Mitosis and the Cell Cycle: Crash Course Biology #29

Summary

In this engaging Crash Course Biology episode, Dr. Sammy explores the cell cycle, the process by which cells grow, divide, and replace themselves. From the bustling activity of interphase to the meticulous steps of mitosis and cytokinesis, the video explains how cells function as the supporting cast in our body's story. It also delves into the importance of cell cycle regulation, highlighting the potential dangers of uncontrolled cell division, such as cancer. The episode is a lively journey through the microscopic world within us, emphasizing the intricate balance that keeps us healthy.

Takeaways

• 🌟 The script personifies the body's cells as co-stars in the 'Sammy Show,' emphasizing the importance of cells in making up who we are.
• 🔬 Cells follow a pattern called the cell cycle, which includes growth, development, and division, similar to the life cycle of an organism.
• 🏠 The cell cycle consists of interphase and the mitotic (M) phase, with interphase accounting for 90% of a cell's life where it performs its functions.
• 📈 Interphase is divided into G1 (growth), S (DNA synthesis), and G2 (preparation for division) phases, each with specific activities and growth.
• 🧬 During the S phase, the cell duplicates its genetic material, ensuring that each new cell will receive a complete set of DNA upon division.
• 🤖 The M phase includes mitosis and cytokinesis, where the cell's nucleus splits and the cell divides into two, each with a full set of genetic information.
• 🧬🔄 The mnemonic 'Pass Me A Taco, Chef' is introduced to remember the stages of mitosis: prophase, metaphase, anaphase, telophase, and cytokinesis.
• 🛑 The cell cycle is regulated by a control system of proteins and checkpoints that ensure the process occurs correctly and is responsive to the cell's environment.
• 🚨 Failure in the cell cycle control system can lead to uncontrolled cell division, which is a characteristic of cancer.
• 🌱 All life, from the simplest to the most complex, undergoes cellular cycles, highlighting the universality of biological processes.
• 🤔 The script ends with a philosophical question about identity, 'Am I me, or am I my cells?', suggesting a deeper inquiry into the nature of self.

Q & A

What is the main character's perspective on their role in their own life story?

The main character considers themselves as the central figure in their own life story, making all their own decisions and acknowledging their cells as co-stars in their story.

What is the cell cycle and why is it important?

The cell cycle is a pattern that cells follow as they grow, develop, reproduce, and eventually die. It is important because it allows for the maintenance, growth, and repair of organisms by ensuring cells divide and replace themselves.

How does cellular division differ between single-celled and multicellular organisms?

In single-celled organisms like an amoeba, cellular division results in a new organism. In multicellular organisms, cells divide to replace old or damaged ones and to allow the body to grow.

What is the role of the Golgi apparatus in a cell?

The Golgi apparatus, humorously referred to as 'The Golgi Girls' in the script, is involved in modifying, sorting, and packaging proteins and lipids for secretion or use within the cell.

What is interphase and why does it matter in the cell cycle?

Interphase is the period of the cell cycle when the cell is not dividing and is instead performing its regular functions. It matters because cells spend about 90% of their time in interphase, growing, preparing for division, and carrying out their specific roles within the organism.

What are the three main steps of interphase?

The three main steps of interphase are G1 phase (first gap), where the cell grows and makes new organelles; S phase (synthesis phase), where the cell duplicates its DNA; and G2 phase (second gap), where the cell prepares for division by making more organelles and molecules.

What is the purpose of the mitotic spindle during cell division?

The mitotic spindle, formed between the two centrosomes, is crucial for cell division as it helps in the separation of chromosomes by attaching to their centromeres and pulling them to opposite ends of the cell.

What is the mnemonic 'Pass Me A Taco, Chef' used for in the context of mitosis?

The mnemonic 'Pass Me A Taco, Chef' is used to remember the stages of mitosis in order: prophase, metaphase, anaphase, telophase, and cytokinesis.

What is the difference between mitosis and cytokinesis?

Mitosis is the process where the nucleus of a cell divides, ensuring each new cell gets a complete set of genetic material. Cytokinesis is the actual division of the cell's cytoplasm and cell membrane to form two separate cells.

What is the role of the cell cycle control system in cell division?

