THE CELL CYCLE
HOW DO CELLS DIVIDE?
Introduction – Answer the following
questions:
1. Why do cells divide? (Try to come up with
multiple explanations)
2. What is the difference between asexual
and sexual reproduction?
3. What conditions and molecules are
necessary for cell division?
4. If you would be a scientist who studies
cell division, what kinds of organisms and
tools would you need?
I. WHY DO CELLS DIVIDE?
Most living things grow by increasing the
number of cells not by increasing the size
of cells.
Larger cells have two demands on them:
“Information crisis” – DNA is not able to fulfill
the demands for information in larger cells (not
able to give enough information for protein
synthesis)
Exchange of materials become inefficient – lack
of nutrients and oxygen, too much waste and
CO2
Cell division is also necessary for healing
the organisms’ injuries.
It provides a way to pass on genetic
information to the next generation and with
that to upkeep the species.
Cell division is a vital component of the
reproduction of organisms as well as the
growth of organisms.
Sexual reproduction is vital for adapting to
new environments and avoiding parasites.
http://www.pbs.org/wgbh/nova/miracle/program.html (watch only parts 1 and 4)
II. CHROMOSOME STRUCTURE
Cells need to supply their entire DNA to
the two new cells – DNA replicates than
forms chromosomes.
Chromosome is a very long DNA molecule
and associated proteins, that carry
portions of the hereditary information of an
organism.
Animation on chromosome packaging: http://www.dnalc.org/resources/3d/07-how-dna-is-packagedbasic.html
DNA needs to be tightly packaged before
cell division, so it can be evenly divided
between the two new cells.
First DNA is wrapped around some small
round proteins called histones, that
wrapped again and again by other nonhistone proteins like a wrapping paper
wraps a present, until we get the X-shaped
chromosome of eukaryotic cells.
Prokaryotic cells don’t have histones only
naked, circular shaped chromosomes.
III. Cell Division in Prokaryotes
Simple asexual reproduction takes place
after DNA replication.
Binary fission only splits the cell in half.
IV. THE CELL CYCLE
The cell cycle is a series of events that
cells go through as they grow and divide.
During the cell cycle the cell grows,
prepares for division and divides to form
two (or four) daughter cells each of which
begins a new cell cycle.


http://www.cellsalive.com/cell_cycle.htm
http://wps.prenhall.com/wps/media/objects/486/498658/CDA8_1/CDA8_1f/CDA8_1f.htm
V. MITOSIS AND CYTOKINESIS
The last stage of the cell cycle when the
nucleus of a cell divides to produce two
new daughter cells (with cytokinesis) each
with the same amount and type of
chromosomes as the parent cells.
Mitosis is divided into four phases:
Prophase
Metaphase
Anaphase
Telophase

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter11/animations.html#

http://www.loci.wisc.edu/outreach/bioclips/CDBio.html
Prophase – The chromatin condenses
into chromosomes. The centrioles
separate, a spindle begins to form. The
nuclear envelope breaks down.
Metaphase – The chromosomes line up
across the center of the cell. Each
chromosome is connected to a spindle
fiber at its centromere.
Anaphase – The sister chromatids
separate into individual chromosomes and
are moved apart.
Telophase – The chromosomes gather at
opposite ends of the cell and lose their
distinct shapes. Two new nuclear
envelopes will form.
The cytoplasm pinches in half. Each
daughter cell has an identical set of
chromosomes -- cytokinesis.
In most animal cells cytokinesis takes
place when the cell membrane pinches in
until the cytoplasm is pinched into two
equal halves.
In most plant cells a cell plate forms
midway between the divided nuclei. This
cell plate gradually becomes a new cell
membrane than the cell wall develops.

http://www.youtube.com/watch?v=VGV3fv-uZYI
VI. CONTROL OF CELL DIVISION
Cell division is a complex process that
needs to be regulated. These regulators
determine when and how the cell should
divide.
External regulators:
proteins produced by other cells, speed up or
slow down the cell cycle.
If the cell touches other cells, the cell cycle
slows down.
If enough space and nutrients are available
growth factors make cells divide or speed up
their cell cycle.
Internal regulators:
Cyclins – proteins that regulate the timing of
the cell cycle in eukaryotic cells when their
concentration increase and they bind with other
proteins called kinases, the cell moves to an
other stage of the cell cycle.
Other regulator proteins – they make sure the
cell is ready to move to the next stage (3 major
checkpoints in the cell cycle).
The age of the cell.



