Lesson video

Hi, I'm Mrs. Hudson.

And today, I'm going to be teaching you a lesson called "Observing mitosis in plant cells using a light microscope." This is a biology lesson and it comes in the unit called Cell division: mitosis and meiosis.

The outcome of today's lesson is, I can use a light microscope to observe plant cells in different stages of mitosis.

So we're going to be asking ourselves, how do I use a light microscope and can I identify the stages of mitosis when viewed using a light microscope? There are some key words in today's lesson that will be really important, and they are, light microscope, mitosis, lens, magnification and focus.

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

A light microscope is a type of microscope that uses visible light and a system of lenses to generate magnified images of small objects.

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

And today, we're going to be using onion root tip cells to observe mitosis.

Lens is a piece of glass or other transparent material with curved sides used in a microscope to magnify objects.

Magnification is making small objects appear larger in order to see in more detail.

And focus, a point where light rays converged to form an image with clarity.

If you need to pause the video to make a note of those words, then please do and then press play ready 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, we're going to be looking at the different parts of a light microscope.

Then we're going to move on to using a light microscope.

And in the final part of the lesson, we're going to be looking at observations of mitosis using a microscope.

Let's get going with the first part of the lesson though, the parts of a light microscope.

Cells are the smallest unit of life.

Every living organism is made from cells.

A light microscope is needed to be able to see the cells that make up an onion.

So here, we've got a picture of an onion and we can see that it's got some roots on it.

They look a little bit dry in this image, but the kind of lines, the brown lines coming out of that onion are the roots.

Now, if we wanted to be able to see the cells that make up the onion, we would have to use a light microscope in order to do so.

So the roots there, viewed under a microscope would look like this image, those are onion root tip cells.

And if we were going to look at the onion skin cells with a microscope, they would look like that.

We have to use a microscope because we wouldn't be able to view the cells with our naked eye.

We have to magnify them using a microscope.

This image here is showing you a light microscope, and you might be familiar with this image.

We're going to label the different parts of this light microscope.

So at the very top, you've got the eyepiece lens.

This is the part of the light microscope that you look down and it magnifies the image.

Then you've got the objective lens.

There are usually three objective lenses on a light microscope.

Again, they magnify the image further.

And then you have the stage.

This is the part of the microscope where you place the slide and the specimen and you can see there's little silver clips there to hold the slide in place.

At the bottom, you've got the light source and this shines light up through the specimen so that you can view it very clearly.

And then you've got two parts of the microscope that help focus the image.

The fine focus wheel, which is the inner part of this wheel here, and the coarse focus wheel is the outer part of the focus wheel.

So there are two parts of the microscope that magnify the image, the eyepiece lens and the objective lens, and two parts that focus the image, the coarse focus wheel and the fine focus wheel.

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

So we have an image here of a microscope and your job is to add the missing labels to the parts of the light microscope.

See if you can do that now.

So the very, very top label is the eyepiece lens and then the missing label on the same side is the stage.

We can see the clips there to hold the specimen and slide in place.

And then they've labelled the coarse focus wheel, but the missing, the focus wheel is the fine focus wheel.

So well done if you recognise those missing parts correctly.

That's brilliant job.

Each part of the light microscope has got a specific function and we're going to look at those now.

So we're going to start with the eyepiece lens.

This is the part of the microscope that you look down and it contains a lens that magnifies the specimen.

And then you've also got the objective lenses.

They also increase the magnification of the specimen and usually, they have multiple objective lenses that each magnify to a different power.

And you can see on the image, we can see two of the objective lenses, but the third one will be kind of hidden behind the one on the right.

And for example, the different powers of magnification, there could be times 4, times 10 and times 40, but both the eyepiece lens and the objective lens magnify the image, it's just that the objective lens usually has three different powered lenses, and the eyepiece lens is the lens that you look down to view the specimen.

Then we've got the stage.

This is where the specimen and the slide are clipped into place.

And the stage can be moved closer to or away from the objective lens.

