Lecture 5e - Physical mutagens

Hi, I'm Dr. Lesley Blankenship-Williams, and we are going to continue our discussion with mutagens

by looking at two physical agents that cause mutations. They are both high energy waves that

are found on the electromagnetic spectrum. So when we think about wavelengths, electromagnetic

wavelengths, we know that wavelengths can be pretty big and low energy, or they can be really

tight and high energy. And we can see that in this image here where, as we move from left to right,

we get to higher and higher energy. Visible light is here, but then directly to the right of it here

is UV light, or ultraviolet light. And then as you move farther over you get into a group

that are called the ionizing radiation, which includes x-rays and gamma radiation.

Now these higher energy electromagnetic waves have a tendency to excite electrons.

What does that mean? So in basic chemistry, electrons exist in an orbital, and, if they accept

or absorb certain amounts of electromagnetic energy, that can be popped out of that orbital

into a higher orbital. When the electromagnetic energy dissipates or disappears, they fall back

down into their original orbital, usually. So, again, the excitation of electrons is popping them

out of their orbital and then they fall back down. It's like, "I'm excited, I came back down to where

I was." Occasionally, when they pop out and then they fall back down, they may not fall back down

in the same way that they started. And that happens when DNA bases, or certain DNA bases, are

exposed to ultraviolet light. So let's first take a look at how UV irradiation causes mutations.

So when UV irradiation hits two adjacent thymines, like I'm showing here,

the two adjacent thymines can have their electrons excited. And when the electrons fall back down,

sometimes they actually fall back down together, such that a new covalent bond is formed

between the adjacent thymines. So let's take a look here, and see what I mean. So we've got UV

light that is going to be hitting these adjacent thymines. The electrons pop up.

When they fall back down, the electrons actually fall back down in a way that they are

shared between the two thymines, which should not happen. This is referred to as a thymine dimer

So UV irradiation is notorious for causing thymine dimers. So again, this is where two adjacent

thymines on the same DNA strand are now sharing four electrons in this double covalent bond.

Now, why is that a problem? If you imagine, for a moment, that I am a thymine and I'm sitting

here by myself, and there's my adenine up there. And I'm looking at my adenine and forming two

hydrogen bonds with my adenine, and totally happy with my adenine. And then my electrons get excited

and when they fall back down, I somehow now get compelled to be linked to the thymine next to me.

What happens is my base ends up kind of getting pulled down a little bit like this, and therefore

is no longer available to be hydrogen bonding with the adenines. So let me draw that real quick.

So I drew these little question marks by the adenines, like, "Where did our partners go? Why

are they all over there, cliquey? How come they're not bonding with us anymore?" Now you might notice

that I have drawn this like a bulge. So the reason why I drew it with a bulge in the thymine dimer

is because it actually happens in the double helix. The double helix should be equidistant

all the way around, meaning that if I were to take a double helix and take my hands and kind of feel

along that double helix, the width of my hand should not change. So if I'm feeling, it should

feel like, yes everything feels right, but imagine if there's a thymine dimer, there's gonna be a

big bulge there. So I actually can utilize that fact to identify where the thymine dimers are.

And so if I can identify where the thymine dimers are, I can remove them and replace them with fresh

thymine before the next round of DNA replication hits. Remember, the mutation is not set in stone

until the next round of DNA replication. So before I get into the repair mechanism, which does exist,

let me briefly talk about why this causes

a mutation. So when DNA polymerase comes along and tries to read that,

we know that it should be putting two adenines there,

but it doesn't see much. It just comes over and it's like, nothing fits because the thymines are

all cloistered down and hiding, and so DNA polymerase may only recognize

one thing in that space or it might recognize that there's two things and it just kind of guesses.

And moves on. And so you can see in my example there should be two A's, but now there's a C and

an A, and this is where the mutation comes in. So it's really important to get rid of the thymine

dimers before DNA polymerase replicates that DNA strand. So let's talk about that repair mechanism.

The fact that thymine dimers cause a bulge like there is fortuitous,

because it gives us an easy way to find them. So there is an enzyme, kind of a correction enzyme,

that E. coli has called photolyase. And photolyase is responsible for finding and replacing those

errant thymines. Now the word photo means light, and lyase refers to ligating which

means cutting and repairing this nucleic acid sequence. So photolyase is going to basically

feel along that double helix, find the bulge, and then it's going to go through and cut out

the region. And then it's going to replace it with fresh thymine so that there is no

mutation that shows up. Now most bacteria have photolyase or something comparable.

In E. coli, photolyase is only activated in the light.

In other words, it has to be light outside for this enzyme to work.

Now why would that be the case? Why not be active all the time? Well, UV light usually

only shows up when visible light shows up. If you think about the sun, for instance, UV

light and visible light are both produced at the same time. When it's dark out, the bacteria are

not going to be exposed to much UV light, if at all, so the risk of thymine dimers is much lower.

So the enzyme is only activated when visible light is there, and then it will start kind of climbing

up and down, I think, climbing up and down like a monkey, climbing up and down this double

DNA helix looking for bulges and replacing them with fresh thymine when it finds them.

Humans also have a comparable type of correction enzyme and that's a good thing, because thymine

dimers happen all the time. So in our bodies, the place that is most likely to get UV irradiation

is our skin, and when our stem cells in our skin, specifically our epidermis or perhaps melanocytes,

get thymine dimers and they don't get corrected in time, those thymine dimers can lead to mutations.

If you get enough mutations accumulating in any of those stem cells, then you can get skin cancer. So

UV irradiation leads to skin cancer. If we did not have a repair mechanism,

then we would all have skin cancer by the time we were like four years old.

So that repair mechanism is really fantastic, but even still it doesn't catch every last, uh,

thymine dimer. Clearly, because skin cancer is still occurring in our population.

So that ends our discussion on UV irradiation. Let's take a quick look at ionizing radiation.

Ionizing radiation is going to be the higher energy x-rays and gamma rays,

and so what we're going to do is kind of briefly talk about how this works.

Now the word ionizing radiation tells you exactly what it does. It creates ions. So

if you go back and take a look at the structure of any nitrogenous base, you will see a lot of

N's and O's and C's and H's, but you will not see a charge. Now DNA is negatively charged,

but only on the sugar phosphate backbone. The actual nitrogenous bases themselves are polar,

but not ions. So ionizing radiation causes the electrons to pop off, so much that they actually

become ions. When they become ions, they're no longer forming hydrogen bonds. So when DNA

polymerase goes to try to match a base that has been ionized, it cannot find a match and, again,

just randomly put something there, crosses its fingers that it's the right thing, and moves on.

And so consequently, ionizing radiation can lead to lots of mutations. It should not be a surprise,

then, that frequent exposure to x-rays can lead to cancer, especially cancer

in the bone marrow, like leukemia. Gamma radiation is even more problematic, and if you are exposed

to very, very high levels of gamma radiation, the actual covalent bond and the double, uh,

in the sugar phosphate backbone end up shattering. And I think of it like somebody spinning DNA like

glass and then throwing it on the floor, and just watching it shatter everywhere.

So you hear about people that die within a couple hours after extreme nuclear exposure, and they're

referring to the shattering of the DNA from gamma irradiation. You cannot recover from it.

Anyway, that concludes our lecture of looking at the kind of morbid but fascinating world of

physical mutagens, and how they cause mutations. Thanks.