AP Biology Mitosis

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In today's lesson, we are going to continue on with our discussion of the cell cycle focusing, now, on mitosis.0003

Before we do that, let's look at where mitosis is placed in the cell cycle.0011

Recall that there are two major phases of the cell cycle: interphase, in which the cell spends about 90% of its time and the M phase, in which the cell spends 10% of its time.0016

Interphase consists of the G1, S and G2 phases.0030

Recall that the G1 phase is also called the Gap 1 phase, and this is the growth phase.0035

The cell is duplicating its organelles, so it is making new organelles, cytoplasm, protein, cell membrane components, everything the cell will need to split into two daughter cells.0043

S phase is the synthesis phase. This is the phase, in which the DNA is replicated, so DNA replication0057

because if the cell is going to create two daughter cells, there needs to be enough DNA for the daughter cells.0065

During the S phase, growth is continuing, so there is still production of new organelles and cell membrane components going on.0073

During the S phase, wall synthesis of DNA is going on.0082

Synthesis of DNA will be covered in detail under the molecular biology lecture.0086

G2 is Gap 2. This is another growth phase, Gap 2 phase: continued growth of the cell.0092

This is all in preparation for the division of the cell into two daughter cells.0102

M phase consists of mitosis and cytokinesis, so we are focusing on mitosis today, and what mitosis is, is the separation of the chromosomes into two sets: one for each daughter cell.0108

Cytokinesis is the actual division of the parent cell into two daughter cells, so everything has been duplicated.0123

The chromosomes are split into two sets, one for each nuclei, and then, during cytokinesis, the cell pinches off in the middle to create two physically separate daughter cells.0131

Recall also from the cell cycle lecture that extremely specialized cells like neurons do not divide.0144

They exist, instead, in a quiescent phase called G0. These are called post-mitotic cells.0152

G0, sometimes, plays here in the cell cycle. Other people, other scientists consider it just an extension of G1, and they will show it somewhere in here.0163

Either way, it is just in a rested phase or resting state, and these type of cells like neurons do not proceed on to the cell cycle.0174

Alright, now, we are going to go ahead and focus on what happens in mitosis. Let's begin with just an overview.0184

Mitosis is the separation of chromosomes into two sets, one for each of the two identical daughter cells that result.0191

And it is important to understand chromosome structure, so if you need to review on that, that is covered in the cell cycle lecture, but just briefly, prior to the S phase.0198

Let's say this cell is in G1, the chromosome will exist in such a way that it has only a single chromatid.0210

After S phase or during S phase actually, the chromosomes, the DNA is all duplicated. It is exactly replicated.0223

And what this is going to create is a chromosome that has two sister chromatids, and these are identical sister chromatids.0233

This is considered one chromosome. This is still one chromosome.0251

It has two chromatids, but it is still single chromosomes, so do not let it throw you off that this looks different than this because this is just duplicated information right here.0256

Alright, what happens is this cell - let's look at this cell right here - has 1, 2, 3, 4 chromosome. It goes through mitosis.0267

After mitosis, the result will be two identical daughter cells, but what you will notice is that those daughter cells do not have sister chromatids.0283

So, S phase has occurred. There has been a duplication of this DNA, so that one sister chromatid from each chromosome goes into each of the daughter cells.0296

No information is lost that way.0308

There are four chromosomes here. There are four chromosomes here.0310

The large purple one, that information is here, the small purple one here, large green here, small green here.0315

Now, what do these numbers mean, 2n here and 2n here?0323

Well, there are two different types of cells that exist in terms of chromosome number.0329

Diploid cells have two sets of chromosomes. Haploid cells have only one set of chromosome.0338

In mitosis, chromosome number is conserved, so here, these two chromosomes are similar in their length and their placement of the centromere.0366

In addition, let's say that these were stained in the laboratory, and you found that they had the same staining pattern, banding pattern - different bands would form and different locations.0377

And if their length is the same, the centromere location is the same, the banding pattern is the same, we call those homologous chromosomes.0390

Homologous chromosomes carry genes for the same traits on them.0402

I am going to call this chromosome no. 1, and these are chromosome no. 2.0413

In this individual, she received one chromosome 1 from her mother and one chromosome 1 from her father.0419

