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, including PMAT.

This is a biology lesson and it comes under the unit called cell division, mitosis and miosis.

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 keywords 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 to a light rays converge 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 focus wheel is the fine focus wheel.

So well done if you recognise those missing parts correctly.

That's a 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 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 four, 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, 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 feed back 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 managed 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 each 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 managed 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 light 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 one to six 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 to 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 turn 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.

Nowhere 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 managed to get those corrections done.

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

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

Great job so far.

We now know the parts of a light microscope and how to use one.

So let's look at observations of mitosis using a microscope.

Before we look at cells undergoing mitosis with a microscope, let's just recap what mitosis is.

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

Mitosis is used in multicellular organisms for growth and repair.

Humans are multicellular organisms, and as part of our lifecycle, we grow.

Particularly in the first sort of 18 years of a person's life, they're growing continuously, and growth needs new cells to be made and this happens through mitosis.

Similarly, when we stop growing and even when we are growing, our cells need to be replaced and repaired, and that happens through mitosis as well.

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

The first stage is interphase, and this is when the DNA is copied and the DNA is held within a nucleus.

And then prophase occurs.

This is where the DNA condenses to form the chromosomes, which become visible and the nuclear membrane breaks down.

And then in metaphase, the chromosomes line up along the middle, the equator of the cell, and then anaphase occurs.

This is where the chromosome splits to form chromatids, and they are pulled to either end of the cell.

And then in telophase, the nuclear membrane reforms around those chromosomes and the chromosomes start to decondense back into DNA.

And then finally, cytokinesis occurs, and this is where the cell membrane and the cytoplasm divide.

So the stages in order: interphase, prophase, metaphase, anaphase, telophase and cytokinesis.

A really good way of remembering this is with a mnemonic, such as I push my amazing trolley carefully.

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

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

And we can see an image here of an onion which is being put into some water, and new roots are growing.

So that onion is sprouting new roots in water and the root tips are growing due to mitosis.

The dividing cells can be viewed using a light microscope.

So if we took some of the cells from that growing tip of the onion root, then we could actually see the cells in different stages of mitosis.

And this is what we're going to be looking at for the rest of the lesson.

This is a microscope image of root tip cells undergoing mitosis.

So first of all, we can see that there are lots of different cells within this image, but we can also see that they look slightly different.

And this is because these onion root tip cells are undergoing mitosis, and they're in different stages of mitosis.

So let's look in more detail at different cells.

So circled here is just one individual onion cell, and you can see that this particular cell has got quite a clearly defined nucleus, which is the darker colour in the middle.

So here we circled that nucleus.

And any cell that has a defined nucleus is going to be sort of in the interphase stage.

So this is where the nucleus is containing DNA.

And at this point, the DNA is being copied.

Cells spend most of their time in interphase.

However, in the next image, what we can see is that the DNA has condensed and it has formed visible chromosomes.

So in this cell here, we've no longer got that clearly defined nucleus, and this is prophase.

What happened here is that the DNA has condensed to become visible chromosomes, and you can see that it's got this kind of like stringy-like property to it.

The DNA is no longer held within a nucleus.

It's condensed to form chromosomes and the nuclear membrane has broken down.

Now, once the chromosomes have become visible, what happens next is metaphase.

And here the chromosomes line up along the middle of the cell, which we can see quite clearly in the circled image.

And then the next thing that happens is that those chromosomes split up to form chromatids and are pulled to either end of the cell, and this stage is called anaphase.

And then finally, cytokinesis occurs where two genetically identical cells are produced.

Let's look in more detail then at those individual stages of mitosis.

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

So in an illustration of interphase, it might look like this picture here.

We've got the parent cell and the DNA is held inside of a nucleus, and the DNA is being copied.

Now, in a microscope image of interphase, we're looking for a cell that has got a nucleus and where the DNA is not visible as chromosomes.

So here the DNA is held inside the nucleus and the DNA in the subcellular structures are copied and the cell grows larger.

So this is interphase.

The second stage of mitosis is called prophase.

This is where the nuclear membrane breaks down and the DNA condenses to become visible chromosomes.

So in an illustration here, we can say in the first stage, which we said was interphase, the DNA is held inside of a nucleus, whereas what's happening in prophase is that nuclear membrane is breaking down, which is being represented here by a dotted line to show that nuclear membrane breaking down and then the DNA condenses to become visible chromosomes.

There is no longer a nuclear membrane.

To start with, we can look at this cell.

This cell is not in prophase because we can see that the DNA is inside of a nucleus, whereas in prophase, we've got this circled cell.

Notice the difference, the DNA now has condensed to become visible chromosomes and the nuclear membrane has broken down.

The third stage of mitosis is metaphase.

This is where all the chromosomes line up along the equator of the cell.

So in the illustration of metaphase, we can see here the chromosomes lining up along the equator of the cell.

