In order to heal an injury, your body needs to replace damaged cells with healthy new ones...and mitosis plays a crucial role in this process! Mitosis is a process of cell division that helps you stay alive and healthy. In other words, in the world of cell biology, mitosis is kind of a big deal!
But like with anything science-related, mitosis can be sort of confusing when you first try to understand it. The key idea is that the process of mitosis involves four phases, or steps, that you need to understand if you want to understand how mitosis works.
In this article, we’re going to do the following things to break down the four steps of mitosis for you and help you get acquainted with the mitosis phases:
- Briefly define mitosis and eukaryotic cells
- Break down the four phases of mitosis, in order
- Provide mitosis diagrams for the stages of mitosis
- Give you five resources for learning more about the phases of mitosis
Now, let’s dive in!
Feature image: Jpablo cad and Juliana Osorio/Wikimedia Commons
(Marek Kultys/Wikimedia Commons)
What Is Mitosis?
Mitosis is a process that occurs during the cell cycle. The role of mitosis in the cell cycle is to replicate the genetic material in an existing cell—known as the “parent cell”—and distribute that genetic material to two new cells, known as “daughter cells.” In order to pass its genetic material to the two new daughter cells, a parent cell must undergo cell division, or mitosis. Mitosis results in two new nuclei—which contain DNA—that eventually become two identical cells during cytokinesis.
Mitosis occurs in eukaryotic (animal) cells. Eukaryotic cells have a nucleus that contains the cell’s genetic material. A crucial part of mitosis involves breaking down the nuclear membrane that surrounds the cell’s DNA so that the DNA can be replicated and separated into new cells. Other types of cells, like prokaryotes, don’t have a nuclear membrane surrounding their cellular DNA, which is why mitosis only occurs in eukaryotic cells.
The main purpose of mitosis is to accomplish cell regeneration, cell replacement, and growth in living organisms. Mitosis is important because it ensures that all new cells that are generated in a given organism will have the same number of chromosomes and genetic information. In order to accomplish this goal, mitosis occurs in four discrete, consistently consecutive phases: 1) prophase, 2) metaphase, 3) anaphase, and 4) telophase.
Our in-depth post on mitosis is coming soon, so you can think of this as an intro to what mitosis is and how it works! What we want to focus on in more detail here are the 4 stages of mitosis: prophase, metaphase, anaphase, telophase, and what happens during those phases! So let’s get down to it.
The 4 Phases of Mitosis: Prophase, Metaphase, Anaphase, Telophase
So what are the stages of mitosis? The four stages of mitosis are known as prophase, metaphase, anaphase, telophase. Additionally, we’ll mention three other intermediary stages (interphase, prometaphase, and cytokinesis) that play a role in mitosis.
During the four phases of mitosis, nuclear division occurs in order for one cell to split into two. Sounds simple enough, right? But different things occur in each step of mitosis, and each step is crucial to cell division occurring properly. That means successful cell division depends on the precision and regulation of each phase of mitosis. That’s why it’s important to be able to understand and articulate the role of each phase in mitosis overall.
Also: you may have seen or heard the parts of mitosis called different things: mitosis phases, the stages of mitosis, the steps of mitosis, or maybe even something else. All of those different phrases refer to the exact same process. As long as you remember that the phases/stages/steps of mitosis always happen in the same order, it doesn’t really matter which of those phrases you use!
Next, we’re going to breakdown the four phases of mitosis in order so you can understand how mitosis occurs through each phase.
(Ph. Immel/Wikimedia Commons)
Interphase: What Happens Before Mitosis
We can think of interphase as a transitional phase. Interphase is when the parent cell prepares itself for mitosis. This phase isn’t considered part of mitosis, but understanding what happens during interphase can help the steps of mitosis make a little more sense.
You can think of interphase kind of like the opening act. They aren’t the band you came to see, but they get the audience warmed up for the main event.
Interphase occurs prior to the beginning of mitosis and encompasses what’s called stage G1, or first gap, stage S, or synthesis, and stage G2, or second gap. Stages G1, S, and G2 must always occur in this order. The cell cycle begins with stage G1, which is a part of interphase.
So how does the parent cell prep itself for mitosis during interphase? During interphase, the cell is busy growing. It’s producing proteins and cytoplasmic organelles during the G1 phase, duplicating its chromosomes during the S phase, then continuing to grow in preparation for mitosis in the G2 phase.
