Prokaryotic Cells
• Prokaryotic cells lack a nucleus and other
membranous organelles.
• Cell division can be utilized by unicellular
organisms for reproduction and by
multicellular organisms for growth and
repair.
• Both prokaryotic and eukaryotic organisms
contain DNA in chromosomes.
Prokaryotic Cells
Prokaryotic chromosome contains DNA and
proteins but much less protein than eukaryotes.
• Prokaryotes have a
chromosome which
is referred to as a
nucleoid.
This chromosome is
a circular loop
attached to the
inside of the plasma
membrane.
Prokaryotic Cells
• Reproduce by binary
fission - produces two
genetically identical
daughter cells.
• First DNA is replicated,
then two chromosomes
separate, cell lengthens
and pulls them apart
• Eukaryotes have a
more complex manner
of division.
Eukaryotic Cell
• In eukaryotes
histone
proteins
organize the
chromosomes.
•When a cell is not undergoing division the DNA in
the nucleus is a tangled mass of threads called
chromatin.
•At cell division, chromatin becomes highly coiled
and condensed and now visible as chromosomes.
Haploid vs. Diploid
• Each species has a characteristic number of
chromosomes.
• Diploid number (2n) includes two sets of
chromosomes of each type.
• Haploid number (n) contains one of each
kind of chromosome.
• Cell division in eukaryotes involves nuclear
division and cytokinesis.
Eukaryotic Cell
• Cytokinesis is the division of
the cytoplasm.
• Somatic cells undergo mitosis
for development, growth, and
repair.
• Nuclear division leaves the
chromosome number constant.
• A chromosome begins cell
division with two sister
chromatids.
Chromosomes
• Sister chromatids are two
strands of genetically
identical chromosomes.
• The centromere is a region
of constriction on a
chromosome where sister
chromatids are attached.
• Centromeres are believed
to be responsible for
organizing the spindle
fibers.
Centromere
Sister
Chromatids
The Mitotic Spindle
• Centrosome organizes spindle.
• Each centrosome contains a pair of
barrel-shaped organelles called
centrioles.
• The spindle contains many fibers,
each composed of a bundle of
microtubules.
• Microtubules assemble when
tubulin subunits join, disassemble
when tubulin subunits become free,
and form interconnected filaments
of cytoskeleton.
Mitosis Overview
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Mitosis is divided into five phases:
1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase
Prophase
• Chromatin condenses and chromosomes
become visible.
• Nucleolus disappears and the nuclear
envelope fragments.
• Already duplicated chromosomes are
composed of two sister chromatids held
together by a centromere.
Prophase
• Kinetochores develop for future
chromosome orientation (attach to the
centromere).
• Spindle begins to assemble as pairs of
centrosomes migrate away from each other.
• Short microtubules radiate out from the pair
of centrioles located in each centrosome to
form starlike asters.
Prometaphase
• Important event during this phase is the
attachment of chromosomes to the spindle
and their movement as they align at the
metaphase plate of the spindle fibers.
• The kinetochores of sister chromatids
capture kinetochore spindle fibers.
Metaphase
• Chromosomes, attached to kinetochore
fibers are aligned at the metaphase plate.
• Non-attached spindle fibers, called polar
spindle fibers, can reach beyond the
metaphase plate and overlap.
Anaphase
• Two sister chromatids of each duplicated
chromosome separate at the centromere.
• Daughter chromosomes move to opposite
poles.
Telophase
• Spindle disappears.
• Chromosomes decondense and return to
chromatin
• Nuclear envelope reforms and nucleoli
reappear.
• Cytokinesis nearly complete.
Plant Mitosis
• We have outlined mitosis in animal cells.
Mitosis in plant cells occurs in a similar
way.
• Although plant cells have a centrosome and
spindle, there are no centrioles and asters do
not form.
Cytokinesis Overview
• 1. Animal cells – Cleavage Furrow
• 2. Plant cells – Cell Plate
• 3. Other eukaryotes – Protists and Fungi
Cytokinesis in Plant Cells
• The golgi apparatus produces vesicles that
move to the midpoint between the daughter
nuclei.
• Vesicles fuse forming a cell plate which
indicates the location of the new plasma
membranes and cell walls.
Cytokinesis in Animal Cells
Cleavage furrow indents the plasma
membrane between the two daughter
nuclei at a midpoint.
• Cleavage furrow deepens as a band of
actin filaments constricts between the
Cleavage Furrow
two daughter cells.
Cleavage Furrow
Cleavage Furrow
Cell Cycle
• Cytokinesis in other eukaryotic cells:
• Protists and fungi also undergo mitosis and
cytokinesis.
• In some organisms the nuclear envelope
does not fragment but divides and one
nucleus goes to each daughter cell.
Cell Cycle Overview
• 4-stage sequence of
events.
• 1. M stage
• 2. G1 stage
• 3. S stage
• 4. G2 stage
• Interphase consists
of G1, S, G2 stages.
Cell Cycle
• 1. M stage (mitosis) is the entire cell
division state
• 2. G1 stage is when a cell grows in size and
organelles increase in number.
• 3. S stage is DNA synthesis
• 4. G2 stage occurs just prior to cell
division; preparation for mitotic cell
division.
Cell Cycle
• Some cells divide continuously throughout
the life of an organism.
• There appear to be stimulatory substances
causing a cell to proceed through two
critical checkpoints:
– 1. G1 stage
– 2. G2 stage
S stage
M stage
Enzymes and the Cell Cycle
• Enzymes regulate passage of cells through these
points. (cyclins and kinases)
• Kinases are enzymes that remove a phosphate
group from ATP and add it to another protein.
• Cyclin proteins activate kinases, which in turn
activate enzymes.
• Growth factors are molecules that attach to the
plasma membrane receptors and bring about cell
growth.
Cancer
• Carcinogenesis is the development of cancer.
• Cancer is a genetic disease requiring a series of
mutations toward developing a tumor.
• Carcinogens are agents that cause cancer.
• A tumor indicates a failure in controlling cell
division.
• The tumor-suppressor gene p53 normally stops
the cell cycle when DNA mutates.
Cancer
• A p53 protein mobilzes repair enzymes and stops
the cell cycle; only when repaired does the cell
cycle start again.
• If DNA repair is not possible the p53 protein
promotes cell death. (apoptosis)
• Apoptosis is a sequence of cellular changes
involving:
– the shattering of the nucleus
– chopping up of chromosomes
– packaging cellular remains into vesicles.
Cancer
• Apoptosis is caused by cells harboring enzymes
called caspases.
• Caspases can be released at two times:
– during development
– in adulthood
Cancer
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Characteristics of Cancer cells:
1. Cancer cells lack differentiation
2. Cancer cells have abnormal nuclei
3. Cancer cells form tumors
4. Cancer cells undergo angiogenesis and
metastasis.
Cancer
• Angiogenesis is the formation of new blood
vessels that bring nutrients and oxygen to a
cancerous tumor.
• Cancer in situ is still in its place of origin and has
not spread to other tissues.
• When metastasis spreads new tumors distant from
the primary tumor malignancy has occured.
Cancer
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Prevention of cancer:
Avoiding carcinogenic agents
Early detection-self examinations
Diet