4. DNA, Chromosomes, and Genomes
Human Genetic Variation
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Hi in this video we're gonna be talking about human genetic variation. So um the first thing is, what is the scope of the human genome. What does it look like and what's in it? Well, we know this because we've been able to sequence the human genome and that's provided this information of course on the size and what's in it. Now. I'm just gonna go through some different numbers. Um now you don't necessary, you don't need to memorize you know, these specific numbers, these genes 9% whatever, but just sort of understand the overall composition how many genes there are and and what those genes actually are encoding for are not encoding for is just sort of an overall important thing to understand. So the human genome contains around 3.2 times 10 to the ninth nucleotide pairs organizing 23 chromosomes. Don't need to know that number. Um But you would like to I think find interesting and also want to know that actually less than 2% of that encodes for proteins. So what is the rest of it encoding for? So the overall composition Of the human genome is around 20-25,000 protein coding genes. I would know this number, know that number and know that number. I'll do red equals No. Just so you can help me just help you out there. Um So there's about 1.2% of the genome encodes for proteins. Um 50% of the proteins or of the genes in here can actually um or either currently or have in the past been able to jump around the genome. So they're considered mobile genetic elements or jumping genes. There are around 9000, probably even more than this. But there are 9000 known functional RNA. And the interesting part of this is actually 5% of the human genome is highly conserved and of other organisms. So 5% of our genomic sequence is really conserved across pretty much most organisms on earth. But we already know that there are only 2% of the genome and codes for proteins. So that leaves 3% Of really conserved genomic material that we don't really know what it does. Um so this is really interesting and sort of excites a lot of scientists, you know, what is this other 3% doing. So if we're to just look at an overall human genome composition, this is kind of, you know, just a pie chart just to give you an overall view. You don't need to know these percentages and or even what these things are but just sort of realize, you know, if we're looking at N tron here, that is this region 26%. N tron that's that's a huge portion of this pie chart. If we're looking at these things called lines, these are mobile genetic elements, elements. You don't need to memorize that just sort of I'm telling you now, you can see that's 20%. That's a huge portion of the genome. But what's not a huge portion of the genome is this protein coding region which is here 2% of the pie chart? It's encodes for proteins. It's really tiny. Not a lot, but it creates everything about us. Yeah. So and they come back and now we're going to talk a little bit about um comparing genomic sequences between humans and other organisms. So um you're gonna see a lot of percentages here. You're gonna see a lot of dates and you don't necessarily need to know that unless your professor has specifically said you don't know these dates. No these percentages. Um But I just want to tell you about them just so that we can really get this understanding of scientific advancement and also differences in protein coding compositions between organisms. So prokaryotes sort of bacteria um they were first sequence in 1995, the first organisms to be sequenced for bacteria. And they found that 90% of the genome is protein coding. So you can imagine that scientists doing this, we're thinking okay well then that means that most organisms genome is protein coding. But you can see as we continue down this sort of list here With these model organisms, we have yeast 1996,. We get worms, 1998,. We get fruit flies, 2,013% and this starts to really decrease and they were like dear goodness! Like what what's going on? What are these all of these huge percentages of the genome that aren't protein coding. Um And then in also in 2000, what happened is they started sequencing Arabidopsis which is a flowering plant. Now they were surprised because it was the first organism that they actually saw an increase in protein coding Um compared to what the trend they had been seeing as they were going throughout the years. And so what they concluded here was that and this is really important. So I'm going to highlight this in a different color. So the size of genome does not dictate organism complexity. I mean a flowering plant, 25% of the genome is protein coding. Um That is that's such a small amount. But it's even I mean it's just when you compare these organisms and the amount of genome that's protein coding. You see that this is just kind of this is a little crazy and a little ridiculous and a little exciting um that the size of the genome doesn't dictate complexity or the amount of protein coding genes that are present. So this is just a simple graph. Or it's actually not super simple. Um So let me walk you through this and here, you have pie charts with different organisms. You have humans, mice, fruit flies, um flowering plants, yeast, yeast up here. And so you have these pie charts here would say, you know how much is coding much is coding. And you can see up here with the yeast, there's a lot coating here down here at the humans. There's a little coating Um you also hear have listed the size of the genome and also about how many genes they have. You can see that Arabidopsis, which I just highlighted in green has about 20 over 25,000 genes uh similar to humans, although they've now sort of figured out that humans have a little less than that. Um and the third thing that is on here is this and this is just generally an example of what a gene would look like in these organisms. So in yeast you have this sort of starting sequence and the rest of its coating, you get more complex as you get more complex sort of in the flowering plants. Fruit flies, what you see is you have a starting sequence but you can have these different Exxon's, here's Exxon's that exists, making them more. Making the gene more complex. And then as you get down to mammals is nice. And humans, you can see you have multiple starting regions. You can have these sort of Exxon's here here and here, you have interjected in here some repeating sequences that either have a known function or generally have a non unknown function. And so the purpose of this is not so that you memorize all of this. You don't need to know these percentages. You don't need to know these numbers, you don't need to know these images but what you do, what I'm trying to convey to you is that genomic size doesn't equal complexity, just sort of as simple as that. You know, genomes of different organisms have different levels of protein coding genes. They have different genes, organizations, they have different things that are present in their genes, but more complex doesn't mean bigger. So with that, now, let's turn the page.
