Hi in this video we're gonna be talking about genome evolution. So the next few videos with this topic are all going to be associated with ways the genome evolves. So the first one we're gonna talk about our mutations and we know this mutation is called evolution. Um And so mutations cause gene evolution. You know a particular gene but it also promotes genome evolution as well for the entire organism genome. And so this can happen in a lot of different ways. One that we're familiar with are point mutations which result in a single nucleotide pair change. Um And we know these can be beneficial. They can help the organism they can be really detrimental to the organisms but generally they all arise through some type of error and DNA replication. Now. Um when we think of genes or mutations within genes, we think of them as just affecting the gene function. But when we think of mutations in the genome, we realize that mutations can occur both in the gene but also in regulatory DNA sequences that control things like transcription or replication. And all these regulatory sequences are just as important if not even more important than mutations in the genes themselves. And so if the mutations in the gene that's of course it's going to affect the activity of the gene, it's going to affect some of the interaction the gene can have. It's going to affect the stability of the gene. And generally these are easy to spot because these are mutations that we you know, there's an immediate effect that we can see. But if the mutation is in regulatory D. N. A. This is not as easy to see because the regulatory mutations um will affect how genes are expressed. And so that could be a complete on or off switch which is going to be very clear but it doesn't have to be just on or off. It could just be a little bit less or a little bit more. And those tiny changes really can accumulate over time to create these big genomic evolution changes that affect gene expression as a whole. Those are two main ways that mutations affect genome evolution. So just as an example of how this would happen is we have here we have two amino assets. It's one to which start from the nucleotide sequence here. And so you can see that um just most of these are the exact same. But if there's a single point mutation here that can actually change this amino acid from an are getting too threatening and that can have huge effects on the protein because you're not only just changing the amino acid but you're changing it from a basic which is gonna be over here amino acid to a polar amino acid which can have huge impacts on protein structure and function. So now let's move on
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Okay, so this video is going to focus on another mechanism of whole genome evolution and that is gene duplication. And so gene duplication is a main driver of genomic evolution. So one of the things that gene duplication creates are things called gene families which are groups of gene with similar sequences but specialize functions. So if you were to look at the sequence that are gonna look really similar but they have very unique and specific functions in the sound. And so how does this relate to gene duplication? Well gene families arise from gene duplication because you have one gene and it duplicates. Now each copy is free and is going to accumulate mutations that are different from the other copy. Um And so those mutations are going to result in different functions despite the fact that they're going to have similar overall sequence. And so like I said, they're the result of gene duplication. So one major example of gene family and arising from gene duplication is actually hemoglobin. So you can see here that there are four sub units, there are two alpha subunits and two beta subunits. And originally though each one of these, each one of these four came from an original gene, we'll just call this the hemoglobin precursor. I don't actually know what the name is. I just made that up. Um So we have this hemoglobin precursor and it um duplicated into two which then duplicated into four. And these four are what we currently have as a humans. Let me back out. So you can see that I think you can see most of it. But so these four, some units are what we currently have um as humans and they all they have all arisen from this hemoglobin precursor. So um how does gene duplication happen? I mean obviously it's not something that the cell means to do, but it does it fairly often. So gene duplication actually arises from improper crossing over during mitosis. So no, we haven't gone over mitosis yet. And it's probably been a little while since you heard that term in your intro bio class. But if you remember crossing over leads to more genetic diversity because it's switching um gene segments between nearby chromosomes or between its chromosome pair. And so how this happens. Is there a process called homologous recombination? You don't necessarily need to know what that means right now. We'll go over it in the future. But pretty much when you have two chromosomes and they're aligned properly, then the genes on this chromosome are going to equally move over to the genes on this chromosome and they're just going to switch genes and it's not going to cause a problem. But if the chromosomes are misaligned, then you're going to have genes that are here on my palm moving two genes that are here in my fingers and that's going to just result in chaos and result in gene duplication. So the result of improper crossing over is you get one chromosome with an extra gene copy and one with no copy. So if let's scroll down to the example and just look at that for a second and then we'll go back up and do some of the more terms. But here you have two chromosomes with the same jeans on them. And what you can see the three genes, A, B. And C. Now when they're lined up like this it makes sense. Um And if they were just to switch this way this A. And A. It would be fine. It wouldn't cause any sort of duplication. But if they're lined up here and here what happens is you get one chromosome that actually just completely deletes this B gene, it's gone it's deleted and you get the second chromosome which has two copies and that results in a duplicated gene. So with duplicated genes we can have these you know unique functions but we can also have um some just sort of side effects. So one of these things is called a pseudo gene. And these are duplicated genes but they really lost their functional ability. They are still present in the genome but they have no function. They've either just accumulated so many mutations that they just can't do anything anymore. Um Or they've been moved to a place that no longer is transcribing such as hetero chroma tin areas. Um But it happens. And so um one of the ways that this happens is through this term called processed pseudo genes. And so what happens is that you know you have this gene it's duplicated and it gets transcribed normally like any other gene. So it gets transcribed into RNA. Um But then it happens where it actually goes back to D. N. A. And integrates itself into a chromosome potentially in a location where it's never gonna be transcribed again. Um So that's what we call process pseudo genes. Now we've been talking about genome or we've been talking about gene duplication in terms of genes. But actually there's this process of whole genome duplication which is exactly what it sounds like where the entire genome of an organism is duplicated. And um you can imagine this can cause just kind of chaos in the cell doesn't always but it can and so the whole genome is copied but it remains inside the cell. And so when you started you had a cell with one genome and now you have a cell with two sort of identical genomes. But um and you would think that you know most things would die if this happened to them. And they do. But some organisms have really evolved to live with these extra copies of genome. And so um we call the or we classify these organisms based on their polyp Lloyd ization which is sort of the number of whole genome duplications. And so this can be common and fungi implants. It can also it also happens in some frog species a lot and you can see that a duplicated genome actually makes the frog pretty much twice the size of its single genome counterpart. Um And it's really interesting process. But the purpose of this is to talk about whole genome genomic evolution in terms of gene duplication, which can happen as single genes or as whole jeans. Um and is mainly the result of improper crossing over. So now let's move over, let's move on.
