Lesson video

Hi.

I'm Mrs. Hudson, and today I'm going to be teaching you a lesson called "The Structure of DNA: Including Nucleotides." This is a Biology lesson, and it comes under the unit titled "Cell Division: Mitosis and Meiosis." The outcome of today's lesson is "I can describe the structure of DNA and how DNA is copied during mitosis.

During today's lesson, there will be some keywords and they are DNA, mitosis, nucleotide, polymer, and complementary based pairing.

Let's have a look at what each of those words mean.

DNA is a polymer with a double helix structure that carries the genetic information.

Mitosis is a type of cell division that produces genetically identical cells.

A nucleotide is an individual subunit that makes up DNA.

A polymer is a long chain of repeating subunits.

And complementary base pairing are bases from pairs with one other base.

A pairs with T, and C pairs with G.

If you need to make a note of those keywords, then please do.

Press pause, and then play when you're ready for me to carry on with the rest of the lesson.

Today's lesson is going to be split up into three different parts.

In the first part of the lesson, we're going to look at the structure of DNA.

Then, we're going to move on to describing nucleotides.

And then in the final part of today's lesson, we're going to be describing how DNA is copied.

Let's start with the first part of the lesson there, which is the structure of DNA.

The nucleus of eukaryotic cells contains a chemical called DNA, and that stands for deoxyribonucleic acid.

DNA is a highly coiled to form chromosomes.

So we can see here that we've got a eukaryotic cell, and then it's zooming in to look at the nucleus and then the chromosomes.

Remember, by definition, a eukaryotic cell is a cell that contains a membrane-bound nucleus.

So that image there is showing you a eukaryotic cell because it contains a nucleus.

And we can see there that we've kind of magnified that nucleus a little bit to look at it in more detail.

And inside of that nucleus, you've got these structures that look like crosses, and those are called chromosomes.

Chromosomes are made up of very highly coiled DNA.

So we can see in that image coming outta the chromosome, you've got the strands, which then form into what is DNA.

So there we've got the strands of DNA.

So the takeaway from this slide is that the nucleus in eukaryotic cells contains chromosomes, and chromosomes are made up of highly coiled DNA.

Let's look in a little bit more detail now at the structure of DNA.

DNA is made up of two strands that wrap around each other to form a double helix structure.

That's a really important phrase in this lesson.

So DNA has a double helix structure.

The reason why it's called a double helix structure is because double means that there are two strands, and helix means that those strands are coiled around each other.

So we can see on this diagram here of DNA, that would be one strand.

And then we've got the second strand, so double stranded, and then the strands wrap around each other to form this double helix structure.

So DNA has got a double helix structure because it's made up of two strands that wrap around each other.

DNA is also a polymer.

A polymer is any substance that's made up of repeating subunits.

DNA is a polymer because it's made up of repeating nucleotide subunits.

So that image there is showing you an example of a nucleotide.

If that was one strand of DNA, that strand is made up of repeating nucleotide subunits over and over again.

Now, we can only see a very small section here of what would be DNA.

In reality, there would be millions of these nucleotide subunits repeated over and over again.

So this is the one DNA strand made from repeating nucleotides.

But remember, DNA is a double helix.

So there's a second strand, and that second strand also is made up from repeating nucleotide units, but the nucleotide units are inverted, which means they're upside down in comparison to the other strand.

So the label there is the DNA strand made from repeating inverted nucleotides.

Let's check our understanding of what we've learned so far.

So the first question is, in the nucleus, DNA is coiled tightly to form, A nucleotides, B chromosomes, or C ribosomes.

This is B, the chromosomes.

Well done.

Nucleotides are single subunits of DNA and ribosomes are subcellular structures that are found inside of cells.

The next question, what is this subunit of DNA called? This is a nucleotide.

Well done if you remembered that.

Next question.

Which term describes the fact that DNA is made from repeating nucleotide subunits, A, polymer B, double helix, or C, highly coiled? This is A, a polymer.

A polymer is a repeating chain of subunits.

And DNA is made from repeating nucleotide subunits.

Double helix is not the correct answer for this because double helix is just the word to describe that there are two strands that wrap around each other in that helix structure.

And highly coiled is the structure that DNA forms to make chromosomes.

We're now ready to move on to the first task of the lesson.

So the first part of this task, you need to label the picture and describe how DNA is arranged in a eukaryotic cell.

