Captioning is on. Click the CC button at bottom right to turn off. Follow us on Twitter (@amoebasisters) and Facebook! DNA. We talk about it so much—it is the
ultimate director for cells and it codes for your traits. It’s a major component of what
makes you, you. When you have a really important molecule like DNA that is ultimately responsible
for 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 DNA
replication, which means, making more DNA. First let’s talk about where and when.First
where—it occurs in the nucleus. If the cell has a nucleus. Remember, not all cells have
a nucleus. This video clip is actually going to focus on the types of cells that do have
a 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 happens
during a stage known as interphase. Interphase is when a cell’s growing, it’s carrying
out cell processes, and it’s replicating its DNA. You know what it’s not doing at
the exact same time? Dividing. You don’t want a cell to be replicating DNA and dividing
at the exact same time. That’s a little bit too much multitasking. So DNA replication
does not happen during cell division (aka mitosis). In fact, the cell replicates its
DNA before division processes like mitosis and meiosis. Because once you make that new
cell, you better have DNA to put in there. I think DNA replication would actually make
a great video game. It’s actually quite exciting. I’m going to introduce the key
players in DNA replication so that you can get some background information. The majority
of these key players that I’m going to introduce are enzymes. In biology, when you see something
end 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 that
they act on. So here we go with the key players. Helicase- the unzipping enzyme. If you recall
that DNA has 2 strands, you can think of helicase unzipping the two strands of DNA. Helicase
doesn’t have a hard time doing that. The hydrogen bonds that hold the DNA strands together
is pretty weak compared to other kinds of bonds. DNA Polymerase- the builder. This enzyme
replicates 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 get
started without something called a primer. Primase makes the primer so that DNA polymerase
can figure out where to go to start to work. You know what’s kind of interesting about
the primer it makes? It’s actually a piece of RNA. Ligase- the gluer. It helps glue DNA
fragments 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 biological
process. There is more involved than what we cover. DNA replication starts at a certain
part called the origin. Usually this part is identified by certain DNA sequences. There
can 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 binding
proteins) bind to the DNA strands to keep them separated. Primase comes in and makes
RNA primers on both strands. This is really important because otherwise DNA polymerase
won’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 really
go 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’ or
3’ to 5’. What in the world does that mean? Well the sugar of DNA is part of the
backbone of DNA. It has carbons. The carbons on the sugar are numbered right after the
oxygen in a clockwise direction. 1’, 2’ 3’, 4’ and 5.’ The 5’ carbon is actually
outside of this ring structure. Now you do the same thing for the other side but keep
in 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 strand
on the right here runs 3’ to 5’. Well, it turns out that DNA polymerase can only
works in the 5’ to 3’ direction. So…the strand that runs 5’ to 3’ is fine. It
is called the leading strand. But the other strand will make it a little tricky. DNA polymerase
can 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, you
tend to get little fragments of synthesized DNA. These are called Okazaki fragments. Okazaki.
What an amazing name. The primers have to get replaced with DNA bases since the primers
were made of RNA. Ligase, the gluing enzyme as I like to nickname it, has to take care
of the gaps in the Okazaki fragments.Now at the end, you have two identical double helix
DNA molecules from your one original double helix DNA molecule. We call it semi-conservative
because the two copies each contain one old original strand and one newly made one. One
last thing. Surely you have had to proofread your work before to catch errors? Well, you
definitely 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—or
no protein. DNA polymerase is just awesome…it has proofreading ability. Meaning, it so rarely
makes a mistake. Which is very good. That’s it for the amoeba sisters and we remind you
to stay curious! Follow us on Twitter (@amoebasisters) and Facebook!