Hi. It's Mr. Andersen and welcome to biology essentials video 35. This is on
mechanisms that increase genetic variation. Genetic variation is the spice of life. It's
played against the environment and causes natural selection or adaptation to local environment.
Now sometimes however in nature we see the opposite of variation. Example would be this
fellow right here. This is Charles the II of Spain. We was of the Habsburg family and
the end of the Habsburg Dynasty which was important all across Europe. And ended kind
of the age of expansion and this wonderful time in Spain's history. He even had what's
called the Habsburg chin. Or this prominent chin. But if you look at his pedigree, it
looks a little different than a normal pedigree. A normal pedigree is going to expand outward
as you bring in new blood. And get wider and wider and wider as you go towards the bottom.
And this one actually gets narrower. It tends to kind of focus down here just on Charles
the II. And the reason why is due to inbreeding. Right here we've got like an uncle and a niece.
Right here we've got like direct siblings. And so we have all of this inbreeding. And
what that does is it aggregates all those traits together. So it got to the point where
his chin, this is called the Habsburg chin, made it impossible for him to chew. And so
there was some physical problems. Now they didn't understand DNA, genetics back then.
They thought it was sorcery. He actually went through an exorcism to figure that out. But
we now know and nature has known this for a long time, that variation is good. And so
where does variation come from? Well like in the last podcast I talked about how it
can arise. But how is it maintained is a better way to think about that. I'm not going to
talk about viruses. I'll talk about that in the next podcast. But we could break life
then into two main types of maintaining genetic variation. In prokaryotes or in bacteria,
and we could throw archaea on with this as well, they show what's called horizontal
acquisition. So there's no sex. They use binary fission to reproduce. So there has to be some
way they can get variation. A lot of this is just going to be replication mistakes.
But they can also share material horizontally. In other words from species that are around
at the same exact time. So three processes I'll talk about are transformation, transduction
and then finally conjugation. In eukaryotes they use sex. So sexual reproduction is the
way we maintain genetic variation. And Charles of Spain could have done that same thing if
his parents would have been marrying outside of the family. And so sexual reproduction
does that in three ways. It's what makes you special. First one is crossing over during
meiosis. Next we've got random assortment or independent assortment. And finally we
have random fertilization. And so that's what I want to talk about. These three strategies
for maintaining genetic variation in these two different lineages. Variation is the spice
of life. In other words variation is selected against our environment to create all of the
different phenotypes that we have today. So these are some bivalves. Donax variablilis
have all these different phenotypes. And they're going to live in sand and so depending on
what the sand is those could be some of the darker ones might not be seen. Some of the
lighter ones might be. But also it's important in prokaryotes. And so in this culture we've
got millions and millions and millions and millions of colonies of bacteria. But each
of those are going to be different as well. And so we don't always see that. But it's
just as important in prokaryotes as it is in us. Now this is actually the tree of life
of all life on our planet. And if you look at how it really probably flowed, it's not
as simple as we might think. They have all of these paths going back and forth and back
and forth. An example that we've talked about this year would be mitochondria. Remember
mitochondria were bacteria that eventually have been co-opted for use eukaryotes. But
in bacteria today there's this, what we call, horizontal transfer. In other words it's transfer
of genetic material between organisms that are around in the same evolutionary time period.
And so in bacteria there are three ways that they can share genetic material between different
organisms. And the first one was pointed out by Fredrick Griffith in his famous DNA experiment
in 1928. Showed us a lot about you know there's something that's some magical thing that can
be passed from organism to organism. If you don't remember this we've got two different
types of bacteria. We've got the rough that are non-virulent. And the smooth that are
virulent or in other words are dangerous. You inject the smooth in. They kill the mouse.
The rough don't do anything. He then heat killed the smooth, what would be the virulent
strain. It didn't do anything to the mouse. But this would be that weird discrepant event
where when he mixed the heat killed smooth bacteria with the normal rough one, something
had been transferred between the bacteria. And so we call that transformation. There
was a transforming factor. Well how does that work? It's pretty simple. When you have one
bacteria like this and it's got its DNA on the inside, when it dies, so when we rupture
that cell wall, the DNA may still be intact. And that can be picked up by another bacteria.
And so that other bacteria can actually take in that DNA and make use of it. Now we actually
use this. We transform bacteria so we can use that in genetic engineering. For example
have them make a protein for us. That's how we started making insulin. But it's a strategy
that they use to pass those traits from one bacteria to another. Next is the idea of transduction.
