Lesson video

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

The title of today's lesson is a model of the structure of DNA, and we're going to be looking at that large chemical molecule, DNA, and the chemical units that make it up.

We're also going to be looking at its unique structure and also how we can model that structure and how we can evaluate it, so looking at the strengths and weaknesses of those models.

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 describe the structure of DNA and evaluate models of it.

So first of all, let's have a little look at our keywords for today's lesson.

So we've got genetic material, DNA, double helix, genome, and model.

So I'm gonna put the definition slide up now.

So you might want to pause it if you want to jot down the definitions, but otherwise, we are going to be going through them as we go through our lesson.

So our lesson today is going to be in two parts.

We've got the structure of DNA and we've got modelling DNA.

So the structure of DNA, we're gonna be looking at the chemical components that make it up and its shape.

And in modelling DNA, we're gonna be looking at the strengths and the weaknesses of models of this molecule.

So let's start with the first part of our lesson, which is the structure of DNA.

So the genetic material of all organisms is made of a chemical substance.

And the chemical substance is called DNA.

Now, DNA stands for deoxyribonucleic acid.

Now you can see that I've just highlighted the DNA in that long chemical name.

You don't need to be able to recall it, but just so that you know what it stands for because it is a word that's used in everyday life and often people don't know what it stands for.

And it is stored in the nucleus of plant, animal, and fungi cells, and in the cytoplasm of bacteria.

Because if you recall, bacteria don't have a nucleus, so therefore, their genetic material is free in the cytoplasm.

So that's where you would find the DNA in a bacteria.

In all those three other cells, they have a nucleus.

So the DNA is stored in there.

So just a reminder of a cell, so we've got an animal cell here.

You can see it's got its normal components.

So it's got its cell membrane, it's got its cytoplasm, and there's the nucleus there.

And if we could see what was packaged inside the nucleus, we've got this very, very long molecule called DNA.

So as we've already said, DNA is a chemical molecule, and it is made of two chains of smaller chemical units.

So those smaller chemical units are joined together in the chain and then we end up with these two chains, okay? And they spiral around each other.

And the thing that holds those two spirals together is these rungs that go across the middle.

So if you imagine you've got a ladder, okay, which is straight, if you could take the two ends of the ladder and twist them, then that's the structure that you would end up with for DNA.

And that shape has got a specific name.

It's called a double helix.

Okay, a double helix.

So how is all of this DNA packaged? So approximately, we've got two metres of DNA in the nucleus of all of our cells.

So can we imagine two metres? So in your classroom, you might have a metre stick, or it's sort of the length of a tube of wrapping paper.

And if you've got two of those and then we package up all of that DNA into the tiny nucleus.

Now you might remember from previous units that a nucleus is very small, so small that we have to be able to see it down the light microscope.

But you can still package two metres of DNA into it.

That tells you how small the chemical molecule must be.

We can't see it with our eyes and we can't see it with a light microscope.

But we can see the package that it's in with a light microscope, which is the nucleus.

All of the DNA of a cell is called the genome.

And the genome gives us our inherited physical features and processes.

So all of that is packaged into the nucleus.

Now you can see our DNA double helix here is a molecule and it's packaged into these little bundles.

And those bundles are inside the nucleus.

Now you may already know what those bundles are called.

They're called chromosomes and we're gonna come back to them later on.

But if you were looking at that, thinking, I know what that's called.

It is called a chromosome.

So it's little packages of DNA that are then packed into the nucleus.

So as we've already said, DNA is made of these two chains, these two chains that are made up of smaller chemical units.

Now scientists call these chemical units A, T, C, and G, and you can see that they are labelled there on this molecule.

These form the genetic code.

And the genetic code provides the instructions to build your inherited physical features and all of your life processes, controlling those life processes, in living organisms. So the amazing thing about this is that code, A, T, C, and G, that genetic code, is the same for all living organisms. So from bacteria all the way through plants, animals, thinking about all your different animals, insects, birds, fish, all of them have the same genetic code, but the order that that code is in will determine their different features and processes.

