Transcript Reproduction of Cells

Reproduction of Cells
and the Cell Cycle
Chapter 12
Functions of Cell Division (mitosis)
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Multicellular organisms: growth, development,
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Unicellular organisms: reproduction.
and repair.
 Cell division allows organisms to grow while
maintaining surface area-to-volume ratio; adds
more cells.
 Also provides more cells that can become
specialized, as well as replacing damaged or
dead cells.
Cell division, also called binary fission, is a form
of asexual reproduction.
 New cells will be clones of the original cell.
Chromosomes
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Eukaryotic cells contain genes (sequences of
DNA) which are organized into multiple
chromosomes.
Chromosomes -- Threadlike structures
composed of DNA and proteins.
 • Each species has a characteristic
chromosome number; Human somatic (body)
cells have 46; diploid number.
 • Gametes (sperm and egg) contain half the
number of chromosomes of somatic cells;
Human gametes have 23; haploid number.
 Chromatin -- a long, thin fiber that is folded
and coiled to form the chromosome.
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Chromosomes (cont)
 Before
division, chromosomes are
duplicated, forming an X-shaped
structure made up of two identical
strands of DNA (sister chromatids)
joined at a specialized region called
the centromere or kinetochore.
The Cell Cycle
Cell cycle -- Sequence of events including mitosis,
cytokinesis, cell growth, and preparation for the next
division.
 Some cells divide each hour, others take more than
24 hours; Nerve and muscle cells never or rarely
divide once they are formed.
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Interphase -- includes most of a cell's growth and
metabolic activities; about 90% of the cell cycle.
 1. G1 phase – cell growth. (G stands for "gap".)
 2. S phase -- DNA is made as chromosomes are
duplicated. (S stands for "synthesis".)
 3. G2 phase – final prep for division and cell
growth.
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Cell Cycle Cont.
phase -- shortest part of the cell
cycle; cell divides.
 1. Mitosis -- division of the nucleus.
 Five stages: prophase,
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prometaphase, metaphase, anaphase
and telophase.
 2.
Cytokinesis -- division of the
cytoplasm (cytosol and organelles).
Processes of Cell Division
1. Replication:
 By late interphase, chromosomes have been
duplicated and take the form of loosely packed
chromatin.
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2. Alignment:
 Prophase, prometaphase, and metaphase.
 Nuclear membrane breaks down, chromatin
fibers condense into observable chromosomes.
 Spindle forms, composed of microtubules.
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Processes of Cell Division cont.
2. Alignment continues:
 Spindle fibers (bundles of microtubules)
extend from each pole toward the cell's
equator; some attach to chromosomes.
 Chromosomes move to the metaphase
plate, the plane between the poles.
 Centromeres of all chromosomes are
aligned on the metaphase plate,
connected to spindle fibers.
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Processes of Cell Division cont.
3. Separation:
 Anaphase, telophase, and cytokinesis
 Centromeres divide as sister chromatids
pull apart into separate chromosomes and
move towards opposite poles of the cell.
 Microtubules shorten as chromosomes
approach the poles.
 The poles of the cell also move farther
apart, elongating the cell.
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Processes of Cell Division (end)
3. Separation continues:
 Daughter nuclei begin to form at the two poles.
 Nuclear envelopes form around the
chromosomes.
 Nucleoli reassemble.
 Chromatin of each chromosome uncoils and the
chromosomes become less distinct.
 Cytokinesis begins with cleavage (animal cells)
or cell plate formation (plant cells).
 In plants, a new cell wall forms as cellulose is
deposited between the two membranes of the
cell plate.
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Cell Cycle Animation
Regulation of the Cell Cycle
External and internal cues control cell
division.
 1. Cell membrane receptors (nutrients,
pH, chemical growth factors).
 PDGF (platelet derived growth factor) is
made by blood platelets; binds to
membrane receptors (tyrosine kinases)
and causes cells to divide to heal wounds.
 2. Cell density -- Crowding inhibits cell
division (density-dependent inhibition).
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Regulation of the Cell Cycle
cont.
3. G1 phase of the cell cycle -- If a cell is
going to divide, it progresses beyond the
R (restriction) point then proceeds with
DNA synthesis. Checkpoints
– If the cell is not going to divide, it may
exit from the cell cycle at the restriction
point and switch to a nondividing state
called the G0 phase.
 4. Cell size -- cytoplasmic volume is the
most important indicator of whether a cell
will pass the restriction point.
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Regulation of the Cell Cycle (cont)
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5. Regulatory proteins – Cyclin and protein kinases.
Cyclin is produced and accumulates during interphase, then
triggers mitosis.
An example of a cyclin-dependent kinase is MPF (maturation
promoting factor).
• Active MPF phosphorylates chromatin proteins, causing
chromosomes to condense.
• Nuclear envelope breaks down when some of its membrane
proteins are phosphorylated.
Near the end of mitosis, cyclin is destroyed by an enzyme that
is activated by MPF.
The destruction of cyclin causes the decline in active MPF at the
end of mitosis.
(ANOTHER EXAMPLE OF FEEDBACK CONTROL)
Cancer cells escape from the controls
on cell division
Cancer cells divide excessively and ignore density-dependent
inhibition.
 Cancer cells will divide indefinitely as long as nutrients are
available. Normal mammalian cells divide about 20 to 50
times before they stop; there is a culture of cancer cells that
have been dividing in a lab since 1951 (HeLa).
 The immune system normally destroys abnormal cells that
have converted from normal to cancer cells.
 If not destroyed, abnormal cells form a tumor.
 • In benign tumors, the cells remain at the original site.
 • A tumor is malignant (cancer) if the cells have the ability
to damage tissue and to spread to other parts of the body.
 Detached cancer cells may spread into other tissues
surrounding the original tumor and may even enter the blood
and lymph vessels of the circulatory system (metastasis).
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Tumor Suppressor Genes
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The protein products of tumor suppressor
genes can directly or indirectly prevent cell
division or lead to cell death.
 Tumor suppressors can be likened to the
brake system in a car.
 Loss of function of tumor suppressors
leads to abnormal cellular behavior.
 Car Analogy
Key Tumor Suppressor Genes
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P53
– A transcription factor that regulates genes
controlling cell division and cell death.
– Important in the cellular response to DNA damage.
– Aids in decision between repair and induction of
cell death.
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BRCA
– BRCA proteins have multiple functions including
repairing DNA damage and regulation of gene
expression.
– Non-functional BRCA leads to compromised DNA
repair and gene regulation. (Animation)
Cancer Treatment
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New cancer drugs target the cell cycle by
inhibiting division internally or externally.
Taxol and spindle formation
Herceptin and membrane receptors
animation