She received one chromosome 2 from her mother and one chromosome 2 from her father.0429

You look at any somatic or regular body cell, not a reproductive cell, all somatic cells are going to have two sets of chromosomes in them, and they are, therefore, diploid.0435

These somatic cells are diploid.0449

Homologous chromosomes, chromosomes that are the same length, have the same centromere placements, same banding pattern, those are more superficial things.0455

The important thing is that they carry DNA for the same traits. In other words, let's say chromosome 1 is the chromosome that codes for eye color.0464

Then, this individual received a gene or a form of a gene, which is called an allele. Allele are alternative forms of a gene.0477

Let's say chromosome 1 encodes eye color, and this individual inherited the blue eye allele from her mother. There is DNA here that encodes for blue eye color.0492

Maybe this individual inherited the allele or form of the gene for brown eye color from her father, so these homologous chromosomes carry information about particular traits in alternative forms.0506

Maybe the gene for height is on chromosome 2, and this individual inherited shorter stature from her mother and taller stature gene from her father.0521

Cells that are diploid contain two complete sets of chromosomes. What is really important in mitosis is that this chromosome number is conserved.0536

The daughter cells are identical in terms of the information they contain to each other and to the parent cell, and they are still diploid.0549

Mitosis, you start out with a diploid cell. You end up with two diploid daughter cells.0556

Now, I have two n written here.0561

Diploid cells in terms of chromosome number are known as 2n. Haploid cells are n.0563

Now, in humans, we have 46 chromosomes. Different species may have different numbers of chromosomes, for example, cats.0573

The diploid cells in cats have 38 chromosomes.0586

Humans are somatic cells. You took out a skin cell and looked at it, you would see 46 chromosomes.0589

And this exists in the 23 pairs of chromosomes. That is the easiest way to look at it, that this is 23 pairs.0596

Chromosome 1 - chromosome 2, there is a pair. Chromosome 3, there is a pair, chromosome 4 and all the way.0614

Somatic cells are diploid. They contain 23 pairs of chromosomes.0624

One thing to note is that the 22 of the chromosomes are just numbered. They are numbered 1 through 22, and these are called autosomes.0637

The 23rd set is the sex chromosomes.0650

We will definitely get into this later on when we talk about reproduction, but just briefly, remember that XX, that chromosome conformation is for a female; XY is male.0656

There are what we call autosomes, chromosomes 1 through 22, and you will have two pairs of those.0675

You will have two chromosome 1s, two chromosome 10s, two chromosome 20s, one from your mother, one from your father.0682

In addition, you have a set of sex chromosomes. Females have XX; males have X and a Y.0687

Alright, in mitosis, you start out with a diploid cell, two full sets of chromosomes. You end up with a diploid cell.0698

We are going to talk about miosis separately, but just keep in mind that in miosis, you start out with a diploid cell, but you end up with a haploid cell.0706

Now, delving into how mitosis works, to understand mitosis, you should have an understanding of the structure of the spindle apparatus.0717

We touched upon this in the earlier lecture on cell structure in the discussion about motility within the cell and microtubules.0726

Focusing here on the spindle apparatus, this consists of centrosomes, microtubules and asters.0733

These are sometimes called spindle microtubules or spindle fibers, and what the spindle apparatus does is it physically separates the chromosomes into two sets.0740

In mitosis, it pulls apart sister chromatids, so that there will be one set on one side of the cell, the other set on the other side.0753

Let's look at each part of the spindle apparatus starting with the centrosome.0761

The centrosome is right here, and within the centrosome, these two brown structures are actually centrioles.0765

There are two centrioles located within each centrosome, and these are microtubule-organizing centers. They are a type of microtubule-organizing centers.0778

Short version of this is MTOC, and the title tells you what they do. They organize microtubules.0797

These spindle fibers here or spindle microtubules are organized right here at the centrosome.0803

Centrosomes are a type of MTOC, and within those exist centrioles.0815

Spindle fibers or spindle microtubules connect the centrosome to the chromosome. Spindle microtubules connect at a specific part on the chromosome called the kinetochore.0824

Kinetochores are located on the centromere, and each chromosome has two.0847

Let's use red. Here is kinetochore.0860

There are two of them, and the reason there is two of them is the goal in mitosis is to separate the sister chromatids.0869

