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Lecture Comments (32)

1 answer

Last reply by: Bryan Cardella
Tue Nov 3, 2015 9:36 AM

Post by Rigers Ahmetay on November 3, 2015

How can I reference this source using Havard system

2 answers

Last reply by: Hossain Khondaker
Thu Jun 25, 2015 9:47 AM

Post by Hossain Khondaker on June 24, 2015

hi I was just wondering is there any text books you recommend for I am about to be a freshman in high school and I got placed at AP Biology.

1 answer

Last reply by: Bryan Cardella
Fri May 15, 2015 11:05 PM

Post by Khalid Khan on May 15, 2015

The funny thing is that you are a better teacher than my own bio teacher...:P Thanks so much for all these amazing lectures!

4 answers

Last reply by: Bryan Cardella
Thu Mar 12, 2015 11:38 AM

Post by Natali Cohen on March 5, 2015

Dear Mr. Gonzalez,
Some point still not so clear for me. Diploid cell contain 46 chromosomes before the division. Thus if DNA, which is chromosomes replicated, we will start with 92 chromosomes in original cell?

1 answer

Last reply by: Bryan Cardella
Wed Dec 31, 2014 1:31 AM

Post by Setayesh Omidian on December 30, 2014

Hi Mr. Cardella
So once we are done with meiosis we have 23 chromosomes,but how do we end up with 46 to go through meiosis again?

1 answer

Last reply by: Bryan Cardella
Sun Nov 23, 2014 7:43 PM

Post by Richard Meador on November 22, 2014

Thanks very much for your answer on my reverse engineering question. It looks like there are almost unlimited number of details and complications.

One final question: What cell or what type of cell would be the most likely candidate to reverse engineer given that all cells are incredibly complicated? Is there a "least complicated cell" that biologists have identified?  

1 answer

Last reply by: Bryan Cardella
Sat Nov 22, 2014 3:17 PM

Post by Richard Meador on November 22, 2014

you stated that all cells come from existing cells.  what's the main technical limitation that keeps biologists from creating a living cell?  In other words, why can't they reverse engineer a living cell such as a simple plant cell given they have total access to study the cell?  

1 answer

Last reply by: Bryan Cardella
Wed Oct 29, 2014 9:27 PM

Post by Judy McAfee on October 29, 2014

How are the cells at the end of mitosis compare to the cells at the end of meiosis I ?

6 answers

Last reply by: Brady Dill
Wed Jul 30, 2014 7:42 PM

Post by Brady Dill on July 25, 2014

Meiosis, huh? So that story about storks and doorsteps was a lie? Apparently I have a bone to pick with my parents...
Great video! In each sperm or egg, are the 23 of the 46 chromosomes totally random? Does this mean, in theory, that two siblings could be completely unrelated?

1 answer

Last reply by: Bryan Cardella
Fri Jun 27, 2014 1:40 AM

Post by David Gonzalez on June 26, 2014

Where exactly is Meiosis occurring if the person isn't born yet? Thanks Mr. Cardella.

2 answers

Last reply by: Duru Bumedien
Sun Mar 30, 2014 7:30 AM

Post by Duru Bumedien on March 29, 2014

Hi Mr. Cardella

I have a question about 2n and n chromosome. So humans have 2n=46 chromosomes. Does it mean that we have 23 chromosomes with two sister chromatids in each cell or do we have 46 non-dublicated chromosomes in each cell?

Cell Division

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Purposes of Cell Division 0:05
    • Growth and Development
    • Tissue Regeneration
    • Reproduction
  • Cell Size Limitations 4:01
    • Surface-to-Volume Ratio
    • Genome-to-Volume Ratio
  • The Cell Cycle 12:20
    • Interphase
    • Mitosis
    • Cytokinesis
  • Chromosome Structure 16:08
    • Sister Chromatids
    • Centromere
    • Chromatin
  • Interphase 21:38
    • Growth Phase #1
    • Synthesis of DNA
    • Growth Phase #2
  • Mitosis 25:13
    • 4 Main Phases
    • Purpose of Mitosis
  • Prophase 28:46
    • Condense DNA
    • Nuclear Envelope Breaks Down
    • Nucleolus Disappears
    • Centriole Pairs Move to Poles
    • Spindle Apparatus Forms
  • Metaphase 32:36
    • Chromosomes Line Up Along Equator
    • Metaphase Plate
  • Anaphase 34:21
    • Sister Chromatids are Separated
    • Sister Chromatids Migrate Towards Poles
  • Telophase 37:17
    • Chromatids Become De-Condensed
    • Nuclear Envelope Reforms
    • Nucleoli Reappears
    • Spindle Apparatus Breaks Down
  • Cytokinesis 39:01
    • In Animal Cells
    • In Plant Cells
  • Cancer in Relation to Mitosis 41:59
    • Cancer Can Occur in Multicellular Organism
    • Particular Genes Control the Pace
    • Benign vs. Malignant
    • Metastasis
    • Natural Killer Cells
  • Meiosis 48:17
    • Produces 4 Cells with Half the Number of Chromosomes
    • Produces Genetically Unique Daughter Cells
  • Meiosis I 52:39
    • Prophase I
    • Metaphase I
    • Anaphase I
    • Telophase I
  • Meiosis II 1:01:04
    • Prophase II
    • Metaphase II
    • Anaphase II
    • Telophase II
  • Meiosis Overview 1:03:39
  • Products of Meiosis 1:06:00
    • Gametes
    • Sperm and Egg
    • Different Process for Spermatogenesis vs. Oogenesis

Transcription: Cell Division

Hi, welcome back to, this is the lesson on cell division.0000

When we talk about the purpose of cell division, what is the point of it happening?0006

Some of the reasons are more obvious than the others but there are really three main purposes,0011

or three main reasons that cells divide.0015

Number 1 is growth and development.0018

Obviously, to get from one cell, the first of life, to a hundred trillion cells.0021

As a developed organism, you got to have cells dividing to make more cells.0026

Not only as growth getting from that first cell, the zygote, to all the cells you have now, but it is also development.0031

Getting those cells to become your liver, your heart, your muscles, to develop into the different tissues and organs of the body.0038

Cell division definitely accounts for that.0047

Tissue regeneration, this is not just injuries that we are talking about.0050

Every second of every day of your life, you are regenerating tissues,0055

whether or not you are still growing or that you are going through puberty.0059

This refers to something like your red blood cells.0063

The average human being loses about a million red blood cells per second, every second of their life.0066

That is just an average approximation, but that is a lot of cells.0075

Guess how many you need to make every second of your life, one million.0078

Some people might be 1 million and ½, might be 2,000,000, depends on your body size, depends on your health, etc.0084

But, you have a lot of tissue regeneration going on.0089

If it does not happen adequately, you are going to have some health problems or worse.0092

