Phases of Mitosis

Phases of Mitosis

Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase. Some textbooks list five, breaking prophase into an early phase (simply 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 theMAT. 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 G2 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. In prophase, 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 finish condensing, 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, and they form the narrow "waist" of a duplicated chromosome). Microtubules that bind a chromosome are called kinetochore microtubules. Microtubules that don’t bind to kinetochores (inventively called non-kinetochore microtubules) can overlap with and grab on to microtubules from the opposite pole, stabilizing the spindle. More microtubules radiate from each centrosome towards the edge of the cell, forming a structure called the aster (Greek for “star”).

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.

• Cohesin, the protein “glue” that holds the sister chromatids together, is broken down, allowing them to separate. Each is now considered its own chromosome.
• The microtubules attached to the chromosomes (kinetochore microtubules) get shorter, pulling chromosomes towards the poles.
• Microtubules not attached to chromosomes (non-kinetochore microtubules) elongate and push apart, separating the poles and making the cell longer.

All of these processes are powered 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 finished dividing, and it starts to re-establish normal internal structures as cytokinesis 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 actually a band of filaments made of a protein called actin, and the pinch crease is known as the cleavage furrow. Plant cells, on the other hand, 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.