AP Biology Meiosis

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This is the third in a series of lectures on cell reproduction, and we will be focusing on the topic of meiosis.0002

To understand meiosis, you also need to understand chromosome number and sets of chromosomes.0013

There are two general types of cells in terms of the number of chromosome sets: haploid cells and diploid cells.0021

Diploid cells have two sets of chromosomes. Somatic cells are diploid, so what are somatic cells?0030

Well, they are any cells in the body that are not reproductive cells.0036

Gametes or sperm and egg are reproductive cells. Other cells are somatic cells, for example cells of the skin, of the kidney, of the eye.0042

Those are all somatic cells. These cells are diploid and contain two sets of chromosomes.0054

The previous lecture covered mitosis, and somatic cells are reproduced via mitosis.0061

If you have not watched that lecture yet, it would be a good idea to start out with that because it explains some basic concepts.0067

And meiosis is more complicated than mitosis, so it is important to have a good understanding of that before you move on.0073

Now, with meiosis, what we are focusing on is cell division that reduces chromosome number and produces gametes.0081

Gametes, as I mentioned sperm and eggs, are produced via meiosis, and these cells are haploid. They contain only one set of chromosomes.0092

It is easiest to understand this by using an example.0101

Each species has a particular chromosome number. Humans have 46 chromosomes in diploid cells and somatic cells.0105

These actually consist of two sets. Each set contains 23 chromosomes.0123

We will often talk about diploid and say "oh, the cell is 2n". 2n equals a diploid cell.0130

Here, since we have, for humans, two sets of 23 chromosomes, n, therefore equals 23. 2n equals 46.0138

A chromosome with n is considered haploid. A chromosome with 2n sets of chromosomes is considered diploid.0157

Looking a little bit more deeply, diploid cells have 46 chromosomes consisting of two sets, and each of those chromosome pairs is given a number.0170

So, 23 sets, and we number 1 through 22 actually, and then the 23rd set are the sex chromosomes.0189

Let's say you are looking at chromosome 2, chromosome 2, an individual would have - say this is chromosome 2 and then, this is another chromosome 2 - two of these0203

chromosome 2s, one maternally derived and then, another chromosome 2 from the father, paternally derived.0220

These two chromosomes together are called a homologous pair or sometimes just homologues.0236

Each chromosome, like chromosome 1, chromosome 2, chromosome 2, has a particular size, centromere placement and banding pattern. They also carry alleles for particular traits.0250

For example, let's say one chromosome looks like this, and that is chromosome 1; and there is another one similar to chromosome 1.0265

Now, maybe I have another chromosome that has very short, short arms and much longer long arms, so different shape, a different centromere placement, and let's say this is chromosome 4.0274

So, homologous chromosomes are going to be the same length. They are going to have the centromere placements so the same proportions, and they will also have the same banding or staining pattern.0288

In the lab, when we want to visualize chromosomes within cells, we can stain them, and when you stain, you will see certain bands like, maybe this one bands up here and here and here and down here.0298

Its homologue is going to do the same, and then, this one might be different. It might have just a band right here and then, one right here.0309

These have similar banding patterns, similar lengths, or same, same banding patterns, not just similar- same lengths, same centromere placements.0319

These are homologous chromosomes.0328

In a diploid cell, chromosomes 1 through 22 are going to be found in homologous pairs, and these are called autosomes.0330

1 through 22, and you have two of each autosome- two 1s, two 2s, two 3s, now, the 23rd set plus 1 set of sex chromosomes.0344

Chromosomes 1 through 22 are autosomes. That 23rd set is the sex chromosomes.0355

So, looking a little bit more carefully at how sex chromosomes work, in a diploid cell in an individual, if their sex chromosome pair that they have is XX, that individual is a female.0361

If the chromosome pair is XY, that individual is a male.0378

This is in diploid cells. There is going to be 1 through 22, two sets- one that the individual got from the mother, one that they got from the father.0386

In sex chromosomes, the individual is going to have XX if they are a female and XY if they are a male.0394

One of these Xs comes from the mother, in a female. The other comes from the father, in a female, in a male.0405

One X came from the mother, and then, the Y came from the father.0413

This is focusing on diploid cells.0421

What we are going to be talking about today is the production of haploid cells or the production of gametes via meiosis.0424

Gametes, sperm and egg, do not contain homologous pairs. In fact, they will only have one chromosome 1, one chromosome 2, one chromosome 3 and either an X or a Y.0433

Egg cells will only carry Xs because the female producing these eggs only has Xs.0452

