Transcript CELL DIVISION - Dr. Annette M. Parrott

The Cell Cycle
Mitosis & Beyond…
THINK:
Of how many cells are you composed?
When an organism grows bigger do you get
more cells or just bigger cells or both?
When do your cells divide the fastest?
Slowest?
Do cells ever stop dividing?
Are all cells capable of division and
replacement?
Why does a cell divide?
-As a cell absorbs nutrients and gets larger, the
volume of the cell increases faster than the
surface area.
-Therefore, the demands of the cell (the volume)
exceed the ability of the cell to bring in nutrients
and export wastes. Solution? Divide into two
smaller cells
When is cell division occurring?
GROWTH -increase number of cells
REPAIR -replace lost cells due to injury, disease
CANCER – Abnormally high rates of cell division
due to mutation
Different kinds of cells divide at different rates:
E. coli – 20 minutes
(What domain?)
Yeast cell – 2 hours
(What domain? What kingdom?)
Amoeba – a few days (What domain? What kingdom?)
Human embryo cell – 15-20 minutes
Human adult cell – 8 hours to 100 days
Aging
All cells die after a certain number of divisions
(programmed cell death-”apoptosis”). At any
given time some cells are dividing and some
cells are dying.
Childhood
Cell division > cell death
Adulthood
Cell division = cell death
Aging
Cell division < cell death
Control of the Cell Cycle
Cell proliferation
Interphase
Interphase ~ 90% of the time.
 G1: Little new cell absorbs nutrients and
grows larger. Does protein synthesis, its job.
 S phase: Synthesis of new DNA (DNA
replication) for daughter cells in preparation
for mitosis.
 G2: Cell continues to grow, do protein
synthesis, do its job. Gets too large, needs to
divide.
Chromosomes exist in 2
different states, before and
after they replicate their
DNA. Before replication,
chromosomes have one
chromatid. After
replication, chromosomes
have 2 sister chromatids,
held together at the
centromere. Each chromatid
is one piece of DNA with its
supporting proteins.
Remember that diploid cells
have two copies of each
chromosome, one from each
parent. These pairs of
chromosomes are NOT
attached together.
Structure of a eukaryotic chromosome
• unreplicated chromosome
arm
arm
centromere
Prior to cell division:
• chromosomes (DNA) are replicated
(duplicated)
• duplicated chromosome
– attached at their centromeres
– as long as attached, known as
sister chromatids
duplicated
chromosome
sister
chromatids
daughter
chromosomes
How long is one cell cycle?
Depends. Eg. Skin cells every 24
hours. Some bacteria every 2 hours.
Some cells every 3 months. Nerve
cells, never. Cancer cells very short.
Programmed cell death: Each cell
type will only do so many cell cycles
then die. (Apoptosis)
MITOSIS
Equal distribution of the 2 sets of DNA
amongst the 2 daughter cells.
4 Stages: “PMAT”
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
How the Cell Cycle Works
Mitosis Animation
Cell Cycle
What is Mitotic Cell Division?
• Division of somatic cells (body cells)
(non reproductive cells) in
eukaryotic organisms
• A single cell divides into two
identical daughter cells
(cellular reproduction)
=> Maintains chromosome ploidy of cell
Ploidy – refers to the number of pairs of
chromosomes in cells
• haploid – one copy of each
chromosome
– designated as “n”
• diploid – two copies (= pair) of each
chromosome
– designated as “2n”
As a cell enters
mitosis from
interphase it has 2
complete sets of
chromosomes because
of replication in the S
phase.
Each set must be rearranged and
distributed into the 2
new daughter nuclei.
This is mitosis.
Prophase…
-Chromatin condenses
(coils) into chromosomes.
Sister chromatids joined
by centromere.
-Nuclear membrane
dissolves.
-Centrioles divide and
move to opposite poles
forming spindle between
them.
chromatin
nucleus
nucleolus
centrioles
condensing
chromosomes
Metaphase
-Sister
chromatids line up on metaphase plate.
-Centromeres lock on to spindle fibre
Anaphase
Centromeres divide.
-Spindle fibres contract
pulling sister chromatids
apart to poles.
-
Telophase:
-New nuclear membranes form
around new nuclei
Mitosis Movie
CYTO KINESIS – Cytoplasm splits
into 2 cells.
-Animal cells: Cleavage furrow forms from outside
in.
-
Plant cells: Division/cell plate forms from inside
out.
Cell now returns to interphase . The
chromosomes uncoil back into
chromatin. The whole cell cycle starts
over again…..
http://www.cells
alive.com/mitosi
s.htm
At any point
in time the
cells in a
tissue will be
at different
stages in the
cell cycle.
Mitosis
Stages
Put these
in the
correct
order..
The Guarantee of Mitosis…
The
2 daughter cells formed are
identical to each other and
identical to the mother cell.
 Why is this so important?
In Mitosis, each daughter cell is exactly the
same as the original mother cell.
Cell Differentiation
Mitosis is also an ASEXUAL
form of reproduction. These are
other examples of the uses of
mitosis to create new organisms
asexually:
Propogation of
plants by cuttings
Runners from
plants like
strawberries
Budding of Yeast
Homologous pairs of
chromosomes:
Each chromosome has a certain gene sequence
on it. Eg. Chrom #1 Has insulin, foot size, and
lactase on it.
