Lesson video

Hi, I'm Mrs. Hudson, and today I'm going to be teaching you a lesson called Making gametes: meiosis.

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 explain how a type of cell division called meiosis makes gametes for sexual reproduction.

There are some keywords in today's lesson, and they are sexual reproduction, gametes, meiosis, chromosomes, and haploid.

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

Sexual reproduction is a type of reproduction that combines the genetic material of two organisms to produce genetically different offspring.

Gametes are an organism's sex cells that contain half the number of chromosomes.

Meiosis is a type of cell division that forms the gametes.

Chromosomes are highly coiled strands of DNA found inside the nucleus of eukaryotic cells.

And haploid is a cell with half the number of chromosomes.

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

In the first part of the lesson, we're going to be looking at sexual reproduction and gametes; and then secondly, we're going to look at how meiosis forms gametes; and then finally, we're going to be comparing mitosis and meiosis.

Let's get going though with the first part of the lesson, sexual reproduction and gametes.

Reproduction is an essential part of an organism's lifecycle and it can happen two ways.

First of all, you can have asexual reproduction.

This is where only one organism is involved.

Yeast cells divide asexually.

The yeasts divides to form two genetically identical cells, which are called clones.

And you can see there's a diagram here of one yeast cell, which is then dividing by asexual reproduction and it's producing two genetically identical yeast cells.

The other type of reproduction is sexual reproduction, and this is where two organisms are involved.

Specialised cells called gametes fuse to form a zygote which is genetically different from both parents.

And you can see here there's a picture of lions, you've got a male and a female lion, and they will reproduce sexually and produce an offspring which is genetically different.

In sexual reproduction, the male and female parents produce specialised cells called gametes.

The gametes are known as sex cells, which fuse together during fertilisation to form a zygote.

So there's going to be some pictures now which are showing you sexual reproduction in humans.

So remember that gametes are the sex cells and they fuse together during fertilisation to make a new cell, which is called a zygote.

The female gamete in humans is the egg cell and the male gamete in humans is the sperm cell.

So the egg and the sperm, the gametes, they fuse together during fertilisation to make a new cell, which is called a zygote.

Plants can reproduce asexually or sexually.

The gametes used in sexual reproduction are the egg, which is the female, and the pollen, which is the male.

So there's a difference there between the gametes in humans and the gametes in plants.

Both have a female gamete called the egg, but the male gamete in humans is the sperm and the male gamete in plants is pollen.

And we can see the plants labelled here.

You've got the female gametes, which are the eggs, also known as ovules, and then the male gametes, pollen, are labelled here, and they are found on the part of the plant called the anther.

During sexual reproduction, pollen from one plant is transferred by vectors including water and wind to the ovules of another plant, and fertilisation takes place.

So here you can see that the pollen grains are being transferred to the stigma of another plant, and then the pollen grains travel down into the ovules, and this is where fertilisation takes place.

Let's do a quick check for understanding.

Which type of reproduction requires two organisms? A, asexual, B, sexual, or C, sexual and asexual? This is B, sexual reproduction.

In asexual reproduction, only one organism is needed.

What are the sex cells known as? A, zygote, B, asexual, or C, gametes? This is C, the gametes.

Well done if you got that right.

Human body cells have 46 chromosomes, or 23 pairs of chromosomes.

They are diploid cells with the full number of chromosomes as they have two copies of each chromosome.

So this image here is showing you a diploid cell.

It's not showing you a human body diploid cell, it's showing you a diploid cell, and we can see that we have two copies of the same chromosome.

There we've got a pair of chromosomes and also here we have a pair of chromosomes, but there's two copies of each chromosome.

The cell is diploid.

Human gametes have 23 chromosomes in total, one copy of each chromosome.

Gametes are haploid cells containing half the number of chromosomes.

So here we have a haploid cell and we can see that there's only one copy of each chromosome.

So a single chromosome and another single chromosome.

Haploid cells have one copy of each chromosome.

They have half the number of chromosomes.

Gametes are haploid cells.

During fertilisation, the haploid gametes fuse together to create a zygote.

The zygote is diploid and it has the full number of chromosomes.

So if this is an egg cell, which is a female gamete in humans, it has 23 chromosomes, one copy of each chromosome.

And then we have the sperm cell, which is the male gamete, and this also has got 23 chromosomes, one copy of each chromosome.

And during fertilisation, the egg and the sperm fused together to make a zygote.