The cell cycle control system regulates cell division by using a set of special proteins and checkpoints to ensure that each stage of the cell cycle proceeds correctly and that the cell does not divide when it shouldn't.

What can happen when the cell cycle control system fails?

When the cell cycle control system fails, it can lead to uncontrolled cell division, which is a characteristic of cancer. Cancer cells bypass normal checkpoints and divide uncontrollably, disrupting the body's balance and causing illness.

Outlines

00:00

🌟 The Star and Supporting Cells of Life

The paragraph introduces the concept of the cell cycle, emphasizing the importance of cells in our bodies as the main characters in our personal life stories. The narrator, Dr. Sammy, humorously compares their role in making decisions to the autonomous functions of cells, which are essential for our existence. The paragraph explains that cells go through a cycle of growth, development, and division, similar to the life cycle of an organism. It also touches on the process of cellular division in response to injury, highlighting the differences between single-celled and multicellular organisms. The role of cells in various body parts, such as the skin and digestive tract, is discussed, along with the potential for research into stimulating cell division to replace damaged nerve or muscle cells.

05:03

🔬 The Cell Cycle and Its Phases

This paragraph delves into the specifics of the cell cycle, detailing the various stages that cells go through before they divide. The interphase, which makes up 90% of a cell's life, is broken down into G1, S, and G2 phases, each with its own functions and preparations for division. The paragraph uses the mnemonic 'Pass Me A Taco, Chef' to describe the stages of mitosis: prophase, metaphase, anaphase, telophase, and cytokinesis. It explains the process of DNA replication and the formation of chromosomes during these stages, as well as the role of centrosomes in organizing cell division. The importance of the cell cycle control system and checkpoints in regulating cell division is also highlighted, with a cautionary note on what can happen when this system fails, leading to diseases such as cancer.

10:06

🤔 Philosophical and Practical Implications of Cellular Life

The final paragraph reflects on the philosophical question of identity in relation to our cells, pondering whether we are our cells or something more. It provides a teaser for the next episode, which will cover meiosis, the type of cell division that produces egg and sperm cells. The paragraph also acknowledges the collaboration with HHMI BioInteractive and offers resources for educators. It concludes with a thank you to the production team and an invitation for viewers to support Crash Course on Patreon to keep the educational content free and accessible.

Keywords

💡Cell Cycle

The cell cycle is the series of events that take place in a cell leading to its division and duplication. It is central to the video's theme as it illustrates the continuous process of growth and reproduction within the body's cells. The script mentions that cells follow a pattern called the cell cycle, which includes growing, developing, and dividing to make more cells with identical copies of DNA.

💡Cellular Division

Cellular division, also known as cell division, is the process by which a parent cell divides into two or more daughter cells. It is a key concept in the video as it demonstrates how organisms grow and repair themselves. The script explains that cellular division occurs in response to injury, like a skinned knee, where new skin cells are created to heal the wound.

💡Interphase

Interphase is the stage of the cell cycle during which the cell is not dividing but is preparing for division. It is a significant part of the video's narrative as it highlights the majority of a cell's life spent not in division but in performing its functions. The script describes interphase as a period where cells are doing their jobs, such as producing insulin in pancreas cells, while also preparing for growth and division.

💡G1 Phase

G1 phase, or the 'first gap,' is the phase of interphase during which the cell grows significantly and produces new organelles and proteins. It is mentioned in the script as an intense growth spurt where cells balloon in size, setting the stage for the subsequent steps of the cell cycle.

💡S Phase

The S phase, or 'synthesis' phase, is the stage of interphase where the cell duplicates its DNA in preparation for division. This concept is crucial to the video's theme as it shows how cells ensure that each new cell will have a complete set of genetic information. The script describes the S phase as the step where DNA replication occurs, doubling the amount of genetic material in the cell.

💡G2 Phase

G2 phase, or the 'second gap,' is the final stage of interphase before the cell enters mitosis. It is important in the video's message as it marks the cell's final preparations for division. The script explains that during G2, the cell makes more organelles and molecules needed for the upcoming division.

💡Mitosis

Mitosis is the process of nuclear division in eukaryotic cells, resulting in two daughter nuclei with the same number of chromosomes as the parent cell. It is a central process in the video as it illustrates the mechanism of cell division. The script details the steps of mitosis, emphasizing its role in ensuring that each new cell receives a complete copy of the organism's genetic code.

