Lesson video

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

Today, we're gonna be looking at the structure of DNA, what chemical groups it's made up of, and a little bit of a recap of 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 structure of DNA.

So we've got some keywords in today's lesson.

And our keywords today are DNA, nucleic acid, polymer, nucleotide, and double helix.

So I'm gonna put the definitions up for these keywords so you can write them down if you'd like to.

So pause the video.

Otherwise, we'll be going through the definitions as we go through today's lesson.

So you might want to do it later.

So our lesson today is in two parts.

The first part of our lesson is the structure of DNA.

So we're going to be looking at that chemical molecule about the small chemical groups it's made up of, and the shape that it forms, and why that's important.

And then we're gonna look at modelling DNA and evaluating models of DNA.

So let's get started with the first part of today's lesson, which is the 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, 'cause 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 the 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.

It's really, really tightly packed and 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 acids.

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 the nucleic acid 'cause it belongs to that chemical group, but it's 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 beige pinky colour peach possibly, is made off 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've 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 the 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.

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 try 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.

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 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 'cause the nucleotides are the groups that make up the polymer.

And DNA is a single chain.

This is incorrect, okay? 'Cause 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 gonna 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 'cause 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, well done.

And 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 computer model of DNA.

Much easier to see that it's made up of much smaller chemical groups in this one.

And that the 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 model's 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 statement 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 that 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 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 do this 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 is 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 time for a task for you.

So here is another model of DNA, and you can see this one is made out of Lego.

And I'd like you to state the strengths and weaknesses of this model, just like you just did.

Describe the part of DNA each part of the Lego is modelling or where an element of the DNA structure might be absent.

So we're looking for strengths and we're looking for weaknesses.

So how is it similar to DNA and how is it different? Look very carefully at this to see what the different parts of the Lego that are being used and pause the video and then we'll come back and we'll see how you've got on.

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

So we've got some strength.

So we've got two chains that are represented and they represent the nucleic acids.

We've got links between the chains across the middle and that represents the nucleotides joining across the middle.

And we've also got the nucleic acid chains are made up of smaller repeating chemical groups, so hopefully you could see that.

And those are the nucleotides.

You see the pattern repeats as you're going up each of those chains.

So the weaknesses of this model is that their chains are not spiralled, they're straight, and we can't see that double helix structure that's not represented in this picture.

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

So it comes now to the summary for our lesson, which is the 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.

The nucleotides in the nucleic acid chain join across the middle, and then the mo molecule spirals to form a double helix shape.

DNA is a polymer of four different nucleotides, A, C, G, and T.

And these repeating chemical groups make up the nucleic acid chains.

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