An introduction to genetic mutations | Biomolecules | MCAT | Khan Academy

November 15, 2019 0 By Kody Olson

Voiceover: Today I’m
going to give you a quick introduction into genetic mutations. But first, let’s go over the central dogma of molecular biology,
which is just the idea that genetic information in a cell is formed in the form of DNA. This DNA is used to
generate complimentary RNA through a process called transcription. That RNA is then used to
synthesize a corresponding protein through the
process of translation. Looking at a quick example, our short DNA strand here will be used to generate an RNA stand. Remember that A pairs with U or T and C pairs with G. Next, our RNA will be used to generate protein through translation. Remember that during this process, RNA nucleotides are
read in groups of three, called codons, in order to generate corresponding amino acids. Just very generally, we say that mutations>>have the affect of
making this synthesized protein not turn out quite right. I’m going to give a quick
shout-out to sickle-cell disease, which is an example of a disease that’s caused by a genetic mutation. You may remember that there is a protein in red blood cells called hemoglobin, which we can also call Hb. Hemoglobin in a protein that coordinates to iron ions in order to
hold onto oxygen molecules and transport them throughout the body. The mutation that causes
sickle-cell disease results in a mutated form of hemoglobin called HbS being formed, where
the S is for the word sickle. The difference between normal hemoglobin and HbS is that one
glutamate amino acid residue is being replaced with a
valine amino acid residue. This small change results
in all of these mutated HbS proteins aggregating
together in a red blood cell, which makes it very difficult
for that red blood cell to transport oxygen effectively. Just a side point, remember
that red blood cells are initially generated from
hematopoietic stem cells through a process called hematopoiesis. Where are mutations found, and how do they come
up in the first place? Let’s look at a couple of
different possible mistakes that could lead to an
incorrectly produced protein. First, we’ll see what happens if a cell makes a mistake during translation. We’ll stick with our example of sickle-cell disease from before. Let’s say that we have
this sample piece of DNA with three nucleotides from the gene coding for hemoglobin. This DNA is transcribed to form the complimentary RNA sequence GAG. That GAG would normally correspond to a glutamate residue during translation, but a mistake during
translation might lead to a valine residue
being translated instead to produce the mutated hemoglobin associated with sickle-cell disease. But notice that if a mutation happens during translation, the cell will only produce one mutated hemoglobin, or HbS, for each overall mistake. Since cells are making tons
and tons of hemoglobin, just one mutated protein might not have that big of an effect on the cell. So we can say that
mistakes during translation probably don’t cause mutations like the one associated
with sickle-cell disease. Next, we’ll look at mistakes
during transcription. Again, we have our CTC piece of DNA, which would normally make GAG on RNA, but maybe a mistake occurs which leads to the
transcription to a GUG instead, which would then code for the valine associated with mutated hemoglobin. If this mistake occurred, the cell would only make a few mutated
hemoglobins for each mistake since an individual
strand of messenger RNA will only be translated a couple of times before being degraded.>>So we can say that
mistakes during transcription probably don’t cause
mutations like the one associated with sickle-cell disease. Finally, we’ll look at
mistakes in the DNA strand. If our CTC in DNA is
mistakenly turned into a CAC, then our corresponding
RNA from transcription will be changed and ultimately a valine would be produced instead
of a glutamic acid. Now, since a cell’s DNA stores all of its genetic information,
that mistake would lead to all future hemoglobins produced from that gene being mutated. So overall, we can say that mutations will usually result from
mistakes in a cell’s DNA and not from the RNA or the protein. So where do these types
of mutations come from? There are two ways a person
can get a genetic mutation. The first is that they
inherit it from their parents. Remember that DNA is passed down from parents to offspring, so if we have a mutated father here, then there’s a good
chance that at least one of his kids will inherit that mutated gene the same way that the child might inherit any amount of that parent’s DNA. The other possibility is that the mutation will come on spontaneously, which is where a person suddenly gets a mutation in their DNA
without their parents having had the same mutation. Spontaneous mutations can come
from many different sources, with just a few examples being from DNA replication errors, environmental factors
like certain poisons. It’s also possible that genetic mutations can come on entirely randomly. What did we learn? First we learned that mutations originate at the DNA level, and not
the RNA or protein level. The effects of the
mutation, like the example we gave of sickle-cell disease, are found with problems with the proteins that are ultimately
expressed by the mutated DNA. Now, like every rule, there are a couple of exceptions to this one, but we can say that the
effects of a mutation are usually found at the protein level. Finally, we learned that mutations are either inherited from a parent, or come on entirely spontaneously.