Lesson video

Hello and welcome to this lesson from the unit DNA and the genome.

The title of today's lesson is Mutations and Genetic Variants.

So we're going to be looking back over what DNA is made up of and how it forms the genetic code, what a mutation is, and how that leads to a genetic variant, and what the impact of that genetic variant might be.

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 mutations in DNA cause genetic variants.

And we've got some key terms for today's lesson.

Our key terms are DNA, nucleotide, mutation, ionising radiation, and genetic variant.

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

So our lesson today is in three parts.

The first one is mutation and what mutations are and how they're formed.

The second one is what causes these mutations.

And finally, we get onto what this definition for genetic variants is and what it means for living organisms. So let's get started with the first part of today's lesson, which is mutations.

So DNA is a chemical molecule and it stores the genetic code in all living organisms. And the genetic code is the instructions for the cell to make proteins to carry out the common processes of all living organisms. Now, a little bit of a recap.

All living organisms includes bacteria, fungi, plants, animals, and within those, they're all made up of cells.

And all of those living organisms have cells which carry out these common processes.

So if you remember, those are movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition.

And that's what makes an organism living.

They don't carry them out at all points in their life, but they will do at some point in their lifecycle.

And we've got an example here of a frog, and we can see that that's one of its animal cells and its DNA there is stored in the nucleus.

So DNA is a chemical molecule, it's a polymer, which means it's made up of many repeating groups.

And the repeating groups that make up this polymer are nucleotides.

And this is what a nucleotide looks like.

These are the little groups that you can see that make up this very common image that we have of a DNA molecule with these two twisted backbones and these chemical groups that stretch across the middle.

So the two groups that make up that backbone there are always the same, and the one that stretches across the middle differs.

So that whole group there is a nucleotide, and when they are bonded together, you form this polymer and it's a nucleic acid chain.

And two nucleic acid chains twist together to form the DNA molecule.

The structure of each nucleotide is as follows.

We have a sugar and we have a phosphate, and we have that base that differs because there's four different base groups.

So the genetic code is actually determined by the order of the bases.

So the sugar and the phosphate is always the same.

So it's the order of the bases that determine the code.

And these are stored in the middle of that double helix structure that helps to protect the code.

So if we look at it zoomed in there, we can see our two strands, so our two chains of nucleotides, and the sugar and the phosphate don't change down the backbone.

But then across the middle there, we can see those bases stretching.

There's our nucleotide, and that order of the bases forms our genetic code.

So the double helix structure of the DNA actually just protects the code.

So those sugar and phosphate groups form the backbone and it's the base across the middle that forms the genetic code.

So the reason it's double stranded is to protect that code, but when we read the genetic code, we actually just read it from a single strand in order for us to use it to make proteins.

And you can see in this example the A, the T, the C, and the G, the order of those determines your genetic code.

Just like the order of letters in an alphabet when you put them together in different orders gives you words.

When you put the order of this A, T, C, and G together in different order, it gives you different genetic codes in genes which will code for different proteins.

So time for a quick check.

Select the chemical group that is not part of DNA.

So pause the video and decide and then we'll see how you've got on.

Okay, so the chemical group that is not part of DNA is an amino acid.

So we've got base, nucleotide, and sugar.

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

Another quick check.

Select the chemical group that makes up the genetic code.

So again, pause the video while you decide and then I'll give you the correct answer.

Okay, so the correct answer is base.

It's the order of bases that determine the genetic code.

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

So let's move on.

So genes carry the genetic code to make proteins.

So we can see we've got our whole genome there and our genome is made up of genes and non-coding DNA.

And the role of the genetic code, as in the gene, is to code for proteins.

Different proteins are coded for by different genes because each of them will contain a different genetic code, so a different order of bases.

Some of the non-coding sections of DNA help to control when these proteins are actually made by the genes.

So it's not that the non-coding DNA doesn't have a role, it just doesn't have a role in the actual production of proteins, but it can control when they are made.

So the genetic code is carried by genes, and when you read the genetic code, you read it in groups of three, and that is called the triplet code.