So you're going to use the lines to label that picture, and then to the left of it, describe how DNA is arranged in a eukaryotic cell.

Then, for question two, use the images and the words below to describe the structure of DNA.

And the words that you've been given are double helix, nucleotide, strands, polymer, and repeating.

So use those words to help describe the structure of DNA.

I'm sure you're gonna do a fantastic job.

Pause the video and then press play.

Ready for me to feedback through the answers.

Let's see how we did.

So labelling this picture to begin with, you should have a eukaryotic cell.

It's eukaryotic because it contains a membrane-bound nucleus, and then that is the nucleus.

And those structures inside of the nucleus are called chromosomes, and they are made up of highly coiled strands of DNA.

And then the final image at the bottom are the DNA strands, which form a double helix structure.

And then to describe how DNA is arranged, DNA is found inside the nucleus in eukaryotic cells.

DNA is highly coiled and form structures called chromosomes.

Looking at question two now.

So we were using these words and the images to help us describe the structure of DNA.

Now, your answer might not be exactly the same as the answer that I'm going to put up, but maybe pause the video afterwards to check that you've got all of the detail in it.

So DNA has a double helix structure.

It is made up of two strands that wrap around each other.

DNA is a polymer.

It is made from repeating nucleotide subunits.

You may also have extended that second part of the answer to say that one strand has got inverted nucleotide chains.

That's absolutely fine if you've added that in.

But the two main things are the DNA has the double helix structure, DNA is a polymer, and then saying why.

Brilliant job if you managed to get all of that right.

Let's carry on for the next bit of our lesson.

Super job so far.

So we have covered the structure of DNA.

So let's move on to describing nucleotides.

Nucleotides are made up of a phosphate, a sugar, and a nitrogenous base.

So I told you previously that this image here is showing you a nucleotide.

Let's have a look then at the three different parts of the nucleotide in more detail.

So we've got the phosphate group, which is this circle that's on the left hand side.

Then we've got the sugar, which is the pentagon.

And then on the right hand side, you've got the nitrogenous base.

Now, there are four different bases that a nucleotide can contain, and they are given letters to represent them.

And those letters are A, T, C, or G.

And the four different bases stand for the names of the bases.

So adenine has the letter A, thiamine has the letter T, guanine has the letter G, and cytosine has the letter C.

You only need to know the letters, so you can just remember A, T, C, and G.

Let's have a look then at what they might look like.

So examples of those nucleotides, that could be A.

And then we've got T, C, and G.

What you should notice is that the only part of the nucleotide that's different is the base.

The phosphate and the sugar is the same.

Let's talk about that sugar phosphate bit a little bit more.

So DNA has got a sugar phosphate backbone.

This contains the sugar and the phosphate parts of the nucleotide.

So if we look at the image that's on the left hand side of this PowerPoint, we've got an illustration of DNA and the sugar phosphate backbone is the strand that we were talking about earlier on in the lesson.

So strand one and strand two, the actual coiled bit, that is made up of the sugar and the phosphate, and it's called a sugar phosphate backbone.

And then you can see between the sugar phosphate backbones, you've got these coloured lines and those are representing the bases.

So A, T, C and G.

Now if we go over to the right hand side, we've got that image again of the nucleotide.

Can you remember which part was the sugar and the phosphate? Hopefully, we remembered that the phosphate was the small circular part and the sugar was the pentagon shape in the middle.

And then the base is on the right hand side.

So the sugar phosphate backbone comprises as the sugar and the phosphate.

The base runs along the inside of the sugar phosphate backbone.

And the two bases on either strand are complimentary to each other, so they join up.

So let's have a look at this using our diagram that we had before.

So the sugar phosphate backbone is going to be comprised of the phosphate and the sugar.

And then we can see that's the bases run along the inside of our double helix DNA strand.

The base on each nucleotide pairs with a base on a nucleotide of the other DNA strand.

The bases always pair up in a specific way.

A pairs with T, and C pairs with G.

And this is called complementary base pairing.

So the base is always pair with one partner, A to T, and C to G.

And the phrase that we use to represent this is complementary base pairing.

So we can see this represented here.

We've got the base pairing within the nucleotides.

A always pairs with T.

And notice here that the T nucleotide is inverted, and C always pairs with G.

And again, note here that the G is inverted.

And again, we can see the diagram.

We've got that double helix structure of DNA with the sugar phosphate backbone and the bases are on the inside.