Sperm, half the sperm will contain an X. Half will contain a Y.0458

Something else to note is these chromosomes and homologous pairs do not just look the same. They are not just the same length and the same banding pattern.0466

Much more important than that is they carry alleles for the same traits. An allele is an alternative form of the gene for a trait, for example, eye color.0478

One could have an allele for blue eyes, so they have DNA that would encode for the eye color to be blue.0501

Another allele could be brown eyes. Another could be green eyes.0509

Hair color: there could be an allele for blonde hair. Someone could have an allele for black hair.0514

Now, let's look at these chromosomes. Chromosome 1, let's say that it does carry the information for eye color.0520

Let's say it is right up here at the tip. Maybe this chromosome came from the individual's mother, and that mother gave a blue eye allele right there.0528

The father might have given a different form, brown eyes.0544

So, diploid cells are carrying two chromosomes of each type - except for the sex chromosomes, and the male can be XY - that carry alleles for the same traits like the eye color allele.0548

The eye color allele is here, and this could be blue eye. This could be brown, or they could both be blue.0563

Maybe chromosome 4 contains the allele for hair color, and this individual may have gotten the allele for black hair from his mother and the allele for brown hair from his father.0567

Diploid cells contain two alleles for each trait. Haploid cells would not.0580

Half of the information has been lost, so if this goes from being diploid to being haploid, that egg cell, for example, may only carry the blue eye trait0588

and the black hair trait, or the blue eye trait and the brown hair trait.0600

This allows for genic variation, which we are going to talk about more in a moment.0604

For right now, the important thing to know is diploid cells, somatic cells, are produced via mitosis. They contain two complete cells of chromosomes.0609

Haploid cells are produced via meiosis, so spermatogenesis, production of sperm and oogenesis, production of eggs that we will talk about later in the course in more detail, are produced via meiosis.0617

Right now, we are just going to focus on that process of meiosis and reduction of chromosome number to create four haploid cells that are non-identical.0632

I am going to go talk about a comparison of mitosis and meiosis to give you an overview before we go into the details of meiosis.0644

Again, if you have not learned mitosis, it would be a really good idea to go back, cover that now, make sure you have that down before you proceed.0652

In mitosis, there is one round of cell division, and recall that there are four phases: prophase, metaphase, anaphase and telophase.0659

In meiosis, there are actually two rounds of cell division: meiosis 1 and meiosis 2.0682

If the cell goes through prophase, which is called prophase 1, then, it goes to metaphase 1, anaphase 1 and telophase 1.0690

And some of the events during this phase are quite different than the events of mitosis.0705

There is also after this occurs, the parent cells sorts out. Meiosis 1 occurs, and you end up with two non-identical daughter cells that, then, continue on into meiosis 2.0712

You end up with four cells because two rounds of cell division.0731

The second round of cell division, meiosis 2, you have prophase 2, metaphase 2, anaphase 2 and telophase 2.0736

This is actually very similar to mitosis, the second round of cell division.0746

Mitosis, one round PMAT, meiosis, two rounds of PMAT resulting in two identical daughter cells with mitosis because this is only one round of cell division.0752

Here, you get 1, 2, 3, 4 daughter cells, and they are non-identical.0764

Extremely important is that the daughter cells of mitosis are diploid.0771

They contain two sets of chromosomes. We often say they are 2n.0776

The daughter cells produced via meiosis are haploid. They are n.0782

They are non-identical, so if you took four different eggs produced by a female, you would see that one may carry an allele from blue eyes.0790

One might carry brown eyes. One egg might have blue eyes but a gene for being short.0801

The other might have brown eyes but a gene for being tall.0807

All kinds of mixing and matching allow for huge amount of genetic variation, and it is the reason that the offspring of a couple do not all look identical.0810

In order for some of this mixing, matching genetic variation to occur, a very important event occurs in prophase 1 that we are going to discuss in just a moment,0821

and that is pairing of homologous chromosomes with exchange of DNA.0829

Crossing over is exchange of DNA between homologous chromosomes.0834

Mitosis: one round of cell division, the result is two daughter cells that are diploid.0841

They are identical to each other. They are identical to the parent cells.0846

Meiosis: two rounds of cell division, non-identical daughter cells with pairing over - excuse - pairing of homologous chromosomes and crossing over.0850

In summary, mitosis and meiosis, here, we have mitosis right here, and on the other side is meiosis.0862

This parent cell is diploid. This individual has, let's call this chromosome 1 and chromosome 2.0880

This individual, this could be, say, a skin cell from someone. That skin cell has a chromosome from that person's mother, chromosome 1 and a chromosome 1 from the person's father.0888