But it gives us loads of evidence for how we've all evolved from common ancestors way back in time.

So it's really amazing thing, this genetic code.

So let's have a quick check.

Which two of these are features of DNA? Is it single stranded, double helix, spiral shaped, or four chains? Just pause the video and then we'll check your answer.

Okay, let's see how we got on them.

So the two features of DNA are double helix and spiral shaped.

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

Let's move on to our next check activity.

So we've got a confidence grid here.

So what we'd like you to do is to read each of the statements and decide whether you think that they are correct or incorrect, right or wrong.

And then once you've decided that, how sure are you of your answer? So do you think it's right, or do you know it's right? Do you know it's wrong, or do you think it's wrong? So once you've made your decision, you can come back.

But for now, just pause the video.

Okay, let's look at what we got for these.

So the first one was the genome is made of DNA.

So that is right.

The genome is all of the DNA that is stored in the nucleus that gives you your inherited physical features and processes.

Next one.

DNA is made of cells.

Okay, that's wrong.

DNA is packaged into the nucleus of cells or in the cytoplasm of bacteria, but it's not made of cells.

Okay, next one, DNA is made of genetic information.

Again, that's wrong because it's the other way around.

DNA makes up a part of our genetic information.

And finally, DNA is alive.

DNA is not alive because it is packaged into cells that are living.

But DNA itself is a chemical molecule, okay? So it is not alive.

So it's time for us now to move onward to our practise task.

So we've got two pupils here, Izzy and Lucas, and they are discussing DNA and the genetic code.

And Izzy says, the DNA is made up of the genetic code, which gives the instructions for all of our features.

And Lucas says, the DNA is not made of the genetic code because it is made of chemicals.

And DNA is a large chemical molecule.

Okay.

So they both actually have some correct ideas here.

But what I'd like you to do is to write an explanation for the difference between DNA and the genetic code.

Now, you need to use your own knowledge from this lesson and previous lessons, but also use some of the correct ideas that Izzy and Lucas have suggested above.

So pause the video and then we'll come back and we'll see how you got on after.

Okay, I hope you managed that okay, and let's have a look at your answers.

So just a reminder that Izzy said that the DNA is made up of the genetic code and gives the instructions for all our features.

And Lucas said the DNA is not made of the genetic code because it is made of chemicals.

And the DNA is a large chemical molecule.

So let's have a look at a correct explanation.

So first of all, Lucas was right in this way.

So DNA is a large chemical molecule and it's made up of smaller chemical units, A, T, C, and G.

But DNA contains the genetic code.

So that's where Lucas was wrong.

And finally, the genetic code gives the instructions for all of our inherited features and processes, which is what Izzy was saying.

So those three ideas, okay, tell us the difference between DNA and the genetic code.

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

So let's move on to the second part of our lesson, which is modelling DNA.

The models are used in science to explain things that are difficult to understand or to see.

So here is a 2D image of DNA, and here is a 3D physical model of DNA.

In both cases, they're models of something that we can't see with our eyes, so therefore we have had to model them to be able to visualise them.

A model is something that represents something in the real world.

It's a simpler version of the real thing.

So here's an example, we've got a real iguana, and then we've got a simple model of an iguana, and then a more detailed model of an iguana.

And you can see in these cases that the level of detail in the models is different, but they're all models of the real thing.

Another type of model, that we've learned about earlier in our lessons, is an analogy.

Analogy is a way of explaining a difficult idea, often a scientific idea, with something that's more familiar to us in everyday life.

So for example, we've got a recipe book and a recipe book is made of paper.

And this is like our genetic material is made up of DNA.

And then within that book, we've got recipes that are written using a code, which is the alphabet, A to Z.

And in our genetic material, we've got the genetic code, which is written with the chemical molecules, A, T, C, and G.

And finally, we will use those instructions, that code in that recipe book, in order to make a cake.

But in our bodies, we'll use that genetic code in order to make a living organism.

So time for a quick check.