And the light source at the bottom, this shines light up through the specimen so that it can be viewed really clearly.

And then we've got the coarse focus wheel.

This is the part of the microscope that moves the stage closer to or away from the objective lens.

And this helps us when we're trying to use it to roughly focus the image.

So we use the coarse focus wheel first to move the stage up to or away from the objective lens to get a rough focus.

And once we've got the image roughly in focus, we then use the fine focus wheel.

So this is used after the coarse focus wheel and it brings the image into fine focus, a really sharp focus to get a very clear image with a high resolution.

Let's check our understanding so far.

Which part of the light microscope roughly focuses the image? A, fine focus wheel.

B, eyepiece lens or C, coarse focus wheel.

Roughly focusing the image, this is C, the coarse focus wheel.

The fine focus wheel is used afterwards and this gives you a really sharp fine focus.

And the eyepiece lens is the part of the microscope that you look down and it magnifies the specimen.

Well done if you got that right.

Next question, which two parts of the microscope magnify the specimen? A, fine focus wheel.

B, eyepiece lens or C, objective lens.

This is B and C.

The eyepiece and the objective lens magnify the image.

The fine focus wheel is the second focus wheel you use to give you a really sharp focus.

And the third question, what part of the microscope moves the stage up and down? A, fine focus wheel.

B, light source or C coarse focus wheel.

This is C, the coarse focus wheel.

So we use the coarse focus wheel to move the stage up towards the objective lens and then to bring it back down when we want to bring it into rough focus.

Amazing job if you managed to get those correct, well done.

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

In the first part, you need to label the diagram to show the parts of the light microscope and the labels are already on there for you.

And then the second part, you're going to complete the table to show the function of each part of the microscope.

And you've either been given the name of the part or the function and you have to fill in the gaps.

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

Pause the video and then press play ready when I can feedback the answers.

Let's see how we did.

So these were the correct labels of the microscope.

You've got the eyepiece lens, the objective lens, the stage, the light source, and then the two focus wheels, the coarse focus wheel and the fine focus wheel.

Well done if you manage to remember all of those.

Now moving on to the second part, completing this table.

The function of the eyepiece lens is that it is the part that is looked down, it magnifies the specimen.

Usually has three that teach magnify the specimen, that's the objective lens because there are three of them.

Where the slide is clipped into place can be moved up and down, this is the stage.

The coarse focus wheel is used before the fine focus wheel to bring the image roughly into focus.

And you might also have written that it moves the stage towards or away from the objective lens.

Brings the image into fine focus, this is the fine focus wheel.

And the light source shines light through the specimen so it can be viewed clearly.

Brilliant job if you manage to get all of those in the table correct.

If you need to pause the video to add in any detail, then please do.

We're going to move on then with the second part of our lesson.

Great job.

We know the parts of the light microscope.

Now let's have a look at how we use the light microscope.

So for the first part of this lesson, we're going to watch a video clip to see how to use a light microscope to observe cells in a specimen.

So watch the video and listen carefully because we're going to come up with a method that we would need to use to be able to view a specimen under a microscope.

Setting up a microscope to view a slide.

So here we can see the different parts of the light microscope we've just been looking at.

And the first thing you need to do is turn the objective lenses to the lowest magnification.

Then you're going to place the slide onto the stage underneath the clips.

Then looking from the side, you're going to turn the coarse focus wheel to move the stage up so that it is close to the objective lens.

Make sure that the slide does not touch the lens though.

Now we're going to turn on the light source.

And then looking into the eyepiece lens, we should see quite a blurry image.

And now we're going to turn the coarse focus wheel, which moves the stage down and bring the image into focus.

This is a rough focus.

To make it a fine focus, we're going to turn the fine focus wheel until the image has more clarity.

The magnification can be increased by changing to a higher powered objective lens.

And we can see here on this image, if we change the objective lens that it magnifies the image.

Let's have a look again at the method for using a light microscope.

So number one, we turn the objective lens to the lowest magnification.

Two, place the slide onto the stage under the clips.

