Voiceover: So, today
we're going to talk about the different types of genetic mutations that you would find in a cell. 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. Nucleotides from the 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 let's say turning this thymine-based into an adenine-based, 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 are the different
types of mutations? Well, the first type of mutations we're going to talk about
are called point mutations. Now, here I've just written
out a random sequence of DNA, which is just a repeating pattern of CTC, which would code for a
repeating sequence of GAG in the RNA strand, and
finally, a protein sequence of three glutamate amino acids. So, a point mutation is
when one of our DNA bases is replaced with another. So, in this example, a thymine-based is being replaced with an adenine-based, which leads to a change
in one RNA nucleotide and ultimately a change in one amino acid. Another type of mutation
is called frame-shift, which works a little differently. So, first I'll write
out the same DNA, RNA, and protein sequences from before, but now, instead of changing
one base to another, I'm going to add one to the sequence, and here I've thrown in
this extra cytosine base that I've written in blue. Now, naturally, this change would lead to an additional guanine base being in the resulting messenger RNA sequence, but what's interesting
is that this mutation will change the reading frame of the RNA. Remember that RNA is read
in groups of three or codons when being translated to form protein, but now, since we've
added an extra G here, all of the codons coming
after that extra G will look a little differently. Now, instead of having three GAG codons, we've swapped out two for GGA codons. This means that two of our amino acids in the final protein will be changed, and in this example,
they'll be changed from glutamate to glycine. So, you can see that frame-shift mutations usually have more significant effects on the final protein
than point mutations do. Now, it's important to recognize that both of these mutations
are classified and named for how they affect the
cell's DNA structure and aren't really named for how they affect the resulting protein. Now, our next type of mutations are non-sense mutations
and missense mutations. Let's say we have a DNA sequence that normally generates RNA and
codes for a cysteine amino acid. A non-sense mutation is any
genetic mutation that leads to the RNA sequence becoming
a stop codon instead. Now, missense mutations
are a little different, and they're any genetic
mutation that changes an amino acid from one to another. So, in this example,
our mutation is changing the resulting amino acid from
a cysteine to a tryptophan. Now, you can see that non-sense mutations probably affect the resulting protein a lot more than missense mutations do, since that new stop
codon that we're creating could chop off a huge
section of the protein, instead of just changing
one amino acid to another. So, now we can divide
the missense mutations even further into a bunch
of smaller categories. Silent mutations are when the mutation doesn't actually affect
the protein at all. Since many different RNA codons can code for the same amino acid, it's possible that the mutation might not
affect the protein at all. So, in this example,
CCA, CCG, CCT, and CCC in the section of DNA will
all end up coding for glycine. So, if you change the third base, it wouldn't affect the final protein. Conservative mutations are
where the new amino acid is of the same type as the original. So, here I have a
glutamate and an aspartate, which are both acidic amino acids. So, a mutation that
swapped out an aspartate for a glutamate would be
a conservative mutation. Finally, a nonconservative
mutation is one with a new amino acid is of a
different type from the original. So, here we have a serine amino acid, which is a small polar amino acid, being replaced with phenylalanine, which is a large, nonpolar,
aromatic amino acid, and this would be an example of a nonconservative mutation, since serine and phenylalanine are
different types of amino acids. Now, I'll point out again
that all of these mutations are classified and named
for how they affect the resulting proteins
and aren't really named for how they affect the cell's DNA. So, let's look at a quick example. Sickle cell disease is a
disorder where hemoglobin or Hb, which is a protein found in human blood, is mutated into a less active form, which we're going to
call HbS, and it results from a single glutamate residue being converted into a valine residue. Now, we can classify this
mutation as a point mutation, since only one DNA base is affected, but we can also say that
it's a nonconservative missense mutation, since
glutamate is being swapped out for valine, and the two are
different types of amino acids, since glutamate is an acidic amino acid, and valine is a nonpolar one. So, what did we learn? Well, first we learned that mutations originate at the DNA level, but show their effects
on the protein level, and second, we learned
that we can classify different types of mutations by either their effects on DNA or
their effects on protein. In reference to DNA, we have point and frame-shift mutations, and
in reference to protein, we have missense and non-sense mutations.