And that can only occur if this one pole of the cells over is here, one pole is over here, and the spindle fibers on this side are connecting to the kinetochore on this sister chromatid.0875

The spindle fibers radiating out from this side are connecting to the kinetochore on the other sister chromatid0889

so that, when these pull, when they shorten up, they will pull apart the sister chromatids.0896

Recall from the cell structure lecture that microtubules can add subunits to one end and remove them from an end.0904

Microtubules are formed from subunits of tubulin protein, so these tubulin subunits can ba added on to make the microtubule longer, so that it can attach to the kinetochore.0913

And then, they can remove subunits to become shorter to pull apart the sister chromatids, so that is how microtubules allow for motility.0930

Alright, we covered centrosomes. Within them are centrioles.0940

These are microtubules and, now, asters. What asters are, if you look right here near the centrosome, this area of microtubules is radiating out.0944

Some people consider this a star shape, and that is where it gets that name aster.0955

These microtubules that are just radiating out from the centrosome are called asters.0959

This is really the situation for animal cells. Plant cells and fungi do not have centrosomes, but they still have microtubule-organizing centers.0967

Centrosomes are one type of MTOC, but there are other types; and plants and fungi do have microtubule-organizing centers.0980

They still go through mitosis, and they have organized microtubules; but the control is not via a centrosome.0989

It is some other type of structure.0996

Now, that you understand the spindle apparatus, it is time to go on to the first phase and to focus on the first phase of mitosis.1002

The first phase is called prophase. Mitosis consists of prophase, metaphase, anaphase and telophase.1014

You should know the major occurrences for each phase, so starting with prophase, during prophase, the nuclear membrane begins to break down, and the nucleoli disappear.1035

In order to separate these chromosomes out into two group for the two daughter nucleoli, the nuclear membrane needs to be removed.1046

This is shown as a dotted line because the nuclear membrane is breaking down.1052

The last cell I showed you to illustrate the spindle apparatus already had a spindle apparatus completely in place.1065

But the spindle apparatus does not exist throughout the whole cell cycle. It has to be formed.1073

The spindle apparatus starts to form during prophase, and here, you can see the centrioles within the centrosomes and the asters.1080

The spindle fibers are starting to radiate out. Here, we have formation of spindle apparatus.1089

In addition, the chromatin condenses.1102

In the previous lecture discussing the chromosome structure, I mentioned that chromosomes consist of chromatin.1105

Chromatin is DNA plus protein, and the type of protein is histone protein.1116

The DNA is wound around histone proteins, and for most of the cell cycle, the chromatin is in a very non-condensed state.1128

And if you looked at a cell in most of the cell cycle, you would not be able to visualize chromosomes via light microscope because they are in their uncondensed form.1135

Now, at prophase, the chromosomes condense. The chromatin condenses and forms, what most people recognizes "OK, that looks like a chromosome".1147

What we are picturing is the state that the chromosomes are in during mitosis, which is their condensed form.1157

And at this point, the chromosomes would be visible via light microscopy.1164

Chromatin condenses, and the chromosomes, now, they form the form of chromosomes that is visible via light microscopy.1171

Three major events: formation of the spindle apparatus, breakdown of the nuclear membrane and the chromatin condenses.1180

One other thing happens, the nucleoli disappear. There were some nucleoli visible in the nucleus, but now, they disappear.1188

OK, that is prophase. The second phase is metaphase, second phase of mitosis.1202

During the metaphase, part of mitosis, the chromosomes line up single file along the metaphase plate.1211

And this single file is very important because it is one thing that differentiates mitosis and miosis.1218

In miosis, chromosomes line up differently, and that makes a big difference in the outcome of that type of cell division.1223

Here, this cell, keep in mind, has 1, 2, 3, 4 chromosomes, and these are two sets of two. These two chromosomes are homologous chromosomes.1232

I will call them chromosome 1, one from this individual's mother, one from his father, chromosome 2 from his mother, chromosome 2 from his father.1242

So, I have two sets of two chromosomes, and the cell is diploid.1251

Now, these chromosomes line up single file along what is called the metaphase plate, and all the metaphase plate is an imaginary plain equidistant from the two centrosome.1257