In the cell cycle, interphase doesn’t just occur before mitosis—it also alternates with mitosis. It’s important to remember that this is a recurring cycle. When mitosis ends, interphase starts up again! In fact, in the grand scheme of the cell cycle, mitosis is a much shorter phase than interphase.
Phase 1: Prophase
Prophase is the first step of mitosis. This is when the genetic fibers within the cell’s nucleus, known as chromatin, begin to condense and become tightly compacted together.
During interphase, the parent cell’s chromosomes are replicated, but they aren’t yet visible. They’re just floating around in the form of loosely collected chromatin. During prophase, that loose chromatin condenses and forms into visible, individual chromosomes.
Since each of the parent cell’s chromosomes were replicated during interphase, there are two copies of each chromosome in the cell during prophase. Once the chromatin has condensed into individual chromosomes, the genetically-identical chromosomes come together to form an “X” shape, called sister chromatids.
These sister chromatids carry identical DNA and are joined at the center (in the middle of the “X” shape) at a point called the centromere. The centromeres will serve as anchors that’ll be used to pull the sister chromatids apart during a later phase of mitosis. And that’s what’s happening inside the nucleus during prophase!
After the sister chromatids form, two structures called centrosomes move away from each other outside of the nucleus. As they move to opposite sides of the cell, the centrosomes form something called the mitotic spindle. The mitotic spindle will eventually be responsible for separating the identical sister chromatids into two new cells and is made up of long protein strands, called microtubules.
Late Prophase: Prometaphase
Prometaphase is often referred to as “late prophase.” (Though it’s also sometimes called “early metaphase” or referred to as a distinct phase entirely!) Regardless, some really important things occur during prometaphase that propel cell division along and that help explain what happens in metaphase.
Prometaphase is the phase of mitosis following prophase and preceding metaphase. The short version of what happens during prometaphase is that the nuclear membrane breaks down.
Here’s the long version of what happens during prometaphase: first, the nuclear membrane or nuclear envelope (i.e. the lipid bilayer surrounding the nucleus and encasing the genetic material in the nucleus) breaks apart into a bunch of membrane vesicles. Once the nuclear envelope breaks apart, the sister chromatids that were stuck inside the nucleus break free.
Now that the nucleus’s protective covering is gone, kinetochore microtubules move near the sister chromatids and attach to them at the centromere (that spot at the center of the “X”). Now these kinetochore microtubules are anchored at opposite poles on either end of the cell, so they’re extending themselves toward the sister chromatids and connecting them to one of the edges of the cell.
It’s kind of like catching a fish with a fishing pole—eventually, the chromatids are going to be separated and drawn to opposite ends of the cell.
And that’s the end of prometaphase. After prometaphase ends, metaphase—the second official phase of mitosis—begins.
Phase 2: Metaphase
Metaphase is the phase of mitosis that follows prophase and prometaphase and precedes anaphase. Metaphase begins once all the kinetochore microtubules get attached to the sister chromatids’ centromeres during prometaphase.
So here’s how it happens: the force generated during prometaphase causes the microtubules to start pulling back and forth on the sister chromatids. Since the microtubules are anchored at opposite ends of the cell, their back-and-forth pulling on different sides of the sister chromatids gradually shifts the sister chromatids to the middle of the cell.
This equal and opposite tension causes the sister chromatids to align along an imaginary—but very important!—line trailing down the middle of the cell. This imaginary line dividing the cell down the middle is called the metaphase plate or equatorial plane.
Now, in order for metaphase to progress on to anaphase, the sister chromatids must be equitably distributed across that metaphase plate. That’s where the metaphase checkpoint comes in: the metaphase checkpoint ensures that the kinetochores are properly attached to the mitotic spindles and that the sister chromatids are evenly distributed and aligned across the metaphase plate. If they are, the cell gets the green light to move on to the next phase of mitosis.
The checkpoint is very important because it helps the cell make sure that it mitosis will result in two new, identical cells with the same DNA! Only once the cell passes the metaphase checkpoint successfully can the cell proceed to the next stage of mitosis: anaphase.
Phase 3: Anaphase
The third phase of mitosis, following metaphase and preceding telophase, is anaphase. Since the sister chromatids began attaching to centrosomes on opposite ends of the cell in metaphase, they’re prepped and ready to start separating and forming genetically-identical daughter chromosomes during anaphase.