Also when you get injured, you need to have tissue regeneration.0096

When you have damage to a part of the body to a tissue, you need to be able to bring back cells through cell division,0100

to replace or make up for the ones that are damaged and going to be gone.0106

Of course, reproduction, this is not just reproduction of sexual reproducing organisms.0112

This is also reproduction of single cell beings, an amoeba, or a perimysium. 0117

It can divide just like that to make two new ones, that is the process of mitosis, 0126

for it to make two new daughter cells, and essentially it doubled itself.0132

We also reproduce, the sexual reproducing organisms, whether you are talking about animals, plants, fungi, etc, 0140

reproduction with them is a little bit different.0148

Having two organisms contribute two sets of DNA, it is making a unique new individual is a little bit different.0151

You have to get half the number of genetic information from each person to make a new whole.0159

You can add 46 chromosomes from one person, 46 from the other, 92, it would not be a new human being able to work out.0165

There is a process where you have to take the chromosome number and divide it in half, and get them to combining.0172

That is the process of meiosis, a little bit different with sexual reproduction, 0178

it will cover both kinds of cell division, mitosis and meiosis, in this lesson.0181

Here, you have a micrograph, an actual image from microscope.0186

There is a little bit of fluorescence just to show us what is what.0190

This is a cell that is at the end of mitosis, anaphase.0193

You actually can see the pulling of the chromosome, you can see the spindle apparatus 0199

that is pulling the genetic information to the sides of the cell.0207

Pretty cool image.0211

Then you have a cartoony version of cell division here.0213

We have prophase, pro metaphase also known as late prophase.0219

Metaphase, anaphase, telophase, and leading to cytokinesis.0225

Eventually, you are back into what is known as interphase, once these two finished dividing.0230

We will cover the details of what these phases are about, coming up.0236

In terms of why cells divide, it is not just the purpose of cell division, like what purpose does it serve in nature for organisms.0243

But why do they ever need to divide, why cannot a cell just keep growing and growing, and get this big, why not? 0249

Cells need to divide because they cannot keep growing indefinitely.0258

There is too much volume for them to manage, that is one of the reasons.0263

The largest cell in the human body is by volume, not by length, would be one single female egg cell.0268

From what I have heard, if you could isolate it by itself on a table, just look at it by itself, it looks like a grain of sand.0276

That is actually very large, the average human cell, you cannot see because it is microscopic.0283

You can see neurons, that are these long but they are so skinny, they are still microscopic.0289

By volume, the female egg is very large, but beyond that, you are not going to see cells bigger than that.0295

One exception would be, at the bottom of the ocean, there is so much pressure keeping cells from bursting,0303

potentially, that you could have certain sea life with large cells that develop.0311

I have seen a video of one of them where they claim that, that particular structure is one cell that has grown to that size.0318

On the surface of the earth, a little bit different, different stuff going on with the physics, but I digress.0325

Factors that require a cell to divide.0333

Like I have mentioned, you cannot keep having a cell just grow and grow, the volume becomes too much to manage.0335

It stretches out the plasma membrane way too much.0341

If we look at surface dividing ratio, this sort of mathematical concept of surface area and volume changing over time shows us that.0344

The average typical animal’s cell is spherical.0353

In a human body, you have a lot of varieties though.0358

You got long ones, you have round one, skinny ones, differently shaped ones, just depending on what the function of that cell is.0360

But instead of talking about a sphere, let us talk about a cubed growing.0370

It is the same basic concept, the formulas required to track surface dividing ratio with cubes 0374

are little easier to talk about than the one that are spheres.0384

Let us talk about a cube growing.0387

I have done this in a lot of classes before with sugar cubes.0392

But, you can do it with any 3 dimensional structures that grows over time.0396

Let us pretend that we got this cube that is growing in, and this is a side.0399

Right there, we are going to use s for side, that is a side.0404

Since this is a cube, all the sides should be the same length.0408

Here we are going to talk about a side length, 1, 2, or 3.0413

It could be mm, it could be cm, let us say it is μm.0423

I know it looks like a u, but it is a fancy looking m.0429

Micrometer is millionths of a meter, very tiny.0432

You would typically use this measurement to measure the average cell, nanometers would be smaller than that.0436

Here is our side, we are going to talk about surface area, volume, and then the ratio.0442

This is a little table we got going here.0456

The formula to come up with the surface area here, it would be side² times 6.0460

Why, because 1² to figure out the area of that is side times side, length times width, side times side.0471

But, times 6 because when we look at a cube, there are actually are 6 sides.0480

We are just measuring the area around the surface of it.0485

We talk about with one, side 1, 1 squared times 6, ends up just being 6 squared micrometers.0488

Volume is a little bit different.0501

Volume, in terms of the measurement of all the stuff that would be inside of it, liquid or otherwise, 0504

the volume is length times width times height, side³, literally, cubed.0510

Of course, that would be 1 micrometer³.0519

The ratio is 6 to 1.0526

That is different when that cubed gets a little bigger, when every side doubles in length, the cube is much larger.0530

When we pull 2 into there, 2² = 4 times 6 = 24 micrometers².0537

When we put a 2 into there, 2 times 2 times 2 is 8.0554

Reduce the ratio, what do we get, 3 to 1, ratio was changed.0561

A pattern is developing here.0567

Finally, when we put a 3 in there, 3² is 9 times 6 = 54 micrometers².0568

Putting a 3 in there, 3 times 3 times 3 = 27 micrometers³.0579

When you reduce that, look what has happened here, the ratio is gradually changing.0588

What is this telling us is that the volume is increasing at a much greater rate.0596

It is catching up to the surface area very fast, it is going to the point where, it is really hard for that surface,0601

in the case of the cell, the plasma membrane to contain that growing volume of the cytoplasm inside.0608

Because of surface to volume ratio, eventually a cell needs to take itself and split in half.0614

Each of those will continue to grow in their own, and the cycle will repeat itself.0620

Those two will eventually divide, so on and so forth.0624

Genome to volume ratio, what is the difference, it has to do with relationship to volume0628

but genome referring to the chromosomes, the DNA inside the nucleus.0632

Think about this, if the genome for humans is 46 chromosomes and it is, 23 pairs, that does not change.0638

Meaning, you do not get new chromosomes, additional chromosomes, to manage a cell that is increasing in size.0647

You have fixed amount of information that is supposed to regulate a cell, however big it gets.0654

Think about this as an analogy, let us say you have a very small town, 5000 people.0660

I came from a small town, and from where I live, it is 60,000 people, that is the smallest in this part 0668

but if we think of a really small town, rural community, a few thousand people.0675

Let us say you have a city council of 5 people managing a 5000 person town, that is doable, totally fine, they can manage that town.0680

Let us say 50 years from now, the town grows to be 2 million people, from 5000 to 2 million, that is a big change.0692

