Lesson video

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

The title of today's lesson is The Chemical Structure of DNA.

So we're going to be looking at that chemical molecule DNA and the smaller chemical groups that make it up and we're going to recap on its function.

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 chemical structure of DNA.

So we've got some keywords in today's lesson, and our keywords are: DNA, nucleic acid, polymer, nucleotide, and base.

Now you can pause the video if you would like to write down the definitions, which I'll put up in a minute.

If not, we'll be going through these words as we go through the lesson.

So you might want to jot them down later.

So in today's lesson, we're in two parts.

And the two parts of today's lesson are the structure of DNA and modelling DNA.

So we're going to be looking at the chemical molecule that is DNA and the smaller chemical groups that make it up, its function and its structure overall and how that helps it to carry out its job.

And then we're going to look at modelling DNA and evaluating some models of DNA.

So let's get started with today's lesson, so with structure of DNA.

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

And the chemical substance you'll be really familiar with is DNA.

So that stands for deoxyribonucleic acid.

Now you don't need to be able to recall that, but it's sometimes useful to know what it's made up of, particularly when you start to look at its smaller chemical groups.

So it's stored in the nucleus of animal, plant, and fungi cells and in the cytoplasm of bacteria, because if you remember, bacteria don't have a nucleus, so they store their DNA in the cytoplasm.

But in those other cells it is stored in the nucleus.

So just a reminder, this is an animal cell and it's got its normal structures in there.

So we've got ribosomes, mitochondria, all inside the cytoplasm with the nucleus, and then the cell membrane around the outside.

And the DNA is packed into that nucleus in those three types of cell.

There's approximately two metres of DNA packed into most animal cells.

And the DNA molecule cannot be seen with a microscope.

Because it's a chemical molecule, you can't look at it down a light microscope.

You might be able to see it when it's really bundled up.

And you perhaps have had the opportunity before to be able to do a practical where you extract DNA, or you might have seen a video of it.

So you can see it when it's all clumped together, but you can't see the individual molecule itself.

It's too small to see.

And all of the DNA in a cell is called the genome.

So the genome, remember, is all the coding and non-coding sections of DNA and most cells will have a copy of this genome.

DNA is in the shape of a double helix, which we'll come to later.

And it's wound up and packaged into these chromosomes.

So it's really, really tightly packed.

That keeps it nice and organised.

And then the chromosomes are stored within the nucleus of those cells, those plant, animal, and fungi cells.

So let's look a little bit more at the structure of DNA.

So DNA is a long chemical molecule called a nucleic acid.

So we said that it stands for deoxyribonucleic acid.

So nucleic acids are groups of chemicals.

There's lots of different types of nucleic acid.

So DNA is just one example of it.

It is a polymer, and polymers are made up of smaller chemical groups.

So all polymers are made up of smaller chemical groups.

That's why they've got the part of the word poly in there, poly meaning many.

So the small chemical groups that DNA is made up of are called nucleotides.

So DNA is both a nucleic acid because it belongs to that chemical group, but its small groups that it's made up of are called nucleotides.

So if you look at those in a little bit more detail, the nucleotides you can see zoomed in on there.

The nucleotide is made of these three parts.

And the bit that goes across the middle, the base, you can see that makes up the ladder that goes across the middle differs, and we'll look at that in a little bit more detail.

But the backbone, so that's that sort of beigey pinky colour, peach possibly, is made up of those other two groups that don't change.

So sometimes when we draw DNA, it's just worth remembering that that backbone is actually also made of chemical groups as well as the ones that reach across the middle.

So time for a quick check.

DNA is a nucleic acid polymer.

Now do you think that's true or false? And when you've decided, 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 check your answer.

Okay, so the correct answer is it is true.

DNA is a nucleic acid polymer.

And the statement that justifies that is it's made up of many nucleotides.

So if you got that right, well done.

Let's move on.

So DNA is actually made up of two nucleic acid chains.