The zygote has 46 chromosomes, two copies of each chromosome.

One copy comes from the female and one copy comes from the male.

So the egg and the sperm are haploid gametes.

Remember that haploid means half the number of chromosomes, whereas the zygote with 46 chromosomes is diploid.

It has two copies of each chromosome, a full set of chromosomes.

Let's do a quick check for understanding again.

Which word describes a cell with half the number of chromosomes? A, diploid, B, haploid, or C, zygote? This is B, haploid.

Well done.

Remember, diploid is a cell with two copies of each chromosome and a full set of chromosomes, whereas haploid is a cell with half the number of chromosomes, only one copy of each chromosome.

A zygote is the cell that is produced after fertilisation.

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

And in the first part you need to describe the difference between asexual and sexual reproduction.

And then in the second part, label these pictures to name the gametes in humans and plants.

Then question three, you're going to complete the table below to fill the gaps.

So you have different cells, the sperm, the egg, and the zygote, and you need to say the number of chromosomes, if it is haploid or diploid, is it a gamete? So that will be yes, it is a gamete, or no, it isn't a gamete.

And is it found in males, females, or both? And then finally in question four, explain why it is important for gametes to be haploid.

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

You can pause the video and then press play ready for me to go through the answers.

Let's see how we did.

So the first question, asexual reproduction only requires one organism, and the resulting offspring are genetically identical to the parent, which we call clones.

Sexual reproduction requires two organisms and involves gametes fusing together to form genetically different offspring.

You might also have written that the gametes fusing together is called fertilisation.

In question two, you've got the human egg cell, which is female, the human sperm cell, which is male.

And then that line there is pointing towards the plant egg cell, which is the female sex cell, and then the line there is pointing to the plant pollen, which is the male gamete.

Well done if you manage to get those right.

Now looking at this table, a sperm has 23 chromosomes, an egg also has 23 chromosomes, but a zygote has 46 chromosomes, or you might have written 23 pairs.

A sperm is haploid and an egg is also haploid.

They have half the number of chromosomes, only one copy of each chromosome.

and a zygote is diploid, it has two copies of each chromosome, the full amount.

And is it a gamete, a sperm? Yes, it is a gamete.

And an egg, that is also a gamete.

A zygote is not a gamete.

And then is it found in males, females, or both? The sperm is the male gamete, the egg is the female gamete, and zygotes can be found in both because that could be male or female.

Number four, explain why it is important for gametes to be haploid.

During fertilisation, the sperm and egg cell fuse together to form a zygote.

The zygote has to have the full number of chromosomes, so either 46 or 23 pairs.

Therefore, each gamete must have half the number of chromosomes to ensure that the zygote is diploid.

Now, you might have written that slightly differently, but maybe pause the video now and just check that you've got all the detail in there that you need, and then we're gonna carry on with the rest of the lesson.

Really great job so far.

We're now ready to look at how meiosis forms the gametes.

So we're thinking now, how are the gametes actually made? Gametes are formed through a process called meiosis.

So meiosis is the process that forms the gametes.

Meiosis involves two stages of cell division, resulting in four haploid cells being produced that each contain half the number of chromosomes.

Now, we know this because the gametes are haploid, and if meiosis is the process that forms the gametes, then it's a process that must produce haploid cells.

So we start off with a parent cell that is diploid.

So this could be a human body cell and it has two sets of each chromosome.

The first part of meiosis is that the DNA is copied, so you end up at one point with double the amount of DNA.

And then during meiosis, what happens is that sections of DNA are swapped, and this results in variation in the gametes.

Then we have the first round of cell division and that produces a diploid cell.

And then we have a second round of cell division that produces four haploid gametes.

Let's just quickly check our understanding of that so far.

So true or false, meiosis results in the formation of two cells, gametes.

And then justify your answer; A, meiosis involves one cell division resulting in two cells, or B, meiosis involves two cell divisions resulting in four cells.

So meiosis results in the formation of two cells.

That is false.

And the justification here is that meiosis involves two cell divisions, resulting in four cells, which is B.

So well done if you remembered that.

So we're going to have a look at the process of meiosis now in more detail.

Before meiosis can occur, the DNA is copied during interphase.

Each chromosome is an exact copy of the other.

So here you've got a maternal chromosome and a paternal chromosome.

The maternal chromosome will have come from mum during fertilisation and the paternal chromosome will have come from dad during fertilisation.

