TA: Alberto Lopez
FALL 2015
Discussion D4
Lectures 13-15 Study Guide
I have organized some terms and topics that I think are important. This does not mean that other topics
mentioned during lecture or in the book will not be tested. This guide is meant to clarify and emphasize certain
points, NOT to list everything you need to know. I will focus on tying things together across lectures, and giving
real-life examples of the biological principles that we are learning. Details that I include that I think will be
helpful, but that you don’t need to know, I will write in green. Questions to think about I will write in blue.
Lecture 13: Cell Cycle, Mitosis
Take a second and think about how amazing it is that you started as one cell. While cell division can be
dangerous, as in cancer, it is also necessary for life, as we know it.
Chromosome = single molecule of DNA. Several levels of structure:
1. Nucleotides
2. DNA double helix
3. DNA wraps around nucleosomes (bundles of histones)
4. Chromatin (DNA + histones) coils
5. Coils of chromatin coils
6. Chromosome
Interphase
1. G1: Cellular components duplicate (macromolecules and organelles). Remember, we’ll need two of
everything when this cell finally divides into two.
2. S: DNA is replicated
3. G2: Checkpoint for cell cycle regulators (more on these in lecture 15) and additional growth
4. When interphase ends, the cell is pretty much normal… it just has two of everything.
Mitosis
Nuclear
envelope
Chromosomes
Mitotic spindle
Intact
Very condensed. Sister chromatids
joined at centromeres, and along the
arms (using cohesins).
Prometaphase
Fragments
Chromosomes in their most
condensed form. Sister chromatids
now have kinetochores at their
centromeres.
Begins to form. Microtubules start to form
and attach to each other. They push the
centrosomes to opposite sides of the cell
The mitotic spindle is fully formed and the
centrosomes are at the opposite poles.
There are 2 distinct types of MT. (One set is
attached to the kinetochore. That
kinetochore is a protein complex that forms
on the centromere and where the MT
attach. This allows the chromosomes to
separate later on. The other type of MT
(non-kinetochore MT) attach to each other
and they push against each other to oblong
the cell, pushing and forcing the cell to
elongate.)
Metaphase
None
Lined up at the metaphase plate
(middle of the cell)
Prophase
Modified from materials prepared by Carley Karsten, 2013
Centrosomes are at opposite sides of the
cell
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TA: Alberto Lopez
FALL 2015
Discussion D4
Anaphase
None
Telophase /
Cytokinesis
Beginning
to re-form
The MT starts to have a tug of war,
pulling the chromosomes. There’s
an enzyme called separase that
cleaves the cohesin protein that
keeps the sister chromatids together.
The chromosomes are drawn to
opposite poles.
Start to decondense immediately.
The cells need access to the genes to
create protein products for the new
cells to be fully functional
Kinetochore microtubules shorten, moving
centromeres away from the center.
nonkinetochore microtubules lengthen,
elongating the cell.
Depolymerization (breaking down) of
microtubules
93stop Experiment: 93stop stops cell cycle progression at a specific stage of the cell cycle.
Dye is used to label the DNA. This allows a researcher to measure the
amount of fluorescence. Basically, more fluorescence = more DNA. In
the figure shown on the left, the Y-axis represents the amount of cells
expressing a particular fluorescence. The X-axis represents the amount
of fluorescence per cell.

A fluorescence of 400 units will have 2x more DNA than a
fluorescence of 200 units.

Be careful with the peak! A large peak doesn’t necessarily
mean more DNA! It just means that you have more cells expressing a
certain fluorescence unit. In this case, most cells are undergoing the
G1 phase (200 units). This is because they have less DNA since the DNA
has not been replicated these cells will have a lower amount of
fluorescence. For C, on the other hand, these cells have undergone
DNA replication in the G2 and should then have 2x more fluorescence.
For S phase (B) you can see a spectrum of fluorescence. This is because some cells are progressing through
DNA replication and they’re at different levels of replication.
In the experimental group treated with 93stop, how can we interpret from the data that 93stop stops the
cell cycle progression after the G1 phase? To help with this, at what stage are most of the cells at?
Lecture 14: Cell Cycle Regulation
G1 checkpoint: Checks if environmental conditions are ok for cell division
Pass: continue to S phase
Fail: exit cell cycle and goes to G0. This is a nondividing state. Most of the cells in your body right now
are in G0.
G2 checkpoint: checks that newly synthesized DNA (from S phase) is not damaged
M checkpoint: checks that all kinetochores are attached to mitotic spindles
Important regulatory molecules:
 Cyclin-dependent kinase (CDK): enzymes that promote cell division; “gas pedal”. ONLY works when
bound to a cyclin.
 Cyclin: accumulates during the cell cycle, and is degraded afterwards  this is important. Remember
that the amounts of CDK don’t change during the cell cycle. The only thing that changes are CDK
activity, and this depends on how much cyclin is around.