And each triplet code codes for a very specific single amino acid.

So therefore, the order of the nucleotide bases is really important.

Because each three codes for a specific amino acid, if there was a change to that three, it may code for a different amino acid.

And putting those amino acids together in the correct order is what gives you the structure of the protein.

Now, the amino acid that that triplet code codes for is always the same whatever living organism is using their DNA in order to code for proteins.

So it doesn't change, it's a universal code.

However, a change in the nucleotide base sequence in DNA can occur.

And when it does, this is called a mutation.

So when the mutation occurs in the DNA, so when the change happens in the DNA, it is called a mutation.

So here's an example.

So you can see in this original strand of DNA, we've got a nucleotide with the base A, and in the second strand of DNA, you can see that that has been substituted for a nucleotide with the base C.

And a mutation may be due to the following thing.

So here's a copy of our original DNA strand.

So it could be that you've got a substituted nucleotide like the example in the slide previously.

So you can see one of the nucleotides has been changed for another one.

So therefore, the base code will be different.

Another one would be that a nucleotide could be inserted in.

So again, that's gonna change the genetic code because it's gonna change the triplet code that that has been inserted into.

And you could also delete one.

So a deleted nucleotide is also gonna change that genetic code.

So again, it's going to change the triplet codes that code for those amino acids.

So a change, a mutation leads to a difference in the genetic code.

So time for a quick check.

The order of amino acids determines the sequence of a gene.

Now first of all, I want you to decide whether that statement is true or false.

And then once you've decided, I want you to choose which of these statements below best justifies your answer.

So pause the video while you read those and decide, and then I'll come back and we'll see if you've got it right.

Okay, so order of amino acids determines the sequence of a gene.

This is false, and the reason is because the order of nucleotide bases within a gene determines the order of amino acids in the protein.

So sometimes there's lots of coding for things in this particular topic, and it's making sure that you get those small repeating groups right in the molecules.

So in DNA, the repeating groups of the nucleotides, which have the bases, and the repeating groups in the protein are the amino acids.

So the order of bases determines the order of amino acids in the protein.

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

So time for a practise task.

So Laura and Izzy are discussing what a mutation is, and Laura says, "A mutation is where the structure of DNA is changed." And Izzy says, "A mutation is when the genetic code of a gene is different." So Laura and Izzy both have correct ideas.

So what I would like you to do is using Laura and Izzy's ideas and your own knowledge, can you write an explanation for what a mutation is? So pause the video while you do this little piece of writing and come back and we'll have a look at a model answer.

Okay, let's see how you got on with that then.

So Laura and Izzy both had some correct ideas.

So Laura said that a mutation is when the structure of DNA is changed, and Izzy said a mutation is when the genetic code of a gene is different.

So we can put those ideas together into our answer.

So a mutation is where the structure of DNA is changed, making the nucleotide sequence different.

This can be due to a nucleotide being substituted, inserted or deleted from the DNA molecule.

This leads to a change in the genetic code carried by the DNA, as the base sequence is different from the original.

So if you manage to get words that are in there into your answer, then well done.

If you want to add to your answer, then now's the time to do it.

So let's move on to the second part of our lesson today, which is what causes those mutations.

So most mutations occur due to errors when the DNA is copied during cell division.

So here's an example of bacteria.

So in bacteria when they reproduce, they copy their DNA and the cell splits in order to form offspring.

So when that DNA is copied, there could be errors in the nucleotides that are put in place when you're making a copy of the DNA, which could lead to a mutation.

In multicellular organisms, such as animals, plants, and some fungi, mutations can occur when cells divide in order to form gametes, for sexual reproduction, for example, in humans and animals, egg cells and sperm cells.

So when that DNA was copied to go into those cells, a mutation could have occurred.

Mutations might also occur when a cell divides.

So we're making new cells all of the time in order to grow, but also to repair any damaged tissue in our bodies.

And every time we need to make new cells, a cell has to copy its DNA in order to divide into two new cells.

