Download 12 Cell Division AP Bio 2015

12 Cell Division; Prokaryotic and Eukaryotic A/P Biology Pages 232-248
Name: __________________________ Date: _____________ Period:________
Mitosis (or some small variation of this) is the mechanism used by all organisms.
Some single celled organisms use mitosis as a method of reproduction. All multi-cellular
organisms use mitosis for growth, repair, and maintenance. Over time (aging) multicellular animals lose the ability to repair perhaps because the cells can not longer
replicate the DNA properly (loss/shortening of telomeres) or perhaps because cells
cannot repair mistakes or damaged DNA effectively. Think about the huge number of
nucleotides that must be replicated. The more times DNA is replicated, the more chances
that the DNA will “make mistakes”. If there is a mistake “bad” enough, then the DNA
may trigger apoptosis (programmed cell death). There is a tremendous amount of work
done on the process of mitosis and certainly in the uncontrolled division of cells (cancer).
The concept of “shortening the telomeres” with each successive mitotic cycle actually
makes some evolutionary mechanical sense. In other words, if you think about the
massive number of nucleotides that MUST be replicated accurately (3 BILLION for the
human genome) each time the cell makes a copy---there is a likely chance that the DNA
as a whole will have accumulated a larger number of “mistakes”. If the cell tries to
automatically limit the number of times the cell replicates by shortening the telomeres
with each cycle-then the cell may prevent the accumulation of too many DNA errors that
may cause a “cancer” (uncontrolled mitosis). So--- the shortening of the telomere
stopping the cell from replicating further may be a “fail-safe” device to prevent cells from
uncontrolled mitosis (cancer). This is a two edged sword. The limiting of the cells’
ability to replicate for growth, maintenance and repair, may be the inherent aging process
that causes the body to ultimately and fail as we get older!
What we have come up with is the sequence of events that occurs when a cell
divides. This choreography of cell organelles and DNA is controlled by cell products
built up over the maturation of the cell (some of these chemicals are called cyclins). All
of the actual cell products that control the cell cycle responsible for mitosis have not been
identified and we have just started to learn the mechanism of the switch that turns the cell
cycle on and off (p53, see Internet source:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellCycle.html).
Originally, it was thought the cell cycle was
broken into two parts. Mitosis (the actual process
of dividing the cell) and Interphase (what was
thought to be the resting stage). We have since
discovered that the cell cycle is more complex. We
can identify up to six stages in the cell cycle (more
if you talk to different scientists) that have a lot of
processes going on in each phase or stage. The
different stages of the cell cycle are often called
phases. We arbitrarily start describing these phases
at G1, then G0, S, G2, M, and finally C.
G1- This is the primary growth phase of the
cell. This is where the cell gets bigger (surface to volume ratio decreases)
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and certain cell products build up (such as p53- thought of as an “anti-cancer protein”).
During this phase the cell performs its “normal” function. Example: a beta cell in the
pancreas will produce insulin. Moreover, the cell continues to process energy, exchange
gasses, and respond to the surrounding environment (as in responding to chemical stimuli
or physical stimuli).
At SOME point in time the cell will get to a certain size and MAY stop growing
larger. This may be a function of the build up of waste products or perhaps cell products
(cyclin-dependent protein kinases-called Cdk’s) that help control the cell cycle (such as
p53). This is an important “check-point” that is used to stop or let the cell cycle continue.
(check pt. #1)
G0 before the cell begins to replicate the DNA in preparation for cell division, the
cell may halt growth and simply continue its “normal” function. This phase can be
thought of as a “resting” state, because based on the process of cell replication, there is
no further movement toward division. Cells receive cell signals through cell surface
receptors and physical contact (see desmosomes, tight junctions - Internet, AND
inhibition of cell division as a result of confluence of cells). If cells get certain signals
to remain in Go phase (and the cells are not cancerous), then there is no cell division.
Cells may remain in the “resting state” (though normal cell function is still proceeding)
for hours, days, or years. At any one time, most
cells in a multi cellular animal are in the Go
phase. At one point in time texts erroneously
claimed that mature cells such as muscle cells
and nerve cells remain permanently in the Go
phase. In reality, scientists are just now figuring
out how to coax these cells into dividing, and
perhaps changing from one cell type to another.