If you only have 5 people managing that 2 million person city, it will be a little challenging for them to manage it.0701

It is the same analogy with a fixed amount of genomic information of chromosomes, 0708

managing it ever increasing cell, in terms of its volume.0713

Because of genome to volume ratio, eventually, cells need to divide.0717

They double the genetic information but it is just copies of what was already there.0721

Those copies are passed on to the two daughter cells, and they will manage themselves.0725

Eventually, copy a DNA again and pass it on.0731

Because of surface to volume ratio and genome to volume ratio, limits cell size.0734

What are the types of the cell cycle, in terms of how the timing of cell subdivision is regulated?0742

All cells are supposed to have a timing in which they grow and divide, and at a particular pace.0747

Depending on what part of the body you are looking in, the pace defers a lot.0752

Skin cells, the cells lower down in the layers of skin, that actually divide.0756

They go through mitosis to make these skin cells that you have here on the top.0763

That divide pretty often, because you are causing losing skin cells, they are subject to the environment.0766

You lose a lot of them.0773

Brain cells, you have some cells in your brain that are quite the opposite, they do not ever divide.0775

It actually go to something called G0 or outside of the cell cycle where they have to be chemically stimulated 0781

to get back into that cycle, or eventually will divide.0788

The actual timing of the cell cycle varies depending on tissue, depending on the organism as well.0792

The cell cycle is made up of three main parts.0799

Interphase is all of this orange part here.0803

This is like a clock, it goes clockwise in this direction, as you could see from the arrow, all of this is interface.0808

In the average cell, a lot of textbook will say that interphase is about 90% of time or 85% of the time, it really depends on the cell.0815

If the cell is dividing a lot in the tissue it is in, interphase may only be 2/3 of the time.0823

But if it gets to be where interphase is too little of the amount of time and the cells dividing way too fast, 0830

that can be a cancerous cell.0838

We will get to that more, later.0840

But, interphase is always orange part, that is in between cell divisions.0841

This little yellow part, that is mitosis, that is actual cell division.0846

That is the actual act of the cell currently dividing.0855

In cytokinesis, we are just not depicted with a letter here but it is depicted with this little illustration.0860

Right at the end of mitosis, you get cytokinesis, the actual splitting of cells.0866

And then, those two split cells go back into interphase, which means it is the phase between divisions.0871

This inner circle is showing you a bit more detail the subphases, because within interphase,0877

you get G1 as G2, more on that in a bit, and leading to mitosis which also has its own individual phases.0882

There are checkpoints along the way that lead to the subsequent phase or not.0892

What I mean by that is, to get from G1 to S, there is either a green light or red light, and that is a chemical signal.0895

If all is going well in G1 and things are looking good to move on in S, there will be a green light, and then S will happen.0904

If all goes well in S and things had been checked inside the cell, chemically, they will green light to go to G2, etc.0910

It is checkpoints that make sure it is working out properly.0918

A red light means that there is something wrong, we are going to continue on this division.0922

Sometimes, the red light is a good thing though.0927

To get from G1 to here, it is a certain kind of red light signal, 0929

meaning we are not progressing with interphase, we are not going to divide.0934

Like I mentioned a moment ago, you have some brain cells and muscle cells, particularly, that do not divide.0938

They have been stimulated to got to G0, there are reasons for that.0945

Some cells you do not want to go in G0, you want them to continue on with their cycle and make new daughter cells.0949

It really depends on the need of the tissue, the needs of the organism at that moment in time.0955

Checkpoints do regulate these, if those checkpoints will get out of control, a cancer could be happening in that tissue.0961

Before we proceed, we have to look at chromosome structure, because you are going to see these chromosome X shaped.0971

Some people say it is like a butterfly looking thing, you are going to see this in the phases of mitosis.0977

We got to know what we are looking at.0982

During mitosis, chromosomes are condensed in packages, that is what you are seeing here.0984

When you are not in prophase, metaphase, etc., when we are what it is called interphase.0989

If you are to look really close to the nucleus, what you are not going to see is these,0998

you are going to see chromosomes unraveled, more or so.1002

Here is the nucleolus of the nucleus, and of course, you got little nuclear pores, I’m not going to draw all of them.1006

This is a nucleus cut in half, so we can see inside of it.1015

All of this stuff, it looks like spaghetti, and I know the nucleolus looks like a meatball in the spaghetti.1022

All of this that I’m drawing is loosely arranged chromatin.1030

I will explain more what a chromatin is, in a second.1036

Here is the analogy that you can keep in mind.1038

This is during interphase, when a cell is not dividing.1041

Think about this, if you are not moving to a new apartment or a house, you are comfortable in your place, 1049

you live there, no plans of moving, you do not have your stuff packed up in boxes.1053

You have your stuff sitting around sitting around where you can use it, loosely arranged, not packaged away.1058

That is what is going on here, all of the stuff that look like pasta, 1065

most of it is loosely arranged to DNA with the proteins that help keep it arranged.1070

You have easy access to all these different genes, you can use these genes and read them in, and make proteins, etc.1075

You will hear more about that in the DNA and RNA lesson.1081

All this is stuff is loosely arranged so you can get to it and use it.1084

When it comes time to pack up and leave, meaning you are going to divide and make two new daughter cells,1087

you got to move this stuff.1093

Then, it is packaged up into little boxes that look like this.1095

It is easier to separate your copy pieces of DNA and move them to their final destination, the daughter cells.1099

And then, they will be moved again, eventually.1107

It is kind of like when you move to a new apartment or house, you pack all your stuff in boxes, easier to move stuff.1110

Once you get there, you unpack, and this is the unpacked DNA.1116

Condensed chromosome structure, when you are in the process of moving, it looks like this.1120

This here, this whole thing, this thing is known as a chromatid.1126

See the label chromatids, plural, this is a single chromatid.1130

This is its sister chromatid and they should be identical copies.1139

The DNA that is here is exactly the same as that.1144

There could be hundreds thousands of genes on a single chromatid.1149

But the point is that, this particular chromatid is an exact copy of that one, sister chromatids.1154

The centromere is the middle portion.1162

The centromere holds together the sister chromatids.1170

It is in the center, more or less.1173

Typically, the top arm of the chromatids is shorter in length than the bottom part.1175

But, it is close enough to the center and it is little knot that is keeping the chromatids attached.1182

Chromatin, all this stuff which is also in here, is DNA plus mainly histones which are proteins that DNA is wrapped around.1189

It is like a yarn or string around this bull, that is a good way to think of it.1199

They illustrate that here, all of this is tightly round compacted DNA.1205

If you spin it to where you see more and more of what is rattled up, you can finally see what is going on here.1213

