The diagram provided shows a
simplified outline of the different types of gene mutations that can occur in a
strand of DNA. Which diagram, one, two, or three,
demonstrates an insertion mutation?
This question is asking us to
remember what an insertion mutation is. Let’s discuss a few different types
of mutations before we return to the diagram and try to answer our question.
In order for a gene in DNA to be
converted into a protein, it needs to go through a couple of steps. The first step is called
transcription. Here, the DNA sequence is copied to
make an mRNA transcript. The sequence of nucleotides in mRNA
can then be converted into a sequence of amino acids during translation to form a
polypeptide which can then fold into a protein.
Let’s look at how mRNA can be
translated into a protein sequence. Here’s a nine-nucleotide sequence
of mRNA. When this sequence is translated,
it’s done so in three nucleotide segments called codons. These codons correspond to specific
amino acids. So the codon AUG corresponds to the
amino acid methionine for example. These amino acids are matched to
the correct codons during translation, and each codon is read consecutively. This is sometimes called the
This mRNA sequence is copied from
the DNA sequence, which you can see here. Remember, uracil in RNA is replaced
by thymine in DNA. A genetic mutation is a change in
the nucleotide sequence of DNA. There’s different types of
mutations that are possible, so let’s look at each of them now.
Insertion mutations insert a
nucleotide into a DNA sequence. Notice how we now have an
additional nucleotide, a thymine, inserted into our DNA sequence. So now there’s 10 nucleotides
instead of nine. This mutation will be carried over
into the mRNA as we can see here.
Notice that the last two codons are
now different from what they were previously. Because the sequence of codons have
changed, this changes the amino acid sequence too. Also notice how multiple amino
acids can be impacted by a single insertion mutation. You can see these two here that are
now different from what they were. This is because the reading frame
has changed from the point of the insertion mutation onwards.
Now let’s reset our sequence and
see what happens in a deletion mutation. Here the nucleotide is deleted, so
all the nucleotides in front of this mutation will be shifted over. You can see that in the DNA
sequence here. Notice how there’s eight
nucleotides instead of nine now. You can see this carried over to
the mRNA sequence as well, which changes the codons and therefore changes the amino
acid sequence in the protein. This kind of mutation also causes a
frameshift, just like the insertion mutation.
Let’s again reset our sequence and
look at the final type of mutation, a substitution mutation. Here, thymine is substituted for a
guanine. Notice how we still have nine
nucleotides in total, which again is copied over into mRNA and changes the
codon. Now the amino acid proline replaces
histidine. Notice how the two amino acids on
either side aren’t affected. This is because there was no
frameshift like in the insertion or deletion mutations. So only a single amino acid is
usually affected with a substitution mutation.
Now let’s look at our provided
diagram and try and figure out which one is an insertion mutation. The easiest way to do this is to
count the number of nucleotides in the mutated sequence. In the original sequence, we have
12 nucleotides. In mutation one, there’s 11
nucleotides, so this is a deletion mutation. In mutation two, there’s 12
nucleotides. We can see the substitution at
position six, where cytosine in the original sequence was substituted for guanine in
the mutation. And in mutation three, there’s 13
nucleotides, so this is an insertion mutation. You’ll notice that a thymine was
inserted in the 10th position.
Therefore, diagram three indicates
an insertion mutation.