Video Transcript
In this video, we will learn about
changes in DNA that we call mutations. We will see how these mutations can
affect the information that DNA carries and learn how to identify the different
types of mutations on both genes and chromosomes. We’ll also learn more about
spontaneous mutations and induced mutations.
Our body has hundreds of different
types of cells, each of which perform very specific functions. Each cell type helps keep our
bodies systems running smoothly, keeping us alive, active, and healthy. Each of these cell types depend on
our genetic material, DNA, for the instructions needed to perform their functions
properly. In this video, we’ll learn about
the mutations in DNA that can change these instructions. These mutations can introduce
errors in the way these cell types function, which can lead to serious medical
problems.
There are two types of mutations,
genetic mutations and chromosomal mutations. We’ll discuss both of these types
of mutations. But first, let’s review the
structure of DNA and how it can be used to provide instructions for cells. Our DNA is stored in the nucleus of
our cells as structures called chromosomes. Chromosomes contain highly
compacted DNA that’s wrapped around special proteins called histones. If we unravel the DNA, we can see
its structure here. DNA is made up of two strands that
twist together to form a shape called the double helix. The colored lines we see in between
these two strands are called nitrogenous bases. And they come as four different
types: guanine or G for short, represented in orange; cytosine, as shown in blue;
adenine, as shown in green; and thymine as shown in pink.
If we zoom in on this section of
the molecule and untwist the helix, we can see that these bases can pair with each
other to form a base pair. Guanine always pairs with its
complementary base cytosine, and adenine always pairs with its complementary base
thymine. So if we write out a sequence of
nucleotides, as shown here, then we can figure out what the complementary strand
will be based on these rules. So it’s often enough to just write
the sequence of a single strand since the other strand sequence is implied.
So how is this DNA sequence
converted into something useful? Well, Throughout DNA are multiple
genes that are made up of a specific sequence of these bases. So let’s pretend that this is a
segment of a DNA sequence for a gene. This DNA sequence can be decoded to
produce a protein with a particular function. To do this, there’s a couple of
steps. First, the gene undergoes a process
called transcription. This produces a copy of the gene
that we call messenger RNA, or mRNA, from DNA. mRNA is single stranded, and it
contains uracil in place of thymine which is in DNA. This sequence of mRNA can then be
translated to give a sequence of amino acids.
To do this, mRNA is decoded in
groups of three nucleotides that we call codons. Each codon codes for a specific
amino acid. The codon GUC codes for the amino
acid valine. The codon for CGG codes for
arginine. And the codon AGU codes for
serine. So the sequence of DNA in a gene
corresponds to a specific sequence of mRNA, which corresponds to a specific sequence
of amino acids. These amino acids can then fold
into a unique shape to form a protein with a specific function.
Now let’s talk a little bit about
mutations in genes or genetic mutations. There’s actually three types of
genetic mutations that we’ll be discussing. There’s the substitution mutation,
the insertion mutation, and the deletion mutation. Let’s start with the substitution
mutation. These sequences on the left are the
same as the sequences on the right. What we’re going to do now is
introduce a substitution mutation into the DNA sequence on the left, and we’ll see
what happens. So we’ll change this G to an A. This is an example of a
substitution mutation because one base was substituted for another. This mutation will then be carried
on into the mRNA.
So now this codon reads AUC,
whereas before it read GUC. AUC codes for a different amino
acid compared to GUC. So instead of the valine amino
acid, we now get isoleucine. So this mutation is also carried
over to the protein, which may now change its shape relative to the unmutated
protein. This might have an impact on the
function of the protein. And it may not even function at
all. Substitution mutations tend to
change single amino acids. So what about the other kinds of
mutations? Insertion mutations insert
nucleotides into our DNA sequence. So here we can see an extra adenine
inserted into the sequence. This is carried over into the
mRNA. And now, if we underline all the
codons in the mRNA, we can see that these first two codons are the same as the
original codons. But this last codon is
different.
Insertion mutations actually cause
what’s called a frameshift, because the sequence of codons that are being read
during translation, called the reading frame, has now changed after the point of
this insertion. This might be easier to understand
with a simple sentence like “the fox and the boy run.” Each of these words are three
letters long, like our codons in mRNA. So when we insert the letter p
towards the beginning of the sentence while keeping each word three letters long, we
can see that this sentence doesn’t make sense anymore because the reading frame has
been altered by inserting this letter, but only after this point of insertion, while
the first two words aren’t affected.
So now, if we get back to our
example, this altered codon changes from producing serine, as over here, to lysine
and also changes all the amino acids from this point forward. This causes our protein to
dramatically change from the original protein because most of the amino acids are
now different. And so it will have a completely
different shape and will be unlikely to function like the original protein. As you may now see, mutations can
be harmful if they change the structure of the protein and the protein becomes
nonfunctional.
The final type of genetic mutation
we’ll cover is the deletion mutation. For this example, I’ve extended our
DNA sequence a bit more. In this mutation, a nucleotide is
deleted from our DNA sequence, which is therefore also deleted from the mRNA
sequence. And now everything moves over. This causes a frameshift, just like
our insertion mutation, except this time we’re missing this adenine nucleotide and
everything is shifting over to the left. So now, when we look at the codons
in mRNA, again, they don’t match but only pass the point of this deletion. Just like the insertion mutation,
this causes all the amino acids to change further on from the point of the mutation,
which again causes a dramatic change in the protein structure. Deletion mutations can therefore
cause proteins to be nonfunctional.
Although this kind of mutation is
more likely to be harmful and leave the protein nonfunctional, it’s also possible
that it’s a beneficial mutation. Here, the protein may be more
effective in its function or may have a new function altogether. In this case, a beneficial mutation
may allow an organism to better adapt to his environment.