These two together are homologues.0900

This individual has a chromosome 2 from their mother and from their father, another homologous pair.0905

Now, in humans, there would be 22 of these pairs plus a pair of sex chromosomes.0911

Mitosis occurs. This diploid cell divides into two daughter cells that are identical and diploid.0917

1, 2, 3, 4 chromosomes, there is still 1, 2, 3, 4 chromosomes. These chromosomes here, because the cell has been through S phase, it has been through synthesis of the DNA,0928

the DNA was copied, so there are sister chromatids that identical to each other; so no information has been lost.0939

One cell got this chromatid. The other got this chromatid.0948

They are identical. Everything here, all of these, contain the same genetic information.0952

If there is a blue-eyed allele here and brown-eyed allele here, this individual still has the blue eye allele and the brown.0956

If there is a tall allele here and a short allele here, tall, short, no information has been lost- very different from meiosis.0963

In meiosis, this individual also started out diploid.0972

This would be a cell, for example, that produces sperm.0977

Spermatogenesis, the production of sperm, starting out with the cell and going through meiosis.0983

M1 occurs here. M2 is occurring right here.0991

This individual started out with two chromosome 1s, two chromosome 2s.0997

By the end of that first division, M1, these cells are already haploid.1004

1, 2, 3, 4 chromosomes, there is only 2 chromosomes by the end of meiosis 1, so information has been lost.1009

Another very important fact is that these sister chromatids are no longer identical, and we will see why in a minute.1019

It has to do with that exchange of segments of DNA between homologous chromosomes.1027

Here, the homologous pairs have been split up.1033

There is one chromosome 1 here. There is one chromosome 1 here.1036

There is one chromosome 2 here. There is one chromosome 2 here.1040

These cells are no longer identical. Information has been lost.1047

Maybe there is a blue eye allele here, and there was a brown eye allele here.1051

Well, now, this cell has the blue eye allele, but the brown allele is gone. This cells the brown-eyed allele, blue-eyed allele is gone.1057

The gene for short stature might be down here, and tall stature might be up here.1067

Not all the information is there, so the cell is different.1074

Then, in meiosis 2, sister chromatids separate.1077

In this phase, we have the reduction of chromosome number. Now, we have sister chromatids separating, and I will go over all this in detail.1082

But just as an overview, you started out with a diploid parent cell. You end up with four haploid non-identical daughter cells.1089

Diploid cell, two diploid daughter cells identical, diploid cell starts out. You end up with four gametes that are non-identical.1099

For spermatogenesis, you actually do end up with four sperm cells.1109

The production of ovum, of eggs is a little bit different. You actually do not end up with four eggs.1114

We will talk about that in detail and the reproduction, but just so you know now, you will actually only end up with one ovum and other cells that are called one ovum plus polar bodies, which are dead ends.1121

They cannot create an offspring.1136

Spermatogenesis, there are four sperm cells produced, but the process is the same as far as the steps of meiosis and mitosis - excuse me - steps of meiosis 1 and meiosis 2.1140

Alright, looking at prophase 1 to start, prophase 1 is extremely important step in creating genetic variation, genetic variability within the sexually reproducing species.1155

During prophase 1 in meiosis, homologous chromosomes pair up.1172

Nothing like this happens in mitosis, and this process of pairing up is called synapsis.1178

Crossing over is the exchange of DNA between homologous chromosomes.1189

Here, looking at these long chromosomes, let's call this one and one, as usual, and the shorter ones two and two.1195

This is a homologous pair. These homologous pairs, this is a homologous pair, and when they pair up like this, they form what is called a tetrad.1201

Homologous pair means two chromosome 1s, two chromosome 2s, two chromosome 3s.1217

Now, paired up like this, we would call a tetrad because there is 1, 2, 3, 4 chromatids.1221

Another word you might hear is bivalent for the two chromosomes together, so a tetrad or a bivalent.1227

Synapses is this pairing up process. Crossing over is the exchange of DNA.1236

The DNA that is exchanged is between corresponding alleles, meaning alleles that code for the same traits.1243

Right here, where these two are crossing over, let's say that eye color is controlled right here.1250

And again, let's say this purple chromosome 1 contains DNA coding for blue eyes, and the green chromosome contains DNA coding for brown eyes. These two could swap.1258

Now, I have this whole chromosome that, maybe, came from my father and this whole chromosome that came from my mother, but they are different now.1272

This chromosome from my father started out with a brown-eyed allele. Now, it has all my father's DNA but with the blue-eyed allele.1280

The maternally derived chromosome has all this DNA derived from my mother but with the brown-eyed allele that started out with my father.1289