So a model is, how would you finish this sentence? A model includes every part and detail of the real thing, is a simpler version of the real thing, or is the real thing? So pause the video while you make your decision.

Okay, how did you get on? So the correct answer is it's a simpler version of the real thing.

Okay, so a model can be a physical model, so it can be a 3D model.

Or it could just be a description using words and diagrams, like this description of DNA.

Or it can be an analogy.

We used an analogy earlier in the lesson when we said that DNA is like a ladder with the rungs across the middle in a twisted shape.

So it's using something that's familiar to model something that is unfamiliar.

So here is a model of DNA.

You see it's got the parts of the DNA molecule labelled that it's modelling.

So it's modelling the two chains.

It's modelling the spiral shape.

It's modelling the fact that the chains are joined together.

And it's modelling the fact that DNA is 3D.

And some models are better than others.

And in science, we can use our understanding to evaluate a model.

So when we evaluate a model, we're doing a few things.

We are looking at the strengths of the models, and that means how similar it is to the thing that it's representing.

In this case, how similar is it to the structure of DNA? We're also looking at the ways that it's different.

So the weaknesses of the model.

And finally, we may be looking at, if we were to do it ourselves, how we could improve that model to make it more similar to the real thing.

So let's have a quick check of your understanding here.

So look at this image of DNA and identify the part that is missing.

Is it the links between the chains? Is it the spiral shape? Or is it a second chain? Pause the video.

We'll come back and see if you've got it right.

Okay, how did you do? Did you get this one right? It is a second chain.

Because you can see it's got the links between the chain and it has got a spiral shape, but it's not got the second chain and therefore isn't a double helix.

So let's have a go at evaluating a model together.

So we're going to do one, and then you're going to do one by yourself and we'll see how you got on.

So let's have a look at this first model.

So first of all, it has curved chains, so that makes it similar to DNA.

That's a strength.

It's got links between the chains.

Again, that's a strength.

It makes it similar.

It's 3D, like DNA.

And it forms a double helix.

At the moment, this looks like it's a pretty good model.

Let's see if we can find a weakness.

It's not fully spiralled.

It's just curved.

You could also say, if you wanted to go into detail, that those chains are not made up of single units, smaller chemical units.

Okay, it's your chance to have a go at this now.

So if you pause the video while you have a go at evaluating the next model on the slide and then come back and we'll see how you get on.

Okay, let's see if you wrote the same as me.

So first of all, it does have a spiral chain, so that is a strength.

And it's got links within the chain, between the chains.

So therefore that is a strength.

And it's 3D.

But there's only one chain present.

So that would be a weakness of the model.

And also, it doesn't form a double helix.

You could also say it's not made of smaller chemical groups as well.

So let's have a go and see if you can do this yourself without the side by side.

I'm gonna look at this Lego model here.

So we're gonna write an evaluation of this model and you are gonna state the strengths and the weaknesses.

So you'll need a little bit of time to write this out.

So pause the video and then we will come back and see how you got on.

Okay, how did your evaluations go? Let's see.

So we're gonna evaluate this model here, and it has got two chains.

So that is a strength.

And the chains are joined together across the middle.

And also, it's not completely straight.

So it's got sort of an idea that the DNA molecule isn't made of straight chains.

However, let's have a little look at the weaknesses of this.

So it's 2D, and we know that DNA is 3D, and the chains are not spiralled, and therefore, it doesn't form a double helix.

So those are the weaknesses of the model.

Okay, so well done because we've got to the end of our lesson on DNA structure and evaluating models.

So let's run through our summary.

So number one, genetic material is made of DNA.

The DNA is a long chemical molecule made of two chains of smaller chemical units joined together.

DNA is usually packed up tightly inside cells.

The chains in DNA are spiralled around one another in a double helix shape, with links across the middle like the rungs of a twisted ladder.

All the DNA in a cell is called the genome.

A model is a simpler representation of something.

Scientific models of DNA can be evaluated for their strengths and weaknesses.

Well done for your work in today's lesson.