And what happens in the first stage of interphase is that the DNA is copied and we have an exact copy of each chromosome, which we can see in the second cell.

After the chromosomes have replicated, the maternal and paternal chromosomes arrange themselves into pairs and line up along the middle of the cell.

So the maternal and the paternal chromosomes can be seen here and they pair up.

And again, another set of maternal and paternal chromosomes pairing up.

And the pairs of chromosomes line up along the middle of that cell.

And we can see at this point that there's mixing of the maternal and the paternal DNA, which causes variation.

What that means is that as the chromosomes line up along the middle of the cell, some of the DNA on the maternal and paternal chromosomes are swapped and mixed over, so the resulting chromosomes have a mixture of maternal and paternal DNA.

And this means that the chromosomes in the gametes at the end of meiosis have a mixture of genes, so there is variation.

The chromosomes are then pulled to each end of the cell and the first cell division occurs.

So here the chromosomes are being pulled to either end of the cell and the first cell division occurs.

Two diploid cells are produced with a mixture of maternal and paternal DNA within the chromosomes.

So each of the cells produced is diploid, it has two copies of each chromosome, but some of the chromosomes contain a mixture of DNA, leading to genetic variation.

The chromosomes then line up along the middle of the two cells again and are pulled apart and a second cell division occurs.

So here the chromosomes line up along the middle and are pulled to either end of the cell again and that second cell division happens.

Four haploid cells are produced, each with half the number of chromosomes.

So we can see now that there are four cells that are produced, and those cells are haploid cells.

They are the gametes.

And you can also see as well that some chromosomes contain a mixture of DNA, leading to genetic variation.

It's really important that we understand that gametes have variation, and this is because DNA from the maternal and paternal chromosomes mix during meiosis.

And this creates chromosomes that have a mixture of maternal and paternal DNA, which we can see in this diagram here, which shows the mixing of DNA during meiosis.

Some genes become changed and it results in genetic variation in the offspring.

No two gametes are genetically identical.

Let's check our understanding of that.

So starting with interphase, which happens before meiosis, put the stages into the correct order, starting with interphase.

So first of all, the DNA's copied to form identical copies of the chromosomes, and then the chromosomes line up along the middle and the DNA is mixed.

Then cell division occurs to produce two diploid cells, and then cell division occurs again to produce four haploid cells.

Well done if you managed to get those into the correct order.

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

And your job is using the illustrations below, describe what is happening at each stage.

So there are some illustrations and you need to say what is happening at each stage.

So these are the first two stages, and then we've got stage three here, and then finally, stage four.

So have a go at that task and then press play when you're ready for me to go through the answers.

Let's see how we did.

So the first stage is happening before meiosis can occur and the DNA is copied during interphase.

And then diagram two is showing you a pair of chromosomes and a pair of chromosomes, and also we've got the first bit there of DNA mixing together.

So after the chromosomes have replicated, they arrange themselves into pairs and line up along the middle of the cell.

Diagram on the left is showing you that.

And DNA mixing between maternal and paternal DNA occurs.

This creates variation within the chromosomes, which is what is happening in the right-hand side.

Well done if you managed to get that right.

In slide three, the chromosomes get pulled to either end of the cell and the first cell division occurs, and this creates two diploid cells.

And then we've got the two diploid cells there, and that line is showing you that some chromosomes contain a mixture of DNA, leading to genetic variation, and the other cell is showing you a diploid gamete that contains the full number of chromosomes.

You could also say it has two copies of each chromosome.

And then in number four, the chromosomes line up along the middle of the cell and are pulled to either end again.

The second cell division occurs, creating four haploid gametes.

So all of those four cells are haploid gametes that contain half the number of chromosomes, one copy of each chromosome.

And then those lines there are pointing towards the fact that some chromosomes contain a mixture of DNA, leading to genetic variation within the gametes.

Fantastic job if you managed to label all of those pictures.

If you need to pause the video to add any detail into your answer, then please do, but we're going to move on now to the third and final part of our lesson.

So now we know all about meiosis, let's look at our final part of the lesson, which is comparing mitosis and meiosis.

Mitosis and meiosis are both types of cell division, but there are key differences between them, and it's really important that we understand the differences between those two processes.

So let's have a look at some of those differences now.

Let's look first at the uses of mitosis.

Asexual reproduction in some unicellular organisms such as yeast.

