Mitosis: Splitting Up is Complicated - Crash Course Biology #12

Summary

This video script explores the fascinating process of mitosis, the cell division responsible for growth, healing, and development in multicellular organisms. It delves into the stages of mitosis, from interphase to cytokinesis, and highlights the importance of DNA replication and chromosome alignment. The script also touches on the mysteries of the underlying mechanisms and the ongoing scientific quest to understand them.

Takeaways

• 🧬 Cells can clone themselves through a process called mitosis, which is crucial for growth, healing, and maintaining life.
• 🌱 Mitosis allows a single cell with 46 chromosomes to divide into two genetically identical cells, each with 46 chromosomes.
• 🔬 The process of mitosis involves several stages: prophase, metaphase, anaphase, and telophase, each with distinct cellular activities.
• 🌀 During interphase, cells prepare for mitosis by duplicating their DNA and centrosomes, and the DNA is in a loosely coiled state called chromatin.
• 🧬 In prophase, chromatin condenses into visible chromosomes, and the nuclear envelope breaks down while centrosomes move to opposite ends of the cell.
• 🧵 Metaphase is characterized by chromosomes aligning at the cell's equator, connected by microtubules regulated by motor proteins.
• 🔄 Anaphase involves the separation of sister chromatids into individual chromosomes, which are pulled towards opposite ends of the cell.
• 🌐 Telophase is the final stage of mitosis where the nuclear membrane reforms, chromosomes decondense into chromatin, and the cell prepares to divide.
• 💥 Cytokinesis follows mitosis, where the cell physically splits into two daughter cells, each with a complete set of chromosomes.
• 🧬 Mitosis is a fundamental biological process that occurs approximately 10 quadrillion times in a human's lifetime, highlighting its importance in cellular reproduction and organism development.

Q & A

What is mitosis and why is it important for multicellular organisms?

Mitosis is a type of cell division that allows one cell with 46 chromosomes to split into two genetically identical cells, each with 46 chromosomes. It is crucial for multicellular organisms as it enables growth, repair, and maintenance of the body by producing new cells.

How does mitosis contribute to the healing of a cut?

When you get a cut, your body needs to generate new cells to replace the damaged ones. This process is facilitated by mitosis, which helps in the rapid production of new cells to heal the wound.

What is the role of the centrosome in mitosis?

The centrosome plays a critical role in mitosis by duplicating itself and organizing the cell's microtubules. It helps in the separation of chromosomes by forming spindle fibers that pull the chromosomes apart during cell division.

What happens during the prophase of mitosis?

During prophase, the chromosomes condense and coil up to form visible structures. The nuclear envelope breaks down, and the centrosomes move to opposite ends of the cell, forming spindle fibers that will later help in separating the chromosomes.

What is the significance of the metaphase in mitosis?

Metaphase is the stage where chromosomes align at the cell's equator, connected by spindle fibers. This alignment ensures that each new cell will receive an equal number of chromosomes during cell division.

How do chromosomes align during metaphase?

Chromosomes align during metaphase through the action of motor proteins like dynein, which pull on the spindle fibers. Dynein essentially plays a tug-of-war with itself, ensuring that chromosomes are evenly distributed to the two new cells.

What is the process called when a cell divides into two identical cells?

The process by which a cell divides into two identical cells is called mitosis. It involves several stages including prophase, metaphase, anaphase, and telophase, culminating in the formation of two genetically identical daughter cells.

What is the difference between diploid and haploid cells?

Diploid cells contain two sets of chromosomes (46 chromosomes in humans), one set from each parent. Haploid cells, on the other hand, contain only one set of chromosomes (23 chromosomes in humans) and are typically found in sex cells or gametes.

What is the role of DNA replication during interphase?

During interphase, DNA replication occurs, creating two copies of every strand of DNA. This ensures that each new cell formed during mitosis will have a complete set of genetic information.

What is cytokinesis and how does it differ from mitosis?

Cytokinesis is the final stage of cell division where the cytoplasm of the parent cell is divided into two new cells. It follows mitosis, which is the process of nuclear division where the chromosomes are separated into two new nuclei.