💡Cytokinesis

Cytokinesis is the final stage of cell division, following mitosis, during which the cytoplasm of a eukaryotic cell is divided into two daughter cells. It is highlighted in the video as the physical separation of the cell into two new cells. The script uses a relatable analogy of the cell membrane 'cinching in' like drawstrings to explain how one cell becomes two.

💡Checkpoints

Checkpoints are control mechanisms within the cell cycle that ensure the proper sequencing and accuracy of cell division events. They are crucial to the video's theme as they represent the regulatory systems that govern cell division. The script discusses how checkpoints control the fate of a cell, deciding whether it proceeds to the next phase or stops based on various conditions.

💡Cancer

Cancer is a disease characterized by the uncontrolled growth and spread of abnormal cells. It is mentioned in the video as a result of the failure of the cell cycle control system. The script explains that cancer occurs when genes that regulate the cell cycle are affected, leading to cells dividing uncontrollably and causing serious illness.

💡Meiosis

Meiosis is a type of cell division that results in four daughter cells, each with half the number of chromosomes of the parent cell, and is essential for sexual reproduction. Although not the main focus of the script, meiosis is mentioned as the topic of the next episode, indicating its importance in the study of cell division and its role in creating genetic diversity.

Highlights

The concept of viewing oneself as the main character in the story of their own life, with cells being the supporting cast.

Cells are the fundamental units of life, with approximately 37 trillion co-existing in the human body.

The cell cycle is a pattern that cells follow, similar to the life cycle of an organism.

Cellular division is crucial for healing and growth, as seen in the rapid response to skin injuries.

Different cells divide at different rates depending on their location in the body.

The exploration of stimulating cell division to replace damaged nerve or muscle cells is an ongoing area of research.

Interphase is the longest phase of the cell cycle, where cells perform their regular functions and prepare for division.

The G1 phase is characterized by intense cell growth and the production of new organelles.

The S phase is when a cell duplicates its DNA in preparation for cell division.

The G2 phase is the final preparation stage before cell division, involving the production of necessary molecules and organelles.

Mitosis is the process of nuclear division, ensuring each new cell receives a complete set of genetic material.

Cytokinesis is the physical division of the cell, resulting in two separate cells from one.

The mnemonic 'Pass Me A Taco, Chef' is used to remember the stages of mitosis: prophase, metaphase, anaphase, telophase, and cytokinesis.

The cell cycle is regulated by a complex system of proteins and checkpoints to ensure proper cell division.

Cancer arises from a breakdown in the cell cycle control system, leading to uncontrolled cell division.

The video discusses the philosophical question of identity in relation to cells, suggesting it for further exploration in Crash Course Philosophy.

The episode concludes with a teaser for the next episode, which will cover meiosis, the cell division process that produces egg and sperm cells.

Transcripts

I have a confession to make:

I like to consider myself the main character in my own life story.

I make all my own decisions:

from what I had for breakfast, to whether or not I sing as part of Crash Course Biology.

Spoiler alert: a dragonfruit smoothie and [singing] I do, I do, I do, I do.

Ok so when are we doing a musical again?

But here’s the kicker: I’ve got about 37 trillion co-stars in my story, called cells.

And without them I couldn’t be…well…me.

Each of my cells has its own little life going on, all starring in their own productions,

from “The Golgi Girls” to  “The Fault in Our Astrocytes.”

The little lives our cells lead follow a pattern called the cell cycle.

Just like how an organism grows, develops, perhaps reproduces, and then dies,

our cells do the same.

They're growing, developing, and many of them are dividing to make more cells with identical

copies of our DNA to replace the old ones.

So, yeah, I might be the star of the Sammy Show,

but today’s episode is all about my amazing supporting cast.

Take a bow y’all!

[applause]

Hi, I’m Dr. Sammy, your friendly neighborhood entomologist,

and this is Crash Course Biology.

Now look here y’all, all of y’all know what’s about to happen with this theme music, right?

[THEME MUSIC]

Ok, now back to the topic at hand.

Have you ever skinned your knee?

It stings, right?