So that picture there is showing you a yeast cell and it is dividing by asexual reproduction, and that is mitosis.

Growth and repair in multicellular organisms also is a use of mitosis.

So we can see here that we've got a seed which is growing and germinating into a plant.

That's growth.

And then an X-ray, which is showing bones that would be repairing and healing, and growth and repair in multicellular organisms both need mitosis.

The uses of meiosis though, it is the process used to form the gametes, which is the sex cells.

So here in humans, the egg, which is the female sex cell, and the sperm, which is the male sex cell, they are the gametes in humans, meiosis is the process which produces those cells in humans.

And same for plants.

The gametes are the egg and the pollen.

So the egg is female, the pollen is the male gamete, and meiosis is the process by which the egg and the pollen cells would be made.

Now let's look at the number of cell divisions and cells produced in mitosis and meiosis.

So in mitosis, mitosis only involves one cell division and produces two cells.

So this illustration here is showing you mitosis, and there is only one cell division, resulting in the formation of two cells.

Whereas in meiosis, there are two cell divisions and it results in four cells.

So here we have one division and then a second division, and you can see that there are four cells that are produced at the end.

Now let's look at the amount of genetic material in the cells produced.

So in mitosis, you produce diploid cells with two copies of each chromosome, a full set of chromosomes.

So you start off with the DNA being copied, and then there is one cell division, which results in two diploid cells.

So the cells produced at the end are genetically identical to the original cell you start with.

So mitosis produces diploid cells with two copies of each chromosome, whereas in meiosis, it produces haploid cells with only one copy of each chromosome, half a set of chromosomes.

Now, remember that interphase happens before meiosis and interphase is where the DNA is copied, which is the same as mitosis.

So we start off with the DNA being copied, but the key difference here is that there are two cell divisions.

So the first cell division produces two diploid cells, but then the second cell division produces four haploid cells that only have one copy of each chromosome.

Let's look now at the genetic variation in the cells produced.

So mitosis usually produces genetically identical cells.

If everything goes correctly in mitosis as it should do, you will produce genetically identical cells.

So there is no mixing of the DNA and the cells produced are genetically identical.

In meiosis, however, the parental DNA mixes, which causes genetic variation in the gametes produced.

So there is mixing of the DNA which occurs, and therefore the gametes have genetic variation.

The chromosomes can contain a mixture of maternal and paternal DNA, which results in variation.

Let's quickly check our understanding of that.

So identify if these statements are true for mitosis or meiosis.

So look at the statements and decide if they are in mitosis or meiosis.

So the first statement involves two cell divisions.

That is meiosis.

Produces two cells.

If two cells are produced, that's only one cell division, so that is mitosis.

Cells produced are haploid.

Gametes are haploid, meiosis produces the gametes, so that is meiosis.

DNA mixing occurs.

That leads to genetic variation, which happens in meiosis.

It's used to make the gametes.

That again is meiosis.

Cells produced are genetically identical.

That is mitosis, because there is no mixing of DNA.

Really great job if you've got those right.

Well done.

Sexual reproduction in humans involves mitosis and meiosis.

So if we look at this image here, which is showing you cell vision in sexual reproduction, you can see that in the first part of this diagram you have meiosis, and this is where the gametes are made through the process of meiosis.

So the human body cells will divide to produce the gametes and that process is meiosis.

And then the next stage will be fertilisation.

So this is where a sperm and an egg, the two sex cells, the gametes, they fuse together to form the zygote.

And then once the zygote is formed, the process of mitosis will occur.

So after fertilisation, the zygote is formed.

The zygote cell divides by mitosis to produce two genetically identical cells, and then these cells keep dividing by mitosis to form an embryo.

And sometimes this can be represented in diagrams. So this diagram shows the different types of cell division during human reproduction.

And we can see that we start off with a female cell, A, and a male cell, B.

And you can see that those two cells have divided once to produce two cells and then divided again to produce four cells.

That whole process there is showing you meiosis, cell division leading to the formation of the gametes.

Cell A and cell B, they would be just female and male human body cells.

They would have the full number of chromosomes, and they have divided once and then they've divided again to produce the gametes.

So the egg cell would be cell C and the sperm cell would be cell D.

They are both haploid.

So cell A has 46 chromosomes, cell B has 46 chromosomes.

They are both diploid human body cells.

Then cell C has 23 chromosomes, because it's an egg cell, it's a gamete.