Why is there still much to learn about mitosis despite its well-documented stages?

While the stages of mitosis are well-documented, the underlying mechanisms that drive these stages are not fully understood. This includes the precise actions of motor proteins and the regulation of microtubule dynamics, indicating that there is still much to discover in the field of cell biology.

Outlines

00:00

🌱 Mitosis: The Life-Sustaining Cell Division

The first paragraph introduces the concept of mitosis, the process by which cells reproduce themselves. It explains that mitosis is essential for growth, healing, and repair in multicellular organisms. The paragraph details the stages of mitosis: prophase, metaphase, anaphase, and telophase, and emphasizes the importance of the cell nucleus and DNA replication during interphase. It also touches on the scientific mysteries surrounding the exact mechanisms of mitosis, highlighting the ongoing nature of biological research.

05:01

🧬 The Mechanics of Chromosome Alignment in Mitosis

This paragraph delves into the specifics of chromosome alignment during metaphase of mitosis. It describes the role of centrosomes and microtubules in structuring the cell and the function of motor proteins like dynein in aligning chromosomes. The historical context of mitosis discovery by Walther Flemming is provided, along with recent insights from Tomomi Kiyomitsu's research on the tug-of-war mechanism involving dynein for proper chromosome alignment. The paragraph concludes with the anaphase stage, where chromosomes are pulled apart, and the subsequent telophase, where new cell structures are rebuilt.

10:01

👶 From Mitosis to Meiosis: Understanding Genetic Inheritance

The final paragraph shifts focus from mitosis to the formation of eggs and sperm through meiosis, which will be discussed in a future episode. It clarifies the difference between being a clone of one's sibling and inheriting half of one's DNA from each parent. The paragraph invites viewers to revisit the video for better understanding or to use the table of contents for quick reference. It also encourages viewer interaction through social media and comments, and hints at the continuation of the topic in the next episode of Crash Course.

Keywords

💡Clone

A clone refers to an organism or cell that is genetically identical to the original organism or cell from which it was derived. In the context of the video, the idea of having a clone to perform tasks like cleaning or attending events is introduced, but it is clarified that humans cannot clone themselves. This concept is used to transition into discussing cell division and cloning at the cellular level.

💡Cell Division

Cell division is the process by which a parent cell divides into two or more daughter cells. It is fundamental to the growth, development, and repair of organisms. The video emphasizes that unlike human cloning, cell division is a natural and essential biological process, highlighting mitosis as a key type of cell division.

💡Mitosis

Mitosis is a type of cell division that results in two daughter cells, each having the same number and kind of chromosomes as the parent nucleus. It is crucial for growth and repair in multicellular organisms. The video script explains that mitosis is responsible for healing wounds, replacing damaged liver cells, and growing from a baby to a child.

💡Chromosomes

Chromosomes are thread-like structures composed of DNA and proteins that carry genetic information. They are visible during cell division. The script describes how chromosomes, initially in a messy form called chromatin, condense during mitosis into distinct structures that can be divided equally between daughter cells.

💡Diploid Cells

Diploid cells are cells that have two sets of chromosomes, one from each parent, totaling 46 in humans. The video explains that most body cells are diploid, containing 46 chromosomes organized into 23 pairs, which is essential for maintaining genetic diversity and stability.

💡Haploid Cells

Haploid cells contain half the number of chromosomes of a diploid cell, typically 23 in humans. These cells are involved in sexual reproduction. The video mentions haploid cells in the context of sex cells, which are produced through a different process called meiosis, to be discussed in a future episode.

💡Interphase

Interphase is the phase of the cell cycle during which the cell is not dividing but is preparing for division. It is characterized by growth and DNA replication. The script describes interphase as a 'limbo period' where cells are in between episodes of mitosis, preparing for the upcoming cell division.

💡Prophase

Prophase is the first stage of mitosis during which the chromosomes condense and become visible, and the nuclear envelope breaks down. The video script uses the analogy of 'dreadlocks' to describe the appearance of chromosomes during prophase, setting the stage for their alignment in the subsequent stages.