The most recent articles have teased out two
proteins that may control which parts of the genome are turned on and off-thus
determining what “specialized” cell it will be or become – see YAP and TAZ. Many
cells in the body CAN go into mitosis phase in response to cell surface signals, where-as
cancer cells (by definition) NEVER seem to go into Go phase!
S- This phase is where the cell replicates the original DNA. This is the only time
there are normally “96 chromosomes” in the human cell*. (Except for liver cells that
are often tetraploids: i.e. 4X the normal number of chromosome.) This “96 chromosome”
presence is brief. We have discussed the replication of Eukaryotic and Prokaryotic DNA.
(i.e. helicase, binding proteins, primase, DNA polymerase, exonuclease, and ligase).
Even though the diagram above does not show a check point here, recent evidence
suggests that there is a check point to make sure that the DNA has replicated properly and
completely. That is all chromosomes have replicated properly. (Check point #2)
G2- This phase finds the cell replicating many of the organelles
(Golgi, mitochondria, rough endoplasmic reticulum, centrioles, etc...) in the
cell, as well as the formation of microtubules, chromosome condensation,
assembling of the spindle fibers attachment of DNA sister chromatids
(homologous chromosomes), and the formation of the kinetochore.
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To list all the changes being made here:
1.) duplication of organelles
2.) formation of microtubules
3.) arrangement of spindle fibers
4.) formation of sister chromatids attached at a point called a centromere
5.) formation of kinetochore where spindle fibers from centrioles attach
6.) and the process called condensation, where the DNA starts to wrap around histones
and begin to fold and super-coil (Check point #3)
M- mitosis phase. This is a series of approximately four steps that begin to move
the chromosomes in a fashion to orderly distribute one complete set of chromosomes to
one daughter cell, and distribute another complete set of chromosomes to another
daughter cell. The steps are: (you need to know these steps in order and what happens in
each step!)
Prophase- Nuclear envelope disintegrates
Nucleolus disappears
Chromosomes condense (now visible with light microscope)
Centrioles begin migration to “poles”(of the cell)
Mitotic spindles begin to form between centrioles
Kinetochore begins to mature and attach to spindle fibers
Metaphase- Kinetochore attach chromosomes to mitotic spindle fibers and
align them (chromosomes) along the metaphasic plate (down center of cell)
check point #4
Anaphase- Kinetochore microtubules shorten (that is they begin pulling the DNA sister
chromatids apart. Polar (meaning those at opposite ends of cell) microtubules begin to
elongate preparing for cytokinesis.
Telophase- Chromosomes segregate to poles of cells
Kinetochore disappears. Polar microtubules begin to elongate (preparing for
cytokinesis)
Nuclear membrane reforms
Nucleolus reappears
Chromosomes unwind (become less distinct and disappear from view of light
microscopes.
Cytokinesis- Plant cells- cell plate forms outward dividing the nuclei
Animal cells- cleavage furrow forms at equator of the cell and pinches inward until
cell divides in two.
Control of the cell cycle is accomplished by “proteins” called “cyclins”. In yeasts
and other single celled organisms, there is only one type of cyclin. In multi-cellular
animals, there are three different cyclins produced to Start/Stop the FOUR major check
points (you need to know these for the test). Understand that there may be other signal
molecules, cyclins, and side reactions going on that help the cell navigate the proper steps
for a successful mitosis. With our present technology we only know a small percentage
of these chemical interactions. There is still so much we do not understand about all the
factors that may control or help control the cell cycle.
Moreover, we are just beginning to learn how individual cells in a multi-cellular
organism may sacrifice itself- a process called apoptosis, to prevent the formation of a
cancer cell. The most recent research indicates that disruption of the cell cycle at
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checkpoints 2 through 4 may cause the cell to self-destruct! Apoptosis in a multi-cellular
organism is an advantageous strategy compared to allowing cells that are not
“functioning properly” to continue to replicate (undergo mitosis) and may kill the entire
organism. The loss of one cell out of millions/trillions is a better bet, than risking cancer!
Below is a diagram of how the cell might control itself:
Answer the following here and on your scan-tron after reading the text... pages 232-248
and this handout.