But check it out, anytime you get a cut or a scrape like that,

boom: a flurry of cell division kicks off, and new skin cells are created to patch it up.

Cellular division is a cell’s way of making more cells.

In a single-celled organism, like an amoeba, that results in a whole new organism.

But in multicellular organisms, like you and me, cells split in two to replace old or damaged ones,

or to allow our bodies to grow.

How often your cells divide varies depending on certain factors, like where they are in your body.

For example, the cells in your skin and digestive tract divide often, throughout your entire life.

In fact, you’ve got different cells in your stomach right now than you did last week.

But, once you’re an adult, many cells in your nerves and muscles don’t divide at all.

The muscle cells will grow bigger if I don’t skip leg day, but at a certain point in our lives,

we typically won’t be getting new ones.

Many biologists have been investigating this limitation,

researching whether it might be possible to stimulate cell division

and replace nerve or muscle cells that have been damaged.

We haven’t quite figured out the secret yet, but hey, maybe that can be your project.

Now, there’s more to a cell’s  life than reproduction.

In fact, cells spend 90% of their time just doing their jobs,

in a period of their cycle called interphase.

So, for example, during interphase,

a pancreas cell might be making and secreting insulin,

which helps other cells access energy from sugars.

Meanwhile, your liver cells are likely running chemical reactions

to help you digest food and remove toxins from your blood.

But while they’re doing their jobs, inside, they’re also preparing to grow and divide.

Which happens across three main steps.

The initial step is called G1, or the “first gap,” and it’s actually an intense growth spurt.

At this point, cells are ballooning in size

while making new proteins and tiny cell versions of organs, called organelles.

The cell cycle is driven by  the activity of proteins,

and there are related  mechanisms called checkpoints,

which we’ll get into more later, that control the fate of any given cell.

For example, say we have a mature muscle cell in an adult platypus.

Since it won’t be dividing anymore, it never leaves this step.

It stays in what we call G0.

But for those cells that do clear the checkpoint,

the next step in interphase is S phase, or the“synthesis” phase.

During this step, the cell makes copies of all your genetic information,

or synthesizes DNA that will eventually be folded into tight bundles called chromosomes.

The S phase marks the only step in interphase

where a cell might slow down and not perform the rest of its job as well

because it’s preoccupied with replicating DNA.

By the end of the S phase, the amount of DNA in the cell has doubled,

and it now contains two  copies of your genetic code.

This is super important, because if the cell divides later,

each replicated cell will need its own copy of all the DNA –

even if it only needs access to certain genes.

Now, the chromosomes aren’t in their tightly bundled form just yet.

At this step, they’re still  just noodles of information, 

hanging out in the soup of the cell’s nucleus.

It’s actually how mitosis got its name.

The Greek word “mitos” means thread and that’s what DNA looks like when it’s not all bundled up.

So, the last of the three interphase steps is G2, or the “second gap.”

At this point, the cell is  getting serious about dividing.

It’s handling the finishing touches,

making more organelles and molecules that it’ll need for the split.

And thankfully, our cells have  some help during this step.

Two little protein complexes called centrosomes organize all the stuff that’s about to be divided

between the cell and its soon-to-be replicant.

By the end of G2, the cell has everything it needs for division.

Now it’s time to move out of interphase and into the mitotic or M phase.

Despite being a lot shorter than interphase,

the M phase accomplishes tons of stuff in its two-step process: mitosis and cytokinesis.

The goal of mitosis is for the cell’s nucleus to split in two,

pushing the copied chromosomes to opposite ends of the cell.

This ensures that each cell ends up with a complete copy of the organism's genetic code.

Then there’s cytokinesis, from two Greek words that literally mean “moving cells,”

the cell’s jelly-like insides split apart.

In animal cells, the squishy cell membrane cinches in like drawstrings on your favorite sweatpants,

until one cell becomes two.

Now, since plant cells have rigid walls, not flexible membranes,

they put a wall down the middle to divide one cell’s stuff from the other’s.

It’s like when you’ve got a plate of food and the juice from your string beans

starts creeping towards your biscuit

so you have to make a barrier out of mashed potatoes!

I haven’t had lunch yet...yeah.

But, let’s back up for a second.