It is swapping up corresponding DNA. Maybe these two are exchanging the allele for height, the DNA that encodes for height.1297

This chromosome might have short stature encoded, and this chromosome might have tall stature; and these swapped.1308

This allows for a huge variability in offspring because traits have been mixed and matched.1315

A child might end up with or one child has brown eyes and black hair. Another child of a couple has blue eyes and blonde hair.1324

And that has to do with obviously the traits of the two different parents, but also within those parents, the fact that the sperm and egg within a parent, the eggs that a female is carrying are not identical.1336

The DNA has been mixed and matched.1349

Now, some terminology, the physical manifestation of crossing over is called chiasmata.1354

This is a physical manifestation. These regions here, where I can literally see, that is where crossing over is occurring.1362

And crossing over occurs at about one to three places per chromosome.1374

Here, crossing over is occurring in two, and on this one, two as well, but it could be on one place, three places roughly, on one to three.1382

And these x-shaped regions here chiasmata are the physical manifestation of crossing over.1395

OK, synapsis is the process of the pairing of homologous chromosomes. Crossing over is the exchange of DNA between corresponding alleles, corresponding segments.1403

And chiasmata are the physical manifestation of crossing over.1418

Prophase 1: crossing over occurs, but I do not want to have you not realize that the usual events of prophase occur as well.1424

Remember, in mitosis, in addition, in prophase, we saw the breakdown of the nuclear membrane.1431

You should realize that in prophase 1 here, P1, there is still a breakdown of the nuclear membrane occurring, formation of the spindle apparatus.1440

Recall that the spindle apparatus consists of centrosomes, and within those centrosomes are the centrioles.1455

Again, this is a review from mitosis lecture.1467

Spindle fibers, which will eventually attach to the kinetochores on the chromosomes.1472

Breakdown of the nuclear membrane, formation of the spindle apparatus, also the nucleoli disappear. Finally, the chromatin condenses.1483

Remember that, for most of the cell cycle, if you took a cell just a G1 or something, you would not be able to see the chromosomes of the light microscope.1502

You can only visualize chromosomes of the light microscope after they have condensed, and this is how we picture chromosomes or this x shape.1512

And that is what you see with the light microscope once prophase has occurred, and a chromatin has condensed.1519

The fifth is crossing over, synapsis and crossing over.1527

Five events in prophase: breakdown of the nuclear membrane, formation of the spindle apparatus, the nucleoli disappear.1531

The chromatin condenses, and finally, crossing over occurs, and the crossing over is the exchange of DNA between homologous chromosomes.1540

Continuing on with meiosis 1, we just discussed prophase 1 in detail, and do not forget crossing over. It is a big thing to remember.1550

Now, let's look at metaphase 1.1561

Immediately, you will notice that this chromosome contains some DNA from the homologous pair. This is the result of crossing over.1565

These homologues are so closely associated, but crossing over is complete.1578

Also notice that these sister chromatids are not identical anymore.1585

If with mitosis, I emphasize "OK, these sister chromatids are identical".1589

If one sister chromatid goes into one cell, and one goes into the other, fine, everything, all the information is there.1593

That is not true here because this has crossed over.1599

It has exchanged DNA with the homologue. This chromatid has not.1603

Therefore, let's say this has the gene for blue eyes, and this green one is brown eyes.1608

These pieces of DNA are not identical.1620

Here, purple, they might have the blue. It still has the blue eye.1622

This one crossed over, and now it has the brown eye.1627

Before, these chromatids were identical so that this segment was brown-eyed, brown-eyed allele, chromatids that have the brown-eyed allele.1630

On this other chromosome 1, blue eye, blue eye- identical.1639

Now, I have got chromatids that are not identical- blue-eyed on one, brown-eyed on the other, blue-eyed on one, brown-eyed on the other, same here with number two.1647

Again, this increases genetic variation a great deal because the offspring are going to have different combination of chromosomes in the parents and then, each other.1659

Now, metaphase 1, you see that chromosomes line up on the metaphase plate but in a much different way than in mitosis.1669

Homologues line up on the metaphase plate.1682

In mitosis, the lining up was single file. Here, it is double file.1689

The chromosome 1s are next to each other. The chromosome 2s are next to each other, and that is because in anaphase, homologues separate.1694

Sister chromatids do not separate. Homologues separate.1710

The spindle fiber are going to attach from one pole to the kinetochore in one chromosome, from the other pole, to the kinetochore and the other chromosome.1716

And they are going to separate those homologous pairs. They are not going to separate chromatids.1728

Here, I have 1, 2, 3, 4 chromosomes.1737

They are attached. They are very closely associated especially in crossing over, but if I count what are separate chromosomes, I have four.1743