And in a microscope image of metaphase, I think this stage is relatively easy to spot.

So the circled cell here.

We can see those chromosomes are lining up along the equator of that cell.

The fourth stage of mitosis is anaphase, and this is where the chromosomes split to form chromatids and are pulled to either side of the cell.

So again, we've got our illustration.

So those are the chromatids, and they have been pulled to either end of the cell.

Again, this is quite easy to spot on the microscope image of anaphase.

So we're looking for an image where the chromatids are being pulled apart, and we can see that is happening in that circled cell there.

The fifth stage of mitosis is telophase where the nuclear membrane reforms at each end of the cell and the chromosomes decondense back into strands of DNA.

So in the illustration of telophase here, we can see that the nuclear membrane is reforming around that DNA and the chromosomes start to decondense again back into strands of DNA.

A microscope image of telophase.

You can sort of see that we've still got some condensed form of the chromosomes, but clearly, those chromatids have been pulled to either end of the cell and it's starting to look like the nucleus is going to reform around each end of the cell.

So the chromosomes start to decondense and the nuclear membrane is reforming.

And then in the final stage, the sixth stage of mitosis, we've got cytokinesis.

This is where the cell membrane and the cytoplasm divide to produce two genetically identical cells.

So our illustration of cytokinesis.

We can see that the cytoplasm and the cell membrane are starting to divide, and we end up with two genetically identical cells.

So our microscope image, we've got two cells here that are circled and the cytoplasm and the cell membrane have divided to form two genetically identical cells.

Let's see if we can apply that knowledge to some quick check for understanding questions.

So here you've got an image of some cells undergoing mitosis, and one of those cells is circled.

First question is which stage of mitosis is occurring in the circled cell? A, interphase; DNA is copied.

B, metaphase; chromosomes line up along the middle.

Or C, cytokinesis; cell membrane and cytoplasm divide.

This is B, metaphase.

We can see that the chromosomes have lined up along the middle of that cell.

Well done if you recognised that.

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

B, DNA has condensed to form visible chromosomes.

Or C, the nuclear membrane is reforming around the DNA.

This is B, DNA has condensed to form visible chromosomes, and we can see that it's kind of got that string-like structure and the chromosomes are becoming visible.

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

B, anaphase; the chromosomes are pulled apart.

Or C, cytokinesis; two genetically identical cells are being produced.

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

And we can see here that there's a nucleus.

Maybe you can also see that the DNA is starting to condense a little bit, but it's none of the other options on here.

So well done if you recognised that.

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

So in the first part, you need to identify which stage of mitosis is circled and explain what is happening.

For question two, you've got two circled cells, A and B, and the question is explain the difference between the genetic material in the two cells circled, A and B, and identify at which stage this occurs.

Question three and four are the same, so you have to identify which stage of mitosis is circled and explain what is happening.

So question five and six, again, same thing.

So identify which stage of mitosis is circled and explain what is happening.

I'm sure you're gonna do a really fantastic 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 for question one, this stage is interphase.

The DNA is held in the nucleus and the DNA and subcellular structures are copied and the cell is growing larger.

So question two, again, the top cell is interphase.

The DNA is held within the nucleus, whereas in B, that is prophase.

And we can see here that the DNA has condensed to become visible chromosomes and that the nuclear membrane has broken down.

Fantastic job if you managed to recognise the difference between those two images.

For question three, this is metaphase.

The chromosomes line up along the middle of the cell.

And then for question four, it's anaphase.

The chromosomes are pulled to either end of the cell.

In question five, what we can see here is telophase.

The nuclear membrane is reforming at each end of the cell and the chromosomes are decondensing to become strands of DNA.

And then for question six, this is cytokinesis, the cytoplasm and cell membrane divide to produce two genetically identical cells.

Really great job if you managed to recognise all of those six different stages.

If you need to pause the video and go back to look at any answers in more detail, then please do.

I'm going to move on now to summarise everything that we've learned this lesson.

So today's lesson, we were observing mitosis in plant cells using a light microscope, including PMAT.

And we said at the start of the lesson that a light microscope uses visible light and a system of lenses to generate magnified images of small objects.

And we said that we needed to use a light microscope to view cells.

There are two parts of the light microscope that magnify the specimen: the eyepiece lens, and the objective lens.

Increasing the power of the objective lens will 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.

Then in the third part of the lesson, we started to look at mitosis, and we said that mitosis is a type of cell division that produces two genetically identical cells.

Multicellular organisms use mitosis for growth and repair, and onion root tips grow through mitosis.

The following stages of mitosis can be seen in dividing cells when viewed under a microscope: interphase, prophase, metaphase, anaphase, telophase, and cytokinesis.

Remember, a good way to remember those stages: I push my amazing trolley carefully.

I've really enjoyed today's lesson.

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