And these chains are often called strands, the two strands of DNA.

And these two chains will spiral around each other to give that quite characteristic look of DNA, that double helix shape.

And across the middle you've got the parts of the nucleotide that connect the bases, connect across the middle, and they form the rungs of a ladder.

So if you were to untwist this, it would look like a ladder with a backbone down one side and then those bases reaching across the middle.

And then if you held the top and bottom of the ladder and gave it a twist, that's what this structure would look like.

So if we look at it zoomed up in detail, again, you can see those chemical groups that make up the backbone and you can see the chemical groups that join across the middle.

So two nucleic acid chains spiral around each other to form the double helix.

And inside you've got the genetic code.

So those chemical groups that reach across the middle, the bases, form the genetic code, and we term those A, T, C, and G.

The structure of the DNA actually protects that code.

Because you've got that backbone made of those chemical groups that are always on the outside, it means that the code is always in the inside.

So that protects the code so that we can use it many times for its function, which is eventually to form proteins.

So the genetic code is actually determined by the order of the different nucleotides and the bases that they have.

And A, T, C, and G make up those chains, and the order that they make up makes up the genetic code.

Those nucleotides termed A, T, C, and G actually stand for four chemical groups: adenine, thymine, cytosine, and guanine.

So time for another quick check.

Select the statements that you think correctly describe DNA.

So looking at the list at the bottom, which ones do you think are good descriptions of DNA? So pause the video while you decide and then we will come back and we will check your response.

Okay, hopefully you got on okay with that.

So the first one, DNA is a polymer.

That is true.

DNA is a polymer.

DNA is a type of nucleic acid, that is also true.

It is a type of nucleic acid.

DNA has a double helix structure, that is true.

And DNA is made of nucleotides, that is true.

So tried to catch you out with that one because all of them were actually correct descriptions of DNA.

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

The nucleotides that we were looking at, those four different nucleotides are the groups that make up the polymer, the monomers that make up the polymer.

And they always are made of the same three things.

So they have a sugar.

So when we think about the name deoxyribonucleic acid, the ribo bit is actually the sugar.

It's a sugar called ribose.

So it's always got the sugar.

And you might have seen in books or your teacher showing sugars often drawn as pentagons or hexagons.

So that's a sugar.

And then we've got a phosphate group.

So there's always a phosphate group connected to that sugar.

And then a base.

And as we've previously talked about, there are four different types of bases and the names are adenine, thymine, cytosine, and guanine.

So those are the four different types of nucleotides, but only because the base alters.

So you can see in this image here that they've all got the sugar, they've all got the phosphate, but they each have a different base.

And the base has a slightly different chemical structure and it has a different name because of that.

So nucleotides in a strand of DNA will pair up in a complementary way with the bases joining across the middle.

So we've already looked at the fact that they have slightly different shapes in those bases and that's what will determine whether those chemical groups join together.

So A, adenine, will always bond with T, thymine, across the middle of the DNA molecule.

And cytosine will always bond with guanine, again across the middle.

Now I like to remember it that the curly letters bond together and the ones that are made of sticks bond together.

So it just helps me to remember and look at them in shape form so you never get mixed up.

So time for a quick check.

So maybe you can use my tip to help you with this.

So select the image that shows the correct pairing of the nucleotide bases.

So pause the video and then we'll check if you've got it right.

Okay, let's see what you chose.

So the correct answer is B, C and G, those curly letters that go together.

So that is the correct base pairing.

So if you got that right, well done.

Okay, time for a practise task now.

So for this practise task, what I would like you to do is take each statement and decide whether you think it's correct.

So are you sure it's correct or you think it's correct, or do you think it's incorrect or are you sure it's incorrect? So for each one it's basically how confident you are about your answer.

So pause while you do this and then we'll come back and we'll see whether you've got the right answers.

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

So DNA is a chemical molecule, so that is correct.

So we need our ticks in either of those two columns.

DNA is a living cell.