Three, looking from the side, turn the coarse focus wheel to move the stage up so it is close to the objective lens, but make sure that it is not touching the objective lens.

Number four, turn on the light source.

Five, looking into the eyepiece, turn the coarse focus wheel to lower the stage and bring your specimen into focus.

This is the rough focus.

Then for six, you turn the fine focus wheel to make the image clearer, and at this point, you have a very sharp focus.

And finally, number seven, the magnification can be increased by changing to a higher power objective lens.

We should be ready now to put these statements in the correct order for using a microscope.

There are six statements here and you need to number them 1 to 6 to show what order you should do them in when trying to view a specimen under a microscope.

If you need to pause the video to give yourself a little bit longer, then please do.

Okay, so let's have a look at the answers.

The first thing you need to do is turn to the lowest power objective lens.

Then we need to place the slide onto the stage.

Then we turn the coarse focus wheel to move the stage up to the objective lens and turn on the light.

Number four, we turn the coarse focus wheel to bring the stage down and the image into rough focus.

Then number five, we use the fine focus wheel to make the image clearer.

And then finally, number six, we change the objective lens to a higher power.

Brilliant job if you got those right, well done.

We're now ready to move on to the second task of our lesson, Task B.

And here, Aisha has written a method to enable her to view a prepared onion slide.

Your job is to rewrite Aisha's method to correct the incorrect statements and add in any missing detail.

So this is what Aisha has said.

Turn the objective lens to the highest power magnification.

Place the slide onto the stage.

Looking from the side, turn the fine focus wheel to move the stage up towards the objective lens.

Look through the objective lens and turn the fine focus wheel to lower the stage and bring the specimen into rough focus.

Turn the coarse focus wheel and bring the image into fine focus.

The magnification can be increased by changing the power of the eyepiece lens.

So you need to rewrite that out and make sure that it's correct and any missing detail is added.

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

Pause the video and then press play when you're ready for me to go through the answers.

Let's see how we did.

So in these sentences, the text that is in the slightly different font is showing you the correction that has been made.

So turning to the objective lens to the lowest power magnification was the correction.

Then place the slide onto the stage, that was fine.

Looking from the side, turn the coarse focus wheel to move the stage up towards the objective lens and turn on the light.

Now where in that method did it mention turning on the light and also the wrong focus wheel was being used? Look through the eyepiece lens and turn the coarse focus wheel to lower the stage and bring the specimen into rough focus.

Turn the fine focus wheel to bring the image into fine focus.

And finally, the magnification can be increased by changing the power of the objective lens.

Brilliant job, if you manage to get those corrections done.

If you need to pause a video to check any of your answers, then please do.

But you've done a fantastic job so far in the lesson.

You've done a fantastic job so far this lesson.

We know the parts of a light microscope.

We now know how to use a light microscope.

So let's move on to the third part of the lesson, observations of mitosis using a microscope.

Before we look specifically at onion cells in stages of mitosis, let's just recap what mitosis is.

The mitosis is a type of cell division that produces two genetically identical cells.

Mitosis is used in multicellular organisms for growth and repair.

So humans are multicellular organisms and as part of our life cycle, we grow, particularly in the younger years, we're growing constantly.

And that means new cells need to be produced for growth that happens through mitosis.

But even when we stop growing, new cells are continuously needed for repair and replacement.

And those new cells, again, are produced through mitosis.

And this image here is showing you the different stages of mitosis.

So we start off with the parent cell and in the first stage that DNA is copied, and then you've got double the amount of DNA and they condense to form something called chromosomes and those chromosomes line up along the middle of the cell.

And then those chromosomes split in half and they're pulled apart to either end of the cell and the nucleus begins to divide.

And then in the final stage of mitosis, the cytoplasm and the cell membrane split to form two genetically identical cells.

In plants, mitosis occurs in the tips of the roots and shoots.

Onion root tips can be used as a specimen to view cells undergoing mitosis using a light microscope.

So we can see here that there's an image of an onion growing new roots in the water, and that onion has been placed in the water and it sprouts new roots.

