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The Cell Cycle
Chapter 12
• Cell division - process cells
reproduce; necessary to living
things.
• Cell division due to cell cycle (life
of cell from origin in division of
parent cell until own division into 2)
• Unicellular organisms - results in
many new members.
• Multicellular organisms - division
helps in development of organism
and repair and renew preexisting
cells
• Requires distribution of identical
genetic material (DNA) to 2
daughter cells.
• Genome - cell’s genetic information
packaged as DNA.
• DNA molecules packaged into
chromosomes.
• Body cells - somatic cells; sex cells
- gametes.
• DNA has proteins – maintains
structure; helps control gene
activity.
• Duplicated chromosome - 2 sister
chromatids (identical copies of
chromosome’s DNA)
• Region where strands connect
shrinks to narrow area
(centromere)
• Processes continue every day to
replace dead and damaged cells.
• Produce clones - cells with same
genetic information.
http://www.s8int.com/images2/cloned.jpg
Cloned cells
• Mitotic (M) phase of cell cycle
alternates with much longer
interphase.
• M phase includes mitosis,
cytokinesis.
• Interphase - 90% of cell cycle.
• Interphase - cell grows by
producing proteins and cytoplasmic
organelles, copies chromosomes,
prepares for cell division; 3
subphases.
• 1G1 phase (“first gap”) - growth.
• 2S phase (“synthesis”) chromosomes copied.
• 3G2 phase (“second gap”) - cell
completes preparations for cell
division.
http://www.fhcrc.org/science/labs/fero/RL_gifs/cycle.jpg
• Mitosis – 5 subphases.
• End interphase - centrosomes
duplicated, begin to organize
microtubules into aster (“star”).
• 1Prophase - chromosomes tightly
coiled, with sister chromatids
joined together.
• Nucleoli disappear; mitotic spindle
forms, appears to push
centrosomes away toward opposite
ends (poles) of cell.
• 2Prometaphase - nuclear envelope
fragments and microtubules from
spindle interact with chromosomes.
• Microtubules from 1 pole attach to
1 of 2 kinetochores (special regions
of centromere), microtubules from
other pole attach to other
kinetochore.
• 3Metaphase - spindle fibers push
sister chromatids until all arranged
at metaphase plate (imaginary plane
equidistant between poles)
• 4Anaphase - centromeres divide,
separating sister chromatids.
• Each pulled toward pole to which it
is attached by spindle fibers.
• 2 poles have equivalent collections
of chromosomes.
• 5Telophase - cell elongates; free
spindle fibers from each
centrosome push off each other.
• 2 nuclei form, surrounded by
fragments of parent’s nuclear
envelope.
• Cytokinesis (division of cytoplasm)
begins.
• Animals - cytokinesis (cleavage) appearance of cleavage furrow in
cell surface near old metaphase
plate.
• Cytoplasmic side of cleavage furrow
contractile ring of actin
microfilaments and motor protein
myosin form.
• Contraction of ring pinches cell in 2.
• Plants, cytokinesis - cell plate
between dividing cells.
• Plate enlarges until membranes fuse
with plasma membrane at
perimeter; contents vesicles
forming new wall material in
between.
Bacteria
• Prokaryotes - binary fission.
• DNA of bacteria coiled, highly
packed.
• Binary fission - chromosome
replication begins at 1 point in
circular chromosome, (origin of
replication).
• Copied regions move to opposite
ends of cell.
• As chromosome replicates and
copied regions move to opposite
ends of cell, bacterium grows until
it reaches 2x original size.
• Cell division involves inward growth
of plasma membrane, dividing
parent cell into 2 daughter cells
with complete genome.
Regulation of cell cycle
• Some cells divide frequently in life
(skin cells), others can divide
(reserve - liver cells) mature nerve,
muscle cells do not divide at all.
• Some control over when cells
divide/how often they divide in
lifetime.
http://www.ii.bham.ac.uk/webs/shuttleworth/bbsrc1.jpg
• Cycle driven by specific chemical
signals in cytoplasm.
• Events of cell cycle directed by cell
cycle control system.
• Checkpoint in cell cycle is critical
control point where stop/go signals
regulate cycle.
• 3 major checkpoints found in G1, G2,
and M phases.
• G1 checkpoint (most important), cell
either get go ahead to finish cycle
and divide, or receive stop signal.
• If stop signal - goes into G0 phase
(remains in limbo waiting to start).
• Most human cells in this mode.
http://www.microscopy-uk.org.uk/mag/imgaug99/01.jpg
• Proteins, kinases, can
activate/deactivate other proteins.
• Kinases always present in cell; need
cyclins (protein) to activate.
• Complex of kinases and cyclin cyclin-dependent kinases (Cdks).
http://www.mie.utoronto.ca/labs/lcdlab/biopic/fig/9.5.jpg
• MPF (“maturation-promoting
factor”) triggers cell’s passage past
G2 checkpoint to M phase.
• G1 checkpoint regulated by at least
3 Cdk proteins and several cyclins.
http://www.uic.edu/classes/bios/bios100/summer2002/cdk02.gif
• M phase checkpoint makes sure
chromosomes are attached to
spindle so each cell ends up with
right amount of chromosomes.
• Cell division influenced by growth
factors, proteins released by 1
group of cells that stimulate other
cells to divide.
http://www.fhcrc.org/science/education/courses/cancer_course/basic/img/growth_factors.gif
• Platelet growth driven by growth
factors.
• Presence of injury - released to
stimulate division of platelet cells
to seal wound.
• Density of cells too high - cell
division inhibited.
Cancer
• Cancer cells divide out of control no regulation.
• Can either produce own growth
factors or have problem in signaling
pathway.
• Can divide indefinitely if they have
continual supply of nutrients.
http://www.sandia.gov/news/resources/releases/2005/images/mitopic.jpg
• Starts when single cell undergoes
transformation to change it into
cancer cell.
• If immune system does not destroy,
can form tumor (gathering of cells).
• If tumor does not invade other
areas - benign.
• If it does - malignant.
• If cells get into blood stream,
travel throughout body
(metastasis).
http://www.livercancer.com/images/metastasis.gif
• http://www.teachersdomain.org/resources/t
dc02/sci/life/stru/dnadivide/index.html