That is incorrect.

DNA is packed into the nucleus of a animal, plant, and fungi cell and in the cytoplasm of a bacteria cell.

It's not a cell itself.

DNA is a polymer.

This is correct.

And DNA is made of nucleotides is also correct because the nucleotides are the groups that make up the polymer.

And DNA is a single chain.

This is incorrect, okay? Because it's made of two chains that twist around each other.

Okay, time for another task.

So in this task we've got Izzy and she's trying to explain to Andeep the correct answers to the DNA statements from the previous slide.

So what you are going to have to do is be able to see those statements in front of you.

So either on your worksheet or flicking back to that slide.

And for that first statement, which was that DNA is a chemical molecule, Izzy's description is: DNA is a chemical molecule as it is made up of smaller chemical groups bonded together.

So that is her extra information in order to back up her answer to why DNA is a chemical molecule.

So what I'd like you to do is to do that for each statement please.

So pause the video while you do that and then we'll come back and we'll have a look at some example descriptions.

Okay, hopefully you managed to finish off those statements with some good explanations.

Here are some examples that you could use.

So DNA is a chemical molecule because it's made of smaller chemical groups bonded together.

DNA is not a living cell as it is a chemical molecule that's stored inside living cells.

DNA is a polymer as it is a long molecule that is made up of smaller repeating chemical groups.

And DNA is made of nucleotides, which are smaller chemical groups that bond together.

There are four versions: A, T, C, and G.

And finally, DNA is not a single chain.

It is made of two chains of nucleotides joined across the middle, forming a double helix.

So if you wrote good explanations to those or words similar to those, then well done.

So it's time to move on to the next part of our lesson, which is modelling DNA.

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

So models can be in 2D, like the diagrams that we use in these slides, or they can be in 3D.

So you can see this is a 3D model, computer model of DNA.

Much easier to see that it's made up of much smaller chemical groups in this one and that each of the chemical groups actually has a 3D shape.

A model represents something from the real world and it's a simpler version of the real thing.

But there's different versions of models that we can use.

So for example, we've got a picture here of a real iguana, and then you could do just a simple model of an iguana, and then you can see a more detailed model of an iguana.

Different models of the same thing can have different amounts of detail, and sometimes it depends on what you want to explain, how much detail that you want to put into it.

An analogy is also a type of model.

We use these quite a lot in science to try to link something from science that's complex to something in the real world that you might understand or see every day.

So for example, a recipe book is made of paper.

And the genetic material, so this models the genetic material being made of DNA.

A recipe book stores instructions written using letters.

So in this case, 26 letters A to Z.

Whereas the genetic material stores instructions written in the molecules A, T, C, and G.

And the recipe instructions can be used to make a cake.

And the genetic instructions from that genetic code, A, T, C, and G, can be used to make a living organism.

So this is an example of an analogy.

So time for a quick check.

A model is used in science to do what? So tick the statements that you think are correct.

Pause the video and then we'll check if you got it right.

Okay, the correct idea is that models are used in science to explain scientific ideas that are difficult to understand.

And also to explain objects that are difficult to see.

So a model can be a physical model, so like a 3D model that you can touch.

So sometimes that's the word that most people would represent with the word model.

Or it can be a description using words and diagrams like this example here.

Or it could be an analogy.

So using something familiar as a model of something unfamiliar.

So the two chains in DNA are spiralled around one another with links across the middle like rungs of a twisted ladder.

So when we model DNA, we can use various materials to do this, and some models are better than others.

We can use our understanding of science to evaluate a model.

To evaluate a DNA model, and to evaluate anything in general, we must identify and explain the ways the model is similar to the thing we're trying to explain.

So in this case, how is it similar to DNA.

And the ways that it is different from DNA.

And in evaluating models, it shows us our understanding of DNA.

So therefore it's a really good activity for us to do, to really show whether we understand what we are talking about.

We also, once we use our knowledge, can maybe explain how it could be improved.