These are being separated to the two poles of the cell.1753

The result is, after telophase 1 and then, subsequent cytokinesis, in which the daughter cells separate, once these separate completely, there will be two cells with only 1, 2 chromosomes each.1756

This cell is now haploid. Therefore, meiosis 1 is called the reductive division.1773

The reduction in chromosome number occurs during meiosis 1.1783

In telophase 1, in some species, the nuclear membrane actually reforms and spindle apparatus breaks down and all, not in all species though1791

because this cell is going to go straight into another round of prophase, metaphase, anaphase, telophase.1801

And then, in some species, the nuclear membrane does not completely reform and everything.1808

It just goes straight into meiosis 2, but in some species, what you would see in telophase is reforming of the nuclear membrane, the nucleoli reappearing and everything. In others, you do not.1812

Prophase 1: cross pairing of homologues and crossing over.1825

Metaphase 1: lining up of homologous pairs on the metaphase plate, double file line.1830

Anaphase 1: homologues separate.1840

Telophase 1: you now have in one of the daughter nucleoli, haploid number of chromosomes, and the other daughter nucleoli, haploid number of chromosomes.1844

I talked about crossing over as a means of providing for genetic variation.1859

Another means is something called independent assortment.1865

What that means is that what one homologous pair does is independent of what another homologous pair does.1871

You see here, the chromosome 1s, the green paternally derived chromosome 1 homologue, went into this cell. Oops, this should be two.1879

For chromosome 2, the purple maternally derived one went into this cell. That was random chance.1892

It could have just as easily bend if these would align up differently that the green chromosome 1 ended up in here, and the green chromosome 2 could have ended up in here.1898

These could have both been green, and for chromosome 3, it could have been a purple one, for chromosome 4, the paternal one, for chromosome 5, the maternal.1907

It totally this is not as so all the paternally derived go into one cell, and all the maternally go into another. That also allows for genetic variation.1914

The homologous pairs separate out independently of what other homologous pairs are doing. It is independent assortment.1925

Now, meiosis 2 is much easier to understand because it is very similar to what occurs in mitosis.1938

Here, I am showing one cell going through meiosis 2 just to keep the picture simple, but in reality, it started out with that parent cell.1950

It went through M1, meiosis 1, and we ended up with two non-identical daughter cells, haploid.1967

Each of these will go on to meiosis 2.1980

I am only showing one, but this is happening a second time so that there will be a total of four cells.1987

This will split, and you will get 1, 2; and this will split, 3, 4, but for simplicity, I am just showing one of these cells.1993

Remember those are happening twice to create a total of four cells.2000

One meiosis 2 for one cell, and then, a meiosis 2 for the other cell.2005

Alright, at the end of meiosis 1, what we ended up with is a haploid cell.2010

In this case, there is only a chromosome 1 and a chromosome 2, so just two chromosomes.2019

And now, I am just going to go on through prophase 2.2025

Prophase 2, very similar to prophase and mitosis.2030

Nuclear membrane, if it is reformed will break down, spindle apparatus formation, disappearance of nucleoli just like in mitosis.2036

Now, the cell will, then, go on to metaphase 2.2046

Just like in mitosis, the chromosomes line up single file along the metaphase plate.2053

They cannot line up as homologous pairs because there are no homologous pairs anymore.2057

There is only one chromosome 1. It does not have a homologue in the cell.2062

Metaphase 2: chromosomes line up on the metaphase plate.2069

Remember that the metaphase plate is an imaginary plane equidistant between the two centrosomes at the two poles of the cell.2082

Anaphase, this anaphase 2 is very similar to anaphase and mitosis. Sister chromatids separate. What is different is that the sister chromatids are not identical.2091

In mitosis, sister chromatids are identical. This and this would be duplicates.2114

That is no longer the case because crossing over occurred back in prophase 1, so sister chromatids have some differences.2122

They have swapped some genes with their homologue.2127

This one has the brown-eyed allele, we said. This one has the blue-eyed allele.2132

Now, this brown is going to go into one cell with a bunch of other genes maybe tall, curly hair.2141

This one is going have blue eyes but still maybe tall and curly hair, so traits have been mixed and matched.2153

OK, metaphase: we had lining up of chromosomes single file.2159

Anaphase 2: sister chromatids separate.2163

Telophase 2: the usual events of telophase meaning breakdown of the spindle apparatus. Nucleoli reappear.2171

So, this is for telophase 2, breakdown of the spindle apparatus. Nucleoli reappear.2186

