Lesson video

Welcome to this lesson from the unit "DNA and the Genome." The title of today's lesson is "Genetic Variants in Genes Can Influence Phenotype." So what we're going to be looking at is the definition of a genetic variant and how when that occurs in genes we form alleles and how these alleles can produce different proteins which lead to different phenotypes.

My name's Mrs. Barnard and I'm going to be taking you through today's lesson.

So by the end of today's lesson, you should be able to explain how a genetic variant in a gene can influence an organisms phenotype.

So we've got some keywords for today's lesson and those are gene, genetic variant, allele, triplet code and phenotype.

Now I'll put a slide of the definitions up, so if you want to pause the video to write those down, then you can do, but otherwise we'll be going through them as we go through today's lesson.

Okay, so our lesson today is in three parts, genetic variants in in genes, protein structure and variation in phenotype.

So let's get started with our first learning cycle today, which is genetic variance in genes.

So a bit of a recap here.

DNA is a polymer.

It's made of two new nucleic acid chains.

So we've got these two nucleic acid chains that are wrapped around each other and then the chains are connected across the middle by the basis of the nucleotides.

If we zoom in and look at that a little bit more closely, we can see that those two chains are made of those nucleotide groups and those nucleotides are the small repeating groups that make up our polymer.

And in the middle across where they join across the middle are the bases and it's the basis that form the genetic code that's carried by the DNA.

The genome is made up of genes and non-coding sections of DNA.

The genes are the regions of DNA that carry the genetic code to make proteins.

So you can see those there.

So we've got those sections and there are over 20,000 different genes in the human genome.

So they all code for proteins, but they only make up a very small amount of the overall genome.

Most of it is non-coding DNA.

So some of this non-coding DNA does have a job.

So even though it doesn't code for proteins, it can control when these proteins are made.

The double helix structure of the DNA protects the genetic code.

So that stops it from being damaged because that genetic code is really important for us to be able to form proteins.

But we read the genetic code from a single strand and we can see that that here, which is formed from the A, T, C and G, which are our different nucleotides because they carry different bases.

There's only four of them and putting them together in different orders and sequences codes for different amino acids.

So the genetic code within a gene will determine the order that amino acids are bonded together within a protein.

So we can see here that for each three bases in the genetic code, we code for one amino acid and those amino acids there are different from each other.

So let's look at this amino acid in a little bit more detail.

So proteins are also polymers.

So as well as DNA being a polymer made of nucleotides, proteins are polymers because they're also made of repeating chemical units.

The repeating chemical units in proteins are amino acids and your body needs 20 different amino acids.

The main structure of all amino acids is the same, but each of them has a variable group.

So that's what makes the 20 different from each other.

So we've got a model image here of an amino acid and you can see three groups of the same.

But then there's one variable group and in the examples where we show the model amino acid, the variable group when it's different will change colour.

So time for a quick check.

So if you could fill in the missing words to complete these two sentences.

So pause the video while you do it and then we'll check that you've got the right answers after.

Okay then.

So let's see what we've got.

DNA is a polymer made up of nucleotides and proteins are polymers made up of amino acids.

So we've got those right then well done.

So the genetic code is read in groups of three, so therefore we call it the triplet code and each triplet codes for a single amino acid.

Now this code does not overlap, so we read one of them, which gives us a certain amino acid and then the next triplet will then code for a different amino acid.

And then when those two amino acids are brought together, they will bond to start the structure of the protein.

The amino acids that those triplet codes code for don't change.

So the amino acid that each triplet codes for always remains the same, it's universal.

Organisms make proteins using the same triplet codes for the same specific amino acids.

However you can see here in this extract of the genetic code table.

Now in the additional materials, you can see a full copy of all of the amino acids and all of the genetic codes.

So this is just an extract of that.

But in this extract you can see that each of the triplets codes for an amino acid.

However, because when you make combinations of three from four bases, you actually make 64 different triplet codes.

Now, because there's only 20 amino acids, what that means is that each amino acid will actually be coded for by more than one triplet code.

But each triplet code does not code for more than one amino acid.

So a change in the nucleotide based sequence in a genome is called a mutation.

And this change to a region of DNA produces a genetic variant.

So this is an example of a mutation here.

So we can see the original sequence of DNA and one of the nucleotides has been swapped for another.

So we've got an A nucleotide that's been swapped for a C, and this would be called a substitution mutation.

This means that this region of DNA now looks different from the original and therefore it is a genetic variant.