💡Metaphase

Metaphase is the stage of mitosis where chromosomes align at the cell's equator, connected by spindle fibers. The video explains that this is the longest phase of mitosis and involves the chromosomes attaching to microtubules and aligning in the middle of the cell, preparing for separation.

💡Anaphase

Anaphase is the stage of mitosis following metaphase, during which the sister chromatids separate and move to opposite poles of the cell. The video script uses the prefix 'ana' to help remember that the chromosomes are moving 'back' to opposite ends of the cell, facilitating the creation of two genetically identical daughter cells.

💡Telophase

Telophase is the final stage of mitosis where the nuclear membrane reforms around the separated chromosomes, and the cell prepares to divide into two. The video script describes telophase as the phase where new cell structures are reconstructed, setting the stage for the physical separation of the two new cells.

💡Cytokinesis

Cytokinesis is the process by which the cytoplasm of a parent cell is divided into two daughter cells following mitosis. The video script explains that cytokinesis literally means 'cell movement' and is the final step in cell division, resulting in two separate cells.

Highlights

Cells can clone themselves, a process fundamental to life.

Mitosis is the cell division process responsible for growth, healing, and maintaining life.

Mitosis occurs about 10 quadrillion times in a human lifetime.

Cells are made of DNA organized into chromosomes, with 46 chromosomes in human somatic cells.

Each human cell's DNA is a combination of maternal and paternal chromosomes.

Mitosis involves the duplication of DNA and the creation of two genetically identical cells.

The nucleus controls cell activities and contains all necessary instructions for cell survival.

Mitosis is divided into stages: prophase, metaphase, anaphase, and telophase.

Interphase is the preparatory phase before mitosis, where DNA replicates and centrosomes duplicate.

During prophase, chromatin condenses into chromosomes, and the nuclear envelope disintegrates.

Metaphase is the longest phase of mitosis, where chromosomes align at the cell's center.

Anaphase involves the separation of sister chromatids into individual chromosomes.

Telophase is the final stage of mitosis, where the cell's structures are reconstructed.

Cytokinesis completes cell division, separating the two new nuclei and creating two daughter cells.

The daughter cells are genetically identical to each other and the original cell.

Understanding mitosis is crucial for future biologists, as many underlying mechanisms are still being discovered.

Meiosis, the process of sex cell formation, will be discussed in the next episode.

Transcripts

Hey, check this out.

Cool, huh?

I bet you wish you could do this have a clone clean up around the

apartment for you go to class, maybe take your Mom

to dinner on her birthday?

Well, you can't do that. And actually there are some really

good reasons why you can't do that.

We're going to talk about those in the next episode.

But, do you know what CAN clone themselves?

Your cells.

Like, almost every single one of them. And in fact

they're doing it right now!

For any creature bigger than a single-celled organism,

all of life stems from cells' ability to reproduce themselves,

because that's what allows organisms

to develop, grow, heal and keep from dying, for as long as possible.

This particular kind of cell division is called mitosis,

and it's responsible for a whole lot of your body's key functions.

If you get a cut, your body needs to make new cells. Mitosis.

Have too much to drink and damage your liver?

You gotta replace those cells. Mitosis.

Tumor growing in your spine? Unfortunately, again mitosis.

While you go from a seven pound baby to a seventy pound child it's not

your cells that are increasing in mass;

you're just getting more of them.

Over and over and over again.

That's mitosis.

This process is so central to your life that it will take place

in your body, over your lifetime, about 10 quadrillion times.

That's 10 thousand billion times!

Like all split ups, it's not easy. It's going to maybe be

a little bit messy, there's a lot of drama,

and it can take a surprisingly long amount of time.

But trust me, after we're done with it we'll all be better off.

So you are made of trillions of cells just like giraffes

and redwood trees.

And remember that inside each cell there's a nucleus that

stores your DNA, which contains all of the instructions

on how to build you.

That DNA is organized into chromosomes

and as we've mentioned before,

in your body cells, or somatic cells, you have 46 chromosomes

grouped into 23 pairs,

one in each pair is from your mom, and the other one's from your dad.

