Download Mitosis and the Cell Cycle

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Cell Structure
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Mitosis and the Cell Cycle
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Mitosis is part of the 'cell cycle' in which an existing cell (the
parent cell) divides into two new ones (the daughter cells).
Mitosis does not result in a change of chromosome numbers
(unlike meiosis): the daughter cells are identical to the parent
cell. Although mitosis is part of a continuous cell cycle, it is
divided into stages (below). In plants and animals mitosis is
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Interphase
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Centrosome, which
later forms the spindle,
is also replicated.
associated with growth and repair of tissue, and it is the method
by which some organisms reproduce asexually. The example
below illustrates the cell cycle in a plant cell. Note that in animal
cells, cytokinesis involves the formation of a constriction that
divides the cell in two. It is usually well underway by the end of
telophase and does not involve the formation of a cell plate.
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Early Prophase
Centrosomes in plant
celis lack centrioles. In
animal celis, centrioles
are associated with the
centrosomes but their
exact role is unclear.
Late Prophase
Nuclear
Membrane
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Nucleolus
Second Gap: the
chromosomes
begin condensing
Synthesis of
DNA to replicate
chromosomes
Mitosis: nuclear division
Cytokinesis: division of the
cytoplasm and separation of
the two cells. Cytokinesis is
distinct from nuclear division.
First Gap: the cell
grows and develops.
Division of the cytoplasm
(cytokinesis) is complete. The
two daughter cells are now
separate cells in their own right.
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Cytokinesis
Metaphase
DNA continues condensing into
chromosomes and the nuclear
membrane begins to dissolve
DNA is replicated to
form 2 chromatids
The mitotic spindle is
formed to organize the
chromosomes. The
spindle consists of fibres
made of microtubules
and proteins.
Anaphase
Two new nuclei form. The cell
plate forms across the midline
of the parent cell. This is where
the new celi wali will form.
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Telophase
Late Anaphase
1. The five photographs below were taken at various stages through the process of mitosis in a plant cell. They are not in any
particular order. Study the diagram above and determine the stage that each photograph represents (eg. anaphase).
(a)
(b)
(c)
(d)
(e)
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2. State two important changes that chromosomes must undergo before cell division can take place:
3. Briefly summarize the stages of the cell cycle by describing what is happening at the points (A-F) in the diagram above:
A.
B.
c.
D.
E.
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Cancer: Cells out of Control
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Normal cells do not Jive forever. Under certain circumstances,
cells are programmed to die, particularly during development.
Cells that become damaged beyond repair will normally undergo
this programmed cell death (called apoptosis or cell suicide).
Cancer cells evade this control and become immortal, continuing
to divide regardless of any damage incurred. Carcinogens are
Normal cell
Cancer: Cells out of Control
Cancerous transformation results from changes
in the genes controlling normal cell growth and
division. The resulting cells become immortal
and no longer carry out their functional role. Two
types of gene are normally involved in controlling
the cell cycle - proto-oncogenes, which start the
cell division process and are essential for normal
cell development, and tumor-suppressor
genes, which switch off cell division. In their
normal form, both kinds of genes work as a
team, enabling the body to perform vital tasks
such as repairing defective cells and replacing
dead ones. But mutations in these genes can
disrupt these finely tuned checks and balances.
Proto-oncogenes, through mutation can give rise
to oncogenes; genes that lead to uncontrollable
cell diVision. Mutations to tumor-suppressor
genes initiate most human cancers. The best
studied tumor-suppressor gene is p53, which
encodes a protein that halts the cell cycle so that
DNA can be repaired before division.
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agents capable of causing cancer. Roughly 90% of carcinogens
are also mutagens, i.e. they damage DNA. Chronic exposure to
carcinogens accelerates the rate at which dividing cells make
errors. Susceptibility to cancer is also influenced by genetic
make-up. Anyone or' a number of cancer-causing factors
(including defective genes) may interact to induce cancer.
Mutations (defects) in one or two of the controlling
genes cause the formation of a benign tumor - a
tumorthat is nonmalignant (not growing in size). As the
number of controlling genes with mutations increases,
so too does the loss of control until the cell becomes
cancerous (a 'renegade cell').
Damaged
DNA
If the damage is too
serious to repair,p53
activates other genes
that cause the celi to
self-destruct.
Tumor-suppressor genes
When damage occurs, the
tumor-suppressorgene p53
commands other genes to
bring cell division to a halt.
If repairs are made,
then p53 allows the
cell cycle to continue.
Proto-oncogenes Genes that turn
on cell division.The mutated form
or oncogene somehow leads to
unregUlated cell multiplication.
Features of Cancer Cells
The bloated, lumpy shape is readily
distinguishablefrom a healthy cell,
which has a flat, scaly appearance.
Cancercells can go on dividing
indefinitely, if they have a
continual supply of nutrients, ~
and are said to be immortal.
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Metabolism may be deranged
and the cell ceases to function
in a constructive way.
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Cancer cells may have
unusual numbers of
chromosomes.
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The diagram above shows a single lung cell that has become
cancerous. It no longer carries out the role of a lung cell, and instead
takes on a parasitic 'lifestyle', taking from the body what it needs in
Cancerous cells lose
their attachments to
neighboring cells.
the way of nutrients and contributing nothing in return. The rate of cell
division is greater than in normal cells in the same tissue because
there is no restingphase between diVisions.
1. Explail11 how cancerous cells differ from normal cells:
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2. Explain why a cell with a faulty oncogene can be likened to a car with a stuck accelerator:
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3. Explain Why a cell with a damaged tumor-suppressor gene can be likened to a car with no brakes:
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4. Define oncogene:
5. Define tumor-suppressor gene:
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Cell Structure
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Root Cell Development
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In plants, cell division for growth (mitosis) is restricted to growing
tips called meristematic tissue. These are located at the tips of
every stem and root. This is unlike mitosis in a growing animal
where cell divisions can occur all over the body. The diagram
below illustrates the position and appearance of developing and
growing cells in a plant root. Similar zones of development occur
in the growing stem tips, which may give rise to specialized
structures such as leaves and flowers.
Zone of
specialization
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Zone of
elongation
Root tip
growing in
this direction
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Zone of
cell division
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The root cap protects the growing tip of the
. ; - - - - root immediately behind it. The cells in the
root cap undergo cell division to replace cells
that are rubbed off as the root grows.
1. Describe what is happening to the plant cells at each of the points labeled (a) to (c) in the diagram above:
(a)_:--
(b)
(c)
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2. Simple ('primitive') plants such as liverworts and mosses lack a high degree of cell specialization in their tissues.
Explain the advantage to a plant of having tissues comprising of highly specialized cells rather than unspecialized cells:
3. Identify which cells undergo cell division and specialization when a tree increases its girth (diameter):