You have a chromosome one from your mom
and one from your dad. So you have 2 genes for
each trait. One from your mom – one from your
dad.
A homologous pair is a pair with the same gene
sequence – one from mom, one from dad.
Cancer
• Cancer is a disease of uncontrolled cell division. It starts with a
single cell that loses its control mechanisms due to a genetic
mutation. That cell starts dividing without limit, and eventually
kills the host.
• Normal cells are controlled by several factors. They stay in the
G1 stage of the cell cycle until they are given a specific signal to
enter the S phase, in which the DNA replicates and the cell
prepares for division. Cancer cells enter the S phase without
waiting for a signal.
• Another control: normal cells are mortal. This means that they
can divide about 50 times and then they lose the ability to die.
This “clock” gets re-set during the formation of the gametes.
Cancer cells escape this process of mortality: they are immortal
and can divide endlessly.
• A third control: cells that suffer significant chromosome damage
destroy themselves due to the action of a gene called “p53”.
Cancer cells either lose the p53 gene or ignore its message and fail
to kill themselves.
Many Mutations Lead to Cancer
All cancer is genetic, in that it is
triggered by altered genes.
Genes that control the orderly
replication of cells become
damaged, allowing the cells to
reproduce without restraint.
Cancer usually arises in a single
cell. The cell's progress from
normal to malignant to
metastatic appears to follow a
series of distinct steps, each
controlled by a different gene or
Even though all cancer is
genetic, just a small
portion—perhaps 5–10%
—is inherited.
Most cancers come from
random mutations that
develop in body cells
during one's lifetime—
either as a mistake when
cells are going through cell
division or in response to
injuries from
environmental agents such
Cancer cells contain several (6-8)
mutated genes. These almost
always include:
• mutations in genes that are
involved in mitosis (oncogenes &
tumor suppressor genes)
• Genes that regulate apoptosis
• Genes that maintain telomeres
• Genes that stimulate angiogenesis
• Metastasis genes
• Genetic Therapy & Breast
Tumors
Cancer Progression
• There are many different forms of cancer, affecting
different cell types and working in different ways.
All start out with mutations in specific genes called
“oncogenes”. The normal, unmutated versions of
the oncogenes provide the control mechanisms for
the cell. The mutations are caused by radiation,
certain chemicals (carcinogens), and various
random events during DNA replication.
• Once a single cell starts growing uncontrollably, it
forms a tumor, a small mass of cells. No further
progress can occur unless the cancerous mass gets
its own blood supply. “Angiogenesis” is the
process of developing a system of small arteries
and veins to supply the tumor. Most tumors don’t
reach this stage.
• A tumor with a blood supply will grow into a large
mass. Eventually some of the cancer cells will
break loose and move through the blood supply to
other parts of the body, where they start to
multiply. This process is called metastasis. It
occurs because the tumor cells lose the proteins on
their surface that hold them to other cells.
Cancer Treatment
• Two basic treatments: surgery to remove the tumor, and radiation
or chemicals to kill actively dividing cells.
• It is hard to remove all the tumor cells. Tumors often lack sharp
boundaries for easy removal, and metastatic tumors can be very
small and anywhere in the body.
• Radiation and chemotherapy are aimed at killing actively dividing
cells, but killing all dividing cells is lethal: you must make new
blood cells, skin cells, etc. So treatment must be carefully
balanced to avoid killing the patient.
• Chemotherapy also has the problem of natural selection within the
tumor. If any of the tumor cells are resistant to the chemical, they
will survive and multiply. The cancer seems to have disappeared,
but it comes back a few years later in a form that is resistant to
chemotherapy. Using multiple drugs can decrease the risk of
relapse.
Resources
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Mitosis  CANCER
http://www.biology.iupui.edu/biocourses/N100H/ch8mitosis.html
http://www.schoolscience.co.uk/content/4/biology/abpi/cancer/cancer3.html
http://www.schoolscience.co.uk/content/4/biology/abpi/cancer/cancer3.html
http://genetics.gsk.com/chromosomes.htm
http://www.horton.ednet.ns.ca/staff/selig/handouts/bio12/cellrepro/cancermitosis.pdf#search=
%22%22cancer%20animation%22%20mitosis%22
How Cancer grows http://www.pbs.org/wgbh/nova/cancer/grow_flash.html
Mitosis: http://pages.csam.montclair.edu/~smalley/mitosis.mov
http://faculty.pingry.org/thata/pingry_upload/movies/cellular_reproduction/plantcell_lily_mit
osis.mov
http://www.zo.utexas.edu/faculty/delozanne/celldivision/Mitosis%20Video.mov
Cell Cycle & Cancer Animations:
http://science.education.nih.gov/supplements/nih1/cancer/activities/activity2_animations.htm
Cell Biology & Cancer Animations:
http://www.learner.org/channel/courses/biology/units/cancer/images.html
Mitosis & Meiosis Interactive Exercise: http://biologyinmotion.com/cell_division/
Mitosis Animations: http://www.sci.sdsu.edu/multimedia/mitosis/navigator.html
Plant Cell Mitosis: http://www.teachersdomain.org/resource/tdc02.sci.life.stru.dnadivide/