Cells with all 46 chromosomes are called diploid cells,

because they have 2 sets each. And that's what we're

focusing on today.

You also have haploid cells that have half as many chromosomes (23).

And those are your sex cells. They're produced in an equally

fantastic process called meiosis, which we'll be talking about

in the next episode.

But for now, the main thing to remember about mitosis is that

it allows one cell with 46 chromosomes to split into

two cells that are genetically identical,

each with 46 chromosomes. All in order to keep the

party of life going.

Now, the nucleus in your cell controls everything that

goes on in the cell.

It has all of the instructions necessary for making

the cell survive so you don't need to duplicate the whole cell.

All you need to do is duplicate the DNA, get it wrapped up,

and then if you have two separate pockets of DNA, that's all you need

to have two new cells.

Mitosis takes place in a series of discrete stages called

prophase, metaphase, anaphase, and telophase.

And you can just say that over and over again,

and let it sink into your head.

And part of what's really amazing about this whole process is that,

while we know what these stages are, we don't always know the underlying

mechanisms that make all of them happen.

And this is part of science. Science isn't all the stuff we know,

it's how we're trying to figure all this stuff out.

Consider it job security if you want to be a biologist;

there is a lot of stuff that future biologists have to still figure out,

and this is one of them.

Alright, let's get our clone on.

So, most of their lives, cells hang out in this

limbo period called interphase, which means they're in between

episodes of mitosis, mostly growing and working and doing all the stuff

that makes them useful to us. During interphase, the long strings

of DNA are loosely coiled and messy, like that dust bunny of dog fur

and laundry lint under your bed.

That mess of DNA is called chromatin.

But as the mitosis process begins to gear up,

lots of things start happening in the cell

to get ready for the big division. One of the more important things

that happens is that this little

set of protein cylinders next to the nucleus, called the centrosome, duplicates itself.

duplicates itself.

We're going to have to move a lot of stuff around in the nucleus

and that's going to be regulated by these centrosomes.

The other thing that happens is all of the DNA begins

to replicate itself too,

giving the cell two copies of every strand of DNA.

To brush up on how DNA replicates itself like this,

check out this episode and then come on back.

Now the cell enters the first phase, or the prophase, when that mess of

chromatin condenses and coils up on itself

to produce thick strands of DNA

wrapped around proteins - those my friends, are your chromosomes.

Instead of dust bunnies, the DNA is starting to look

a little bit more like dreadlocks.

And the duplicates that have been made don't just float around freely;

they stay attached to the original, and together they look like little X's;

these are called the chromatids and one copy is the left leg

and arm of the X, and the other copy is the right leg and arm.

Where they meet in the middle is the centromere.

Just so you know, these X's are also called chromosomes

sometimes double chromosomes, or double-stranded chromosomes.

And when the chromatids separate, they're considered individual

chromosomes too.

Now, while the chromosomes are forming, the nuclear envelope

gets out of the way by completely disintegrating.

And the centrosomes then peel away from the nucleus, and start heading

to opposite ends of the cell. As they go, they leave behind

a wide trail of protein ropes called microtubules running from

one centrosome to the other.

You might recall from our anatomy of the animal cell

that microtubules help provide a kind of structure to the cell;

and this is exactly what they're doing here.

Now we reach the metaphase, which literally means "after phase"

and it's the longest phase of mitosis.;

It can take up to 20 minutes.

During the metaphase, the chromosomes attach

to those ropey microtubules right in the middle,

at their centromeres.

The chromosomes then begin to be moved around, and this seems to be

being done by molecules called motor proteins.

And while we don't know too much about how these motors work,

we do know, for instance, that there are two of them

on each side of the centromere.

These are called Centromere-associated protein E.

So, these motors proteins attach to the microtubule ropes and

basically serve to spool up the tubules' slack. At the same time,

another protein, dynein, is pulling up the slack from the other ends

of the ropes near the cell membranes. After being pulled in this

direction and that, the chromosomes line up, right down

the middle of the cell.

And that brings us to the latest installment of Bio-lography.