Transduction requires a phage. And a phage is going to be, here's a picture of a phage
right here, it's a virus that infects bacteria. So we call that a bacteriophage. What it does
is it injects its DNA in. It then hijacks the cell, so it will make more copies of the
virus. So it's viral replication. But what happens is sometimes when one of these viruses
escapes it will actually carry some of that bacterial genome with it. It'll then inject
that into another bacteria where that bacteria can actually make use of it. And so transduction
is another way that we can actually move DNA from one bacteria to another, horizontally.
And we have to have a vector. In this case it's a bacteriophage that's doing that. And
then the last way that we can actually share genetic material is the closest we have to
bacterial quote unquote sex. And that's usually through the transfer of a plasmid. And so
a plasmid, remember this would be the chromosomal DNA. This would be the chromosome of the bacteria.
But a plasmid is another little bit of auxiliary DNA. In this case we are talking about the
fertility plasmid. I could do this with hand puppets. Essentially you have two bacteria.
This one has the fertility plasmid. It will connect to that other bacteria and it will
share some of its DNA. After it's done that, this one now has a pilus. It has the fertility
plasmid and it can pass that on. Now this seems a lot like sex, but again we don't have
that meiosis. We're just transferring that DNA, that plasmid from one to another. Now
all of these things are important. In other words let's say I were to wash my hands every
morning with a really intense antibiotic. Well if I were to do that, I would kill all
of the bacteria on my hands, except those that are resistant to that antibiotic. Now
those bacteria can get together between my hand washings. They can actually share that
gene, the ability to resists antibiotics. And so they get stronger and stronger and
stronger over time. And so bacteria aren't as weak as we might think they are. And they
also, even though they lack meiosis, they can share genetic material. And also there's going
to be mutations going on. Those are prokaryotes. If we talk about eukaryotes, there are really
three things that make you special, make you the way you are. And this first one would
be crossing over. And so during meiosis, during meiosis 1, you get a chromosome from your
dad. Let's say this is the chromosome you get from your dad. And this is the chromosome
that you get from your mom. And each of these chromosomes has a number of genes that are
on them. Go all the way down. This would be after DNA replication. And so this would be
the sister chromatid on the other side. It's going to have the exact same genes on this
side. So it's simply just copied it. But if we didn't have crossing over, all the genes
that are found on that one chromosome, which is you know hundreds of genes, would just
kind of travel together through time. Except we have crossing over. And in crossing over
what happens is the homologous chromosomes, in other words the chromosome from dad and
the chromosome from mom, will actually wrap around each other and they'll share some of
that information. And so genes that were on this chromosome from dad will be swapped to
a chromosome from mom and vice versa. And so that gives us variability. Next thing that
gives us variability is random assortment or we sometimes call this independent assortment.
And so example, let's say we're talking, a real example, let's say we're talking about
Huntington's Disease. Huntington's Disease is a dominant disease. It's a dominant trait.
It affects this portion of your brain and eventually causes death in middle age. If
you get the gene, your going to get the disease. And so if we look at the genotype of this
individual, this is an individual who has Huntington's Disease. And then we look through
meiosis. So this would be meiosis 1 and meiosis 2. Well let's say that the Huntington gene
is found on this chromosome. But the good gene, the non-Huntington's gene is found on
the other one, like that. Well it's going to duplicate itself. So if it's got a gene
on this side we're going to duplicate it again. And likewise we're going to have the good
gene on this side. But at this point right here, it's going to line up. It's going to
line up along the metaphase plate. And eventually that bad Huntington's Disease is actually
going to go in this direction and it's going to end up in this sperm. And it's going to
end up in that sperm. And so right here there is a flip of the coin. In other words if it
goes in that direction, you're going to get sperm that have the Huntington's Disease in
it. And if it goes in that direction you're going to get sperm that don't. And so that
random assortment determines what happens to the offspring that you have. And remember
we have, you know, thousands and thousands of genes. And each of those are going to assort
independently. And so just with that imagine flipping thousands and thousands of coins,
how often are you going to have the same exact coins come up. It's going to be really really
rare. And that's because each of these are going to assort independently. On top of,
and here we also see crossing over occurring right there. And so those two things are creating
a lot of genetic variability. And then the last one is random fertilization. During the
course of your lifetime males are going to create trillions and trillions of sperm. And
females have already created all the eggs that they'll have. But this one egg getting
together with this one sperm is really, really rare. And so that also leads to genetic variability.
And so what are the odds with a couple, where it's not identical twins and they create the
same exact child? It's incredibly rare. It's like one in 8 trillion or something like that
they would ever be the same because of those three things. And so that's eukaryotic variation
in genetics. The lights just went out and so this must be the end of the show. So I
hope that's helpful.