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.