During anaphase, the centromeres at the center of the sister chromatids are severed. (It sounds worse than it is!) Remember how the sister chromatids are attached to the mitotic spindle? The spindle is made up of microtubules, which start shrinking during this phase of mitosis. They gradually pull the severed sister chromatids toward opposite poles of the cell.
Anaphase ensures that each chromosome receives identical copies of the parent cell’s DNA. The sister chromatids split apart down the middle at their centromere and become individual, identical chromosomes. Once the sister chromatids split during anaphase, they’re called sister chromosomes. (They’re actually more like identical twins!) These chromosomes will function independently in new, separate cells once mitosis is complete, but they still share identical genetic information.
Finally, during the second half of anaphase, the cell begins to elongate as polar microtubules push against each other. It goes from looking like one round cell to...well, more like an egg as the new chromosome sets pull further away from each other.
At the end of anaphase, chromosomes reach their maximum condensation level. This helps the newly separated chromosomes stay separated and prepares the nucleus to re-form . . . which occurs in the final phase of mitosis: telophase.
Phase 4: Telophase
Telophase is the last phase of mitosis. Telophase is when the newly separated daughter chromosomes get their own individual nuclear membranes and identical sets of chromosomes.
Toward the end of anaphase, the microtubules began pushing against each other and causing the cell to elongate. Those polar microtubules keep elongating the cell during telophase! In the meantime, the separated daughter chromosomes that are being pulled to opposite ends of the cell finally arrive at the mitotic spindle.
Once the daughter chromosomes have fully separated to opposite poles of the cell, the membrane vesicles of the parent cell’s old, broken down nuclear envelope form into a new nuclear envelope. This new nuclear envelope forms around the two sets of separated daughter chromosomes, creating two separate nuclei inside the same cell.
You might think of the events of telophase as a reversal of the events that occur during prophase and prometaphase. Remember how prophase and prometaphase are all about the nucleus of the parent cell starting to break down and separate? Telophase is about the reformation of the nuclear envelope around new nuclei to separate them from each cell’s cytoplasm.
Now that the two sets of daughter chromosomes are encased in a new nuclear envelope, they begin to spread out again. When this occurs, it is the end of telophase, and mitosis is complete.
Cytokinesis: What Happens After Mitosis
Like interphase, cytokinesis isn’t a part of mitosis, but it’s definitely an important part of the cell cycle that is essential to completing cell division. Sometimes, the occurrence of the events of cytokinesis overlaps with telophase and even anaphase, but cytokinesis is still considered a separate process from mitosis.
Cytokinesis is the actual division of the cell membrane into two discrete cells. At the end of mitosis, there are two new nuclei contained within the existing parent cell, which has stretched out into an oblong shape. So at this point, there’s actually two complete nuclei hanging out in one cell!
So how does one cell become two cells? Cytokinesis is responsible for completing the process of cell division by taking those new nuclei, separating the old cell in half, and ensuring that each of the new daughter cells contains one of the new nuclei.
Here’s how the separation of the old cell is accomplished during cytokinesis: remember that imaginary line running down the middle of the cell and dividing the centrosomes, called the metaphase plate? During cytokinesis, a contractile ring made of protein filaments develops where that metaphase plate used to be.
Once the contractile ring forms down the middle of the cell, it starts shrinking, which pulls the cell’s outer plasma membrane inward. You can think of it like a belt that just keeps tightening around the middle of the cell, squeezing it into two sections. Eventually, the contractile ring shrinks so much that the plasma membrane pinches off and the separated nuclei are able to form into their own cells.
The end of cytokinesis signifies the end of the M-phase of the cell cycle, of which mitosis is also a part. At the end of cytokinesis, the division part of the cell cycle has officially ended.
5 (Free!) Resources for Further Study of the Steps of Mitosis
Mitosis is a complex process, and the mitosis phases involve a lot of big words and unfamiliar concepts that you might want to learn more about. If you’re interested in diving more deeply into the 4 stages of mitosis, take a look at our five suggested resources for further study of the steps of mitosis, explained below!
#1: Mitosis Animations Online
Reading all about mitosis can definitely be helpful, but what if visuals really help you understand how things work? That’s where web animations of mitosis might come in handy for you. Watching mitosis in action through web animations can help give you an idea of what all those verbal descriptions really mean. They can also help you picture what the phases of mitosis might look like under a real microscope!
