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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!

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