The nuclear membrane reforms, so I will just put membrane - but it is a nuclear membrane - reforms, and also the chromosomes decondense.2209

Here is finishing up cytokinesis, which starts in late anaphase but finishes up after mitosis.2221

The result is going to be these two cells, and remember that, another daughter cell also went through meiosis 2, so there will be actually a total of four cells:2229

one daughter cell here, one daughter cell went through its own meiosis 2, and the result is going to be actually 1, 2, 3, 4, 1, 2, 3, 4 daughter cells.2244

They each have a chromosome 1 and 2, but some information has been lost. Some things have been mixed and matched.2255

This sperm might encode for traits of blue eyes, curly hair and short, and this one has brown eyes, straight hair and tall. It does not have both alleles in it.2267

That allows for a huge amount of genetic variation, which confers a survival advantage on species that reproduce sexually.2283

Alright, to sum up meiosis is the production of four non-identical daughter cells that are haploid.2296

This occurs for gamete production, production of sperm and egg.2304

The cell starts out as a diploid cell, in this case, four chromosomes, two of one type, two of the other, so two pairs of chromosomes.2310

The cell will go through meiosis 1, and after meiosis 1, the result will be two non-identical daughter cells that are haploid.2321

Remember, meiosis one is the reductive division. There has been a decrease in chromosome number.2332

During prophase 1, crossing over between homologous chromosomes occurred.2339

So, these sister chromatids are no longer identical because crossing over, exchange of DNA, has occurred between the homologous chromosomes.2343

Meiosis 1, we get two non-identical haploid daughter cells.2352

Both of these daughter cells proceed through meiosis 2 if we are talking about spermatogenesis.2359

Oogenesis, production of eggs, is a little more complicated. The polar bodies do not proceed through.2368

We will talk about that in more detail in the reproduction section. Right now, let's focus on spermatogenesis.2376

Spermatogenesis, the result is going to be separation of the sister chromatids during meiosis 2, and now, we have four daughter cells that are haploid and non-identical; and these are haploid.2384

They have one set of chromosomes. In humans, each of these sperm cells would carry 23 chromosomes.2409

Now, let's go a step further.2418

A sperm cell in a human is going to be haploid. It is going to have 23 chromosomes.2421

Fertilization will occur, so the sperm will fertilize an egg to form a zygote.2431

The zygote is diploid because the sperm contains 23 chromosomes. It was n.2441

The egg contains 23 chromosomes, n, so a chromosome 1, a chromosome 2, all the way down and then, either an X or a Y for the sperm and then, just an X for the egg.2450

So, 23 and 23 gives 46 back or 2n. The zygote is now diploid- 46 chromosomes or 2n.2462

The zygote is diploid through the fertilization, which combines the set of chromosomes from the sperm and from the egg to form, now, a single cell that is diploid.2474

The zygote will undergo mitosis to form an embryo, continue on with more mitosis and specialization of cell types to form a fetus, eventually a child and then, adult, so continued mitosis for growth.2493

But, as you can see here, the only time you get meiosis in an animal is to form sperm or egg, to form the gametes.2519

I just want to note that this is actually different than what occurs in plants.2530

We will talk about this more in the plant section, but in plants, haploid cells can actually undergo mitosis.2533

The result is a multicellular organism, so a plant called a gametophyte.2544

This is an actually multicellular organism, but it is haploid. We do not see that in animals.2554

There are no people walking around who are haploid. Everyone you see walking around has diploid somatic cells.2566

The only cells in their body that are haploid are gametes, whereas plants, if you look in a map of moss, what you are primarily seeing are haploid plants.2572

Most plants around, trees and things, are actually diploid, the adult form, but in moss, the haploid gametophyte is just the regular plant form that you mainly see.2584

This is just important to know to take a broader view, not just of the human life cycle or the animal life cycle in biology,2596

but also the different life cycles such as plant life cycle, which we will delve into later in the course.2602

Another fact or factor to keep in mind about meiosis is that if things do not go correctly, the result will be an incorrect chromosome number, either too many or too few chromosomes.2610

And, therefore, the cause can be a genetic syndrome.2626

Mutations of just a single gene can cause diseases such as sickle-cell anemia. However, here, we are talking about something different- actual change in chromosome number.2632

A good example is Down syndrome. Down syndrome is also commonly known as trisomy 21- tri meaning three.2645

An individual, their somatic cells, as you know, should have two of each chromosome.2659

These individuals with Down syndrome actually have three chromosome 21s.2664

This is because there was a nondisjunction meiosis, which created either an egg or a sperm that had an extra chromosome 21 that was then, there when fertilization occurred and zygote was formed.2672

