Mitosis! It explains why a tiny seed eventually grows into a huge plant, why a
cut heals, and why a small population of pond protozoa can suddenly explode in size.
Today, we're going to learn what the purpose of mitosis is, what organisms do
it, how it works and what happens when it goes wrong. Okay here we go!
The overall purpose of mitosis is to create new cells. However, the specifics depend on
the context. Mitosis is involved in growth, repair, and sometimes in
reproduction. Somatic or body cells undergo mitosis in order to help an
organism grow. The cells divide and make identical copies of themselves.
For instance, skin cells divide and give rise only to new skin cells, and intestinal
epithelial cells in the gut only give rise to new intestinal epithelial cells.
(Which is good because you probably wouldn't want ciliated columnar
epithelial or goblet cells on your face secreting mucus... that would probably be
pretty awkward) Some cells, like mature red blood cells, don't divide at all and
never undergo mitosis. Mitosis is also involved in repair of damaged tissues.
A shallow cut or a burn will heal over in a few days as new cells divide and cover
the area. Some cells like epidermal, skin, gut, and liver cells regenerate very
quickly. The jury is still out on whether other cells like heart and nerve cells
divide very, very slowly, or just don't divide at all once mature.
There are organisms that use mitosis in order to reproduce. These organisms, such as
protists, reproduce by cloning themselves. Remember that gametes
(sperm and egg cells in humans) are not produced by mitosis. They're produced by
a related process called meiosis. We will cover meiosis in another video.
Nearly all organisms do mitosis. This includes all eukaryotes, so animals, plants, fungi and protists.
Prokaryotes, also known as bacteria, do divide and that looks a LOT
like mitosis. However, because they don't have a nuclear membrane, TECHNICALLY it
isn't actually mitosis. So, with that out of the way, let's move on to the actual
process of mitosis itself. It's important to remember that mitosis,
the process of cell division, is only a tiny portion of a cell's lifespan.
Cells are actually in interphase most of the time. The order of phases are
Interphase, Prophase, Prometaphase, Metaphase, Anaphase, and Telophase. We'll begin by discussing
interphase. During interphase, the genetic material is in the form of chromatin. It
will be packaged more efficiently later on. Interphase consists of three parts; G1
phase, S phase, and G2 phase. In G1 or "Gap 1", the cell grows, creates mRNA, and
manufactures proteins. At the end of G1, the cell checks for growth and for
damaged DNA. If the DNA has suffered damage, the cell will attempt to repair
it, or will stop dividing in order to contain the problem. In S phase the cell
grows and duplicates its DNA. There are now two copies of every strand. When the
cell divides in half, one DNA copy will be in one daughter cell and the other
DNA copy will be in the other daughter cell. In G2, the cell grows and
manufactures organelles that are necessary for mitosis. Perhaps most
importantly, a set of organelles called centrosomes will duplicate. At the end of
G2, the cell checks one final time for growth and for damaged DNA. Now the cell
is ready for mitosis. First, it enters Prophase. During Prophase, the nuclear
membrane begins to dissolve so the DNA can eventually be divided. The two copies
of the DNA are still in their messy chromatin form. However, they need to be
packaged efficiently so we can ensure that it divides properly. The DNA begins
coiling itself very tightly around proteins called histones to form
structures called chromosomes. Chromosomes look like this and consists
of two chromatids held together by an attachment point known as a centromere.
The centrosomes from G2 are also moving towards the poles of the cell. Second, we
enter Prometaphase. In Prometaphase, the chromosomes continue to condense until
they are coiled as tightly as possible. The centrosomes continue moving towards
the poles of the cell, and the microtubules begin to emerge. Third we
enter Metaphase, the longest phase of the cell cycle.
The chromosomes are pushed into alignment along the equator of the cell by special
proteins called motor proteins, which are attached to the centromere of each
chromosome. The microtubules also attach to the centromeres. Later on, these will
shorten and pull the chromosomes to each side of the cell. Fourth we enter
Anaphase. this is where we will separate the chromosome copies from one another.
The motor proteins exert so much pressure on the two chromosome copies
that they split apart. Each copy is then dragged back towards one of the two
poles of the cell as the microtubules shorten. Finally we enter Telophase.
The separated DNA copies are pulled to each pole of the cell. The DNA is put to work
almost immediately to begin rebuilding and reorganizing the important cell
components. The nuclear membrane begins to reappear and the chromosomes unravel
back into chromatin. The cytoplasm also divides in two; in a process known as
cytokinesis, yielding two identical daughter cells. The cell then re-enters
interphase and the cycle begins all over again. In summary, the genetic material is
duplicated during Interphase, then during Prophase and Prometaphase, it condenses
into chromosomes for easy separation. In Metaphase, the chromosomes are pushed
into alignment on the Metaphase plate, and then are pulled to opposite poles of
the cell during Anaphase. The cytoplasm then splits during Telophase, and the
chromosomes then unravel back into chromatin. We've taken a diploid parent
cell and created two identical diploid daughter cells. Phew! So, that's mitosis, now
we have to ask ourselves what happens when mitosis goes wrong. Well, that
depends. There are two basic categories of incorrect mitosis; incorrect division
of genetic material and too frequent division. Incorrect division of genetic
material most frequently occurs during Anaphase. Usually what happens is the
chromosomes are stuck together very very tightly, and so they don't separate
correctly when the spindle fibers begin to shorten. This results in a condition
called aneuploidy where one daughter cell has too many chromosomes and the
other resulting daughter cell has too few. Depending on the type of cell and
which chromosome did separate correctly, aneuploidy can
actually be fatal. Usually cells divide a measured pace and
stop dividing if there aren't enough nutrients available. However exposure to
certain environmental factors such as drugs, chemicals, ultraviolet light, or
just a simple DNA copying error can cause mutations that impact the
frequency of cell division. When these cells divide uncontrollably, they can
form a growth called a tumor. Some types of mutated cells only form growths in
one area, but others will metastasize and begin to spawn growths in other parts of
the body. When metastasis combines with uncontrollable cell division, we call it "cancer".
All right, that pretty much finishes up
our discussion of mitosis. If you found this video useful, I hope you'll consider
subscribing to my channel and checking out some of my other videos. Thanks again
for watching and please remember to like, comment, and subscribe!