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Okay, so now we're gonna talk more about specifically human evolution. So chimpanzees and humans diverged From a common ancestor. And because they did that they're actually, our genomes are very similar to chimpanzee genomes are 98% similarity between the two genomes. So we look quite different. We do things quite differently than chimpanzees. So what is different? Well, a major region that's different are these regions called human accelerated regions. And they are conserved areas of the genome that had underwent rapid evolution between the chimpanzee and human diversion. So in human genome there's around 50 sites that are considered human accelerated regions. Um and around 25% of these are actually in genes that control brain development. So just sort of looking at the chromosomes and human versus chimpanzee, you can see that it really looks so similar. There's this kind of difference here. But other than that, I mean it's hard to tell just looking at these pictures differences between the human chromosomes and chimpanzee chromosomes. So um that's human and chimpanzees. What about human variation? So human variation exist between individuals. That's what gives us different hair colors, different eye colors, different heights, maybe even potentially different personalities. And so but we're all still human. So what how much variation is acceptable to still be a human while around one in 1000 nucleotides differs between one individual person and another. So between me and you, we have about one and every 1000. So that totals about three million genetic differences between me and between you and these differences are really um can be termed a lot of different things. There are a lot of different differences. One of the differences are called single nucleotide polymorphisms or snips. And they are differences in the genome of one population and another. So that population can be sort of a group of people who live in a certain area. So that can be you know, population of Kenyans versus populations of alaskans. Um single nucleotide polymorphisms of course would exist between those two groups. So if we were to choose rant to people randomly, me and you for instance, um we would differ by about 2.5 times 10 to the six steps, huge number of these like single nucleotide polymorphisms are single nucleotide changes within the genome. Now there is other differences that we can have. These are called copy number variations. So there we've already talked about gene duplication, a lot different number of gene copies. Well um that can sometimes in certain genes they need all these gene copies, they need 10 gene copies over 15 gene copies. Um and between individuals, the number of gene copies present can differ. I can have one, you can have five. Um So that would be a copy number variations. There's another thing that's really similar but instead of jean it's actually just nucleotides and these are called C. A repeats. And so these are strings of repeating C. And a nucleotides and of course they're very prone to mutations But they also differ between different people. I could have 300 c. A repeats. You could have 600 repeats. And these are because they're so prone to mutation, they actually differ between every person. And so this is how DNA fingerprinting works. So if we go to our Crime scene shows and they say oh we're going to use DNA to determine who did this. Um this this crime. Well what they're doing is they're doing DNA fingerprinting. So they're looking at sea A repeats and saying well that person the person the D. N. A. We collected from the crime has 300 C. A. Repeats and this person has 289. So they didn't do it but this one has 300. So they're really linked to the crime. So that's C. A repeat. So um I'm gonna show you this sort of map of the world. Here's the title model of genetic variation of different human populations. And let me back up and see human populations exist throughout the world. And all of these here these sort of boxes sort of are just different genes that exists in these populations. And you can see that you know these uh these colors are are similar. They exist in all of the organisms or all of the humans on earth but the order and the length and things these things can differ. Um And these can be single nucleotide polymorphisms. They can be repeats. Um They can be copy number variations. But in these different populations of people, there is this variation between humans on Earth. So with that, let's let's move on.
Which of the following is true regarding genomic genetic variations?
Genomic size is proportional to genomic complexity
Mobile genetic elements make up a very small proportion of the human genome
Sequence variations between one individual and another occurs once every 1000 nucleotides
CA repeats are extremely stable genetic elements found in the human genome
The majority of the human genome encodes for proteins.
Which of the following genomic variation refers to different number of gene copies between individuals and populations?
Single nucleotide polymorphisms
Copy number variants