And, one other term you will hear is the pole of the cell, so this is one pole of the cell, one end; and this is the other.1282

And this is centrosome is located at one pole, and remember that the spindle fibers are attaching to the kinetochore on one sister chromatid.1289

The spindle fibers on the opposite pole are attached to the kinetochore on the other chromatid all the way down.1299

It is going from centrosome to spindle fiber to kinetochore.1309

In this drawing, the sister chromatids are shown for clarity as having a little bit of space between them, just so you can differentiate that there are two sister chromatids.1327

But, in reality, they are actually held pretty tightly together by proteins called cohesins. OK, cohesins are proteins that hold sister chromatids together.1337

These have to be cleaved, so that the two chromatids can separate, but generally, sister chromatids are held together via cohesins.1360

We did prophase and metaphase. Next phase is anaphase.1371

This is the time when the two...we had a chromosome that had two sister chromatids held together.1375

Now, what is going to happen is the spindle fibers will shorten up. These are microtubules, so they are capable of becoming longer or shorter.1385

The cohesins, therefore, will have to be cleaved, so the cohesins are cleaved. The sister chromatids separate and are pulled to opposite poles of the cell.1393

Remember, I started out with 1, 2, 3, 4 chromosomes, and I can see, i still have 1, 2, 3, 4 chromosomes.1407

No information was lost because each chromosome had been through S phase, synthesis phase, and the DNA had been replicated.1416

If this, right here, was information encoding for blue eyes, that blue eye information is going to go on to this daughter cell.1423

This blue eye information is going to the daughter cell.1432

If this was brown eye allele, this is going into one cell. This is going into the other.1435

The gene for a particular height, an allele for another height, all the information is here- blue eyes, brown eyes, tall, short, blue eyes, brown eyes, tall, short.1441

So, no information is being lost.1451

Remember that also during the growth phases, G1, synthesis phase, as well and G2, organelles, cell membranes, cytoplasm, cell components were all copied more, then, were made.1453

The cell increased in volume, so you can see that the daughter cells will each have everything they need.1465

Finally, telophase and cytokinesis, I put these together because cytokinesis actually begins in the late anaphase. It overlaps, also, with telophase and then, continues on after.1476

It is not a clear telophase ends, and then cytokinesis begins, so I put these together.1486

During telophase, some of what happens is the reverse of prophase.1491

During prophase, a nuclear membrane broke down. Now, the nuclear membrane is reforming, except now, there is two nuclear membranes because there is two nuclei, one for each daughter cell.1495

Chromosomes become less condensed. I mentioned that during prophase, the chromosomes condense.1507

The chromatin condenses, and the chromosomes become visible under light microscopy.1512

Now, the chromosomes are going to decondense, and you are not going to be able to see them with the light microscope anymore.1518

Nucleoli reappear. They disappeared during prophase.1524

They are reappearing now, so we have nuclear membrane reforming. Chromosomes becoming less condensed.1528

The nucleoli reappear, and the other thing, notice, is that the spindle apparatus is going to break down.1534

It formed in prophase. It is going to break down now.1542

It is not needed anymore.1545

Cytokinesis began during anaphase, and cytokinesis is the division of the cell into two physically separate cells by dividing the cytoplasm.1551

In animal cells, this occurs by the formation of a cleavage furrow. This is the cleavage furrow.1559

It is an indentation or a groove in this parent cell. Actin and myosin will pinch off this parent cell, so that it will pinch off and form two daughter cells.1564

When this is completed, when cytokinesis is completed, the end result is going to be two diploid daughter cells.1579

The parent cell was diploid. It had 1, 2, 3, 4 chromosomes.1590

This pinches off. We have still got four chromosomes.1594

Recall from a previous lecture discussing cytokinesis that formation of a cleavage furrow occurs in animal cells. It does not occur in plants and fungi.1598

Plants and fungi have cell walls. Instead of forming this cleavage furrow, they form, what is called, a cell plate.1611

The cell plate starts out as a coalescence of vesicles, and these vesicles are derived from the Golgi apparatus. They contain the materials needed to form a cell wall.1624

Cytokinesis in plants and fungi involves the formation of a cell plate via coalescence of vesicles derived from the Golgi apparatus.1644