And notice that it's colour coded here that we can see A always joins with T and C always joins with G.

So those are representing complimentary base pairing.

Let's see how much of that we can remember.

Which four letters are used to represent bases, A, A, T, P and D, B, A, T, C and G or C, I, T, C and G? This is B, A, T, C, and G.

Great job if you remember that.

Let's move on to the next question.

Which phase describes how bases only pair with one other base, A, complimentary base pairing, B, double helix, or C phosphate pairing? This is A, complimentary base pairing.

Double helix is the structure of DNA.

And phosphate pairing isn't anything that we've spoken about yet this lesson.

And then the next question, which bases pair together in complementary base pairing, A, A to C, G to T, B, C to T, A to G or C, A to T, C to G? You should have C, A to T and C to G.

Brilliant job if you remembered that.

Now, that we know more about nucleotides and base pairing, let's have a look at complementary based pairing in more detail.

So complementary based pairing can be used to predict the sequence of the bases in the opposite DNA strand.

So we can see in this diagram here we've got one strand of DNA.

We've got some repeating nucleotides.

The other strand will have inverted repeating nucleotides.

But because we know that A always joins with T and C always joins with G, we can predict the sequence of bases in the opposite DNA strand.

So looking here, A always bonds with T.

So we'll have A to T, and then the next nucleotide will be C to G.

T always bonds with A.

So we'll have T to A.

G always bonds with C, so we'll have C to G.

And then the next one A always bonds with T.

So that will be T.

So we can see there we've used that single strand of DNA to predict the sequence of nucleotides in the opposing strand.

Your turn to have a go, now.

Write the sequence of base pairs that will match this DNA strand.

So this is a DNA strand.

Can you write the sequence of base pairs for the opposite strand? So we should have G, T, C, A, and C.

Fantastic job if you manage to get those right.

Well done.

We are now ready for the second task in the lesson, Task B.

In the first part of Task B, you need to annotate the labels to show the structure of a nucleotide.

So you've got the nucleotide there with labels.

You just have to label that to show the structure.

And then for part two, using letters to represent each base, show how the four base pairs pair together.

Don't worry too much about remembering which shape belonged to which letter.

As long as you've got the two pairs correct, that's great.

Then, question three, one DNA strand has the following order of bases.

Complete the order of basis for the second DNA strand? I would write this underneath to show which bases joined together.

And then question four, only one strand of DNA is needed to predict the order of bases in the second strand.

Explain why this is the case.

Give this your best go.

I'm sure you're gonna do a really great job.

Pause the video and then press play, ready for me to feed back the answers.

Right, let's see how we did with this.

So labelling this nucleotide, you've got the phosphate, the sugar, and then the nitrogenous base.

Excellent here.

If you owe A, T, C or G, 'cause that could be any one of those four.

And then for question two, using letters to represent each base, show the four bases, and how they pair together.

So we should have A and T, and C and G.

Don't worry if you put C and G in the top one, and then A and T in the bottom one.

As long as you've got A and T paired together and C and G paired together, that is fantastic.

For question three, we had to predict the other DNA strand.

So you should have T, A, C, G, A, A, T, C, G, G, A, C, T.

If you need to pause the video to just check that sequence, please do.

And then question four, only one strand of DNA is needed to predict the order of bases in the second strand.

Explain why this is the case.

So one strand of DNA is made up of repeating nucleotide subunits.

There are four bases.

Each nucleotide could have A, T, C or G.

Each base only pairs with one other base A to T and C to G.

This is called complementary base pairing.

If we know the sequence of bases on one DNA strand, we can work out the order of bases on the second strand.

And this is because of complementary base pairing.

Again, you might not have that word for word the same, but pause the video and check that you've got the main points of that answer in your answer.

You've done a great job so far.

We're onto the final part of our lesson now, which is describing how DNA is copied.

Mitosis is a type of cell division that produces genetically identical cells.

So we are multicellular organisms. We're made up of more than one cell.

We use mitosis for growth and also for repair of cells.

And during mitosis, cell division occurs, and you end up with two genetically identical cells.

Interface is a stage of mitosis where DNA is copied.

Interface is the very first stage of mitosis.

So if this diagram here was showing you the cell cycle of mitosis, interface is the very first phase.

The cell spends most of its time in interphase, and it's at this point where the DNA is copied.

And then what happens is you end up with chromosomes with double the amount of DNA.

