Mutagens and carcinogens | Biomolecules | MCAT | Khan Academy
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Mutagens and carcinogens | Biomolecules | MCAT | Khan Academy


Voiceover: So, today
we’re going to talk about mutagens and carcinogens and how they can cause DNA damage, but first I want to
review the central dogma of molecular biology and
how the genetic information of a cell is stored in the form of DNA, which is then transcribed to form RNA and then translated to generate protein. Now, nucleotides from DNA are transcribed to their complementary forms on RNA, which are then read as
codons, or groups of three, to code for specific amino
acids in a larger protein. Now, if you mutate one of
the nucleotides on DNA, like turn this thymine
base into an adenine base, then that will affect the RNA sequence and ultimately the protein that follows. So, we say that mutations
are mistakes in a cell’s DNA that ultimately lead to
abnormal protein production. So, what is a mutagen? Well, a mutagen is any chemical substance or physical event that can
cause genetic mutations. Chemical substances, like certain poisons, could be mutagens or physical
events, like UV light or different kinds of radiation
could also be mutagenic, and we classify mutagens into
two different categories. So, let’s say we have a person over here. A mutagen could be
classified as endogenous, if it comes from inside
this person’s body, and it’s some mutagen that’s
already found in the organism, but an exogenous mutation
is one that comes from outside the affected organism, something that’s from
the external environment. So, what are some examples
of some endogenous mutagens? Well, the most significant
endogenous mutagens are what we call reactive
oxygen species or ROS, and ROS are naturally
occurring metabolites in the human body that are produced by mitochondria during
oxidative phosphorylation. So, if we have this guy here, who’s about to chow down on a big meal, you can expect that during
the metabolism of the meal, his mitochondria will
produce a bunch of ROS, like O2 dot minus, which
we call superoxide, which is an oxygen molecule
with one extra electron, as well as some hydrogen peroxide, which is another ROS that
your body can produce. Now, reactive oxygen species,
as you may be able to tell by their name, contain
oxygen, like both of these examples do, but they’re
also highly reactive with different cell components, including DNA, and by reacting with DNA,
they can actually cause significant damage to
a cell’s genetic code. One example of this type of damage is the double-strand break,
and ROS can actually break a DNA’s double helix
into two smaller pieces, and you can see why
this type of a reaction could cause a mutation,
since it quite significantly changes the structure of the cell’s DNA. The next type of DNA damage
that ROS can cause is base modification, and that’s when the nucleic acid bases are
changed or swapped around, and that can pretty readily
cause point mutations or maybe even other kinds. Now, you may be wondering why would a cell ever make something that
could damage itself? Well, it turns out that ROS actually have a couple of beneficial effects on a cell, and cells actually have a couple of ways to make sure that they don’t cause damage, but sometimes ROS levels get really high, and cells can’t deal with them anymore, and we call this oxidative
stress, and antioxidants are something that your
doctor might have told you that are good for you, and it
turns out that part of what antioxidants do is help make sure that ROS don’t damage your DNA. Now, let’s look at a couple examples of exogenous mutagens, and there are many different types
of exogenous mutagens, but we’re really only
going to talk about two. Now, intercalators are one example, and one of them is
called ethidium bromide, which you may be familiar
with if you’ve ever done a PCR experiment before,
and what ethidium bromide will do is it will jump
into a DNA double helix and stick itself between the two strands, and when these intercalators
intercalate into DNA, they can deform the structure of the DNA and cause some serious problems. Base analogs, like 5-bromouracil, which we also call 5-BU,
pretend to be a certain base, but then act differently than
that base normally would. So, in the case of 5-BU,
it’s an analog of uracil and looks a lot like it,
but once it’s incorporated into DNA, it can shift
between two different forms. In its keto-form, it will
pair best with adenine. While it’s in enol-form, it
will pair best with guanine. Now, if you’re familiar
with organic chemistry, you might know that
5-BU can convert between its keto and enol form
through something called a tautomerization reaction,
and overall you can see how this base analog might be able to induce mutations in a DNA strand. Now, the last thing we’re
going to talk about is what a carcinogen is. Now, carcinogens can be mutagens, but not all of them are, but in general, you can say that a carcinogen is something that can lead to cancer,
which, if you remember, is when cells in an organism
divide uncontrollably and can form big masses
of cells, called tumors. Now, some carcinogens will
work by making mutations in DNA that lead to cancer,
but sometimes they might carry out their effects
simply by increasing the rate at which a bunch of cells divide, without actually affecting their DNA, and some examples of carcinogens are tobacco, which come from cigarettes, asbestos, which used to be
used as home insulation, and even UV radiation. So, what did we learn? Well, first we learned that mutagens are chemical or physical substances or events that can increase the probability of genetic mutations occurring. And next we learned that
carcinogens are things that lead to cancer, and while they can be mutagenic as well, they
aren’t necessarily mutagenic.

9 thoughts on “Mutagens and carcinogens | Biomolecules | MCAT | Khan Academy

  1. I really like how Khanacademymedecine explain things detail by detail in a simple way so you can understand most of it.. Thanks for the videos keep it up.

  2. chemical carcinogenesis-highly reactive electrophile groups that directly damage DNA-mutations—-cancer
    it has 2 stages-
    1. initiation-
    -results from exposure of cells to asufficient dose of a carcinogenic agent—permanent DNA damage(mutations) it is rapid&irreversible and has memory
    initiators are of 2 categories-
    a.direct acting-req no metabolic conversion—alkylating agents
    b.indirectly acting- req met conversion—-polycyclic hydrocarbons(fossil fuels,smoked meats&fish),benzopyrene(tobacco smoke),azo dyes,aflatoxin
    2.promotion-
    -process of tumor induction in previously initiated cells
    there effect is short lived&causing irreversible DNA damages

  3. No, this person made a lot of wrong points and i'll explain how. Most importantly, he stated that carcinogenics, "can be mutagenic". They aren't anything else. However the most common genetic mutation that causes cancer is acquired mutation. "Cancer is a disease caused by genetic changes leading to uncontrolled cell growth and tumor formation." These genetic damages and changes are a form of genetic mutation. You can even see some sources saying that cancer can be caused by genetic (or)…environmental or constitutional characteristics of the individual as described in Stanford Health Care website. Let me clear this out, environmental factors that cause cancer, are causing cancer by damaging and altering genes, which is genetic mutation, via environmental sources. This is genetic related; it is not separate in any way whatsoever. "constitutional characteristics of the individual." would describe when someone which is born with or has genes which are susceptible to cancer; if not caused by carcinogens before or after birth in the first place, and having these genes susceptible to cancer after birth is a means of effected genes from parents being passed on to their offspring; and even improper birth results from using substances that are carcinogenic while the baby is in the womb, directly altering their genes while they are in the womb or other things that may cause these genetic damages pre-birth. This is all genes related and not seperate, in any way whatsoever.

    Now, where he differentiates cancer from mutagens because he was confused. Cancer is still a mutagen even though mutagens cause other things, that was a silly thing to get confused about. Mutagens; genetic mutations can also cause genetic disorders and so on. An example of a genetic disorder includes down's syndrome. And because, again, carcinogens cause cancer by means of genetic mutation, they are a mutagen, a subsection of mutagens, just as others are their own. They are mutagens just as mutagens that cause down's syndrome in that example are also a mutagen. To say cancer isn't a mutagen because mutagens also cause other things makes absolutely no sense at all whatsoever, you should not be taking this man seriously.

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