Hey it’s Professor Dave, I wanna tell you about mitosis.
So we know about the cell cycle, which means we understand that cells copy the genome and
grow in preparation for cell division.
The actual process of cell division, which is called mitosis, is happening all over your
body right now, and it’s quite complex, so let’s take a look at how this works.
Mitosis is divided into five phases.
There’s the prophase, prometaphase, metaphase, anaphase, and telophase.
At the completion of telophase, there is also cytokinesis.
Once all this is finished, we end up with two identical cells, each with all the genetic
information pertaining to that organism.
But let’s start at the beginning and learn what happens in each phase.
Before mitosis begins, when the cell is still in the G2 phase of the cell cycle, we have
two copies of all the chromosomes sitting in the nucleus, but they are loose and strewn
about.
In addition, the centrosome of the cell, which as we recall, contains two centrioles, has
duplicated, so there are two pairs of centrioles.
Then, as mitosis begins, during the prophase, the chromatin becomes tightly coiled, and
forms the shape we are familiar with for chromosomes, with sister chromatids linked by a centromere.
It is also in the prophase that something called the mitotic spindle begins to form.
This is made up of the two centrosomes and a number of microtubules that begin to form
between them.
Each centrosome also has a radial array of microtubules surrounding it called an aster.
As the cytoskeleton disassembles, the microtubules between the centrosomes grow and grow, which
pushes them apart.
Then, in the prometaphase, the nucleus breaks apart and the growing microtubules cover the
area where the nucleus used to be, so that they can attach to special proteins called
kinetochores, which have assembled on the chromosomes at their centromeres.
Things are starting to get organized as a kind of tug of war plays out.
Then in the metaphase, the centrosomes have settled at the poles of the cell with the
asters attaching to the plasma membrane, and all of the chromosomes have been arranged
nicely along a plane in the middle of the cell.
This imaginary plane is called the metaphase plate.
At this stage, there is a checkpoint to ensure that each pair of sister chromatids is firmly
attached to opposite ends of the mitotic spindle.
Once all the kinetochores are attached to the spindle and everything is lined up nicely,
a regulatory protein complex becomes activated, allowing the cell to pass through the M checkpoint,
which means we are ready for the anaphase.
In this phase, the shortest of all the phases, the enzyme separase cleaves the cohesins that
keep the sister chromatids together, and the kinetochores attached to the two sister chromatids
pull the chromatids apart on each chromosome, thus generating the two separate sets of the
genome.
These chromosomes are then pulled by motor proteins that attach to the kinetochores,
which reel them in by their centromeres to opposite ends of the cell, with the microtubules
they are attached to coming apart as they go.
The cell also elongates during this phase, until the two sets of chromosomes are far
apart.
Then in the telophase, two new nuclei form, rebuilt from the fragments of the original
nucleus that came apart in the prometaphase.
The chromosomes loosen up a little, the microtubules finish coming apart, and mitosis is complete,
with two genetically identical nuclei.
To finish things up, cytokinesis will occur, which is where the cytoplasm, which has already
begun dividing the cell into two smaller ones, will continue until the cells are distinct
and separate.
This starts with a cleavage furrow at the metaphase plate, caused by actin microfilaments
that pull the cell inwards like a drawstring, which eventually pinches the cell in two.
Your body is constantly producing new cells by mitosis, to make new skin, heal a wound,
or when you grow rapidly in childhood.
Every single somatic cell in your body was produced by mitosis, except the very first one.
This first cell is an egg cell that has been fertilized by a sperm cell, and these reproductive
cells, or gametes, are produced by a different process, so let’s learn about that next.