And cell D has 23 chromosomes because again, it is the sperm cell and it is also a gamete.

But that whole process there is meiosis.

We've got the same diagram now, but this time we're going to be focusing on when cell C and cell D, they fuse together to form cell E.

So that process there is called fertilisation, and cell E would be the zygote.

So fertilisation is where the gametes fuse together to produce a zygote, which in this case is cell E.

Sperm D is haploid.

It has got 23 chromosomes.

And also the egg, which is cell C, is also haploid.

It has 23 chromosomes.

However, the zygote, which is E, is diploid, and it's got 46 chromosomes, two copies of each chromosome, one from each of the parents.

And then finally, if we look at cell E downwards, so we're looking at the zygote, and then that divides to form two cells and then they divide again to form four cells, and eventually you will end up with an embryo.

That whole process from E dividing to form an embryo is mitosis.

So this is cell division after fertilisation is occurring by mitosis.

All the cells are diploid.

So they all have 46 chromosomes.

Cells replicate to form the embryo, and this occurs through mitosis.

All of the cells are genetically identical to the original zygote.

Let's check our understanding of that.

So we've got the same diagram here.

How many chromosomes does cell A have? A, 46, B, 23, or C, 64? This is A, 46.

They are diploid cells.

They are human body cells.

That is before any cell division has occurred.

Which process makes cell D from cell B? A, mitosis, B, fertilisation, or C, meiosis? This is C, meiosis.

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

Mitosis and meiosis are processes that produce new cells.

Compare the process and outcomes of mitosis and meiosis.

And then you see here we've got the diagram shows different types of cell division that occur in human reproduction.

Use the diagram to answer the questions.

So question two, name the type of cell division that produces cell C from cell A.

How many chromosomes are in cells A and B? And then question four, how many chromosomes are in cells C and D? Why is it important for cells C and D to have this number of chromosomes? Then question six, what process happens to produce cell E from cell C and D? Number seven, what is cell E called? And number eight, what process happens to produce cell F from cell E? Sure you're gonna do a really great job.

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

Okay, let's see how we did.

So comparing mitosis and meiosis, multicellular organisms use mitosis for growth and repair, whereas multicellular organisms use meiosis to form the gametes.

Mitosis involves one cell division creating two cells, whereas meiosis involves two stages of cell division creating four new cells.

The two cells produced in mitosis are diploid, they contain the full number of chromosomes, whereas the four cells produced through meiosis are haploid, they contain half the number of chromosomes.

Cells produced through mitosis are genetically identical, whereas DNA mixing occurs in meiosis, which produces gametes with genetic variation.

You might have written those in a different order, so please pause the video if you need to to just check you've got all of the detail into your answer.

We're going to move on to question two now.

The type of cell division that produces cell C from cell A is meiosis, so it's the production of the gametes.

And there are 46 chromosomes in cells A and B.

They are diploid cells, they are human body cells.

Then for number four, there are 23 chromosomes in cells C and D.

They are the gametes.

They are haploid.

And why is it important for cells C and D to have this number of chromosomes? The gametes fuse during fertilisation to produce a diploid zygote.

Therefore, gametes need half the amount of DNA.

They need one copy of each chromosome because then the zygote inherits one maternal and one paternal chromosome.

What process happens to produce cell E from cell C and D? That is fertilisation, the fusion of gametes.

What is cell E called? It is a zygote.

And what process happens to produce cell F from cell E? That is mitosis, producing genetically identical cells, and they are all diploid cells with two copies of each chromosome.

Fantastic job if you managed to get all of those right.

Well done.

Really great job today.

Let's summarise our lesson on Making gametes: meiosis.

So we started off by talking about the difference between asexual and sexual reproduction.

We said that sexual reproduction involves two organisms and the fusion of gametes to create genetically different offspring.

We said meiosis is a type of cell division that creates the gametes, the sex cells.

And during meiosis, the chromosomes are copied, and then DNA mixes, creating genetic variation.

We also said that there are no two gametes that are the same because of this DNA mixing.

Once the chromosomes have been copied, the cell undergoes two rounds of cell division to create four haploid gamete cells.

The gametes are haploid because they have one copy of each chromosome.

Gametes fuse together during fertilisation to form a zygote, and the zygote then divides by mitosis to produce genetically identical cells, and we looked in a lot more detail about the difference between mitosis and meiosis.

I've really enjoyed today's lesson, I hope you have too, and I look forward to seeing you next time.