 Tumor suppressors: inhibit cell division; “brakes”
Modified from materials prepared by Carley Karsten, 2013
2
TA: Alberto Lopez
FALL 2015
Discussion D4
*G1 and G2 checkpoints both use cyclin + CDK. The one we talked about most in class was for G2: the
accumulation of a cyclin that allows MPF to be active. MPF is just another name for a particular combination of
cyclin + CDK (cyclin B + CDK1, if you’re interested).
What happens to cells that fail the G2 or M checkpoints?
What do CDKs actually *do*? Start by figuring out what kinases in general do. Then figure out what’s special
about CDKs.
Cell Cycle Regulation and Cancer
Here I will describe some characteristics of normal cell cycle regulation, and how they can be messed up to
lead to cancer. Remember that cancer, in simple terms, is uncontrolled cell division.
1. Presence of growth factors
a. Normal cells only divide when there are growth factors (such as PDGF or BDNF) around
b. Cancer cells can get around this restriction by two possible methods…
i. They make their own growth factor, so that they don’t need to rely on the body to
provide it for them
ii. They have mutated signaling pathways that are always on even if there aren’t growth
factors around
*example: tyrosine kinase receptors are growth factor receptors. In many types of cancer,
tyrosine kinase receptors are mutated so that they are always stuck together (dimerized), and
therefore always active. This tricks the cancer cell into thinking that there are always growth
factors around.
2. Anchorage dependence
a. Normal cells only divide when they are attached to a surface
b. Cancer cells can divide even if they’re unattached. This leads to problems like metastasis, where
cancer cells break off from tumors and travel through the bloodstream to start new tumors
elsewhere in the body.
What type of membrane protein that we talked about earlier in this class might be mutated in
cancer cells to mess up anchorage dependence?
3. Density-dependent inhibition
a. Normal cells stop dividing when they feel crowded by other cells
Why is this beneficial? Think about things like nutrients and waste…
b. Cancer cells don’t care. They form dense tumors, and keep growing anyway.
4. Things we didn’t talk about in class, but are really cool if you’re interested…
a. Apoptosis
i. Normal cells are destroyed if their DNA gets too damaged. This process is called
apoptosis.
ii. Cancer cells are often mutated in a way that allows them to escape apoptosis signals,
and survive even with major damage.
b. Angiogenesis
i. Normal cells have to be really close to blood vessels in order to get all the nutrients they
need. This is part of the reason they exhibit density-dependent inhibition.
ii. Cancer cells can sometimes grow their own blood vessels, allowing even super-dense
tumors to get nutrients to all of their cells.
c. Limitless replication
Modified from materials prepared by Carley Karsten, 2013
3
TA: Alberto Lopez
FALL 2015
Discussion D4
i. Normal cells can only divide a set number of times (usually ~60 times). When they’re
done, they’re done. This has a lot to do with telomeres, which we will be talking about
later on in this course. When telomeres get too short, the cell won’t divide any more.
ii. Cancer cells are often “immortalized”, meaning that they don’t ever stop dividing. This
often involves mutation of telomere-related enzymes, such that telomeres never get
short. The book “The Immortal Life of Henrietta Lacks” is about a woman whose cancer
cells was harvested for research decades ago, and is still being used in labs all around the
world because they will divide forever.
More on Cancer: Proto-Oncogenes vs. Oncogenes
This can be confusing. The most important thing to remember is that proto-oncogenes do not cause cancer.
However, they have the potential to cause cancer if they were to be mutated. When proto-oncogenes are
mutated, they are called oncogenes. In general, proto-oncogenes are genes that promote cell division, and
they turn into oncogenes when they are mutated in a way that makes them active all the time.
Example: The gene for cyclin B (part of MPF cyclin/CDK complex) is a proto-oncogene. Why?
 Because if you mess it up, you could get cancer.
If you mutate the gene for cyclin B so that it can’t be degraded by the proteasome, it is now called an
oncogene. Why?
 Because it causes cancer.
How does it cause cancer?
 Since it can’t be degraded, it is always present, and always bound to CDK1. When CDK1 is bound to
cyclin B, it is active, and promotes cell division by allowing passage through the G2 checkpoint.
Your turn. Looking at the characteristics of normal cells and cancer cells above, what are some other protooncogenes you could think of? How could these proto-oncogenes be mutated to become oncogenes?
I’ll help get you started with an example (also described above):
-- proto-oncogene: tyrosine kinase receptor
-- to make it into an oncogene: mutate it so that the receptors are always dimerized, and therefore
always active. This will trick the cell into thinking that there is always growth factors present, allowing
the cell to divide even in unfavorable environments.
Can tumor suppressor genes be considered proto-oncogenes? Why or why not?
Modified from materials prepared by Carley Karsten, 2013
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