And mistakes can happen when that DNA is copied.

Different nucleotides can be put in place or deleted or inserted during DNA replication.

So that's when most mutations might take place.

However, there are also some substances that might cause mutations, and those substances that might cause mutations in DNA are called mutagens.

So chemical mutagens can occur naturally or they might be produced synthetically.

So tobacco smoke is known to contain many mutagenic chemicals, so chemicals that could cause changes to your DNA.

And chemicals known to cause changes to DNA are carefully controlled.

Ionising radiation such as gamma rays and X-rays is also a mutagen.

So there are always background levels of ionising radiation, but it's the high doses that we need to be careful of.

These should be avoided or limited, so only use when you really need to.

And here are some examples where you may really need to use a high dose of radiation because the outcome of that is more important than any potential harm it may have.

So for example, gamma rays are used in radiotherapy in order to treat cancer.

And if we have broken bones, we need to be able to check which bones are broken and how they can be fixed, so X-rays might be used.

If you've ever been to a hospital to have an X-ray, you'll know that the people who are dealing with X-rays all day long often are behind a screen or have to wear a special apron to stop the X-rays from affecting them because obviously they're dealing with those all day every day.

Also, high energy ultraviolet is an ionising radiation and a mutagen.

So this is one of the reasons that we have to wear sun cream because sun cream can protect us from those high energy UV rays because it's emitted by the sun.

It's also emitted by tanning beds, and therefore, it is advised that you are time controlled.

And again, companies that use the tanning beds as their main source of income would say to people that you can only be on them for certain amount of times and they'll keep a register of how long people have used them because it can cause harm.

So which of these can cause mutations? So pause the video while you decide and then we'll come back and we'll check.

Okay, so the examples here that can cause mutations are UV radiation, chemicals, e.

g.

in tobacco smoke, and ionising radiation, i.

e.

X-ray.

Chemicals in tanning lotions don't.

Okay, we've got a confidence grid now.

So what I would like you to do is for each of the statements that that are there, I'd like you to choose whether you think it is correct or incorrect.

And once you've decided whether it's incorrect or correct, how sure are you of your decision? So tick one box for each of the statements.

So pause the video while you do that, and then we'll see if you've got them right.

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

So DNA structure is affected by mutagens.

This is correct, so either of those two in your right.

Mutagens can affect the nucleotide base sequence.

This is also correct because that's what a mutation is.

All mutagens are controlled by the government.

That is incorrect.

They're not all controlled.

And mutations can occur naturally during cell division.

That is correct.

So following on from that first test, what I'd like you to do is take each of the statements and add an explanation.

So I've written those statements out again here, so that then you can easily write an answer to them underneath.

So pause the video while you do that, and then we'll come back and we'll see what your explanations look like.

Okay, hopefully you got on all right with that, let's have a look at what you've got.

So for the first one, DNA structure is affected by mutagens.

So an explanation of that would be, "It causes the sequence of nucleotides to change by substitution, deletion, or insertion." Mutagens can affect the nucleotide base sequence, and your explanation for that would be, "Changes in the nucleotide changes the sequence of bases." Our next one, all mutagens are controlled by the government.

"Mutagens, such as UV radiation from the sun, cannot be controlled.

Cigarette smoke is also not controlled in adults." And mutations can occur naturally during cell division.

And the explanation for that would be, "As the DNA is copied to make gametes or new cells for growth and repair, mistakes can occur." So if you've got those explanations correct, then well done.

And let's move on to the third part of today's lesson, which is genetic variants.

So mutations are changes in the nucleotide base sequence, and these changes create a genetic variant.

A genetic variant is a region of DNA in which the sequence of nucleotide bases has been changed.

So you can see in this example here, we have had a substitution, which we would call a mutation because the nucleotide has changed, but what that has led to is a change in the base sequence, so it is a genetic variant.

So we can see here an example of a genetic variant caused by a substitution mutation.

One nucleotide, A, has been substituted for another, nucleotide C, and that has led to one triplet code being changed.