_____ 1.) Mitosis is used for:
a.) sexual reproduction only
c.) growth, maintenance, and repair
b.) programmed cell death (apoptosis)
d.) reanimation of dead tissue
_____ 2.) Why do some cells lose the ability to undergo normal mitosis?
a.) they have lost all their DNA
b.) their telomeres are too short
c.) their telomeres are too long
d.) DNA replicates independently of the cell cycle
Page 5 (Cont. Handout #12 AP Bio.)
_____ 3.) A single human muscle cell has divided many times and is finally unable to
proceed past Go check point. One of the reasons this might be a good thing is…
a.) the body no longer needs that type of cell
b.) the body has lost its ability to send out cell signals
c.) the end of this process “saves” the body a lot of energy
d.) the DNA may have accumulated too many errors
_____ 4.) A 12 year old boy falls and breaks a bone in his leg. The bone will heal in
about 6-8 weeks. An 80 year old may fall and breaks the same bone, but takes 8-20
weeks for the bone to heal. One possible reason for the difference might be…
a.) there are less bone cells in the 80 year old man that can divide and help heal the
bone
b.) there are too many bone cells in the 80 year old man that can help heal the bone
and there is confusion as to which cells will do the job
c.) there are no bone cells in the 12 year old that have long telomeres
d.) none of the above are reasonable explanations of the question
_____ 5.) What are cyclins?
a.) these are chemicals or signals that help regulate the mitotic cell cycle
b.) these are pieces of DNA that code for the production of certain proteins in the cell
c.) these are part of the cytoskeleton of all cells
d.) these are parts of the cell membrane that break up when the cell divides
_____ 6.) What is the “resting stage” called? (Mark all that apply!)
a.) Go b.) Interphase c.) when the cell does its job d.) apoptosis
For the questions #7-#11, match the term in column “A” with the best description in
column “B”. On your scan-tron, blacken the best answer for the multiple choice.
Column “A”
Column “B”
_____ 7.) “S” phase
a.) this is where one cell becomes two cells
_____ 8.) Go
b.) this is when the cell gets larger
_____ 9.) G1
c.) this is when the cell “does its job”
_____ 10.) “M”
d.) this is when organelles replicate
_____ 11.) G2
e.) this is when DNA replicates
___________________________________________________________________
Page 6 (Cont. Handout #12 AP Bio.)
_____ 12.) The “M” phase really consists of the following steps (in the proper order)…
a.) Anaphase, telophase, metaphase, and prophase
b.) Telophase, anaphase, metaphase, and prophase
c.) Prophase, metaphase, anaphase, and telophase
d.) Prophase, metaphase, octophase, and telophase
_____ 13.) In which phase do the chromosomes line up in the center of the cell?
a.) Anaphase b.) Prophase c.) Telophase d.) Metaphase
_____ 14.) Why replicate DNA at the “S” phase?
a.) So each cell as a complete set of instructions to run each daughter cell
b.) Just to use up excess ATP energy in the cell
c.) To satisfy check point #4
d.) All of the above are true
_____ 15.) Apoptosis is:
a.) Programmed cell death
b.) A way for the cell to self-destruct to possibly prevent cancer
c.) Possible the result of a failed checkpoint
d.) All of the above are correct
Answer the following questions on this handout only!
16.) What are nucleosomes composed of, and how do they participate in the coiling of
DNA?
17.) Which phase(s) of the cell cycle are specifically associated with the process of cell
division (i.e. making two complete cells)? What is the time difference between cell
division of Prokarytotic cells and Eukaryotic cells (of a multi-cellular organism)?
18.) What events occur during metaphase? What events indicate the end of metaphase?
Page 7 (Cont. Handout #12 AP Bio.)
19.) How does the cell regulate (produce) the cyclin proteins? That is, what is the
process? Speculate why this is a common way to regulate processes in the cell.
20.) What are the four main checkpoints in the cell cycle? Speculate why these check
points are ideal for such regulation.
Date: ________________________
Lesson Plan for Handout #12 AP Biology
Objective: TLWD ability to identify the various parts of the cell cycle, be able to explain
what happens in each phase of the cell cycle, and begin to explain how the cell cycle is
controlled at check points using cyclins and P53 when given handout #12
Content: Mammalian/plant cell cycle and single cell cycle. Begin to explain the
differences between mitosis and meiosis.
Method: Power point, white board, discussion
Homework: Complete #12
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