There are probably a lot of web animations of mitosis that you could take a look at, but we recommend these three:
- John Kyrk’s Mitosis Animation
- The Biology Project’s “Online Onion Root Tips”
- Cells Alive’s “Animal Cell Mitosis”
We particularly like Cells Alive’s “Animal Cell Mitosis” animation because it allows you to pause the animation as it loops through the phases of mitosis in order to take a fine-grained look at how mitosis works. Cells Alive’s version also juxtaposes its animation of the mitosis phases with footage of mitosis occurring under a microscope, so you’ll know what you’re looking for if you’re ever tasked with observing cell mitosis in the lab.
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#2: “Mitosis: Splitting Up Is Hard To Do” by Crash Course
If you’re a bit exhausted from reading dense material and need someone else to put the stages of mitosis into more accessible terms, head over to YouTube and watch Crash Course’s 10 minute video on mitosis, called “Mitosis: Splitting Up Is Hard to Do.”
The nice thing about this video is that, while being a bit more thorough than some of the other YouTube videos you might find out there on mitosis, it’s also really funny. More importantly, it explains mitosis in terms of familiar, everyday biological processes, like when you get a cut and need your body to make new cells to heal.
If you need help thinking about the real-world relevance of the mitosis phases beyond just being something you have to memorize for a lab or exam, this is a great resource.
#3: “Phases of Mitosis” by Khan Academy
Here’s another YouTube video, but the tone and style of this explanation of the steps of mitosis by Khan Academy is a little different. Watching this tutorial on the mitosis phases feels a bit like you’re sitting in biology class and your teacher/professor is drawing out diagrams of mitosis while talking you through the entire process (except in this case, your teacher is sort of cool and only uses neon colors to draw the diagrams).
If you’re looking for a step-by-step tutorial that takes a slow pace and deals with the steps of mitosis thoroughly, Khan Academy has you covered!
#4: Creating a Mitosis Flip Book
For some learners, the process of creating something to show your knowledge can help with memorization of difficult concepts and/or developing a thorough understanding of how things work. That’s why we suggest trying out some old-school tactics to build your knowledge of the 4 stages of mitosis! A tried-and-true approach to learning the mitosis phases, vetted by biology teachers, is creating a mitosis flip book.
Post-It provides a step-by-step guide on how you can create a mitosis flip book on your own, but it’s really pretty simple: you get something to draw with, grab small note cards or sticky notes to draw on, and draw what each phase of the cell cycle looks like on individual note cards/sticky notes!
When you’ve finished drawing your version of the stages of mitosis on your cards, you either stick, tape, or staple them together, and voila! You can flip through your mitosis flip book from beginning to end and watch the progression of mitosis through the four phases.
Activities like this one can help imprint on your memory what each step of mitosis looks like. Plus, when you finish your flip book, you’ve got a pocket-sized resource that you can carry with you as a part of your study guide or a quick resource for review before a quiz or exam!
#5: “Mitosis Study Set” by ProProfs Flashcards
Maybe you’re feeling pretty good about your knowledge of the stages of mitosis but you want some help in testing that knowledge before a formal quiz or exam. That’s where ProProfs Flashcards’ “Mitosis Study Set,” an online study guide that provides an array of flashcards to help you test your knowledge of the stages of mitosis, comes in.
What’s fun about this flashcard set is that you can choose different assessment styles depending on where you are in your knowledge of mitosis. The flashcard set provides traditional question-and-answer flashcards, a flashcard function specifically geared toward memorization, a multiple choice quiz, and matching. If you want to practice being tested on the steps of mitosis before the actual test, check out this resource!
ProProfs Flashcards provides several study sets on other topics related to or involving mitosis, so if you need to test your knowledge of mitosis beyond just the four phases, this resource could help out there as well.
If you want more traditional resources to help you learn about the cell cycle, our list of the best AP Biology study guides has you covered.
You’ll probably need to know about more than just mitosis to ace your AP Biology exam. Here’s an expert guide to the test to get your studying started off on the right foot!
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Ashley Sufflé Robinson has a Ph.D. in 19th Century English Literature. As a content writer for PrepScholar, Ashley is passionate about giving college-bound students the in-depth information they need to get into the school of their dreams.