And remember the triplet codes code for amino acids.

So that one change changing the triplet code could also change the amino acid that it codes for.

A deletion mutation will alter every other triplet code that comes after it.

So you can see that in this example here.

So we've got a deletion of that A nucleotide at the top and shown by the dotted line that it's been removed.

So that means that every triplet code after it will also have been changed 'cause it's all shifted down one.

So that's not only gonna affect the triplet code leading to one amino acid possibly changing, it could lead to every amino acid after that that's coded for changing, which would lead to a completely different protein.

An insertion mutation, again, will alter every triplet code that comes after it because you're putting in a nucleotide, which means you're putting in a different base into the sequence and putting a different base into that sequence will change that immediate triplet code, but it also shifts down every triplet code afterwards.

So you can see that in the example shown below.

So with the substitution, you're only affecting one triplet code.

But with a deletion and an insertion, you're affecting every triplet code that follows.

So mutations can take place in genes or in the non-coding DNA.

So if a mutation takes place in a gene, then the genetic variant of this gene will have been produced, and we call that an allele.

So you can see an example below.

So we've got our DNA that's made of gene and non-coding DNA, but we're focusing on the gene here.

So with the gene, if we get a change, so for example, again, we've gone to a substitution, we can see we've got two different genetic sequences, so two different sequences of bases in these two versions.

So therefore, we call them alleles.

So they are different versions of the same gene and therefore could produce slightly different proteins.

So different alleles may code for different proteins to the original gene, or they may code for a non-functional protein.

So you could get different structures.

Now the reason for that is because if you get a different triplet code, you could get a different amino acid, not always because there are more triplet codes than there are amino acids.

So sometimes if the code changes, you still code for the same amino acid.

But if you do code for a different amino acid, then that is gonna alter the structure of your protein.

Now, that protein may still work in its different structure, or it might be non-functional, so that protein might not work at all.

If a genetic variation is created in the non-coding DNA, this is not an allele because alleles are specifically talking about different versions of genes.

But just 'cause it's not in a gene doesn't mean it can't affect the proteins that are produced because some of the sequences in the non-coding regions control whether a protein is produced or not.

So therefore, if you are changing the code in that non-coding sequence, you could change whether that protein is produced or when it's produced.

So time for a quick check.

Select the types of mutation that can occur in DNA.

So pause the video while you decide and then we'll check your answers.

Okay, so the correct answers are a deletion, a substitution, and an insertion.

They're all types of mutation that can occur in DNA.

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

So time for a task now.

Add labels and descriptions to this diagram to explain how a mutation in a gene can lead to two different protein structures.

So you might want to draw this out, or you might have a printout of it, but want more detail on it and more labels on it to show how you get two different versions leading to two different proteins.

So pause the video while you do this and then we'll come back and we'll look at a model answer.

Okay, so let's see how you got on with that then.

So here's one that I've done.

So you can see that I've added some labels, original gene and new allele.

I've labelled that mutation.

So the mutation in the DNA where one nucleotide base has been substituted for another.

I've labelled here that that means that the triplet code has been altered, so therefore a different amino acid may be coded for.

Different sequences of amino acids produce different proteins.

And at the bottom I've put there that the sequence of the bases in the genetic code of the gene is different.

So it is a new allele.

Now, you might have written these in slightly different orders, but having these general ideas correct is what we are looking for.

So if you've got a good labelled diagram with all of the key terms and descriptions, then well done.

So now it brings us to the end of our lesson, and here is our summary.

A change in the sequence of nucleotides in the DNA of the genome is called a mutation.

Mutations can be caused by some chemical substances and ionising radiation.

Most mutations are caused by errors in copying DNA when cells divide to make new cells.

Substitution, insertion, or deletion of nucleotides changes the base sequence in DNA causing mutations.

Mutations create genetic variants.

These are regions of DNA in which the sequence of nucleotide bases has been changed.

Genetic variants of genes are called alleles and may lead to the formation of a different protein.

So well done for your work in today's lesson, and we'll see you soon.