So it's growing roots in the water.

And those root tips are growing due to mitosis.

And the dividing cells can be viewed using a light microscope.

So if you take some of the cells from those growing root tips, you will be able to see the cells, but really importantly, you can actually see the different stages of mitosis.

And that's what we're going to look at now.

So this is a microscope image of root tip cells undergoing mitosis.

So let's just sort of familiarise ourselves with different stages of mitosis and what that looks like.

The first thing, we're gonna circle on individual onion cell.

Now this is just one onion cell.

There are lots of different cells in this image, we have circled one cell.

And you can see that there is the nucleus, which is the darker part, and it's quite a defined part of that cell.

So here, we've got the nucleus circled, and that nucleus contains DNA.

And in the first part in cell growth of the cell cycle and mitosis, the DNA is copied.

Now we can't see the chromosomes at this point, you can just see that there's a nucleus, there will be a nuclear membrane, but the chromosomes aren't visible.

And this happens after the DNA is copied.

So we can see the cell that is circled here, the DNA has condensed here to become visible chromosomes.

And you can see that the chromosomes almost look a bit more stringy-like, there's no definite nuclear membrane that has broken down.

And there is a difference between the visibility of the chromosomes here.

Now, once those chromosomes are condensed and become visible, the next stage is, that the chromosomes line up along the middle of that cell.

And we can see that really clearly in the circled cell here.

And then once they've lined up, the chromosomes split in half and are pulled to either end of the cell and the nucleus begins to divide.

And then finally, what you end up with are two genetically identical cells being produced.

Now we're going to look in more detail now at each stage.

The first stage of mitosis is cell growth, and this is where the DNA and the subcellular structures are copied.

So in an illustration of mitosis, this is what cell growth might look like.

You have got the parent cell, and then inside of that cell you've got the DNA, which is held inside of a nucleus.

And during cell growth, the DNA will be copied.

Now, in a microscope image of mitosis, we're looking for a cell where there is a visible nucleus.

So this cell here has got a visible nucleus.

The DNA is held inside of that nucleus.

And during cell growth, the DNA and subcellular structures are copied and the cell grows larger.

There are also other cells within this image that have visible nucleus as well, we've just identified one particular cell.

Once cell growth has occurred, the DNA will condense to become visible chromosomes and the nuclear membrane breaks down.

So this happens once the DNA has been copied.

So in an illustration of mitosis, we can see here that we've got the DNA, which is held inside of the nucleus.

So this was the image that we saw on the previous slide where you've got a cell with a very clear nucleus.

But then what happens is that the nuclear membrane starts to break down and the DNA condenses to become visible chromosomes, there's no nuclear membrane anymore.

So on a microscope image of mitosis, the circled cell there shows DNA, which is held inside of a nucleus, the DNA hasn't condensed yet.

Whereas, the circled image there, you can see that the chromosomes are starting to become more visible, the nuclear membrane is broken down, and you've got that sort of stringy-like property, which is the chromosomes.

The DNA has condensed to form the chromosomes.

In the second stage of the cell cycle, the chromosomes line up along the middle of the cell and are pulled apart, then the nucleus divides.

So here we've got the illustration of mitosis.

You can see the chromosomes are lining up along the middle of the cell, and then there's chromosomes which split in half are pulled to either end of the cell.

So let's look at this on a microscope image of mitosis.

I think these two stages are probably the most easy to recognise.

So we can see in this circled cell, the chromosomes are lining up along the middle of the cell.

And then we've got this cell here where the chromosomes are pulled to either end of the cell.

In the final stage of the cell cycle, the cell membrane and cytoplasm divide to produce two genetically identical cells.

So here, we've got the illustration where we can see the cell is starting to divide to begin with, the nucleus is reformed around the chromosomes, but the cell membrane and cytoplasm are started to divide, and then you end up with two genetically identical cells.

And in a microscope image of mitosis, we can see here, we've got two cells that have been circled, and that has occurred because the cytoplasm and the cell membrane have divided to produce two genetically identical cells.

