The Cell Cycle (and cancer) [Updated]

Captions are on! Click CC at bottom right to turn off. You can find us on Twitter (@AmoebaSisters) and Facebook! Have you ever been sitting in class and thought
to yourself, “I wonder what my skin cells are doing right now at this very moment?” This kind of pondering may be unique to
me…maybe…but wouldn’t we at some point wonder what are cells are doing right
now? Because if you remember, as part of the cell
theory, you are made of cells. All living things are made of 1 or more cells! Many multicellular organisms, like you, have
cells that work together. Working together as part of body tissue. Body tissues working together as part of an
organ. Organs working together as part of an organ
system. Your cells are specialized to work in these
different levels of organization—you have skin cells, stomach cells, muscle cells just
to name a few—and their functions need to be regulated. These cells actually are regulated as part
of something called the cell cycle and that is going to relate to my question of, “I
wonder what my cells are doing right now.” Cells themselves can grow in size. But let’s put it in perspective now: a multicellular
organism isn’t growing because each individual cell is getting bigger. A multicellular organism itself grows by making
more cells—by the cells making more cells by dividing. That’s cell reproduction. One reason that you’re bigger than you were
when you were 5—unless you are 5—is because your cells have divided to make more cells. Mitosis, and the cytokinesis that follows
that splits the cytoplasm, allows you to make new body cells but you don’t want that cell
division happening all the time. Why? It is likely that the term ‘cancer’ may
have relevance to you. We have had family members that have battled
cancer before- it is definitely a relevant topic for all of us. Cancer is in part due to cells that divide
too frequently. The cells are not regulated; they are uncontrolled. Cancer cells can have other problems too—they
might not be able to communicate with other healthy cells, they may not be able to carry
out normal cell functions, they may not securely anchor themselves like other cells do which
can make them more likely to travel somewhere else. Some cancer cells have the ability to secrete
their own growth hormone. that makes blood vessels divert over to
those cancer cells and supply the cancer cells with nutrients, which can take nutrients away
from healthy cells. Why do cancer cells become this way? Well, there is a lot of research in
this area. With some cancers, there may be genetic links
making some cells more susceptible to having problems—these genetic factors might run in
families. Exposure to toxins, radiation, or excessive
exposure to UV light can be risk factors for some cells to become cancerous. The uncontrolled growth that cancer cells
have gives rise to more cells like them, which can develop into a tumor. Some tumors stay put but some do not. Now the good news is that scientists continue
to develop better treatments which include destroying the cancer cells with radiation
or medication—such as chemotherapy— which will target cells that divide frequently. Maybe someday you will be part of helping
to meet the challenge of trying to eliminate cancer, because the fact remains that these
cells are not participating in the cell cycle like they should. So what is the cell cycle? The cell cycle is often represented as a pie
chart like this. Cells are either in one of two different phases:
a phase called interphase where the cells themselves are growing, replicating their
DNA, doing their cell functions—- or they are in M phase which includes mitosis and
the actual splitting of the cytoplasm – cytokinesis. So this M phase is where cells actually divide
to make more cells. But cells spend most of their time in interphase. So most of the time, they’re not dividing. Now, depending on what kind of cell, it might
do mitosis more or less often; for example, your hair follicle cells do mitosis frequently
which is why your hair can grow at the rate that it does. It’s also why many cancer drugs may also
target hair follicle cells, because many cancer drugs go after cells that do cell division
frequently. It’s a big deal for cells to hit this M
phase. If a cell has an error—a harmful mutation
for example—you do not want it to divide because then it will create another cell
that has this same issue (harmful mutation). That’s where check points come in handy. Along the cell cycle there are check points
to check that the cell is growing well and replicating its dna correctly and doing
everything it’s supposed to correctly before it divides. To better understand those checkpoints, let’s
further divide this cell cycle pie chart. We have G1 (Gap1), S (synthesis), G2 (Gap
2)—all three of those are part of interphase. Then we have M phase where mitosis will happen. During G1, the cell individually itself grows. Then it replicates its DNA in S phase…you
can remember that because the “s” is for “synthesis” which means to make something, and it’s making
DNA. Then G2, the cell grows some more in preparation
for mitosis. So let’s take a look at checkpoints. We got one here in G1—this checkpoint checks,
“Is the cell growing well enough?”—“Is its DNA damaged?” Because if it is, you definitely don’t want it to move
on to S phase where you would replicate DNA. Does the cell have the resources it needs if it
were to keep moving on? This checkpoint in G2 checks if the DNA was
replicated correctly back in S phase. Is it growing well enough—does it have the
resources it needs to continue? Okay then, moving on, this next checkpoint
in M phase is my favorite checkpoint. It checks in the stage metaphase to make sure
that chromosomes, which are made up of DNA, are lined up in the middle correctly—that
they’re all attached to the spindle correctly. Because if they’re not, the chromosomes will not be separated correctly. So now you may have two big questions. First, what happens if the cell doesn’t
meet the requirements of the checkpoint and second, what is doing the regulating of this
cycle anyway? To address the first question- if the reason
the cell can’t go past the checkpoint is a reason that can be fixed, the cell may kind
of pause here until it can fix the issue. But if it can’t be fixed? Then the cell does something called apoptosis
which basically means the cell self destructs. This ensures that a cell that is damaged beyond
repair will not go on to divide. So what is doing the regulating anyway? We’ve mentioned before that proteins are
a big deal. Genes in your body can code for proteins that
do an assortment of functions, and there are many proteins involved with regulating the
cell cycle. Some of them are positive regulators because
they allow moving forward in the cycle and some are negative regulators that may make
things stop. The proteins themselves can be sensitive to
cues inside and outside of the cell. So, for example, two proteins that are involved
in positive regulation are cyclin and Cdk. Cdk is specifically an enzyme protein—a
fancy kind called a kinase which is worth a google. Cdk can have different forms of cyclin protein
bound to it. Different types of cyclin rise and fall throughout
the cell cycle, and the rising and falling is based on a variety of signals to determine
when the cell should move on to the next cell cycle phase. Typically each cell cycle phase—G1, S, G2,
M—- will tend to have a different cyclin binding with the Cdk. The rise and fall of cyclin types and the
role CdK has when it’s active is a fascinating subject to explore. Remember that vocabulary word we said, “Apoptosis?” Proteins that are negative regulators -for
example, a protein called p53- can be involved in initiating apoptosis. Again, we encourage you to explore beyond
this video. One last thing to mention. There are some cells that don’t go through
the phases we mentioned, because they’re actually in G0. That’s a zero by the way and not an o…because
if it was an o then it’d say go and G0 is kind of the opposite of that. G0 is a resting phase. Now cells here are still performing cell functions,
but they’re not preparing to divide. Some cells go here temporarily, maybe
if there’s not enough resources around for example. But some—like many types
of neurons in your brain and spinal cord may stay here permanently. If they stay here permanently, they’ll never
get to M phase so they will not divide. This can be one reason why a major injury
to the brain or spinal cord can have challenges with healing as many of those cells may not
be able to replicate. A topic that definitely continues to be researched. Well, that’s it for the amoeba sisters, and
we remind you to stay curious.


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