http://www.learner.org/courses/biology/archive/animations/hires/a_cancer5_h.html
http://nobelprize.org/educational/medicine/2001/
To review everything: http://www.youtube.com/watch?v=Q6ucKWIIFmg&NR=1
VI. CANCER
Cancer cells
lack normal checkpoints and continue to
grow without inhibition
do not respond to normal signals
within the cell
are not inhibited by other cells – form
tumors
will divide indefinitely. (Cancer cells
isolated from a woman in the
1950s continue to grow today.)
Tumor Progression
1. Tumor growth
2. Blood vessels
feed tumor
3. Tumor cells
enter blood and
lymph vessels
4. Secondary
tumors form in
other parts of
the body


http://www.youtube.com/watch?v=LEpTTolebqo
Movie clips on cancer, its nature and experiments to treat it (Parts 2 and 6)http://www.pbs.org/wgbh/nova/cancer/program.html

http://www.youtube.com/watch?v=HonoQ6mE6dY&feature=related
Treatment of Cancer
 Surgical removal of tumor – Most effective when
tumor is in a defined area
 Chemotherapy – Medicines that disrupt the process
of mitosis in rapidly growing cells
 Radiation Therapy - High energy gamma radiation
is aimed at the growing tumour. This damages the
DNA in rapidly dividing cells and helps to destroy
the tumor.
VII. CHROMOSOME NUMBER
Every species has a determined number
of chromosomes. If a cell has two sets of
these chromosomes the cell is diploid.
If a cell has only one set of its
chromosomes, this cell is haploid.
Chromosomes that carry the same kinds
of genes are called homologous
chromosomes.
VIII. MEIOSIS
Meiosis is a division of the nucleus in
which the number of chromosomes per
cell is cut in half through the separation of
homologous chromosomes in a diploid
cell.
Meiosis results in four haploid daughter
cells that are genetically somewhat
different from the parent cells.
Prophase I – Each chromosome pairs
with its corresponding homologous
chromosome to form a tetrad. The tetrads
overlap and exchange some of their
genetic material – crossing-over.
Metaphase I – Spindle fibers attach to the
chromosomes.
Anaphase I – The fibers pull the
homologous chromosomes toward
opposite ends of the cell.
Telophase I and cytokinesis – Nuclear
membranes form the cell separates into
two cells.
Prophase II – Meiosis I results in two
haploid (N) daughter cells, each with half
the number of chromosomes as the
original cell.
Metaphase II – The chromosomes line up
in a similar way to the metaphase stage of
mitosis.
Anaphase II – The sister chromatids
separate and move toward opposite ends
of the cell.
Telophase II and cytokinesis – Meiosis II
results in four haploid (N) daughter cells.
http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
Crossing over in Prophase I results in
great diversity because new genetic
variations can result from it.
IX. WHY DO CELLS NEED TWO TYPES
OF CELL DIVISIONS?
 Mitosis and meiosis serve different purpose in
living organisms.
 Mitosis is used for asexual reproduction in single
celled organisms or to growth in multicellular
organisms. Human body cells reproduce by
mitosis and damaged tissues are also repaired
with the help of mitosis.
 Meiosis is used for sexual reproduction in
organisms. Human’s egg and sperm cells for by
meiosis.
X. GAMETE FORMATION
In females:
In males:
X. KARYOTYPES
The chromosomes from a cell that is in
prophase or metaphase of mitosis, can be
separated and lined up.
This method is used to detect various
chromosomal disorders in a dividing cell
and frequently used to diagnose various
chromosomal abnormalities in fetuses
during pregnancy.
XI. MUTATIONS
A mutation is a sudden change in the
number or structure of chromosomes or in
a small section of the chromosome.
We are going to further organize
chromosomal mutations on a concept
map.