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GROWTH, REPLACEMENT and REPAIR of CELLS
All living things are composed of at least one cell. Cells are the smallest unit of living things. These cells
must take in food and communicate with other cells to maintain homeostasis. While prokaryotic cells have
no membrane-bound organelles, there are many organelles inside eukaryotic cells with special functions.
Two that we have just studied in depth are chloroplasts and mitochondria. These structures work together
to make the cell run. Chloroplasts and mitochondria can only function because the membranes that
surround these organelles, and the membrane that surrounds the cell allow selective transport of molecules
into and out of the cell through diffusion, osmosis, facilitated diffusion and active transport (both protein
and vesicle). Membranes of cells really act to allow food, water, wastes and important gases (CO2 and O2)
to move into and out of cells (and in multicellular organisms, like ourselves, we have systems that move
these molecules either to or from our cells.
Cells are busy. Cells and organelles within cells act like factories and machines within the factory. And
like factories and machines, components that make of the cells and organelles can wear out, become
damaged, and ultimately be unable to function. Additionally, organisms grow (get larger) and/or
reproduce. So, there needs to be a cellular process that is responsible for Growth, Replacement and
Reproduction of cells (GRR….). Growth, Replacement and asexual Reproduction is accomplished
through mitosis.
Mitosis cannot occur unless cells are properly nourished. So, food and water must enter the cell through the
process of osmosis. Waste material must also be removed. Proteins in the phospholipid membrane are
selective and determine whether materials may enter or leave. Depending on the size of the molecule, it
may enter or leave the cell through osmosis, active transport, or endo/exo-cytosis.
So, let’s recall what we know before moving ahead with mitosis.
Take a moment to recall how the following get to or are made in the cells of large multicellular organisms
like plants and animals. Hint: use diagrams you have already created
Animals:
Glucose, amino acids, fatty acids, O2, H2O, lipids, vitamins and minerals, hormones, waste
Plants
CO2, H2O, minerals, vitamins, energy, glucose, amino acids, fatty acids, hormones, waste
You should be able to identify whether these molecules are produced as a result of anabolic or catabolic
reactions, what the name of the system or process is in which they are formed, how they get from where
they are “created” to where they are utilized, what type of reaction (catabolic or anabolic) is utilized to USE
molecules (like glucose) to create energy, waste products that result from this process, and how the
organism gets rid of the waste. Additionally, some cells (not all) produce proteins or lipids such as
hormones that need to be transported to other cells.
Cell division is another process that requires ATP , enzymes and chemical signals to work.
Only cells that are ‘well “fed” and healthy are capable of replicating and producing new cells.
Production of new cells (that are genetically identical to the original cell) has different names
depending on the type of organism and whether or not the organism is producing a new cell (as a
part of a multicellular organism, or a new organism (reproduction).
Binary Fission
Prokaryotic organisms are single celled, with no nucleus. Cyanobacteria are small prokaryotes that
can photosynthesize. They divide via a process termed binary
fission. Binary fission involves the following steps:
1.
2.
3.
The cell grows
The cell then duplicates its single circular chromosome
The cell grows again, increasing the distance between
the duplicated chromosomes
4. The cells divide by creating new cell walls, which begin
as a structure called the septum.
EVOLUTIONARY NOTE: Chloroplasts share many similarities
with Cyanobacteria. Chloroplasts are organelles in plant cells
that can photosynthesize, contain their own DNA and ribosomes
and divide via binary fission. The similarity of chloroplasts to
cyanobacteria provides evidence for the theory of endosymbiosis,
which is an explanation for how eukaryotes may have “formed”.
Eukaryotic Reproduction
Eukaryotes are much larger than prokaryotes and have many organelles. Besides organelles like
chloroplasts and mitochondria, a key differentiator between prokaryotic and eukaryotic cells is
the presence of a nucleus. The nucleus is a “container” for DNA. Remember that even the
nucleolus (which creates ribosomes) is a specialized area of DNA that includes proteins and rRNA
and is responsible for the formation of ribosomes.