So a mutation may be due to three things.

We could have a substitution like in that example there where we substitute one nucleotide for another, which means we've got a different base in our sequence.

We could have an insertion.

And an insertion is where a new nucleotide is placed in.

And if a new nucleotide is placed in, that means we change the genetic sequence and actually we change the sequence after that point because everything is shifted down by one.

The same thing happens with the deleted nucleotide.

When one is removed, the genetic sequence is affected from that point forward because every nucleotide is shifted up one.

So let's have a quick check.

So what does a mutation in DNA always produce? So read those options and select the one that you think is correct and then we will check back.

Okay, so what does a mutation in DNA always produce? So the correct answer is a genetic variant.

So if you got that right, then well done.

So if mutations take place in a gene, then a genetic variant of the gene is produced.

Now, when we make a genetic variant of the gene, we give it a specific name and that is an allele.

So an allele is a different version of the same gene.

So we've got an example in our image here.

So our DNA being made up of genes and non-coding DNA.

So if our mutation has occurred in the gene, like in this example, we've put a substitution in, we can see that the nucleotide has changed.

So that sequence has changed.

Now, in some cases, both of those sequences will still operate to produce proteins.

So we say that these are alleles.

So different alleles may code for different proteins to the original gene or they might code for a non-functional protein as they have different triplet codes.

So I've written the triplet codes underneath just to make them easier to read.

Often we do write our bases in groups of three when we are reading our DNA 'cause it helps us to convert them when using the genetic code table.

So you can see here where we have substituted one nucleotide, the C for the G, how that's affected that third triplet code.

So it's gone from CGT to GGT.

And you can see the triplet codes either side of it have actually are actually not affected, but that particular code is affected.

So time for a quick check, a genetic variant of a gene is called an allele.

Is this true or false? Now, once you've decided whether it's true or false, which of the statements below do you think best justifies your answer? So pause the video while you decide and then we'll come back and we'll see how you've got on.

Okay, so a genetic variance of a gene is called an allele.

This is true.

And the statement that best justifies this is the second one, a change in the nucleotide sequence caused by a mutation will change the genetic code in the gene.

So if you've got that right, then well done.

And it's time for a practise task.

So we've got two alleles of the same gene, but they have different base sequences.

So what I would like you to do is to write out these triplet codes for each allele and then circle where the mutations have occurred.

Then what I would like you to do is to use the genetic code that's in the additional materials and write the sequence of amino acid that each of those triplets will code for.

So that we should have a sequence of amino acids for allele 1 and a sequence of amino acids for allele 2.

Now with our amino acids, we do just tend to use a three letter code.

So you don't need to know the full names of the amino acids and you never have to remember the genetic code table because it's always there to refer to.

Okay, so pause the video while you do this and then we'll come back and we'll see how you've got on after.

Okay, let's see how you've got on with this one then.

So hopefully you decoded this.

So our mutations have taken place here, so we can see that that's been substituted.

And again, we've got another mutation here which has been substituted.

Then we're gonna have a look at our amino acid sequence.

So this is the amino acid sequence that you should have.

Met, Leu, Tyr, Arg, Arg, His, Glu and Stop.

And then at the bottom we should have Met, Val, Tyr, Arg, Arg, Asn, Glu and Stop.

So if you got those right, then well done and hopefully you didn't have too much trouble in reading that genetic code table.

Okay, it's time for us to move on to the second part of our lesson, which is protein structure.

So proteins are formed at a ribosome and transfer molecules or tRNA carry specific amino acids, which are based on the triplet code that is displayed on the mRNA.

So remember the mRNA is a copy of the gene.

So where it says that they carry a specific amino acid, that's when you refer back to that genetic code table that you just used in the last task.

So dependent on the triplet code that's on the mRNA, that specific amino acid will have to be brought to that triplet code by the transfer tRNA.

Now, the reason that the tRNA can bring the specific amino acid is because they also have bases as part of their molecule which bind to that triplet code.

So they have to match.

So that's how we make sure that the correct amino acid is brought to the mRNA.

And you can see that the amino acids joined together.

So as though all those tRNA molecules come with their specific amino acid, they're going to bind to those triplet codes on the mRNA, which is our copy of the gene that's come from the nucleus, and they're gonna leave their amino acid behind and then they're going to leave and they're going to go and collect another amino acid, the same amino acid.

And then as those amino acids are left behind, they're gonna join together to form the first stage of protein synthesis.

And the first stage of protein synthesis forms this amino acid chain also called a polypeptide.