These little balls, these are protein units called histones.1222

This line wrap around it, that is your DNA double helix, your DNA strand.1227

There is the DNA that we all know and love.1231

But, all of this is wrapped around these little histones.1234

That is what you are zooming on here.1237

Here is your histones, each of these little units that is DNA + histones is called a nucleuzome.1239

One single unit of histones with DNA wrapped around, nucleuzome.1250

All of these nucleozomes, they take these wrapped and tight, take a whole unit, a whole fiber of those, wind that up again.1255

It keeps getting wind until you get this highly compacted wrapped up, boxed up, chromatid.1266

Those are chromosome structure, you see these starting in prophase, the first part of mitosis.1275

By the time it is in the telophase, you start to get back to this look.1280

Interphase in between mitosis events or meiosis events, you are not going to see those chromosomes.1285

You are going to see them loosely arranged because they have already moved and unpacked those boxes of DNA.1292

With interphase, this is the phase that is in between divisions.1299

Interphase, between phase is really what it means.1303

This is a cell between divisions.1307

The cell grows and prepares for mitosis.1310

The cells just doing its thing, doing its this, eating, making parts, eating some more and making parts.1313

If it is a plant cell which is autotrophic, it is doing photosynthesis in making its own food and building parts, etc.1318

It is just doing what a cell does and it is preparing for the next time that it is going to divide.1326

Its gaining volume gradually, it is making new parts, probably increasing its surface area, of its plasma membrane.1331

The cell walls maybe gaining some parts, if it is a plants cell.1339

As you go through interphase, there are three sub phases, based on what is going on.1344

Here we go, once again, this is that orange band of the cell cycle.1349

Within there, you got this blue section, little more than 1/3, close to 1/2, of interphase.1354

It is growth phase 1, growth phase 1 G1, as it is known.1364

The cell is just gaining mass, it is definitely doing a lot of eating, making of parts,1370

preparing for when it is going to eventually duplicate its DNA.1377

There is a checkpoint there that allows you to go to S.1380

If you get the green light, chemically, you end up going into S.1384

S in red, S stands for synthesis.1389

What really happens is the duplication or replication of DNA.1393

This phase in interphase could have been called the D phase for duplication or R phase for replication,1398

it got the name S, the synthesis making of new DNA.1405

Whatever DNA you had going in, whatever chromosomes, whatever genes, you duplicate them, 1411

make an identical copy because you are going to divide and make two daughter cells, that is the whole point.1417

There is a checkpoint here that if, the DNA has been copied adequately, there are no weird mutations that standout,1422

you will get the green light to go into G2.1430

G2, growth phase number 2, happens right before cell division begins, after you duplicated the DNA.1432

There are more preparations here for cell division.1441

If everything goes okay, you have produced proteins that are needed to actually do mitosis.1444

You probably need to get a green light to go on to N phase which is known as mitosis.1451

Keep in mind that in interphase, the DNA is loosely arranged.1456

I drew that little image for you on the last slide, how it is loosely arranged 1459

compared how it is in prophase, metaphase, anaphase, telophase, of mitosis.1463

Individual chromosomes are not typically visible here.1468

If you see this image or looks more like this, often times, karotypes can have weird, 1471

you will learn more about karotypes in the genetics lesson.1485

But, anything that looks like this, you have duplicated condensed chromosomes which would be in prophase or metaphase.1488

Here, when you are talking about interphase, if S is happened, 1497

you definitely have duplicated DNA or replicate DNA, but not condensed looking like this.1501

It is more loosely arranged, as that chromatin that looks like spaghetti in bowl.1507

Now time for mitosis, once interphase has completed, the phases of cell division begin.1515

There are 4 main phases, prophase, metaphase, anaphase, telophase1521

PMAT is a way that you can remember that.1526

A lot of textbooks will have a 5th phase right in here known as prometaphase.1531

Literally, the phase between pro and meta, other refer to as late prophase.1537

The reason why it does not matter is what prophase, metaphase, anaphase, telophase,1544

are just snapshots of this process that is very fluid and dynamic kinetic.1548

It is the same thing with comparing a storyboard to a movie.1555

The filmmakers are preparing to make a movie, they draw little images, 1559

little sketches of what the scenes are going to look like, so you can keep track of the planning for it.1565

When you look at the images of prophase, they are just little shots that represent a whole scene, like a storyboard does.1571

I’m going to narrate how you get from one scene to the next.1579

Even though, I’m not going to talk specifically about prometaphase, you will know it happens.1583

It is really late prophase leading into metaphase.1588

By the time you get to telophase, you are done with division.1592

You have something that is called cytokinesis, that actually concludes the process.1596

The purpose of mitosis is to correctly separate the genetic information, the chromosomes, into the two daughter cells.1600

It makes, it is supposed to make two identical copies of the parent cell.1606

If they are not identical, you have a mutation occurred, that was not intentional.1612

This is an actual micrograph of onion root tips cells.1617

This is a very common lab in a bio class, looking at a frozen in time image of this actively dividing section of the onion root tip.1622

This is from what is called the root apical meristem.1632

Root because there is actually a shoot apical meristem that would be at the top part of it that is growing up.1646

Apical, apex, or apex is at the tip of something, when you are talking near the tip of the root, and meristem, an actively dividing region.1653

When you are looking at this part, you would less interphase than other parts the plant.1663

They maybe other parts of the onion plant that hardly dividing at all.1669

Interphase might be 98% of the time or 99% of the time.1673

Here, it might be closer to 75% of the time.1677

You have more mitosis going on.1682

Here you have what appears to be late interphase, prophase may be just beginning.1685

You still see the nucleolus cell, you know prophase has not completed yet.1694

Chances are this is interphase.1699

Here, you have metaphase almost being done.1702

It is hard to say because it is the shot.1708

You can see that chromosomes are lined up.1710

You are going to get an image of that later on.1712

Here, you can actually see that this is not a nucleus that is fully intact as here.1714

This is probably a prophase just beginning.1720

Prophase, this is the first phase of mitosis.1727

The main occurrences are, what actually happens in prophase, condensing of DNA.1731

That is when you first start to see DNA, not that image like I talked about before,1737

where you got your nucleolus and all kinds of loosely arranged DNA.1742

You are not going to see this in prophase.1747

Once you see these little guys as you see here, you are definitely in prophase, the beginning of mitosis.1749

All that DNA gets wrapped up real tight.1757

You can see what is known as the diploid number, I’m going to talk about that more with meiosis.1761

The number of chromosomes that have pairs, we have 1, 2, 3, 4, chromosomes, 2 pairs.1766

You are going to see that by the time they divide, we should have 4 chromosomes in each daughter cell, all went well.1776

Nuclear envelope breaks down, you can see that this little barrier here1784

that is keeping all that nuclear information in there is starting to come apart.1788

There is a little gap there, eventually by the time you get to metaphase this would be gone.1792

They will not just be like little nuclear pores, this will break up even more.1798