If we slow things down, we’ll find that there are actually five smaller steps

happening in quick succession during mitosis before we even get to cytokinesis.

Luckily, we can remember them with this simple mnemonic device:

“Pass Me A Taco, Chef.”

Oh! Thank you, Chef!

Oh, that’s good.

Shoutout to former biology student Elijah Baker for that mnemonic.

Let me explain.

First, starting mitosis off  with a bang, is prophase.

Remember how all that doubled DNA was just noodling around in the nucleus?

Well, those loose threads are joined at little points called centromeres,

and now condense into identical, connected bundles, called chromatids.

So together, they form  chromosomes that look like an X.

At the same time, a bunch of long, tiny strands start to form between the two centrosomes,

which are drifting toward  opposite ends of the cell.

We call this the “mitotic spindle.”

Coincidentally: also the  name of my pet coconut crab.

Anyway, next comes metaphase.

Now the spindle slides into the middle of the cell where the chromosomes are hanging out

and attaches to their centromeres, lining them up across the cell’s middle.

After that, we get anaphase, which only lasts a few minutes.

Here, the proteins holding the chromatids together split apart—

so they’re now two separate chromosomes.

They get pulled backwards until they’re at opposite ends of the cell,

and the spindle has mostly broken apart.

The final step in mitosis is telophase.

Two new nuclear membranes form around two groups of chromosomes, sealing them up.

Sing it with me now, “telo  from the other siiiiide.”

Y'all didn’t sing.

Ahem. The nucleus has split at this point.

Inside their new nucleus homes, the chromosomes let loose again,

until those tight bundles of DNA and protein spread out in a noodly tangle.

Like this.

Once cytokinesis does its thing, and the cells move apart,

we've officially got two identical cells containing two matching sets of DNA,

which means M phase ends, and the cell cycle starts all over again.

Prophase, metaphase, anaphase, telophase, and cytokinesis –

Pass Me A Taco, Chef!

And with that, mitosis is complete…

Now, the whole operation of cell division is regulated by a set of special proteins

called the cell cycle control system.

Remember those checkpoints I mentioned earlier?

They are a big part of this system.

The proteins check in at different intervals to make sure everything is going as planned.

Based on what they find, they  can either stop the cycle 

or give the go-ahead to move to the next step.

Like, one set of proteins checks that the cell has made copies of its chromosomes

before it moves onto mitosis.

And another confirms that the cell doesn’t enter anaphase

before the spindle fibers have grabbed hold of the chromosomes.

Still, other proteins can sense what’s happening outside of the cell.

They can signal that it’s  time to step up the pace, 

or stop the cell cycle completely.

Which Is Bonkers!

I mean, how are they better coordinated than I am when they are what I am!

But what happens when the cell cycle control system fails?

Like, if the cells in one organ suddenly stop growing?

Or if the cells in another part of the body just keep dividing out of control?

That loss of regulation can  result in serious diseases.

In fact, that’s what cancer is.

Cancer cells arise when there’s a problem in the genes that regulate the cell cycle,

so the control system breaks down.

Cancer cells blast through the normal checkpoints, dividing uncontrollably,

even when proteins aren’t signaling them to do so.

And as cancer cells spread, they use up nutrients other cells need,

throwing off the body’s whole balance and causing serious illness.

The good news is, for the most part, our cells complete their cycles

without any hiccups thanks to our bodies' awesome regulatory systems –

and it’s not just us!

All life as we know it undergoes cellular cycles,

from the simplest bacterium to the coolest Crash Course host.

Our bodies are vast, complex systems filled with trillions of tinier complex systems.

Which might make you wonder “Am I me, or am I my cells?”

But that’s a question that I’ll leave for the eggheads over in Crash Course Philosophy.

In our next episode, we’ll get into the other kind of cell division, meiosis,

that makes egg and sperm cells.

Peace!

This series was produced in collaboration with HHMI BioInteractive.

If you’re an educator, visit BioInteractive.org/CrashCourse

for classroom resources and professional development

related to the topics covered in this course.

Thanks for watching this episode of Crash Course Biology

which was filmed at our studio in Indianapolis, Indiana,

and was made with the help of all these nice people.

If you want to help keep Crash Course free for everyone, forever,

you can join our community on Patreon.