Down syndrome is caused by an extra chromosome 21, and when chromosomes do not separate correctly, that is called nondisjunction.2687

They separate out incorrectly. Either they will have too many chromosomes or they will be missing a chromosome, and then, that will be passed down to the offspring.2699

Many times, nondisjunction would result in an offspring that would not be viable, would not make it very far past the zygote stage, but some of these differences in chromosome number are viable.2710

Down syndrome is one example. Turner syndrome is another.2725

Individuals with Turner syndrome have only one X chromosome.2732

In their cells they have 45 chromosomes. They have the complete set of autosomes, so they have their 44 autosomes, 1 through 22, two sets of those plus an X.2739

These are females with just a single X chromosome instead of XX.2758

So, you can see the medical applications of some genetics and molecular biology and nondisjunction can result in an alteration of chromosome number.2762

First example: has the cell pictured below already undergone DNA replication? How can you tell?2778

In other words, has it been through the S phase, the synthesis phase? Yes.2785

How can I tell? I am looking at these chromosomes, and it is showing actually metaphase.2791

This is showing a cell in metaphase, and I also know that it is metaphase 1 of meiosis because homologous pairs are lined up; and I see that there are sister chromatids.2799

That means that if there are sister chromatids, S phase has occurred.2815

Chromosomes contain sister chromatids. That is how I can tell.2824

How many chromosomes will each daughter cell have after meiosis 1? Will these chromosomes contain sister chromatids?2838

Alright, this cell is going to undergo meiosis 1. It is actually right here.2851

What they are depicting is metaphase 1.2856

Meiosis 1 will occur, and it will go through anaphase and telophase.2861

And you remember in anaphase 1, these sister chromatids are going to separate, so the result is going to be two daughter cells.2867

Homologous chromosomes will separate, so I will end up with one homologue.2877

This one will go here. This one will go here.2882

This one will go here. The other side will end up with this big green one, the medium purple and the small green.2886

Each daughter cell will now be haploid at the end of meiosis 1, and they will each contain three chromosomes. The homologues have separated.2893

Will these chromosomes contain sister chromatids? Yes.2904

The sister chromatids do not separate in meiosis 1. They are still together.2907

Homologues have separated.2911

Now, after meiosis 2, how many chromosomes will each daughter cell contain?2915

After meiosis 2, M2 will occur, and now, sister chromatids are going to separate.2920

So, this cell will still have three 1, 2, 3 chromosomes, but they will not contain sister chromatids anymore, and the same thing will occur down here.2928

At the end of meiosis 2, how many chromosomes will each daughter cell have? Three.2944

This was the reduction division, M1. There is no reduction in chromosome number during meiosis 2.2951

Will these chromosomes contain sister chromatids? No.2957

When one of the gametes resulting from meiosis is fertilized, how many chromosomes will the resulting zygote have?2964

Alright, let's take this and redraw it here.2973

This is the gamete. It is haploid.2977

It has three. It is going to unite with another gamete via fertilization, and those two sets of chromosomes will be brought together in the nucleus of the zygote.2979

And the result is going to be the diploid zygote containing six chromosomes.2992

The parent cell of the gametes start out with 1, 2, 3, 4, 5, 6 diploid cells, six chromosomes. The gametes have three each.3003

Diploidy is restored upon fertilization. 3 + 3 gives 6 2n diploid cell.3012

Example two: match the events of meiosis with the phase during which they occur.3023

One: sister chromatids separate and move to opposite poles.3030

Remember, there are two rounds of meiosis- M1 and M2.3036

During M1, there is separation of homologues. Homologues separate.3041

Meiosis 2 is very similar to mitosis, and sister chromatids separate.3052

If sister chromatids separate, I know I am dealing with meiosis 2, so I have to look for one that is part of the second round of division.3065

Then, I just have to think "OK, which phase, prophase, metaphase, anaphase or telophase 2, during which phase does the actual separation occur?".3075

Well, I know that the separation of chromosomes occurs, separate out into two groups during anaphase.3086

During anaphase 2, it is very similar to mitosis, anaphase and mitosis. Sister chromatids separate.3094

So, the answer for one is E.3101

If we use that one, synapsis and crossing over occur. This is one of the first events of meiosis, so it is the very beginning during the first step, which is prophase 1 right here.3110

Remember, during prophase 1, homologous chromosomes pair up via synapsis. Crossing over occurs.3125

Also, the usual events of prophase are occurring like breakdown of the nuclear membrane, appearance of the nucleoli and formation of the spindle apparatus.3134

Three: homologous pairs line up along the metaphase plate.3145

Well, lining up on the metaphase plate is metaphase, and in metaphase 1, homologues separate - excuse me - in meiosis 1, homologues separate.3149