Nucleolus disappears, the nucleolus looks like a mini nucleus inside of it.1804

That is where you make ribosomes, that unravels.1813

Do not think that it completely vanishes like the matter goes away.1816

Same with the nuclear envelope, the parts of the envelope just breakdown into pieces that are so tiny, 1821

you can no longer see them visually through a microscope, same with the nucleolus.1827

Centriole pairs move to poles, that is what you got here.1831

Here and here are your two pairs of centrioles.1835

During earlier parts of interphase, they are typically would be one pair of centrioles.1839

By the time you get to the end of G2, you got to duplicate these.1847

We have two pairs or 4.1851

As they move apart, they are starting to move to the poles, meaning up here, down there, like the poles of planet earth.1853

When they move to the poles, this will get increasingly large.1862

This is the spindle apparatus, that is the last part that is important in prophase.1866

This is also known as late prophase, by the time the spindle apparatus actually attaches itself to the chromosomes, 1871

here are the duplicated chromosomes.1879

The spindle apparatus, what it really is, is microtubules.1881

Microtubules in interphase are just a part of the cytoskeleton, helping give the cell its shape, structure, scaffolding.1890

It also serves as little highways for moving organelles and parts along these protein rods in the cell.1900

A lot of these little protein rods or ropes, if you want to think of them that way,1908

they get reorganized to have a specific function here.1913

And that is attaching to the centromeres, the middle of the chromosomes there, 1916

to eventually pull them apart and to separate the little duplicates that have been formed.1922

The spindle apparatus forms in between the two centriole pairs, like anchors.1927

The rest of what you are seeing here is very important in cell division but not to focus.1933

These are mitochondria, here is your ER with ribosomes, and there is a lot more going on here.1937

As cell division progresses, the mitochondria and ER, all of that will be split as well.1944

But the process focuses mostly on the genetic information.1950

Metaphase is next, it is the second phase of mitosis after prophase.1957

The main occurrence of this phase is that the chromosomes, thanks to the spindle apparatus doing this,1962

they have lined up along the equator.1967

Here is the equator, if you remember that onion root tip cell that I showed you near the beginning,1969

the chromosomes look very similar to these and how they were lined up.1976

You should have lined up their along the equator, imagine your line and see that they are lined up.1980

Here you have the little centriole pairs, these parts right here known as asters.1986

Asters that means stars, looks like little star.1996

Those are just little bits of microtubules that are not actually attached to the centromeres.2003

The reason why they ended up in the equator region or which also known as the metaphase plate,2010

by plate, they mean it is just a lined up region in the middle of the cell.2016

The reason why it ended up there is like a tug of war.2021

The microtubules attached to either side, here you have the centromere.2025

I will do the microtubules in black.2034

There is like a tug of war where it is a tie.2040

Where they end up being just moved towards the center, and there is a slight pause, and eventually these microtubules,2044

each get a shortening of them that makes them snap in half, to the point where each sister chromatid to be separated.2051

That is coming up in anaphase.2059

Anaphase is the third phase of mitosis, the main occurrences here are that2063

the sister chromatids are separated by shortening microtubules.2066

Look what happened here, you have all of these microtubules attached to the centromeres, 2070

they gradually get shortened.2078

The way they get shortened, let me redraw what we just saw in metaphase.2079

Microtubule on each side of that centromere, at this end here, you have a shortening.2089

Tubuline units are chopped off, it is like crude to say but it is true.2102

Tubuline is what microtubules are made up of.2115

It is the main protein that is wound up to make this whole thing called the microtubule.2117

Research has been done to show that there is an enzyme, a little chop of an enzyme 2123

that hangs out here and here, and actually cuts off bits of tubuline.2130

It does stay attached to the centromeres still but that causes the shortening of these microtubules.2135

And that gradually pulls the chromosome duplicates, the sister chromatids to the opposite sides of the cells, or the poles.2143

As that is happening, you have what are called non kinetic core tubules.2152

That is more of an AP biology term.2159

The kinetic core describes this interaction, I will write it down for you.2161

Non kinetic core tubules meaning the microtubules that are not even attached to the chromosomes, you can see them here and here.2170

As the kinetic core tubule get shortened and the chromosomes are pulled apart, or chromatids are pulled apart, 2179

you get a lengthening of non kinetic core tubules.2185

That lengthening actually pushes on the cell a little bit.2188

This cell image compare to the previous ones is a little bit longer.2191

The reason why the lengthening of the cell is important, is imagine that the cell does not like that all and you just divide it.2196

You would see that the daughter cells would like half the width of what they should be.2203

There is a gradual lengthening in anaphase and through telophase, so that when you split them apart,2206

pretty close in size to the original parent cell.2215

Finally, the sister chromatids migrate towards the poles.2219

Late anaphase, you would see them hanging out over here.2223

Once you get the new nucleuses, the nuclei forming around them, you are no longer in anaphase.2226

You are in what is called telophase, coming up next.2234

Telophase, the fourth phase of mitosis.2238

In many ways, this is the opposite of prophase, it is the flipped version.2240

The things that we saw disappeared in prophase, they come back in this phase.2246

Chromatids gradually become decondensed.2251

What happened in prophase, the chromosomes got condensed, they got wrapped up into those little x that we are used to seeing.2253

This image is showing you that there is a little unraveling that starts to happen.2262

Eventually, it is going to look more like that spaghetti look, because they are done moving now.2266

Like we said earlier with the analogy of unpacking your boxes once you finish moving to new place, that your home.2273

Nuclear envelope reforms, look what do we have here, nuclear envelope coming back.2279

Those pieces that made up the nuclear envelope for the original parent cell,2285

they gradually reform around these poles of the cell that are formed where the new chromosomes exist.2289

They are not really new but they are separate chromosomes, they are copies.2298

Nucleoli which is plural for nucleolus, the nucleoli reappear.2302

Here they are, the little meatballs inside of those are going to be the bowl of spaghetti.2307

The spindle apparatus breaks down, you no longer see those lines that were here.2311

The microtubules are still there, they are still now the part of the cytoskeleton here.2317

They are no longer arranged in a way the pulling of chromosomes, because they do not have to do that anymore.2321

The chromosomes have gotten to their final destination.2328

The only thing you see happening here is what is called the formation of cleavage, 2331

that is technically a part is cytokinesis which comes right as telophase is ending.2335

Cytokinesis, this is the actual separation of cytoplasm in the two daughter cells.2342

Once you have actually moved your chromosomes, your genetic material, to the poles,2347

and form a new nucleus around each of those sets of chromosomes, it is time to split the cells up.2352

It occurs differently in animal cells and plant cells.2359

Here we have the look of cytokinesis in an animal cell, a cell that belongs to you and me.2362

Here is a look inside of the plant cell, a bit different, and here is what it comes down to.2368

