ShmoopTube

00:04

And here's your Shmoop du jour brought to you by radioactive nitrogen.

00:08

So uh…maybe don't get too close to the screen. [Man sitting watching television and grows an extra head]

00:10

We don't want to be responsible for any extra heads this video may cause…

00:14

Okay, here's our question…

00:16

Cells were given radioactive nitrogen so that their entire DNA became radioactively labeled.

00:21

The cells were then given non-radioactively labeled nitrogen and allowed to replicate

00:25

by binary fission.

00:26

A sample was taken at this point to quantify what percentage of DNA was radioactively labeled.

00:32

What percentage of the DNA would one expect to have radioactive nitrogen?

00:36

And here are our answers…

00:40

Have you heard of the phrase, “you are what you eat?” [Man eating an apple and a man made from apples appears beside him]

00:42

It's actually pretty true…

00:43

…At the atomic level, that is.

00:45

We don't see a lot of people walking around made of burritos, which is good…no one wants [Burrito's walking and bump into each other]

00:49

an uptick in the cannibal population.

00:52

In this question, cells were fed radioactive nitrogen, which if we're not mistaken, is [A cell eating radioactive nitrogen]

00:57

the start of almost every cell-based superhero movie.

01:00

The way it behaves in the cell is basically the same, but since it’s radioactive, scientists [A superhero cell wearing a red cape]

01:04

can keep track of where it goes among all the other nitrogen found in the cell.

01:09

In this question, the researchers feed the cells with 100% radioactive nitrogen, which [Scientist feeding a cell radioactive nitrogen]

01:13

means that all of the nitrogen in the cell will have this special nitrogen label.

01:18

That's also why these cells need to adopt an alter-ego in their crime fighting lifestyle.

01:22

And since DNA is made of nitrogen, all the nitrogen in the DNA will be radioactively [Nitrogen in a DNA helix radioactively labeled]

01:27

labeled.

01:28

Now, when the cells are allowed to divide, they're fed nitrogen that doesn’t have the

01:31

special radioactive sauce, which we think might be the same stuff put in A1...

01:36

So basically, this unlabeled nitrogen won’t give off any signal when incorporated into

01:41

the cell.

01:42

So the question is, what happens to the specially-labeled DNA when binary fission occurs? [A man fishing]

01:47

A better question is, what the heck is binary fission?

01:50

Binary fission is when a single cell divides into two daughter cells. [A cell divides into two cells]

01:54

Imagine if when you grew up, you decided to split yourself in half…and then those halves

01:58

grew into two full versions of yourself.

02:00

That’s basically what binary fission is, except less painful and less…creepy. [Man splits in half and two identical people appear]

02:05

But what happens to the DNA when binary fission occurs?

02:08

Of course, both daughter cells need a full set of DNA. [daughter cells hold hands and walk away]

02:11

This means that before binary fission can occur, the cell must perform…

02:15

…The role of the Phantom in The Phantom of the Opera.

02:19

But after that's done, the cell has to perform DNA replication. [Cell replicating it's DNA]

02:22

When DNA replicates, the original DNA double helix separates, and DNA polymerase comes

02:26

in to add new bases to each single strand, resulting in TWO new helices. [DNA polymerase adding bases to two single strands of DNA and creates two new strands]

02:32

So going back to our original question, let’s imagine the parent DNA, completely labeled

02:36

with the radioactive nitrogen.

02:37

Now, when the cell is being fed with normal nitrogen and it prepares to divide, what happens

02:42

when DNA replication occurs?

02:45

Each daughter cell will get one strand of the parent’s original DNA, and one strand

02:49

that was freshly made by DNA polymerase. [DNA polymerase adding bases to two strands of DNA]

02:51

In this case, that means that each daughter will inherit one strand with the special sauce…mm,

02:56

A1… …which means that after one round of cell

02:59

division, 50% of the DNA will be radioactively labeled.

03:03

Which means our answer is C, 50%!

03:06

Which is coincidentally, our steak to A1 ratio. [Man eating steak with A1 sauce]

03:09

…More or less.

Full Transcript