And that is our first stage of forming a protein, the sequence of amino acids that are in that chain.

So as I've already said, a chain of amino acids is called a polypeptide.

Now this is only the first stage in protein synthesis because this then has to fold to form the protein structure.

And the folding of the protein is really important because the shape of a protein allows it to carry out whatever job or function that it has within a living organism.

So the folding of the polypeptide is determined by the order of amino acids.

So the order that those amino acids take place when it folds and bonds will determine what its 3D shape will end up as.

So polypeptides with different orders of amino acids fold in different ways, and this produces a variety of proteins with different 3D shapes.

So here we can see that I've used the model of amino acids again here across the top.

So we're not looking at the chemicals of them, we're just looking at them as repeating units with that variable group on the bottom.

And then these are computer models of what 3D protein molecules look like once they are folded.

So the 3D structure of a protein is important because that allows it to be adapted to its function.

So here's a couple of examples.

So with an enzyme, so we've got a diagram model of an enzyme there with its active site, you can see that this is then a 3D computer generated model of a folded polypeptide or a number of polypeptide chains that have been folded and bonded together.

And you can see that its shape is very specific and that's important because substances will fit into the active site of that enzyme.

And then again, we've got a computer model here of collagen.

And collagen is a protein that builds up the structure of your skin and tissues, different tissues in your body like cartilage and it gives support.

So you can see its shape is quite different to the shape of that enzyme.

It's very long and fibrous, and that's because of its rolling structure.

So choose the statements that could correctly complete this sentence.

A change in the DNA triplet code may.

Now there could be more than one answer, so don't worry if you find that there is, pause the video while you do this and then we'll come back and we'll check that you've got the right answers.

Okay, so completing this sentence correctly, a change in the DNA triplet code may change the triplet code carried by the mRNA.

That's true 'cause it's a copy of the DNA.

Change the tRNA molecules that combined.

That's true 'cause if the triplet code is different, then different tRNA molecules will bind.

Change the amino acid that is carried by the ribosome.

Again, that is true because depending on the tRNA molecule that combined, that will depend on the amino acid that's joined into the amino acid chain.

Change the location of protein synthesis.

That's not the correct answer because protein synthesis always takes place at the ribosome.

So if you've got those right, then well done.

So time for another matching up task.

So if you can match up the word to its correct definition, and then we'll come back and check you've got those definitions correct.

Okay, let's check.

We've matched these up correctly, otherwise you can correct them as we go.

So amino acid should be matched up to small chemical group that makes up proteins.

They're 20 different types.

Triplet code should be matched up to three nucleotide bases that always code for the same amino acid.

Polypeptide should be a polymer of amino acids, the first stage in a protein structure.

And tRNA should be matched up to a molecule that carries a specific amino acid to the ribosome based on the triplet code.

And then a protein.

The final one should be a large polymer made up of amino acids, which has a 3D shape.

So if you've got those right, well done.

So correct them as you go 'cause you're going to need them as we move on.

So now we're on the third stage of our lesson, and the third stage of our lesson is variation in phenotype.

So proteins are made according to the instructions in the genetic code.

And these proteins produce our inherited features and control our life processes.

So these features and processes are called an organism's phenotype.

So therefore the phenotype is the physical characteristics of an organism.

So here's an example, we've got a protein structure and then we've got a caterpillar.

And an example of the phenotype would be the colour.

It's not the only example of the phenotype in that caterpillar, we could talk about its pattern or the number of segments in its body or the way that its legs are formed.

All of those things are examples of phenotype.

We often use colour 'cause it's easy to see.

And then another example here would be in this dog, again, we've got ear shape as an example, but again, it could be fur colour or how wavy the fur is or shape of eyes.

All of those characteristics are all phenotypes.

So proteins can be structural or functional.

So structural proteins will form structures that are visible in the phenotype.

And functional proteins are involved in chemical reactions and biological processes.

So examples of structural proteins that we can see are collagen in the skin and keratin in your hair.

Examples of functional proteins are like enzymes.

Enzymes control our chemical reactions in our body and even though we can't see them, they are made of proteins.

So they are part of our phenotype.

So functional proteins may be involved in things like making colour pigments, which eventually is something that we can see.

But the chemical reaction that produces them, we can't.

So genetic variants of genes, as we already said earlier in the lesson, alleles may code for a different protein.

So we've got our first allele here that's got a substitution mutation.