In animal cells, a cleavage furrow, these arrows are pointing to the cleavage furrow.2372

The reason why they say furrow is, it actually tends to look like this when you zoom into a micrograph.2378

The actual look of the cell dividing, in an animal cell.2387

A furrow like, if I furrow my brow, I’m doing it right now, I’m furrowing my brow.2390

That is a furrow in my brow, you actually see a furrow here.2395

The cleavage just means the in depth thing there2399

A cleavage furrow is formed by micro filaments, parts of the cytoskeleton, pulling in on the plasma membrane in the equator region.2402

And that would take some ATP, it will take some energy.2410

The pulling in is like, if you had a rope attached to the ball, and you could somehow pull in on it and the wall would come in.2413

This is not a cell wall, it is a plasma membrane.2420

The pulling of those microfilaments that are attached to the inside of the plasma membrane does that.2423

It will continue until that separate.2429

And then, you are back into interphase and these two daughter cells.2434

In plant cells it is a little bit different.2437

It is not that easy to pull in on a cell wall made up of cellulose, that is very rigid, thick.2440

There is a plasma membrane but it is hard deal with the cell walls.2445

In plant cells, vesicles, little membranous sac containers, filled with cellulose, the main ingredient of cell walls, 2449

this is a polysaccharide or large sugar, they align along the equator.2459

This little jagged line is representing that but let me give you a better image.2464

Here are the vesicles, each of them is filled with cellulose.2470

They line up along the equator and then they fuse.2473

They gradually come together fused to have just a continuous connector.2476

You would see this.2485

And then, you finally have a cell wall here, a cell wall here, and they are officially separated.2498

The pinching is hard to do, instead a new cell wall is built by the fusing of those cellulose filled vesicles.2505

Once cytokinesis is done, these two daughter cells are back in G1 of interphase.2513

When we talk about mitosis, cancer is another topic that definitely relates to it.2522

Cancer relation to mitosis, all cells are supposed to have a particular pacing of cell division.2527

This is discussed earlier with the cell cycle.2533

Some tissues interphase is 95% of the time, some tissues it is 75% of the time, and that pacing,2536

that timing of when they divide is crucial to cells not overcrowding an area, or not having enough cells in that area.2543

If cell division gets out of control, cancer can occur in a multicellular organism.2551

Cancer is just cell division out of control.2557

The reason why it tends to be a problem is, the cancer itself, that tumor that is spreading through the tissue2561

is not only robbing the healthy cells of nutrients, it is hogging nutrients coming in that region, 2569

it is also physically crowding them out, physically damaging the healthy cells as it grows.2576

This is a reason why cancer, once it get to a vital organ, those cancer cells can cause death.2584

Particular genes control the pace, there are genes in every multicellular organisms and 2590

even in single celled organisms that control the pace of cell division.2595

How quickly certain chemicals or compounds accumulate to get you from G1 to S, to S to G2, and so on.2599

Those genes, if they are changed, if they are damaged, if they are mutated, you can have some problems.2608

The reason why those mutations occur is radiation.2614

Everyone is exposed to radiation, that is unavoidable.2618

Radiation comes from outer space, it comes from things around us.2622

Radiation, UV, ultraviolet radiation is a common source.2628

Everyone is exposed to that, whenever you go outside you are exposed to UV radiation.2632

It is a matter of how much radiation have you been exposed to.2636

At a certain point, the amount of radiation can be the point where it is damaging DNA in your skin cells, that can cause skin cancer.2640

Carcinogens are cancer causing agents, these are chemicals ending up in your body that tend to damage cells,2650

to the point where they can be changed into cancer cells.2657

We are not talking just nicotine here, there are probably 1000 carcinogens that have been identified more than that.2661

Carcinogens are cancer causing agents, avoid those if you can.2670

Enzyme errors, every tiny DNA is duplicated, DNA replication.2674

Every time that happens, mistakes can happen, mistakes can be made.2679

For every few million bases, DNA polymerase, the enzyme that actually copies DNA, it can make mistakes.2684

You actually have enzymes, DNA repair enzymes, that go and change those to the appropriate bases.2692

You are all good, those enzymes are not 100% accurate all the time, mistakes do happen.2699

Enzyme errors that are unchecked can definitely damage the particular genes that control the pace.2707

The terms benign versus malignant, benign means a tumor that is not actively growing.2713

Benign tumor oftentimes can be removed via surgery, and with radiation and chemotherapy, usually, the cancer will go away.2723

Malignant is a bit different, this is actively growing to point where it is a major concern.2734

Malignant tumors are more likely to spread to the parts of the body which is known as metastasis.2740

Research says that radiation can be used to destroy cancer which seems ironic, because radiation can cause cancer.2748

In western medicine, radiation and chemotherapy, if used in right way and targeted at specific regions that have cancer can be effective.2756

Radiation is targeted at specific cancer cells to destroy those cells, 2766

you will have some cells around there being damaged in the process but if it cures cancer, it is worth it.2772

Chemotherapy are chemicals introduced in the bloodstream that tend to target cells that are growing very fast, like cancer cells.2777

That is why people tend to lose their hair and have issues with even their skin during chemotherapy sessions.2787

Their skin can be extra sensitive, as usual to that.2796

If they are working, then it is working.2799

Metastasis, this is when cancer cells migrate to another part of the body.2803

This is what causes people problems.2810

When it starts in your skin and metastasizes and goes to the brain, to the liver, to the lungs, etc.,2812

then you are going to a problem where it can be a terminal thing where it can lead to death.2821

This happens via those cells getting dislodged and ending up in your lymph, in your bloodstream.2829

And those parts, if you a look at the anatomy and physiology lessons on,2835

those parts really connect all parts of the body.2840

Those are highways that would allow cancer cells to migrate to other parts of the body.2844

The earlier cancer cells are caught, the better.2848

The good news is that, natural killer cells which are found in the body naturally, are meant to destroy cancer cells.2851

Some people live their whole lives and not necessarily eating really healthy and exercising a lot, they do did not get cancer.2859

They live to be 90 to 100, and they do not get cancer that causes death.2868

It could be that genetically, they just have natural killer cells.2873

These cells are designed to go around the body and eat up cells that are not growing correctly,2878

that seem to be abnormally growing.2883

Their job is to destroy those, and get the problem to bud.2887

If that is happening in your body, you are not going to notice a malignant tumor growing.2890

Now we get to meiosis, meiosis is cell division, for the purpose of sexual reproduction.2899

Mitosis, when we just talked about, can be for reproduction but that is in single cell beings,2904

that want to copy their DNA, make identical copies in their daughter cells.2910

The purpose of sex production is to have two individuals contribute half of their genome, half of their DNA, to make a unique organism.2915