This is really important because remember we're trying to go from one cell to two cells and before the cell can split, we have to double the amount of DNA, so that the two resulting cells have the same amount of DNA as the original cell.

And then what happens is that half of each chromosome, which is called a chromatid, travels to each end of the cell and then the cell divides.

And then you end up with two genetically identical cells where each cell has the same DNA as the original parent cell.

So we're going to look a little bit more in detail about how DNA is copied during interphase.

So when DNA is copied during interphase, the DNA strands unzip to form two separate strands.

And we can see that we've got a diagram here, which is showing you on the left hand side you've got the original double helix structure of DNA.

And then what you can see happening is that that helix unwinds.

So you end up with two separate strands.

We've got the original DNA double helix, and then that double helix is unwound.

What this then means is that both strands can be used as a template due to complementary base pairing.

So those two strands there can be used as a template because we know the base on the strand of the DNA, we can use that to predict the sequence of bases on the opposite strand.

Free nucleotides are then added to each DNA strand using complementary base pairing.

So you've got free nucleotides that aren't attached to anything.

And because we know the order of bases on one strand, we can predict the order on the opposite strand and add free nucleotides to make that second strand of DNA.

And this creates two identical copies of the same double helix DNA.

So what we can see here is that each strand is acting as a template and free nucleotides are being added.

And we end up with two identical copies of the same DNA.

Complimentary base pairing is fundamental to the idea of new DNA being made.

So if we unwind or unzip the double helix, each strand acts as that template.

We can use the sequence of bases to predict the sequence in the second strand, and then that happens because the free nucleotides are added.

Let's check our understanding of that.

So at what stage of mitosis does the DNA get copied, A, prophase, B, metaphase, or C, interphase? <v ->This is C, interphase.

</v> This is the very first part of the cell cycle.

And then put these steps of DNA replication in order.

So you need to put a number one, two, three, or four, next to the description.

And these are the descriptions.

Each single DNA strand acts as a template.

Double helix parent DNA unwinds to form two separate strands.

Two copies of the same double helix DNA are created.

And, free nucleotides are added to each single DNA strand.

So have a go now at putting a number in to show the stages of each description.

So we should have number one as the double helix parent DNA unwinds to form two separate strands.

Then number two is each single DNA strand acts as a template.

Three, the free nucleotides are added to each DNA strand.

And then four, two copies of the same double helix DNA are created.

Fantastic job if you managed to get that right.

We're now ready to move on to the last task of the lesson, Task C.

In the first part, you need to use the diagram to help explain how DNA is copied during interface.

And then in the second part, explain why it is important for DNA to be copied during mitosis.

Give this your best go and then press play when you are ready for me to go through the answers.

Let's have a look at how we did.

So explain how DNA is copied during interphase.

First of all, the double helix DNA unwinds to form single strands of DNA.

Then secondly, the single strands act as templates to form new strands.

Then, three, nucleotides are added using complementary base pairing.

And then finally, you end up with two identical copies of the same double helix, DNA.

Amazing job, if you remember those different stages.

If you need to pause the video to add anything into your answer, please do.

But moving on to the second part, explain why it is important for DNA to be copied during mitosis.

Mitosis is a type of cell division that creates two genetically identical cells.

The new cells contain the same DNA as the original cell.

The copied DNA forms chromosomes that split to form chromatids and are pulled to either end of the cell.

When the cell divides, each new cell contains the same DNA that was in the original cell.

We've got to the end of the lesson now, and you've done an absolutely amazing job.

So today we've been learning about the structure of DNA, including nucleotides, and we said that DNA is the chemical that carries the genetic information in eukaryotic cells, and it's highly coiled forming chromosomes in the nucleus.

We also said that DNA is a polymer with a double helix structure.

It's a polymer because it's made up of repeating nucleotide subunits.

And it's a double helix structure because you've got the two strands that coil around each other.

DNA is formed from repeating nucleotide subunits.

And nucleotides contain a phosphate sugar and a nitrogenous base.

Nucleotides can contain four bases, A, T, C or G, and the bases pair with each other, A to T and C to G.

And this is called complementary base pairing.

During mitosis, the DNA is replicated, and this involves the DNA unwinding to form two single strands.

Nucleotides are added to each strand, which results in the formation of two identical double helix strands of DNA.

You've done an amazing job today.

Well done.

I hope you've enjoyed it.

I have, and I look forward to seeing you next time.