Let's see if we can recognise some of those stages with some quick questions.

So here, we've got an image of mitosis and one cell has been circled.

Which stage of the cell cycle is occurring in the circled cell? A, cell growth; DNA is copied.

B, chromosomes line up along the middle or C, cell membrane and cytoplasm divide.

This is B, chromosomes line up along the middle.

We can clearly see in this image that you've got the visible chromosomes and they've lined up along the middle of that cell.

Well done if you recognise that.

Next question, what has happened to the DNA in the cell that is circled? A, the DNA has decondensed.

B, DNA has condensed to form visible chromosomes or C, the nuclear membrane is reforming around the DNA.

This is B, the DNA has condensed to form visible chromosomes.

Great job if you got that right, well done.

And then the third question, which stage of the cell cycle is occurring in the circled cell? A, cell growth; DNA is copied.

B, chromosomes are pulled apart, or C, two genetically identical cells are produced.

This is A, cell growth; the DNA is copied.

Brilliant job, well done.

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

In the first part of Task C, you have to identify which stage of mitosis is circled and explain what is happening.

In the second question, there are two cells that are circled and you need to explain the difference between the genetic material in the two circled cells.

For question three and question four, you have to identify which stage of mitosis is circled and explain what is happening.

And then you do the same thing for question five, so identify which state of mitosis is circled and explain what is happening.

And then in question six, explain why mitosis can be viewed using onion root tip cells.

I'm sure you're gonna do a fantastic job, it's always fun to look at the stages of mitosis under a microscope.

So give it your best go, enjoy it, and then press play when you're ready for me to go through the answers.

Let's see how we did.

So for question one, this is stage one, cell growth.

And what's happening in cell growth is, the DNA is held inside the nucleus and the DNA in subcellular structures are copied and the cell grows larger.

For question two, we've got the two cells that are circled in the top cell.

This is actually the same as the stage one, which is in question one.

So the cell is growing at this point, and the DNA is held within the nucleus.

So the key point here, 'cause it's asking about the genetic material, is that the DNA is held within a nucleus, whereas in the other circled cell, the DNA has condensed to become visible chromosomes and the nuclear membrane has broken down.

So well done if you manage to get that right.

Question three and question four are both stage two.

So what is happening specifically in question three is that the chromosomes line up along the middle of the cell.

Question four is still stage two, but this time, the chromosomes are being pulled to either end of the cell.

In question five, we've got stage three, and this is where the cytoplasm and the cell membrane divide to produce two genetically identical cells.

And then for question six, why can mitosis be viewed using onion root tip cells while multicellular organisms use mitosis for growth and repair? And onions are multicellular organisms. Growth occurs from root tips; therefore, mitosis is taking place in root tip cells.

And when viewed under a microscope, the stages of mitosis can be seen.

You might have your answer written in slightly different way, but as long as you've got the main points, that is great.

If you need to pause the video to go back and check some of the earlier answers, then please do, but I'm going to go on now to summarise everything that we've learned in the lesson.

So today, we were observing mitosis in plant cells using a light microscope.

And in the first part of the lesson, we said that a light microscope uses visible light and a system of lenses to generate magnified images of small objects.

So we also said that cells are the smallest unit of living life.

And for us to be able to view cells, we need to use a light microscope.

We said there are two parts of the light microscope that magnify the specimen, the eyepiece lens, and the objective lens.

And increasing the power of the objective lens will increase the magnification.

But we said there were three different objective lenses with three different powers, and we have to increase the power to increase the magnification.

The coarse focus wheel roughly focuses the image and the fine focus wheel is used to bring the image into sharp focus.

So we use the coarse focus wheel first, roughly focus, and then we move on to use the fine focus wheel to make it a very high resolution image.

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

Multicellular organisms use mitosis for growth and repair.

And then we said the onion root tips grow through mitosis and the stages of mitosis can be seen in dividing cells when viewed under a microscope.

And we looked specifically after identifying what those stages look like when viewed under a microscope.

I've really enjoyed today's lesson.

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