has a specific role and they all need to perform their job in specific order otherwise replication is not possible.  The seven major enzymes and their actions are as follows: helicase unwinds parent double helix, single strand binding proteins binds to single stranded DNA, topoisomerase works in front of helicase to stabilize nucleotides, primase lays down RNA primers, DNA pol III makes new DNA strand from parental strand template, DNA pol I puts DNA nucleotides on 5' end and DNA ligase joins strands and pieces. 

The reason for the leading and lagging strand is because the two DNA strands are mirrors of each other and replicating each strand (which goes 3' to 5') is slower on the lagging strand because it has to do the replication in "chunks" which is slower.

Tracy  Hajek

Riley Hajek

I learned that the primase puts down RNA I did not know that.  These are later removed by DNA pol I and replaced with DNA.

Madison Boggan
January 22, 2015 at 5:31pm

Parent: There are many enzymes that play a role in DNA replication...so many to remember! But my two favorite analogies were:

-Topoisomerase: stabilizes the DNA like you stabilize your zipper so that it doesn't break (zip it too fast) 

-Ligase: is like permanently holding hands. L for ligase and L for love. Ligase links fragments together. 

A leading strand goes towards the replication fork, normal, but the lagging strand goes the opposite way. This is because of directionality. 

Madison Boggan
January 22, 2015 at 5:35pm

Parent: There are many enzymes that play a role in DNA replication...so many to remember! But my two favorite analogies were:

-Topoisomerase: stabilizes the DNA like you stabilize your zipper so that it doesn't break (zip it too fast) 

-Ligase: is like permanently holding hands. L for ligase and L for love. Ligase links fragments together. 

A leading strand goes towards the replication fork, normal, but the lagging strand goes the opposite way. This is because of directionality. 

Student: Other analogies that I taught my mom were: Helicase: unzips like a Human (double H) unzipping a zipper. SSBP's keep the fork open like road cones keeping a path open. Primase starts the show for polymerase like an opening band. DNA polymerase III lays DNA down like a person putting train tracks down normally while DNA poly I goes over the tracks and replaces RNA primers or puts down extra pieces of track to replace temporary pieces. 

My dad and mom will both buy the same product (replicate DNA) but my mom is LEADING--> very efficient and has one path while my dad is LAGGING--> goes backwards, picks things out in pieces (fragments). They both get the job done though! 

Mary Fuller
January 22, 2015 at 8:53pm

Student: DNA Replication has seven major enzymes that function in order to make the replication occur. Helicase unzips the double helix (breaking the hydrogen bonds between the base pairs). Single-stranded Binding Proteins (SSBPs) sit on the unzipped strand behind the helicase, holding the strand open like a door stop. Topoisomerase sits in front of the helicase on the DNA strand, stabilizing the strand by breaking and rebinding the hydrogen bonds momentarily to relieve stress. Primase lays down RNA primers which are like the starting blocks for replication. DNA Polymerase III lays DNA base pairs from 5’ to 3’.  DNA Polymerase I replaces the Okasaki fragments (chunks of RNA on the lagging strand) with DNA. DNA ligase binds together the Okasaki fragments to make one continuous strand of DNA. There has to be a leading and a lagging strand because the leading strand can start synthesis at the 3’ end and continuously replicate itself. But, the lagging strand is in the opposite direction and so in order to work properly from 5’ to 3’, it must do so in Okasaki fragments, and continue to lay down RNA primers.


Parent: Mary informed me about the process of DNA replication. It was especially interesting that the two separate strands have to duplicate in different manners. We talked about how directionality is vital to the process and that the DNA must always be duplicated 5’ to 3’. There are also seven key enzymes involved in the process.

Chloe Hobson
January 25, 2015 at 6:08pm

Parent: Chloe explained the roles of the seven enzymes in DNA replication. Helicase-unzips the helix by undoing the base pairs/hydrogen bonds. SSBPs-hold open single strand and act as road cones. Topoisomerase-stabilizes helix ahead of fork and prevents breakage. RNA primase-acts as the starting block for RNA. DNA polymerase 3-lays DNA 5'-3'. DNA polymerase 1-replaces RNA with DNA. Ligases-link together okazaki fragments. It was interesting to learn about  the leading vs. lagging strand and the differences between them due to the direction they are moving.

Student: It was helpful to remind myself of the roles of each enzyme because I understand that they are very important to the AP exam. There is a leading and lagging strand because the two run in opposite directions, and the DNA on the lagging strand has to be built in segments. I also know the importance of understanding directionality, which I explained to my mom. 

Sarah Lincoln
January 25, 2015 at 8:19pm

Parent - Scott Lincoln

In sitting down with Sarah, she did a great job in presenting how the 7 major enzymes are involved in replicating DNA.  1.) Helicase – this enzyme is used to unzip the double helix. 2.) Single Strands Binding Proteins (SSBP’s) sit on the strand and latch onto the DNA to prevent the helix from reattaching.  3.) Topoisomerase – this enzyme sits in front of the Helicase and relieves the stress and so stabilizes the helix.  4.) Primase - this enzyme lays down the “primer” or starting block. 5.) DNA Polmerase 3 - lays down DNA from 5 prime to 3 prime. 6.)  DNA Pol-1 removes the primers and replaces with DNA fragments.  7) DNA Ligase takes the different fragments and creates one big strand “the stapling of strands together”

When the strand unwinds, the lead strand builds 5 prime to 3 prime direction and is easier to replicate while the lagging strand is more complex due to the need of additional steps in order to allow 5 to 3 replication

Student - Sarah Lincoln

Being able to explain the steps and the uses of the different enzymes really reassured me that I knew what I was talking about and I was glad that I was capable of explaining this to someone else. It helped me to understand how the leading and lagging strands really worked as well and their different sequences in replication.


Hannah Johansen
January 25, 2015 at 9:44pm

Stig: My daughter explained that replicating DNA is very complicated. She said that there are many different enzymes involved in replicating DNA, but the class only focused on 7 different types. 1. Helicase unwinds the DNA and looks like a bubble. 2. Topoisomers stabilize the DNA. 3. SSBP's hold open the strand. 4. Primase is an enzyme that lays down the "starting blocks". 5. DNA Pol 3 lays DNA backwards. 6. DNA Pol 1 removes the primers and places DNA. 7. DNA ligase links together the fragments

Hannah: I was surprised that my dad knew that DNA replication involved enzymes and actually knew about RNA. I understand the process of DNA replication, I just have to study the definitions more because I get some of them confused. 

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