So, in order to create a copy of a eukaryotic cell, a copy of the DNA must be created first. This step is
called replication. This happened in the prokaryotic cell as well. But the prokaryotic cell has only a
single circular strand of DNA. Eukaryotic cells can have many linear strands of DNA (we happen to
have 46). Because of the length and number of DNA strands, copying the DNA and ensuring a copy
of each strand of DNA goes into each cell is a more difficult process than replicating a single circular
chromosome. In eukaryotes, the organization and the separation of these replicated strands of DNA
into two groups of identical DNA is termed mitosis.
Mitosis involves coiling the replicated DNA into structures called duplicated chromosomes and
dissolving the nuclear membrane (prophase), moving these structures to the center of the cell
(metaphase) and then splitting each duplicated chromosome to form two “unduplicated”
chromosomes (anaphase). These structures are moved to opposite ends of the cell, and a nuclear
membrane forms around each of the two sets of chromosomes (telophase). This signals the end of
mitosis.
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In many cells the cell membrane divides (cytokinesis) either during or immediately after telophase
and the cell moves back into interphase until it is again ready to divide. In some cells (like muscle
cells) cytokinesis does not occur and the cell becomes longer with multiple nuclei.
Draw a picture of the cell cycle below. You must include interphase (and its three “subphases),
mitosis and cytokinesis as a cycle, with appropriate amounts of the circle allocated to each diagram.
Then draw a representative picture of each stage of mitosis (use website below, next page or the
book)
http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__how_the_cell_cycle_works.html
Cell cycle
Stage of Mitosis (Draw/Label)
Description
Cell Cycle and Cancer.
https://www.youtube.com/watch?annotation_id=annotation_4012916109&feature=iv&src_vid=gwc
wSZIfKlM&v=lpAa4TWjHQ4.
The cell cycle has many checkpoints to ensure the DNA is being copied and divided correctly. Use
the questions attached to this handout (and the cartoon video above) to explore the link between the
cell cycle and cancer. If you are having trouble understanding the phases of mitosis, view the second
video (below) and use the notes sheet for the second video to help to identify the key stages of
mitosis.
https://www.youtube.com/watch?v=JcZQkmooyPk
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QUESTIONS – CANCER and THE CELL CYCLE
1. There are many types of cancer. But this video identifies one underlying cause of all cancers.
What is it?
2. What are factors that can contribute to or increase your risk of cancer?
3. Is cancer contagious?
4. All cells in your body are in one of two phases which are
a. _______________________________ or b. _________________________________
Most somatic (body) go through a cell cycle. This cycle is listed as having either 2
(interphase and mitosis) or 3 (interphase, mitosis, cytokinesis) major phases. This
video divides the cell cycle into two phases, interphase and mitosis. We will be
dividing the cell cycle into THREE phases.
5. What could the cell be doing during interphase (list three things).
6. During mitosis, the DNA found within the nucleus replicates. The cell then divides (a
process called cytokinesis).
7. What percent of a cell’s life (on average ) is spent in interphase? How much time in mitosis?
8. Do all cells divide at the same rate? Provide an example of a human cell that divides rapidly,
what about one that never divides?
9. What is a disadvantage of having cells that don’t divide?
10. Provide three reasons that mitosis is important
11. What is the purpose of a cell cycle checkpoint? What occurs when a cell fails the checkpoint?
12. Why is “self-destruction” a good thing?
13. How is cancer cell division different from “normal” cell division?
14. Go back to question 2, and add to your list of risk factors.
15. What type of cell is targeted by chemotherapy? Can “normal cells” be impacted by
chemotherapy? Explain.
16. What happens if cancer cells are able to produce growth hormone? (be sure to get all the
details?
17. New chemotherapies are able to specifically target key processes, such as hormone
production, or to target key mutations (DNA/RNA sequences) which help cancer to divide
and grow. Why is it helpful to develop medications that have specific targets (rather than
just kill rapidly growing cells).
End of video 1