DNA Replication: The Cell's Extreme Team Sport

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!