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Follow us on Twitter (@amoebasisters) and Facebook! DNA.
We talk about it so much—it is theultimate director for cells and it codes for your traits.
It’s a major component of whatmakes you, you.
When you have a really important molecule like DNA that is ultimately responsiblefor controlling the cell…it would make sense that when you make another cell (like in mitosis), you would also have to get more DNA into that cell.
And that introduces our topic of DNAreplication, which means, making more DNA.
First let’s talk about where and when.
Firstwhere—it occurs in the nucleus.
If the cell has a nucleus.
Remember, not all cells havea nucleus.
This video clip is actually going to focus on the types of cells that do havea nucleus though known as eukaryote cells.
Prokaryotes, which are cells that lack a nucleus, do things a little differently.
Next When does this happen—this typically happensduring a stage known as interphase.
Interphase is when a cell’s growing, it’s carryingout cell processes, and it’s replicating its DNA.
You know what it’s not doing atthe exact same time? Dividing.
You don’t want a cell to be replicating DNA and dividingat the exact same time.
That’s a little bit too much multitasking.
So DNA replicationdoes not happen during cell division (aka mitosis).
In fact, the cell replicates itsDNA before division processes like mitosis and meiosis.
Because once you make that newcell, you better have DNA to put in there.
I think DNA replication would actually makea great video game.
It’s actually quite exciting.
I’m going to introduce the keyplayers in DNA replication so that you can get some background information.
The majorityof these key players that I’m going to introduce are enzymes.
In biology, when you see somethingend in –ase, you might want to check as it is very possible that it’s an enzyme.
Enzymes have the ability to speed up reactions and build up or break down the items thatthey act on.
So here we go with the key players.
Helicase- the unzipping enzyme.
If you recallthat DNA has 2 strands, you can think of helicase unzipping the two strands of DNA.
Helicasedoesn’t have a hard time doing that.
The hydrogen bonds that hold the DNA strands togetheris pretty weak compared to other kinds of bonds.
DNA Polymerase- the builder.
This enzymereplicates DNA molecules to actually build a new strand of DNA.
Primase- The initializer.
With as great as DNA polymerase is, poor DNA polymerase can’t figure out where to getstarted without something called a primer.
Primase makes the primer so that DNA polymerasecan figure out where to go to start to work.
You know what’s kind of interesting aboutthe primer it makes? It’s actually a piece of RNA.
Ligase- the gluer.
It helps glue DNAfragments together.
More about why you would need that later.
Don’t feel overwhelmed.
We’ll go over the sequence in order.
Please keep in mind, that like all of our videos, we tend to give the big picture but there are always more details to every biologicalprocess.
There is more involved than what we cover.
DNA replication starts at a certainpart called the origin.
Usually this part is identified by certain DNA sequences.
Therecan be multiple origins within the DNA strand.
At the origin, helicase (the unzipping enzyme)comes in and unwinds the DNA.
SSB proteins (which stands for single stranded bindingproteins) bind to the DNA strands to keep them separated.
Primase comes in and makesRNA primers on both strands.
This is really important because otherwise DNA polymerasewon’t know where to start.
Now comes DNA Polymerase.
Remember, it’s the important enzyme that adds DNA bases.
Now you have 2 strands right? But they’re not identical.
Remember they complement each other.
They also are anti-parallel so they don’t reallygo in the same direction.
With DNA, we don't say it goes North or South.
The directions for the DNA strands are a little different.
We say that DNA either goes 5’ to 3’ or3’ to 5’.
What in the world does that mean? Well the sugar of DNA is part of thebackbone of DNA.
It has carbons.
The carbons on the sugar are numbered right after theoxygen in a clockwise direction.
1’, 2’ 3’, 4’ and 5.
’ The 5’ carbon is actuallyoutside of this ring structure.
Now you do the same thing for the other side but keepin mind this strand is flipped just because DNA strands are anti-parallel to each other.
So let’s count these—again, clockwise after the oxygen.
1’, 2’ 3’, 4’ 5’.
And the 5’ is out of this ring.
This strand on the left runs 5’ to 3’ and the strandon the right here runs 3’ to 5’.
Well, it turns out that DNA polymerase can onlyworks in the 5’ to 3’ direction.
So…the strand that runs 5’ to 3’ is fine.
Itis called the leading strand.
But the other strand will make it a little tricky.
DNA polymerasecan only go in the 5’ to 3’ direction.
(NOTE: Reads in 3' to 5' direction).
Primase has to set a lot of extra primers down to do that as shown here.
It takes longer too.
This strand is called the lagging strand which is pretty fitting.
On the lagging strand, youtend to get little fragments of synthesized DNA.
These are called Okazaki fragments.
What an amazing name.
The primers have to get replaced with DNA bases since the primerswere made of RNA.
Ligase, the gluing enzyme as I like to nickname it, has to take careof the gaps in the Okazaki fragments.
Now at the end, you have two identical double helixDNA molecules from your one original double helix DNA molecule.
We call it semi-conservativebecause the two copies each contain one old original strand and one newly made one.
Surely you have had to proofread your work before to catch errors? Well, youdefinitely don’t want DNA polymerase to make errors.
If it matches the wrong DNA bases, then you could have an incorrectly coded gene…which could ultimately end up in an incorrect protein—orno protein.
DNA polymerase is just awesome…it has proofreading ability.
Meaning, it so rarelymakes a mistake.
Which is very good.
That’s it for the amoeba sisters and we remind youto stay curious! Follow us on Twitter (@amoebasisters) and Facebook!.