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.