How a cell divides to make two genetically identical cells. Prophase, metaphase, anaphase, and telophase.
What do your intestines, the yeast in bread dough, and a developing frog all have in common? Among other things, they all have cells that carry out mitosis, dividing to produce more cells that are genetically identical to themselves.
Why do these very different organisms and tissues all need mitosis? Intestinal cells have to be replaced as they wear out; yeast cells need to reproduce to keep their population growing; and a tadpole must make new cells as it grows bigger and more complex.
What is mitosis?
Mitosis is a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to itself. In the context of the cell cycle, mitosis is the part of the division process in which the DNA of the cell's nucleus is split into two equal sets of chromosomes.
The great majority of the cell divisions that happen in your body involve mitosis. During development and growth, mitosis populates an organism’s body with cells, and throughout an organism’s life, it replaces old, worn-out cells with new ones. For single-celled eukaryotes like yeast, mitotic divisions are actually a form of reproduction, adding new individuals to the population.
In all of these cases, the “goal” of mitosis is to make sure that each daughter cell gets a perfect, full set of chromosomes. Cells with too few or too many chromosomes usually don’t function well: they may not survive, or they may even cause cancer. So, when cells undergo mitosis, they don’t just divide their DNA at random and toss it into piles for the two daughter cells. Instead, they split up their duplicated chromosomes in a carefully organized series of steps.
Phases of mitosis
Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase. Some textbooks list five, breaking prophase into an early phase (called prophase) and a late phase (called prometaphase). These phases occur in strict sequential order, and cytokinesis - the process of dividing the cell contents to make two new cells - starts in anaphase or telophase.
You can remember the order of the phases with the famous mnemonic: [Please] Pee on the MAT. But don’t get too hung up on names – what’s most important to understand is what’s happening at each stage, and why it’s important for the division of the chromosomes.
Let’s start by looking at a cell right before it begins mitosis. This cell is in interphase (late G
phase) and has already copied its DNA, so the chromosomes in the nucleus each consist of two connected copies, called sister chromatids. You can’t see the chromosomes very clearly at this point, because they are still in their long, stringy, decondensed form.
This animal cell has also made a copy of its centrosome, an organelle that will play a key role in orchestrating mitosis, so there are two centrosomes. (Plant cells generally don’t have centrosomes with centrioles, but have a different type of microtubule organizing center that plays a similar role.)
In early prophase, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes.
- The chromosomes start to condense (making them easier to pull apart later on).
- The mitotic spindle begins to form. The spindle is a structure made of microtubules, strong fibers that are part of the cell’s “skeleton.” Its job is to organize the chromosomes and move them around during mitosis. The spindle grows between the centrosomes as they move apart.
- The nucleolus (or nucleoli, plural), a part of the nucleus where ribosomes are made, disappears. This is a sign that the nucleus is getting ready to break down.
In late prophase (sometimes also called prometaphase), the mitotic spindle begins to capture and organize the chromosomes.
- The chromosomes become even more condensed, so they are very compact.
- The nuclear envelope breaks down, releasing the chromosomes.
- The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes.
Microtubules can bind to chromosomes at the kinetochore, a patch of protein found on the centromere of each sister chromatid. (Centromeres are the regions of DNA where the sister chromatids are most tightly connected.)
Microtubules that bind a chromosome are called kinetochore microtubules. Microtubules that don’t bind to kinetochores can grab on to microtubules from the opposite pole, stabilizing the spindle. More microtubules extend from each centrosome towards the edge of the cell, forming a structure called the aster.
In metaphase, the spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide.
- All the chromosomes align at the metaphase plate (not a physical structure, just a term for the plane where the chromosomes line up).
- At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles.
Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step. If a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed.
In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell.
- The protein “glue” that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards opposite ends of the cell.
- Microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer.
All of these processes are driven by motor proteins, molecular machines that can “walk” along microtubule tracks and carry a cargo. In mitosis, motor proteins carry chromosomes or other microtubules as they walk.
In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (division of the cell contents) takes place.
