During its life (cell cycle), a cell grows to a
maximum size, then it undergoes DNA
replication and mitosis.
Why do cells divide by mitosis?
To replace damaged or old cells (repair)
To grow (cells can’t get too large or they become inefficient in
transporting material)
To reproduce a whole organisms asexually (without a mate)—
clones! Hardly every happens in complex organisms like
most plants and animals.
e.g., a plant grown from a cutting
yeasts dividing in grape juice
starfish regenerating after you tear
it in half
Somatic (body) cells divide by mitosis.
Somatic cells are diploid in animals.
Mitosis starts with a diploid cell and ends with a
diploid cell.
• Diploid=2n
• 2 X n where n is the number of types of
chromosomes a species contains
• E.g., humans n=23
• Body cells have 2 X 23 chromosomes before
and after mitosis
To make sex cells (gametes/sperm or eggs),
body cells in the gonads (testes & ovaries)
divide by meiosis to produce gametes.
Meiosis begins with diploid body cells, 2n=46.
Meiosis begins with haploid gametes, 1n=23.
Mendel’s law of segregation: It’s random for every
chromosome (and every allele on it) whether a person’s
maternal or paternal homologous chromosome is
given.
Mendel’s law of independent assortment: The set of
23 chromosomes that each parent sorts into any
particular gamete is random.
Human body cells are diploid with 2n = 46.
Why? Body cells are produced by asexual mitotic division.
Human gametes are haploid with 1n = 23.
Why? Gametes are produced by sexual meiotic cell division.
Asexual reproduction or growth/repair
of organisms utilizes mitosis (think
My—two—sis)
1 cell divides into two genetically identical
daughter cells
Mitosis generates two genetically identical cells because
the 2 copies of each chromosome are separated into two
new daughter cells. The original cell is called the mother
cell.
Stages of mitosis from mother cells to
genetically identical to 2 daughter cells.
http://highered.mcgraw-hill.com/sites/0073031216/student_view0/exercise13/mitosis_movie.html
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.html
The only reason mitosis generates
genetic differences is mistakes in
copying DNA
• Mutations
• Mutations are sometimes bad, but sometimes
they generate helpful or harmless
differences alleles giving unique traits.
For sexual reproduction specialized
cells called gametes (male sperm,
female eggs) are required
These cells must have only 1 copy of each
chromosome, not 2
Meiotic (meiosis, think my-o-sex) cell division
generates 4 gametes with only 1 copy of each of
the parent’s original pair of homologous
chromosomes
How? Copy the DNA once, then divide twice.
Meiosis is used ONLY for sexual reproduction—it results in
cells that are haploid (only 1 of each chromosome, 1n).
Copy DNA once. Divide twice! For genetically reduced
(half as many chromosomes) gametes are produced.
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html
Human body cells are diploid with 2n = 46.
Why? Body cells are produced by asexual mitotic division.
Human gametes are haploid with 1n = 23.
Why? Gametes are produced by sexual meiotic cell division.
Cell cycle regulation—mitosis and
meiosis
http://outreach.mcb.harvard.edu/animations/ch
eckpoints.swf
Requirements for passing cell cycle
checkpoints in normally regulated cells
1 growth factor stimulation to activate exit from G0 or G1
2
Anchorage to a substrate (usually the extracellular matrix of another cell or
with a culture dish in vitro)—anchorage dependence
3
At least some part of the cell membrane NOT in direct contact with another
cell (open space)—contact inhibition
4
At the G1 checkpoint, minimal size & correct pairing of cyclins (made at
specific points in cell cycle) and cyclin dependent kinases, cdks (made
throughout the cell cycle but only activated by a particular cell cycle phase
regulated cyclin)
5
At the G2 checkpoint, DNA all replicated, not broken, well aligned via
complementary base pairs; cyclin/cdk combination termed “mitosis
promoting factor) is activated, and tumor suppressors like p53 or BRCA are
NOT activated
6
At the anaphase (spindle) checkpoint, Spindle fibers connected to
kinetichores/centromeres on both sides
When a checkpoint is not met, proteins like tumor suppressors stall the cell at the
stage to allow for correction. If correction is not possible, apoptosis (cell
suicide) is initiated.
Many proteins expressed in a cell cycle dependent
manner monitor the checkpoints
http://lsresearch.thomsonreuters.com/sta
tic/maps/472_map.png
Cyclins made in a cell cycle stage dependent manner activate
cyclin dependent kinases at key transitions
Carcinogens
Substances (like
Components of cigarette smoke)
or physical agents (like UV
radiation) that are associated with high risk
oncogene or tumor suppressor
gene mutations.
http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL206019/1940.jpg
1st step in development of cancer: Immortalization—telomerase enzyme
allows cells to continue dividing long after telomere degradation would
normally have signaled senescence (inability to divide again
http://www.quia.com/files/quia/users/lmcgee/mitosis/AP_Cha
pter_12/Density_Dependent_Inhibition_and_Cancer.jpg
As errors go unchecked and as cells become
progressively more aneuploid (wrong
chromosome #s), mutations in more cell cycle
regulatory genes occur.
Tumors are termed malignant when mutations
allow them to migrate via blood and lymph
vessels to new locations, then develop a
tumor in the new location.
Cancer treatments either:
remove the tumor by surgery OR
induce apoptosis after radiation causes DNA damage OR
target/inactivate proteins needed for cell cycle progression
(chemotherapy) OR
stimulate the patient’s immune system to recognize unique
surface antigens of cancer cellscomplement mediated lysis
(immunotherapy)
Advantages of using bioengineering—scans of human genome and its expression leads
to detection of which genes are mutated & possible means of creating drugs that
target these in a oncogene specific way
Oncogene—proteins with dominant mutations which allow cells to skip a checkpoint
Proto-oncogene—normal version of an oncogene (side effects of chemotherapy are due
to these also being targeted by the drugs--> high cell death in rapidly dividing cell
populations like stomach & intestinal lining or hair follicles
Tumor suppressor—regulators that are inactivated by recessive mutations, allowing cells
that ought to undergo apoptosis to escape (inherited cancers affect tumor
suppressor genes, making a mutation in a 2nd allele more likely)
Flow cytometric analysis of noncancerous cells shows most cells in
G0/G1
What is the length of G1, of S, and of
G2/M in these cells? How many pg
DNA in a diploid cell?
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch
apter3/animation__random_orientation_of_c
hromosomes_during_meiosis.html
• http://highered.mcgrawhill.com/sites/9834092339/student_view0/ch
apter11/unique_features_of_meiosis.html
Stages of meiosis
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapt
er28/animation__how_meiosis_works.html
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapt
er28/animation__stages_of_meiosis.html
http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapt
er30/meiosis_with_crossing_over.html
Meiotic events are the basis for
Mendel’s law of random segregation &
independent assortment
•
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter3/animation__random_orientation_of_ch
romosomes_during_meiosis.html
http://www.sumanasinc.com/webcontent/ani
mations/content/independentassortment.html
http://www.dnalc.org/view/16192-Animation5-Genetic-inheritance-follows-rules-.html