So again, let's have a quick check.

So looking at this image of DNA, identify the part that is not represented.

So is it the links between the chains, the nucleotides, the double helix, or the two chains? So pause while you decide and then we'll come back and we'll see if you've got it right.

So the correct answer is the nucleotides.

You can't see those separate chemical groups.

We can see the links between the chains and we can see the double helix.

Okay, we can see that it's made of two chains.

So let's evaluate this model together, and then I'll ask you to have a go at evaluating one by yourself.

So, to evaluate this model of DNA.

It has curved chains representing nucleic acid.

So we know that it's made of these two nucleic acid chains, and it's got those.

It's got links between the two chains.

So that represents where the nucleotides will join across the middle.

And it is 3D and it forms a double helix.

So all of those things are strengths of this model.

They represent the DNA well.

So now let's look at how it doesn't represent the DNA well.

It's not fully spiralled, and there are no individual nucleotides represented in this model.

So those are the places where it doesn't represent DNA very well.

Okay, let's have a go at another one.

So I'd like you to do this by yourself, and then we will come back and we will see how you've got on.

So if you pause the video while you do this, and then we'll look at your strengths and weaknesses after.

Okay then, so the positive things about this model are that the spiral chain represents the nucleic acid, and we have got links between the chain representing the nucleotides joining across the middle, and it is 3D.

But the weaknesses of this model is there is only one chain shown.

So those links are within the same chain, and there are no individual nucleotides represented in this one.

So your task now is to evaluate this model of DNA.

So what I would like you to do is state the strengths and weaknesses of the model.

And what that means is describing the part of DNA that each part is modelling or where an element of the DNA structure is absent.

So look very carefully at this image in order to find your strengths and weaknesses and make sure you link it to your knowledge of the DNA molecule itself.

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

Okay, so there's quite a lot that you could identify in this.

So first of all, it's strengths.

We've got the two chains representing nucleic acids with links between the chains representing the bases.

So we can see we've got the two chains made of Lego, and we've got some bits that go across the middle and they represent the bases.

You can see the nucleic acid chains are made up of smaller, repeating chemical groups.

And that chemical group is identified there at the bottom of the model so that you can see that that is the group that's repeating in both of those nucleic acid chains.

You can see that the nucleotides are represented in a three-part model with a sugar, a phosphate, and a base.

And the same two bases always connect across the middle.

So you can see that blue always connects with white, and red connects with purple, and that's to represent the A and T and C and G.

So adenine and thymine and cytidine and guanine are always complementary across the middle of the DNA.

And the sugar is the same in each one.

Can you see that the sugar is a white Lego brick in each one? So it's always represented by the same colour brick.

So now let's look at some of the weaknesses of this model.

So we have got: the chains aren't spiralled.

It's pretty difficult to do with Lego, so that is a weakness of this model.

And the double-helix structure is not represented.

And finally, the phosphate is a different colour each time.

It should be the same in each nucleotide.

So you can see that the same colour brick has been used for the sugar, but a different colour brick for the phosphate.

So really we could improve this model by making that the same.

So it's quite a lot to find there.

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

So now it's time for the summary from our lesson, the chemical structure of DNA.

So DNA is a type of biological molecule called a nucleic acid.

It is made of two strands, each a long polymer of nucleotides.

DNA is a polymer of four different nucleotides: A, C, G, and T, adenine, cytosine, guanine, and thymine.

These repeating chemical groups make up the nucleic acid chains.

The nucleotides in the nucleic acid chains join across the middle, as the complementary bases A, T, and C and G pair.

The nucleotides are made up of the same sugar and phosphate group and one of each of the four bases.

The order of the four nucleotides makes up the genetic code.

The double-helix structure of DNA is strong and it protects the genetic code.

Scientific models explain difficult ideas in science.

Models can then be evaluated for their strengths and weaknesses at representing the idea or the object accurately.

Well done for your work in today's lesson.