And the materials in them are used to make a cell wall down the middle of the cell, and that will eventually divide the cell into two daughter cells.1654

Summing up mitosis starting out with two sets of two chromosomes. I had chromosome 1, chromosome 2, one maternally derived, the other paternally derived- maternally derived, paternally derived.1668

This cell is, therefore, 2n or diploid. It has been through S phase, so here are sister chromatids.1685

Each one of these chromosomes has enough information to supply two daughter cells.1694

Prophase, metaphase, anaphase and telophase occurred, and after that, the result is two daughter cells.1699

They are identical to each other. They carry all the information that the parent cell had, and they are diploid.1712

That is the most important points of mitosis. It is separation.1719

It is cell division resulting in two identical daughter cells with conservation of chromosome number.1724

Alright, to practice, we are going to look at four examples.1732

Label the parts of the spindle apparatus below. What is the name of the structure on the centromere to which the spindle fiber is attached?1736

Starting out with this first part, here, I have a centrosome. Within it, what is really shown here is the centriole, but the centrioles are a part of the centrosomes.1745

Remember that the centrosome is a microtubule-organizing center, and within each one is two centrioles- same thing on the other side.1758

Radiating out in a star pattern is an aster, so there is an aster on this side and an aster on the other side.1768

Here, I have spindle fibers or spindle microtubules. These are composed of microtubules, hence, the name, and these are attaching to the sister chromatids.1778

This next part of the question asks me what is the name of the structure to which the spindle fiber is attached.1792

The centrosome is here in the middle holding the sister chromatids together, and on the centrosome are two kinetochores.1798

The spindle fibers from one pole attach to one kinetochore. The spindle fibers on the other pole attach to the other kinetochore giving the setup that will allow the sister chromatids to be pulled apart.1811

The answer to this question is that the spindle fiber is attached to the kinetochores, and the kinetochores are located on the centromere.1825

Second example, example two: What is the last phase of mitosis called? What occurs during this phase?1839

The last phase of mitosis is telophase. Remember, there is PMAT: prophase, metaphase, anaphase, telophase.1850

Some of the events here are actually just a reverse of prophase.1862

Remember in prophase, the nuclear membrane broke down. Now, the nuclear membrane reforms, and one nuclear membrane will form around one set of chromosomes at one pole of the cell.1869

The other nuclear membrane will form around the other nucleus at the other pole of the cell, so now, there are two nuclei.1887

The second event: recall that chromatin condensed in prophase. Now, chromatin becomes less condensed.1896

Once is decondenses, the chromosomes are not going to be visible via light microscopy.1915

During prophase, the spindle apparatus formed. Now, spindle apparatus breaks down.1921

In addition, the nucleoli reappear. Also, you should note that cytokinesis is continuing on.1936

It began in late anaphase. It is also continuing on during telophase, but it is often treated as a separate phase.1946

So, for telophase, we are going to put these four events as telophase and keep cytokinesis considered as a separate event.1953

Example three: list three events that occur during prophase of mitosis.1962

Remember, we just talked about telophase. That makes this question easy.1974

It is the opposite. Here, we are going to have breakdown of the nuclear membrane.1977

The next event is going to be that the chromatin condenses. It becomes wound more tightly.1990

Now, the chromosomes are going to be visible via light microscopy.1998

The spindle apparatus begins to form in preparation for anaphase when sister chromatids are going to be separated, so the spindle apparatus begins to form.2004

During this phase, the nucleoli disappear. They are no longer visible.2017

OK, prophase is the first phase of mitosis, and these four events occur during prophase.2024

Example four: What phase is the cell below shown in? What events occur during this phase?2036

Looking at this situation here, I see that the sister chromatids are being pulled apart. Since they are being pulled apart, I know that this cell is in anaphase.2045

The main event of anaphase is separation of sister chromatids.2061

This cell started out with four chromosomes, with sister chromatids. Now, the spindle fibers are going to shorten.2075

They are going to pull just towards each chromatid, towards the opposite pole of the cell resulting in daughter cells that have 1, 2, 3, 4 chromosomes but only one sister chromatid.2082

That concludes this lesson on mitosis here at Educator.com.2096

Thanks for visiting.2101