- The mitotic spindle is broken down into its building blocks.
- Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear.
- The chromosomes begin to decondense and return to their “stringy” form.
Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase.
In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with a drawstring. The “drawstring” is a band of filaments made of a protein called actin, and the pinch crease is known as the cleavage furrow. Plant cells can’t be divided like this because they have a cell wall and are too stiff. Instead, a structure called the cell plate forms down the middle of the cell, splitting it into two daughter cells separated by a new wall.
When cytokinesis finishes, we end up with two new cells, each with a complete set of chromosomes identical to those of the mother cell. The daughter cells can now begin their own cellular “lives,” and – depending on what they decide to be when they grow up – may undergo mitosis themselves, repeating the cycle.
Want to join the conversation?
- So is mitosis the same as asexual reproduction?(160 votes)
- Asexual reproduction = formation of one or multiple genetically identical individuals from one parent. Mitosis = duplication of the cell's chromosomes, after which two identical cells are formed, so not whole individuals. ... Asexual reproduction involves only one parent. All the offspring are identical to the parent(3 votes)
- In the last paragraph, it's said that you end up with 2 "new" cells, but wouldn't one of those new cells be the parent cells? The diagram could be read like that too.(40 votes)
- The 'original' cell, before it divides, is called the parent cell. Both new cells are called daughter cells. (The 'parent' cell ceases to exist after mitosis.)(28 votes)
- In plant cells the "celll wall" separates the cell into two daughters at the end of mitosis right? then they split into two or they remain together?
(sorry if there's a mistake my native language is not english)(24 votes)
- In plant cells, the first part of mitosis is the same as in animal cells. (Interphase, Prophase, Metaphase, Anaphase, Telophase). Then, where an animal cell would go through cytokineses, a plant cell simply creates a new cell plate in the middle, creating two new cells. The cell plate later changes to a cell wall once the division is complete.(22 votes)
- why does nucleolus disappear during cell division and then reappear again?(18 votes)
- The nucleolus is a region in the nucleus where the genes encoding rRNA (for ribosomes) are found. In fact, the structure of the nucleolus relies on transcription of these genes. The rRNA genes are found on several chromosomes. During mitosis, the chromosomes each condense and separate, so clearly the nucleolus can't stay around the whole time during mitosis. I would guess that there is more control to its disassembly though than just the surrounding DNA being pulled away during condensation.(9 votes)
- Is actin in cytokineses also the same protein as the actin which plays a role in our muscle fibers and their contractions.(16 votes)
- Yes, it is, you are exactly right! Actin is an important part of the cell's "skeleton" and is used in many different cellular processes that need strong fibers.(20 votes)
- Are motor proteins found in all living creatures? Do they all serve a similar function or can they have many varied or specific roles? Examples?(11 votes)
- Yes motor proteins are essential proteins for all organisms - they have lots of important roles such as muscle contraction, transporting cargo around the cell and cell motility (e.g. enabling sperm to swim!). Nice question. https://en.wikipedia.org/wiki/Motor_protein(6 votes)
- How does the cell "know " to carry out Mitosis ? Is it directed by its DNA ?(8 votes)
- The details of what causes this or that to happen is probably still being studied. Whereas we know how proteins are made from genes, many questions remain in other areas like mitosis. Like, how does the mitotic spindle system know that all chromosomes have been connected?(6 votes)
- What would happen in anaphase if one or more of the chromosomes didn't pull apart?(3 votes)
- Good question!
Under normal circumstances this is relatively rare, but if the sister chromatids from a chromosome fail to separate during anaphase they will typically both end up in one daughter cell.
This mistake is known as mitotic nondisjunction and results in the daughter cells being aneuploid (having an incorrect set of chromosomes).
Aneuploidy is often associated with severe developmental defects, cancer, or death.
You can start learning more about nondisjunction and aneuploidy here:
- What is the purpose of mitosis?(3 votes)
- Mitosis allows organisms to grow and it repairs damaged cells. This is because it creates more identical cells.(5 votes)