Video Transcript
In this video, we’ll learn what
mutations are and how they can have negative, neutral, or even sometimes beneficial
effects on organisms. And we’ll also take a look at how
mutations have sparked the diversity of life on Earth.
Mutations affect the structure and
function of DNA. So let’s begin by reviewing DNA
structure. If you line up your DNA molecules
from one cell from end to end, they’d be about two meters long. And that means that the average
length of one of your DNA molecules is longer than two centimeters, which is really
long for a molecule. These long molecules are made up of
tiny little units called nucleotides. Each molecule is made up of three
parts, including a sugar, a phosphate, and one of four types of bases.
Nucleotides bond together from base
to base, forming base pairs. The base pairs make up the width of
a DNA molecule, but the long length of a DNA molecule that goes far beyond the
screen is made up of millions and millions of nucleotides bonded from sugar to
phosphate group. The sequence of the four bases
along our DNA molecules make up our genetic code. The bases include A or adenine, T
or thymine, C or cytosine, and G for guanine. There are only two kinds of base
pairs because the base adenine only bonds with thymine and the base cytosine only
bonds with guanine. DNA molecules twist into the
familiar structure called the double helix.
The structure of DNA with its
lengths of genetic code leads to its function, which is to provide instructions for
organizing living systems, in other words, organisms. Notice the similarity between the
words organism and organizing. Their ancient word root means to
do. And that’s appropriate because
organisms have to stay pretty busy doing things to stay alive. But the instructions in DNA are not
as straightforward as those that you would find in some product that you have to put
together yourself or even the blueprints of a house. Instead, DNA codes for protein
molecules, which are made out of amino acids. And DNA also has start and stop
switches that control when and where these proteins are made.
Our bodies contain tens of
thousands, if not more, types of proteins, and they have many different important
functions. Sections of DNA, called genes,
contain the instructions for how to build these proteins. The genetic code in a gene is
copied in a process of transcription that produces a molecule called mRNA — and the
m stands for messenger because it’s a messenger that delivers the genetic code — to
little protein factories called ribosomes that use the instructions from the DNA to
build the proteins. This process is basically the same
in all organisms, but not all DNA is sectioned off into genes. Parts of DNA that don’t code for
protein are called noncoding DNA.
Scientists are still working out
the functions of noncoding DNA. But one function it does have is it
includes some of the start and stop switches that turn genes on and off. Next, let’s see what a mutation in
this gene would look like. Mutations are changes in the
genetic code, and it can be something as simple as replacing one base for
another. Changing one base out of six
billion may seem insignificant, and sometimes it is. But other times it can have a big
effect on an organism. Things that can cause mutations are
called mutagens, and they can be chemical, biological, or they can be some kind of
radiation, like sunlight or other types. And oftentimes, mutations come
about just because a mistake was made during DNA replication and the wrong base was
added.
Next, let’s look at a few different
types of mutations and the effects they might have. Here’s another example of a gene,
except instead of using the A, T, C, and G letters that represent the DNA bases,
this gene can use any letters. And it’s arranged into three-letter
words because genetic code is read in three-letter chunks. So while this isn’t a real gene, it
will help us to understand how changes in code can alter the meaning of a
message. The gene says that the hat was big
for the fat cat, and while that’s pretty silly, we can understand what it means. But let’s say there is a mutation
and the C is substituted for with a B. Now we have a FAT BAT with a BIG
HAT.
And next, we could have a mutation
that switches places of the A and the F in the word FAT. So it becomes AFT. Now our BAT is not FAT, but it’s on
the AFT or rear end of a boat. Now a mutation might come along
that inserts a base, forcing the rest of the bases to shift over one space. Now our sentence says THE HAT WPA
SBI GFO RTH EAF TBA, and the drawing is just an old-fashioned hat and some
meaningless scribbles. The same major changes occur if a
base is deleted. The reading frame of each word
changes, and we call these frame shift mutations. Now our sentence reads GTH EHA WPA
SBI GFO RTH EAF TBA and we’re left with only meaningless scribbles.