So we've gone from a C base to a G base in our genetic code and we can see that that triplet in the middle has changed.

So in this example, the triplet codes CGT has been changed to GGT and the change in the triplet code codes for a different amino acid.

And again, we've just got a example extract from that genetic code, but you can refer to your code in your additional materials.

So our CGT, the amino acid that it coded for was Arg.

But in our new allele where this mutation exists, now we are GGT.

So our amino acid now will be Gly.

So therefore a different polypeptide would be formed because the amino acid sequence would be different.

So we can see here that in our picture model that we've gone from a purple amino acid to this light green amino acid.

So time for a quick check, in a change in the triplet code may code for a different what in the polypeptide? So pause the video while you decide.

And then we'll see if you're right.

Okay, so the correct answer is amino acid.

So we get a different amino acid in the polypeptide.

Remember the polypeptide is the chain of amino acids.

A change in the 3D structure of a protein may lead to a different what in the organism? Okay, so the correct answer is phenotype.

So if you got that right, then well done.

So a change in the amino acid sequence, the polypeptide may lead to a change in the 3D structure of the protein.

The altered protein may not function properly or it might function differently.

So for example, if an enzyme's active site changes shape, the substrate won't fit.

So perhaps that enzyme won't be able to operate at all.

A protein producing a functional colour pigment pink, and then a non-functional colour pigment white where there is no colour pigment present.

The change in a characteristic cause by the change in a protein could lead to long-term variation in a species.

So many alleles for a gene may evolve leading to lots of different phenotypes.

And here's an example of eye colour.

So different proteins controlling colour pigments produce different eye colour phenotypes.

Changes to structural proteins like keratin and functional proteins which control colour pigments lead to variation in hair phenotypes.

So the structural protein could give you curly hair or straight hair or wavy hair or even different texture of hair, whereas the colour pigments are going to change the colour of your hair.

So many alleles exist for this phenotype, and as changes don't affect a human's chances of survival, it means that lots of variation can occur without any negative consequences.

However, sometimes a change in a protein structure might lead it to be non-functional.

And in those cases, sometimes it can have serious consequences for the functioning of an organism, and it can be the cause of genetic disorders.

So true or false, different alleles for the same gene may produce different phenotypes for the characteristic.

Now, once you've decided whether that's true or false, which of the statements below do you think best justifies your answer? So pause the video while you decide and then we'll have a look at what you think.

Okay, so the correct answer is true and the statement that best justifies that is the first one, alleles are genetic variance of genes.

They may code for different amino acids, which lead to different protein structures.

So if you got that right, then well done.

And it's time to move on to a practise task.

So Izzy and Jacob are discussing the colour phenotype of these two Labrador dogs.

And Izzy says, "The fur colour pigment is controlled by proteins." And Jacob says, "The dogs must have different proteins that control fur colour." So what I would like you to do is using your knowledge of genetic variants of genes alleles, explain why these two Labradors have two different colours of fur.

Now you're gonna have to bring all of your knowledge from the lesson together for this because I'd like you to include the following key words, mutation, nucleotide, base, triplet code, gene, allele, amino acids, protein, phenotype.

So you'll need a little bit of time to do this extended writing task.

So if you pause the video and then we'll come back and we'll have a look at a model answer.

Okay, how'd you get on with that? Hopefully you managed to include all of those key words, quite tricky, lots of scientific words in that.

So let's have a look at a model answer.

So a mutation has occurred changing the nucleotide sequence in the gene that codes for a protein that controls fur colour.

This change altered the base sequence, which created a genetic variant of this gene producing an allele.

The two alleles have different triplet codes, and so code for a different sequence of amino acids in the proteins.

The two different proteins produced will lead to the two different fur colour phenotypes.

So if you got that right and you've included all of those keywords in your answer, then well done 'cause it's pretty tricky stuff.

So time for the summary from today's lesson.

A mutation in a gene produces a genetic variant of it called an allele.

There is a change in the sequence of nucleotides, which can affect the triplet code.

If a triplet code in a gene is changed, this may or may not change the amino acid sequence in the protein that is made.

A change in the amino acid sequence of a protein can change the first stage of a protein structure, the polypeptide, and therefore the 3D shape of the protein when it folds.

The 3D shape of a protein is important to its function and it determines the phenotype produced by the gene.

A change in the 3D shape of a protein can change its activity, e.

g.

by changing the shape of an enzyme's active site.

These changes can have impacts on an organism's phenotype.

Well done for your work in today's lesson, and we will see you soon.