The other added interesting thing about meiosis is in the process of the male and female individual, 2925

whether we are talking about animals or plants producing their sex cells, is that the sex that they produce are all genetically unique.2932

What is produced, 4 cells with half the number of chromosomes.2942

Very different from mitosis, in mitosis, we had two daughter cells with the same number of chromosomes that we started out with.2946

If the number was 4 in the parent cell, each of the daughter cells have 4.2953

In this case, if the parent cell has 4 chromosomes, the daughter cells each have 2.2959

You have doubled the number of daughter cells.2968

Instead of producing 2 cells, you will get 4.2970

Two divisions occur to make that happen, that is how you get to that number,2974

where it is twice the number of daughter cells, half the number of chromosomes.2978

One diploid cell forms 4 haploid cells, what does these terms mean.2982

Let us talk about it, diploid.2986

Diploid also known as 2N and haploid also known as N.2994

N is any variable, any positive integer, any number above 0 could fit in here.3009

For humans, N equals 23.3017

Our haploid number for our species is 23.3023

23 chromosomes without their pair, their corresponding pair, because when they get all paired up, that is when you get 2N.3028

Put a 2 in front of 23, 2 times 23 is 46, that is the diploid number for humans.3037

Here is where we look at it, the 46 chromosomes you have in your average bodily cell, not sperm or egg.3046

Any other cells, brain cells, muscle cells, liver cells, skin cells, kidney cells, etc.,3052

you have 23 chromosomes from your dad and 23 chromosomes from your mom.3058

And they all go together, they are known as homologous.3064

That word will come up again later on in the lesson.3067

Chromosome 1 from your dad goes with chromosome 1 from your mom, all the way down to chromosome 23 from your dad 3073

which is going to be X or Y chromosome, sex chromosome, chromosome 23 from your mom an X chromosome.3079

That really makes that diploid number.3086

The purpose of meiosis is making the haploid number, making sperm or egg with half the number of chromosomes.3089

They will pair up though.3098

Once sperm and egg come together, 23 + 23 hits you back to 46, this is how you make a new person.3100

Once that person starts making sex sells, they will make haploids is again.3107

That process continues through out the eons.3112

With meiosis, genetically unique daughter cells.3117

The amazing thing is you have this, what is called crossing over, I’m going to tell you about momentarily, 3120

where you take your dad and your mom's genetic information in your body, when you make sperm or egg, and you shuffle that stuff.3125

You have millions and millions of possibilities with sperm and egg, it is pretty incredible.3133

This genetic recombination happens, thanks to one event in prophase 1 known as crossing over.3140

Genetic recombination is responsible for a lot of genetic variation in humans.3147

If it was not there, we would have not quite as many unique looks with human beings.3152

Since, meiosis has two phases, there is meiosis 1 and meiosis 2, to get you to that 4 daughter cell with half the number of chromosomes.3161

Meiosis 1, this is meiosis I or numeral 1.3170

Meiosis 1 contains the same phases as mitosis but each phase has a 1 next to it.3175

We do that PMAT, prophase, metaphase, anaphase, telophase, but 1 will be next to all of them, then, it will be prophase 2, etc.3181

Meiosis 2 comes after it, we got to keep this straight.3191

Prophase 1 happens right after interphase.3193

The reason why I'm not talking about interphase in detail here is, interphase for meiosis is exactly the same as interphase for mitosis.3196

They both have G1, S, and G2, they both duplicate all the DNA, it is exactly the same.3205

It is just that the cells in the human body that are doing meiosis, they are making sperm and egg.3211

They are located in a particular region of the body, all the other cells, their interphase leads to mitosis.3216

Prophase 1, the first phase of meiosis1, here is where that genetic combination occurs.3222

Crossing over occurs between homologous chromosomes.3227

Homologous, that means same form, mean they are the same type of chromosome.3233

Chromosome 1 from your dad, chromosome 1 from your mom.3244

The genes on chromosome 1 from your dad are the same kinds of genes, 3247

meaning they have the same purpose in the body as the ones from your mom, but different forms of those genes,3253

because your mom and your dad are different people.3259

But if there is a gene on chromosome 1 that has to do with metabolism, 3262

your dad's metabolic activity in his body is a little bit different in your mom's metabolic activity.3267

The way that does gene parts interact has to do with genetics, but that is in a different lesson.3273

Homologous chromosomes mean they go together.3279

The reason why they show different colors is, it is the patrilineal-matrilineal, paternal-maternal side.3282

Let us pretend that red equals male and green equals female.3289

Male symbol, female symbol, these are in one person's body.3299

We are not talking about sperm and egg combining here that had to happen to make this person.3303

But this person is taking the chromosomes that they got from their parents.3309

In prophase 1, after the chromosomes have been duplicated, that is why you have sister chromatids.3314

They lineup and they have this opportunity to switch over parts.3320

Scientists, with all of our knowledge, would not have been able to figure out how to predict where the crossing over points take place.3326

The crossing over points, one of them is called, if you cannot see that, it is a chiasma and plural would be chiasmata.3334

The reason why I say plural is, with homologous chromosomes getting together during prophase 1,3353

they can have two overlapping points, 3, 4, 0, 1.3359

It is unpredictable how they decide to switch over the arms of their chromatids.3365

That is amazing because the amount of the chiasmata corresponds to the amount of crossing over and the amount of switching3372

in genetically combination between your paternal and maternal chromosomes.3379

This gives you more and more variety in the resulting sperm or egg cells.3383

It is pretty amazing that this is happening.3388

You get so many different combinations, there is approximately 25,000 genes in the human genome.3390

If you think about all the combinations of those gene parts being switched around, 3398

you get millions of combinations of a potential unique sperm or egg.3403

This happens and you get this result.3410

Once these little parts switch, once they can let go of each other, look what happen, we switched the parts.3412

For this the to be truly accurate, what I wish was depicted here is that, if this little red piece was here and this blue piece was here,3422

it looks like the chromosomes switched places after they move.3432

The purpose here of what I’m trying to say is that, the chromatids that are next to each other when they line up, 3436

those are the ones that can switch over.3443

Here and here, those 2, they are not adjacent, they are not touching.3445

It is going to go and switch there, the resulting chromosomes look like this.3452

It is like your dad's chromosome has a little piece from your mom, and vice versa.3456

That gives you more of variety.3461

Metaphase 1, you would see the homologous chromosomes that are now switched to pieces, line up.3464

It is going to take a long time if I draw all the greens and reds.3472

Just to keep it easier, more fluid for our lesson, metaphase 1, if we pretend that the diploid number is 4,3474

we would see this lining up along the equator.3489

You can see that instead of lining up single file like they did in mitosis, in mitosis we saw something like this.3494

Why these are not up there and those are not there?3504

It does not matter, the equator can be here, it can be there, it can be here.3506

Depending how you tilt your head and look at the drawing, it is a three dimensional cells.3511