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Okay, so now we're gonna talk about another way that the genome evolves and that's through um entrance and splicing events. So do we remember what introns are first from either previous videos you've seen or your intro class? Right, so in Theron's are the non code are the non coding regions of a gene and they are not used and cut out during processing, whereas Exxon's are the coding regions of the dream. And so most organisms contain introns and exons and most of their genes but it's not completely universal. Actually histone proteins don't seem to have introns or at least not a lot of them. And so uh yeah, so it's mostly universal but not entirely process. Now. There are two um concepts that I want to really mention that can play a big role in evolution and that's alternative splicing and exon shuffling. So first alternative splicing, this is combining of Exxon's from one gene in new orders. So this occurs in about 50 to 90% of human genes. And so what happens is um so we'll just go down to the example here we look. And so we have these four genes 1234. And here are the entr ons these sort of line sections here with no genes in them and are no Exxon's These are Exxon's and these are n trains. Sorry from this spoke. And what happens is that through alternative splicing you get you get 13 and two. So these are obviously in a different order than what they were before. And so this creates what is known as an ice a form. So these are different forms of the same protein produced through alternative slicing. So we have this one form that's 132, we could have another one that's 142. We could have another one, that's 134 or 214. Any of these different combinations here of Exxon combinations and each one of these would be an ice A form. So this is considered the same protein. Just sort of different different forms of it. Now. Exon shuffling is different because this is actually combining the exxons from two um different jeans. And so that you can imagine that's a huge process of gene of gene evolution because you're actually combining these different regions from these different genes into the same protein. So um this can happen, this can happen mainly through um Exxon duplication. So it's some duplication of. Well of course you know there's be an extra copy and that extra copies free to do whatever it wants to an evolution. And that could involve exon shuffling. Um Or the Exxon can be moved to some type of different genomic location. I'm gonna talk a lot about gene jumping in the future future topics. Um You don't need to know about it now. Just sort of know that exon shuffling is the combining of Exxon's into two from two different genes. So now let's move on
Repetitive DNA Sequences
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Okay. So in this video we're going to talk about another way that the genome evolves. And that's through actually repetitive DNA sequences which are exactly what they sound like. Um But surprisingly, even though you may not have heard of them or at least not talked about them very much in previous classes, they're actually extremely common in the human genome. And in other organisms genomes as well. So one of these repetitive DNA sequences are actually just simple sequence repeats. So there can be thousands I mean thousands of copies of these just sort of very short sequences. 1 2 500 nucleotides long just sort of dispersed throughout the genome. So in fruit flies, the sequence one of these sequences is here this a C A C. T. You don't need to know, you don't need to memorize that specific sequence. I'm just kind of using this as an example. But if you were to look at the fruit fly genome, what you would see is you would see this sequence repeated a ton of times just throughout. And it's so unusual because they really they have no genetic information. They're not transcribed. So what's their function? What are they doing? Scientists have ideas. Um But there's no kind of really big consensus on what these simple short repeats are doing other than sort of taking up space in the genome. So that's one way you can imagine that the genome evolves because it has to work its way around all of these short sequence repeats that are just sort of found throughout the genome. Now, another way is through mobile genetic elements. And so these are D. N. A sequence that can move through the genome. So what do I mean by moving? Well I actually mean moving they actually can sort of jump through the gene the genome and change locations and insert themselves into a gene or remove themselves from a gene and insert themselves into another one or regulatory sequences. So they can actually just sort of move throughout the genome and by themselves these aren't necessarily repetitive DNA sequences. But one common feature of these elements is that they contain repetitive DNA flaking regions. And so these are really important in allowing the gene to move. But also so for gene evolution because they have all of these repetitive sequences that are just sort of interspersing throughout the genome. So one form of mobile genetic element that we're going to talk about more in the future. R transpose sins and they can actually move as RNA or DNA. And how they affect evolution is what I've already said is they can just insert in a gene. Um They can affect gene structure or regulation they can insert in regulatory regions. Um All of these um sort of insertions you can imagine have drastic effects on the evolution of the genome. So now let's move on
Which of the following is not a driver of genome evolution?
Which of the following genomic changes are most likely to cause pseudogenes?
Protein isoforms are created through which process?