Mutations in regions of DNA that
don’t code for proteins may have no effects at all, unless that noncoding region is
a switch for a gene that turns the gene on or off, or if it’s a frame shift mutation
that runs into the gene. The bottom line here is that when
you change the code, it might change the instructions. And if the instructions are
changed, whatever you’re trying to make might not come out the same. Let’s start to take a look now at
how mutations in DNA affect proteins instead of how changes in sentences affect
cats.
Although actual genes are much
longer, here are a couple copies of the same gene, so in other words, identical
sections of DNA that code for the same protein. Let’s add a mutation to one of
these genes so that two of the bases switch places. The first step of protein synthesis
is to transcribe or copy a molecule of mRNA from the DNA gene, where we should have
a CG in our mRNA. Because of the mutation, we now
have a GC. The second step of protein
synthesis is called translation. And that’s when the genetic code in
the mRNA is read three bases at a time to determine the sequence of amino acid
monomers that make up the protein.
The sequence of amino acids from
the normal gene reads M, T — not the base T, but the amino acid T— C, and then a
stop signal, while the sequence from the mutated gene reads amino acid M, T, and
then L, which is different, and then a stop signal. The shape of these proteins depends
on the sequence of amino acids. So the shapes can come out
differently if one of the amino acids has changed. It’s important to note, though,
that not all mutations will change the amino acid. Here, ACC codes for amino acid T
and so does ACG. So some mutations have bigger
effects than others. Now we should probably review why
protein shape is important at all.
The reason is that structure leads
to function. Think about all the functions that
your hand has, and if your hand was like a horse’s hoof, would it have the same
functions? Let’s take a look at cell transport
proteins as an example. They’re shaped pretty much like a
tunnel, and their function is to allow certain things to enter or exit cells. But if their shape is affected by a
mutation, that may not be able to happen. An example of this is the mutation
that causes the disease cystic fibrosis.
Enzymes are also made out of
proteins most often. And they have a special area called
an active site, where they catalyze or speed up chemical reactions. If a mutation causes the active
site to lose its shape so it can no longer function, then active site won’t be
active, and those chemical reactions that it catalyzes won’t happen nearly fast
enough. Maple syrup urine disease, which
can be fatal, is an example of a disease that’s caused by a mutation that affects
the shape of an enzyme’s active site. Not all mutations have to be
negative. A few might be positive. An example is our lactase enzyme,
which has evolved since the agricultural revolution to allow people with this
mutation to digest milk throughout their lifetime instead of just as infants.
Most commonly, though, mutations
have no effect on the protein’s function. Sometimes they don’t even affect
the protein shape. But even if the shape does change,
it may not necessarily cause the function to decline. Another important example is what
happens when the shape and function of proteins that regulate the cell cycle are
affected by a mutation. The cell cycle is basically the
life cycle of a cell, and part of it is called mitosis. Mitosis is the division of the cell
so that there are then two cells. And that occurs over and over again
at a certain rate. Each cell will divide in two. But certain mutations in proteins
that affect the cell cycle can cause this rate of division to increase. And uncontrolled cell division like
this is what cancer is. So mutations can also lead to
cancerous cells.
So mutations usually have no effect
at all on protein function. But sometimes they do cause changes
to protein shape that affect function and that can cause genetic diseases or
cancer. But on the amazing and spectacular
side, mutations are also raw material for evolution. Mutations, which are mistakes in
genetic code, can lead to variation in a population such as a flower being yellow
instead of white. If the individual with the mutation
and the different trait reproduces at a higher rate than the others, it will leave
behind more offspring that look like itself than the others.
Without mutations, there could only
be one form of life. But since Earth keeps on changing,
it would probably go extinct once Earth changed enough. So mutations allow enough variation
for populations to adapt as Earth changes. And that way, life keeps going. A mutation is a change to the
genetic code of an organism. Mutations that happen within a
gene, which is a section of DNA that codes for a protein, can result in proteins
that still have same normal function or proteins that have decreased or a total loss
of function or occasionally they might even improve the function of a protein.
Proteins with decreased function
can lead to problems like genetic disease or cancer. But proteins with improved function
increase the variation in a population and can lead to evolution. Types of mutations include when one
base is substituted for another and inversions when two bases switch places. Entire sequences of DNA bases can
shift if a base is deleted or inserted.