As long as they line up along the center, it is fine.3515

But in mitosis, metaphase is also single file, when we are splitting those sister chromatids from the get go.3518

Here, we need to separate the homologous chromosomes because you only want one of the pair in the resulting sperm or egg,3525

because the corresponding cell is going to fertilize it, hopefully.3533

The homologous will join back together, once fertilization happens.3538

But you want to them side by side, not single file, for metaphase 1.3542

The homologous chromosomes align.3547

Anaphase 1, these get splits up.3550

Anaphase 1 would look like this.3553

We did not see this in anaphase of mitosis.3564

See how they are not just that single chromatid, you still have both chromatids intact.3570

They would look like they have different colors on them.3576

Do they really look like that in a living person cell?3579

No, if color was used to discover this, you can cause florescence, you can tag DNA 1 chromosome vs. another, and see it happen.3583

But really, the different colors are not going to be visible on a living persons cell, when it is actually happening in your body.3594

But nonetheless, you do get the sister chromatids still attached here.3601

Homologous chromosomes have been separated in anaphase 1.3607

Different from anaphase and mitosis, we saw the sister chromatids being separated.3611

Finally, telophase 1, you get to the point where we see this.3616

I have read that in some organisms, the nuclear envelope will temporarily reform 3628

around these daughter cells before going into prophase 2.3638

In some organisms, it does not.3643

It seems pointless to reform a nuclear envelope just to break it down again to do prophase 2, but happens.3647

There is what you would see in telophase 1.3654

Following that is not another interphase, we are not going to reduplicate chromosomes.3657

You go straight into meiosis 2.3661

Meiosis 2, the second division of meiosis.3665

Prophase 2, crossing over does not occur.3668

If it happened here, it will be disastrous because you have already separated the chromosomes 3672

that go together, the homologous chromosomes.3677

They already switched parts that happened in prophase 1.3680

Prophase 2, you get a reconnecting of the spindle apparatus and you want to get them to metaphase 2.3684

Metaphase 2, now that we have two cells that are doing this, it is going to look like that.3693

You are going to have half the number of chromosomes and it is going to be single filed along the equator,3703

if I did a little diagonal line here.3709

I know I’m not drawing the spindle apparatus, it is okay, you know it is there.3714

Just wanted to show you how the chromosomes are aligned in metaphase 2.3719

Chromosomes line up single file, not with their homologous anymore.3723

Anaphase 2 resembles anaphase of mitosis, except you would see half the number of chromosomes.3728

This is what you would see sister chromatids being separated.3740

I will draw your little microtubules here.3745

Centrioles will be there.3752

Sister chromatids separate much like anaphase of mitosis.3757

That brings us to the point where we can start to see 4 daughter cells or gametes, sperm or egg coming together.3761

The chromatids by themselves just look like little lines but there they are.3780

If you remember, we said in the beginning that the diploid number for whatever organism this is, is 4.3787

You have duplicated those 4 chromosomes, in preparation for getting 4 daughter cells with half the number of chromosomes.3800

After cytokinesis finishes, 1, 2, 3, 4 sperm or egg with the haploid number which is 2 chromosomes.3807

A meiosis overview shows us a summary of what we just saw.3821

Here we are saying, the diploid number is actually 2.3826

Based on this drawing, 2N = 2, meaning N = 1, the haploid number is 1.3831

This is the 1 paternal chromosome, this is the 1 maternal chromosome.3842

But it duplicated, DNA replication that is during interphase.3848

This is thanks to the S part, synthesis.3851

Then you have this, that brings them to meiosis 1.3858

That meiosis I is meiosis 1.3867

Here we are going to have the homologous chromosome lined up, they are going to be separated.3869

This is the product of meiosis 1.3877

Then, they get lined up, they are going to be separated again in meiosis 2.3881

That is along way but you will get what I’m showing here, meiosis 2.3892

Look what we have, one chromosome in each daughter cell N = 1, that is the haploid.3897

They can combine to make another one of these.3907

The one critique I have about this image is, what are we missing?3909

We are missing the result of crossing over.3914

During the beginning of meiosis 1, you should see some overlap, these chiasmata coming together.3917

The chromatids, you are making those crossing over points and exchanging pieces of their chromatids.3926

Do all homologous pairs overlap and do those crossing over? No.3933

If you did see this in an organism like ourselves, with those 23 pairs, it is going to happen.3939

There is a lot of crossing over points.3946

If I were to make this illustration, I will have some parts of this red chromosome with the little blue pieces.3949

Some parts of this blue chromosome with the little red pieces.3955

Nonetheless, it is a good overview.3958

The products of meiosis, what do we get when we look at particularly animals, by the end, there are 4 haploid daughter cells.3963

We went over that, also known as gametes.3972

Gametes is sperm and egg.3974

In animals, known as sperm or egg.3977

In plants, pollen and egg.3980

When we look at animals, particularly, there is a very interesting process where we compare spermatogenesis, 3984

the formation of sperm ,and oogenesis the formation of an oocyte or an ovum, an egg cell.3992

Here is a good image of what happens with the female process in making gametes.4001

Here is the primary oocyte, these little dots inside of the nucleus represent the chromosomes.4006

They get duplicated and they get split.4016

When you have meiosis 1 and 2 condensing, the other cells you got on the other side of the division, they call the polar bodies.4020

By the end, we should have 4 cells, we already mentioned that.4029

That is a part of the meiosis, 4 daughter cells.4032

3 of them are known as polar bodies, they are useless, in terms of fertilization.4035

The reason why an egg, this is an exaggerated as it could be.4041

There is a siphoning or shunting, in a sense, of cytoplasm and cellular contents to this one.4045

Here is your mature ovum, this is the one that you want fertilize by a sperm.4053

These 3, they hang off to the side and eventually disintegrate.4058

If this gets fertilized, it is not going to work out.4063

Your mitochondria, your extensive ER, a whole bunch of ribosomes, cytoplasm, that volume of the cell ends up in here.4066

In females, those are shunting of all that cytoplasm because when you compare the size of the sperm to the size of an egg, 4076

it is something like 1 to 1000 ratio of volumes.4084

The sperm is close to about half the diameter of an ovum, of that circle.4088

Since it is a sphere filled with voluminous contents, there is a lot more going on volume wise, in an ovum than a sperm.4096

This is meant to make one big egg, and the 3, it is alright, they just end to the sides and disintegrate.4105

They still have the same genetic information but this is the one that is very important.4115

Sperm, it will be the same process but you would not see this polar bodies that you would see for sperm of equal size.4121

They all have the same small amount of cytoplasm.4133

The head of the sperm is mostly a nucleus, there is very little cytoplasm.4138

The front of it is called an aquasome or little enzyme sac.4143

You can hear more about that if you get to the anatomy physiology lesson on

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