Human beings all over the world apparently are so different from each other. We look different, we have different cultures, we eat different foods. However, 99.9% of our DNA is similar among all of us. So it's only 0.1% of the differences in our DNA which result in the variations which we see around us. And a lot of these variations are called DNA polymorphism.
So in this video, we are going to take a look at what DNA polymorphism is and what it results from. Poly means many and morphism refers to forms. So DNA when it is present in many forms in different individuals of the same species is said to have polymorphism.
So let's take some examples. Let's say we have one DNA structure which is present in let's say 99% of the population. Let's say it looks something like this A, T, T, A, G, C, A and in the remaining 1% we have something like this A, T, T, G, G, C, A. So you see in this nucleotide there is a difference right? Only this nucleotide, one nucleotide is different and the rest is the same. So this is a type of polymorphism and in fact it has a name.
The name is single nucleotide polymorphism. So the variation is in base which is a part of one nucleotide hence it is called a single nucleotide polymorphism or in short SNP. So it's not necessary that there has to be only two types of variations. So in this case I've shown you that there are only two types of sequences one has A in the in this position and the other one has G.
There can be more variants in fact. For example we can have something like this. Let's say there's another sequence which is there in one percent of population and let's say this percentage is 98 percent so uh let's say in this case the sequence is something like a t t c g c a so again over here we see that this is a variation so then we have three variants at the same position and there can be many more in fact so i'm just showing an example where there are three variants but for it to be a polymorphism there has to be certain criteria.
First of all there have to be at least two variants and secondly each variant should be in at least one percent of the population. Okay, so let's take another example. Let's say we have five variants. Variant one has, variant one is there in let's say 98% of the population. Variant two in 0.5% of the population.
Similarly, there are three more variants. Variants three, four and five. They are there in 0.5 percent of population each. So here do we have a DNA polymorphism? The answer is no because only one of the variants is present in more than one percent of the population which is 98 percent.
All of the rest of the variants are present in less than one percent of the population. So this is not a DNA polymorphism. So one of the types of DNA polymorphism we saw is the single nucleotide polymorphism. Let's take a look at another type of polymorphism.
Sometimes there are some sequences which are repeated over and over again. Let's look at this example. A, T, T, C, A, T, T, C. So these types of sequences are called tandem repeats. the word tandem here means one after another so each repeat follows the previous one attc attc attc this is called a tandem repeat and the unit that repeats over over again can be just a few bases or it can be many bases let's say hundred or even thousand bases sometimes. So this is another type of polymorphism.
One sequence may have these repeats and the other sequence may not or one sequence may have let's say two of these repeats whereas another sequence may have 50 of these repeats. So this forms another type of polymorphism. So where do they come from? Where do polymorphisms come from?
Do you know where variations come from in DNA? Well the answer is mutation. Now mutations can happen in any body cell right.
So what are the two types of body cells that mutations can affect? The one type is called somatic cells and the other type is called germ cell. Somatic cell is any body cell except a few which are the germ cells the germ cells are the cells that give rise to gametes sperm and ova so can you tell me mutations should happen in which of these two cells so that dna polymorphisms can happen so dna in which of these cells do you think will be inherited by the next generation germ cells right it's the germ cells that give rise to the sperm and ova and when they fertilize they form the next generation the dna is transferred somatic cells however don't matter to the next generation. So mutations in these don't matter.
They don't result in DNA polymorphisms. So DNA polymorphisms have to happen in germ cells in order that they can be transferred to the next generation and the generation after. And that's how more and more people get such variations, right?
So any random variation can happen in any germ cell in any person. But it has to happen in more than 1% of the population in order to count as polymorphism. And how does that happen?
As more and more people get it, right? And more and more people get it if the first person who got it reproduces and produces many offspring and the offspring then again reproduce and produce many more offspring and each of them have that variation. That's when we call the variation a DNA polymorphism.
So yes, the mutations have to happen in the germ cell and they have to be passed on to the next generation. However, the mutation can't be done. deadly.
What do I mean by this? So let's say the mutation happens in a gene which makes such a defective protein that the person doesn't live for let's say more than five years. So obviously they can't pass on the gene to the next generation. So that won't work right that won't result in a DNA polymorphism. Another thing is the mutation should not affect the reproductive potential of that person so it should allow normal reproduction only when these two criteria are fulfilled by the mutation can it potentially give rise to a DNA polymorphism now you know about the central dogma right DNA is transcribed to form mRNA and then that is translated to form protein.
That is how the information in the DNA is expressed. It's through the proteins. Only 1% of DNA codes for proteins. The rest 99% of our DNA does not code for any protein. So 1% is coding and 99% is non-coding DNA.
Now when mutations happen Mutation, as you know, is any random change in the DNA sequence can result from UV radiations, from errors in DNA replication during meiosis. It can happen due to different reasons. So, when mutation happens, does it happen in the coding regions or the non-coding regions?
It doesn't matter, right? It's totally random. So, it can happen in both coding and non-coding regions. Now, tell me which of the two, coding and non-coding, DNA will acquire more DNA polymorphisms.
It should be the non-coding regions because why is it more in the non-coding regions? There is more DNA, right? It's 99% of our DNA that's non-coding. So more DNA. Another thing with non-coding regions is since they don't directly result in any protein, any random variation that happens in them usually may not result in disastrous consequences.
consequences. So what do I mean by that? Let's say there is a random change in the sequence in a coding portion of the DNA. So the protein that may result from it may not be functional at all whereas the chances of something that bad happening in a non-coding region is less. Bad things can still happen.
A non-coding region in a DNA may still be the regulatory regions of the genes which you have studied about or they may fall in the introns which may affect the splicing of the genes. All that can happen. but the probability is much less.
The probability of it being disastrous is much less. Hence, there are way more DNA polymorphisms in our non-coding regions than in the coding regions. Now, DNA polymorphisms are very important. As you saw, they result in variations and that is extremely important for evolution. And they also have practical applications in different fields.
one of which is DNA fingerprinting which we will look at in some other video.