So how do those chromosomes line up like that? We know that

there are motor proteins involved but like, how?

What are they doing? Well, remember when I said earlier

that there are a lot of things that we don't totally understand

about mitosis? It's sort of weird that we don't, because we can

literally watch mitosis happening under microscopes, but chromosome

alignment is a good example of a small detail that has only

very recently been figured out, and it was a revelation

about 130 years in the making.

Mitosis was first observed by a German biologist by the name of

Walther Flemming, who in 1878 was studying the tissue of

salamander gills and fins when he saw cells' nuclei split in two

and migrate away from each other to form two new cells.

He called this process mitosis, after the Greek word for thread,

because of the messy jumble of chromatin, a term he also coined,

that he saw in the nuclei.

But Flemming didn't pick up on the implications of this discovery

for genetics, which was still a young discipline. And over the

next century, generations of scientists started piecing

together the mitosis puzzle, by determining the role of

microtubules, say, or identifying motor proteins.

Now, the most recent contribution to this research was made by a

postdoctoral student named Tomomi Kiyomitsu at MIT.

He watched the same process that Flemming watched, and figured out

how at least one of the motor proteins helps snap the

chromosomes into line.

He was studying a motor protein called dynein, which sits

on the inside of the membrane.

Think of the microtubles as tug-of-war ropes, with the

chromosomes as the flag in the middle.

What Kiyomitsu discovered was that dynein plays tug of war with itself.

Dynein grabs onto one end of the microtubules and pulls the tubules

and chromosomes toward one end of the cell.

When the ends of microtubules come too close to the

cell membrane, they release a chemical signal that punts

the dynein to the other side of the cell. There, it grabs

onto the other end of the microtubles and starts pulling,

until SMACK it gets punted back again.

All of this ensures that the chromosomes will line up

exactly in the middle, so that they will be split evenly.

That discovery was published in February 2012, a couple of weeks

before I sat in this chair, and 134 years after mitosis was

first observed.

If you want to join the ranks of scientists who are answering

the many questions left about mitosis, and lots of

other things about our lives maybe someday I'll do a

Bio-lography about you.

Now so far we've gone through the interphase, when the

centrosomes and DNA replicate themselves and get ready for

the split;

the prophase, when the chromosomes form and the centrosomes

start to spread apart;

and metaphase, where the chromosomes align in the

middle of the cell.

And now it's time to separate the chromosomes from their copies.

This time, motor proteins start pulling so hard on the ropes

that the X-shaped chromosomes split back into their individual,

single chromosomes. Once they're detached from each other, they're

dragged toward either end of the cell.

The prefix 'ana' means 'back' that may help you remember

the name of this phase, called anaphase.

After this, it's just a matter of using all of that genetic

material to rebuild, so that the copied genetic material has all

the accouterments of home.

In the last phase, telophase, each of the new cell's structures

are reconstructed.

First, the nuclear membrane re-forms, and nucleoli form within them.

And the chromosomes relax back into chromatin.

Then a little crease forms between the two new cells,

which marks the beginning of the final split. That division

between the two new cells is called cleavage.

All that's left is to make a clean break.

This is done by cytokinesis literally "cell movement"

by which the two new nuclei move apart from each other,

and the cells separate.

We now have two new cells, each with the full set of

46 chromosomes.

These clones are called the daughter cells of

the original cell, and like identical twins they are

genetic copies of each other and also of their parent.

But, that's obviously not the case for you.

Even if you are an identical twin.

Shout-out to identical twins!

See me in the comments.

while you kind of are a clone of your sibling you are not

a clone of your parents.

Instead, half of your DNA in each of your cells is from your mom,

and half is from your dad.

To understand why that is, we have to understand how eggs

and sperm are formed. And that is meiosis, and that's what we're

going to be talking about next week on Crash Course.

Until then, you can just watch this video over and over again

or you can just watch the bits that you want to re-watch

using our table of contents, which is also available

in the description for people who are using iPhones

and can't click annotations

If you have any questions, you can reach us on Facebook

or on Twiiter or of course,

in the comments below.