Now that we’ve covered genetic
mutations, let’s start looking at chromosomal mutations. Mutations in DNA can happen in the
sequence of bases, while chromosomal mutations can be much larger and affect the
whole chromosome. As we had seen earlier, most cells
in our body contain structures called chromosomes that are made up of DNA. This X-shaped structure is actually
a duplicated chromosome and is what we see after the chromosome has duplicated and
it’s preparing for cell division. When the cell isn’t preparing to
divide, the chromosome exists as a single unduplicated structure that’s represented
here. In reality, the chromosome is
actually a very long and thin structure. And it’s not compacted to look like
this. But we’re gonna draw it this way to
explain chromosomal mutations.
So here’s a chromosome. And on this chromosome, we have a
few genes indicated as these blue bands. In reality, chromosomes can contain
hundreds to thousands of genes. So you can imagine that if this
chromosome itself were mutated, then many genes may be altered, and this can cause
serious health issues. So now, we’ll look at three
different types of chromosomal mutations.
A chromosomal duplication is when a
region of a chromosome is duplicated. You can see this here. This makes the chromosome larger
because it now contains extra genetic material. It also has two copies of all the
genes that were contained in that region. This can increase the production of
those corresponding proteins, which can cause problems for the cell. In a chromosomal deletion, a region
of the chromosome is missing. We can see this here. Now this chromosome is missing
genes that were in that region. And this can cause problems for the
cell.
The last type of chromosomal
mutation that we’ll discuss is an inversion. Instead of representing the genes
as blue lines, as in the previous diagrams, we’ll represent them here as letters,
which will help us see how this kind of mutation works. In an inversion, a chromosome
segment breaks off and reattaches in an inverted orientation. So now instead of DEF, it flips
around to FED. In this type of mutation, the
amount of genetic material doesn’t change. Because of this, these kinds of
mutations may not lead to serious health problems, but they can still cause issues,
for example, with fertility.
Now that we understand how genetic
and chromosomal mutations work, let’s see what some of the causes are. There are two major types of
mutations, spontaneous and induced or acquired mutations. In spontaneous mutations, errors in
DNA replication can occur that introduce mutations in the sequence of the DNA being
copied. If this occurs in gametes, like our
egg or sperm cells, then this kind of mutation can be transmitted to every cell of
the embryo and therefore the human that develops. In this case, this mutation can
then be passed on to future generations. Induced or acquired mutations occur
because of exposure to external factors called mutagens, for example, radiation or
chemicals. These kinds of mutations can cause
single mutations in the DNA sequence or can cause breakage of different sections of
chromosomes.
Only the cells that are exposed to
these kinds of factors are at risk of having that mutation. These kinds of mutations are
generally not passed on to the next generation. As mentioned, mutations can be
harmful or even beneficial if a mutagen happens to alter a gene. But they can also be neutral if
they happen to occur in an area of DNA that doesn’t contain any genes. This is actually more likely to
occur because about 99 percent of all our DNA doesn’t contain genes. Now let’s apply what we’ve learned
to a practice question.
The diagram provided shows a
simplified outline of the different types of chromosomal mutations that can
occur. Which diagram, 1, 2, or 3,
demonstrates an inversion mutation?
This question is asking us about
chromosomal mutations. So what exactly is a chromosome
anyway? Inside most of our cells is our
DNA. Our DNA is organized on structures
called chromosomes. These are long linear pieces of
compacted DNA that contain a number of genes. We’re indicating these genes here
as a few colored orange lines, whereas in reality there can be thousands of genes on
a single chromosome. These genes provide the
instructions our cells need to properly function. A chromosomal mutation is when the
structure of this chromosome is somehow changed. This can cause problems for the
cell because these mutations might have an impact on the genes contained within the
chromosome.
There are several types of
chromosomal mutations, duplications, deletions, and inversions. Let’s take a closer look and see
how each of these can happen. To draw these, we’ll be labeling
different segments of the chromosomes with different letters to help us see what
impact these mutations have on the chromosome structure. In a duplication, a part of the
chromosome is duplicated or copied. In this example, we now have two
segments that contain the letter E. You’ll also notice that this makes
the chromosome larger because it now includes extra genetic material. It also has two copies of all the
genes that were contained in that region. This can increase production of
those proteins, which can cause problems for the cell.
In a chromosomal deletion, a part
of the chromosome is now missing. Now, this chromosome is missing
genes that were in that region, which can also cause problems for the cell. Inversion mutations, which is what
this question is asking us about, involve a segment of a chromosome breaking off
then reattaching in the inverse orientation. We can see this here, how the
section DEF is inverted to FED.
Now let’s turn our attention to the
diagram on the left and see if we can figure out which of the three mutations
represents a chromosomal inversion. Let’s make sure that we pay
attention to these colored bands on each segment of the chromosomes. We can use these to help determine
what happened to each mutated chromosome. In mutation 1, this section of the
chromosome is missing. This is an example of a chromosomal
deletion. We can also see a chromosomal
deletion in mutation 3. In addition, we can also see these
two sections duplicated in mutation 3. In mutation 2, we can see that
these two bands have been inverted. Therefore, mutation 2 demonstrates
an inversion mutation.
Now let’s go over some of the key
points that we covered in this video. Mutations are changes in the
sequence of DNA. Depending on what part of our DNA
is affected, mutations can be harmful, beneficial, or neutral. Genetic mutations include
substitutions, insertions, or deletions, while chromosomal mutations include
duplications, deletions, or inversions. Spontaneous mutations occur as
mistakes in copying DNA, while induced mutations are caused by external factors like
radiation or chemicals.