In order for that to occur, homologous pairs line up double file and so on.3163

In metaphase 1, homologous pairs line up double file, and then, in anaphase 1, there is going to be the separation of these homologous pairs; so this is metaphase 1A.3177

Four: the cleavage furrow forms, and the cells separate into two daughter cells each with one set of chromosomes that do not contain sister chromatids.3194

Cleavage furrow forms during telophase, and there are two rounds of telophase- telophase 1 and telophase 2.3207

This is made easy by the fact that I only have one round here, so it must be C, but let's think a little deeper about this.3215

During meiosis 1, it goes through meiosis 1, and at the end of telophase, homologous pairs have separated. Sister chromatids are not separated.3222

At the end of M1, there are still sister chromatids on each chromosome.3237

At the end of M2, then, you are going to end up with three chromosomes here but each with only one chromatid, so that is telophase 2.3242

Finally, homologous pairs separate and move to opposite poles- anaphase 1.3253

Really important thing to remember is that during meiosis 1, homologues separate, thus reductive division.3259

During meiosis 2, sister chromatids separate.3265

Label the following in the figure below, so chiasmata, tetrad, spindle fibers, centrioles and sister chromatids.3271

What we see here is prophase 1. That crossing over is occurring.3280

The physical manifestation of crossing over is in the form of chiasmata.3285

Now, these two together where I see 1, 2, 3, 4 are what is called a tetrad.3299

These homologous chromosomes paired up, formed a tetrad.3311

Sometimes, they are also known as a bivalent with the two chromosomes.3314

Alright, so it is tetrad.3321

Spindle fibers, you can see, this is just prophase, so the spindle is not completely formed, but the spindle fibers are starting to radiate out or spindle microtubules is the other name.3323

They are starting to radiate out from the centrosome.3335

The centrosome is located in this region, and it is organizing the spindle fibers. It is the MTOC.3340

Within the centrosome lie the centrioles.3346

Finally, sister chromatids, each chromosome, at this point, contains two sister chromatids.3355

They are identical for now, but once crossing over is done, they will not be identical anymore because they have exchanged DNA with their homologue.3365

Here is another set of sister chromatids. Here we have 1, 2 sister chromatids, and they are connected via the centromere.3373

Sister chromatids and then, 1, 2 sister chromatids here, and those four form a tetrad.3385

That covers centrioles, sister chromatids.3394

Describe four differences between mitosis and meiosis.3399

In meiosis, prophase 1, synapsis and crossing over occur.3405

Remember, synapsis is pairing of homologous chromosomes, homologous pairs, and crossing over is the exchange of corresponding segments of DNA between homologous chromosomes.3423

This is unique to meiosis. It does not occur in mitosis.3436

Number one difference: mitosis, no crossing over, no synapsis. That is one difference.3441

Second difference: in meiosis, remember that there are two rounds of cell division- meiosis 1/M1, prophase, metaphase, anaphase and telophase 1; and meiosis 2, prophase, metaphase, anaphase and telophase 2.3454

In mitosis, there is only one round of cell division, so just one round of prophase, metaphase, anaphase, telophase- not two.3481

The result of meiosis is four non-identical daughter cells. Mitosis- very different result: two identical daughter cells.3499

And then, a fourth difference: in meiosis, there is a reduction in chromosome number.3534

The result is that the daughter cells are haploid.3543

In mitosis, there is conservation of chromosome numbers. Therefore, the daughter cells, if the cell starts out diploid, it will continue being diploid.3554

I mentioned in plants, you can have haploid cells and undergo mitosis and stay haploid.3572

What we are talking about focusing on animal cell division, this statement is true.3580

The daughter cells are diploid, but the important point is that with mitosis, it is conservation of chromosome number,3585

whereas with meiosis, it is reduction of chromosome number- having.3597

That makes it a little bit of a broader point that applies.3605

Alright, four differences between mitosis and meiosis: synapsis and crossing over in meiosis, not in mitosis; meiosis, there are two rounds or cell division, PMAT twice,3611

mitosis, there is only one round of cell division, prophase, metaphase, anaphase, telophase 1s; in meiosis, the result is four non-identical daughter cells, mitosis, two identical daughter cells;3625

in meiosis, a reduction of chromosome numbers occurs, so a diploid cell, the daughter cells are haploid, mitosis, the daughter cells are diploid, if you started out with a diploid cell,3638

in other words, there is conservation of chromosome number.3649

This concludes the lecture on meiosis here on Educator.com.3654

Thank you for visiting.3658