Transcript The Cell Cycle

Chpt. 12 ~ The Cell Cycle
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Cell division results in genetically identical daughter cells.
The mitotic phase alternates with interphase in the cell cycle.
The cell cycle is regulated by a molecular control system.
Key Roles of Cell Division
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When a unicellular organism divides and forms duplicate
offspring, the division of one cell reproduces an entire
organism
Cell division on a larger scale can produce progeny from
some multicellular organisms such as plants that grow from
cuttings
Cell division also enables sexually reproducing organisms to
develop from a single cell~ the fertilized egg or zygote
Cell division continues after an organism is fully grown to
repair and replace cells that die from normal wear and tear
or accidents
Cellular Organization of Genetic Material
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Genome: cell’s genetic information
• Prokaryotic single circular or elongated
DNA
• Eukaryotic genomes made of DNA
molecules
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DNA molecules are packaged into
chromosomes making the replication and
distribution more manageable
• Eukaryotic species chromosome # unique by
species
• Somatic cells - body cells
• Gametes – sperm & egg cells
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Eukaryotic chromosomes are made of
chromatin a DNA-protein complex
Chromosome duplication &
distribution during cell division
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When a cell is not dividing each
chromosome is in the form of a long, thin
chromatin fiber
After DNA duplication the chromosomes
condense
Each duplicated chromosome has:
• Two sister chromatids
– Each contain identical DNA
– Attached at centromere a narrowing “waist” of sister
chromatids
– Each sister chromatids have a kinetochore, a
structure of proteins associated with specific
sections of chromosomal DNA at the centromere
– Later in cell division the sister chromatids separate
and move into two new nuclei
– Once sister chromatids separate they are called
chromosomes
Cell Division
 Mitosis
• Division of the nucleus
• Each new nucleus receives a group of chromosomes
identical to the original group
 Meiosis
• Division of gametes (egg & sperm cells)
• Yields non-identical daughter cells that have only one set
of chromosomes
• Daughter cells have half as many chromosomes as the
parent cell
The Cell Cycle
 Interphase (90% of cycle)
• G1 phase - growth
• S phase - synthesis of DNA
• G2 phase - grows and prepares
for cell division
 Mitotic
phase
• Mitosis - nuclear division
• Cytokinesis - cytoplasm
division
Cells Alive Animation!
http://www.cellsalive.com/cell_cycle.htm
A typical human cell
might undergo one
division in 24 hours!
Mitosis
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Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Interactive
http://www.sci.sdsu.edu/mul
timedia/mitosis/
Mitotic Spindle
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Constructed of fibers made of
microtubules and associated
proteins
 Kinetichore - chromosome
attached
 Nonkinetichore or Polar – no
chromosome attached
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Begins to form in the cytoplasm
during prophase at the
centrosome
 The spindle includes the
centrosomes, the spindle
microtubules, and the asters.
http://www.meta-library.net/media/mito2-sm.jpg
Centrosome
http://www.biology.ucsc.edu/mcd/images/centrosome.jpg
In animal cells, a pair of centrioles is located at the center of
the centrosome but are not essential for cell division
 Most plant cells centrosomes lack centrioles
 Centrosome replicates during interphase and
remain together near the nucleus
 The two centrosomes move apart from each other during
prophase and prometaphase of mitosis as spindle
microtubules grow out from them
 By the end of prometaphase, the two centrosomes, at are
opposite poles of the cell with an aster radiating
from each centrosome
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http://www.fotosearch.com/comp/LIF/LI
F113/SA202005.jpg
http://www.bio.unc.edu/faculty/peifer/documents/
people's%20pages/lab%20website/RusanColor2s
mall.jpg
Prophase
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Chromatin fibers condense
into visible Chromosomes
Nucleoli disappear
Each duplicated chromosome
appears as two identical Sister
chromatids joined together
Mitotic spindle begins to form
Centrosomes move away from
each other
Prometaphase
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Nuclear membrane fragments
Spindle interaction with
chromosomes
Kinetochore develops at
centromere of each chromatid
Microtubules making up
spindle
 Kinetochore microtubule
connect to kinetochore
 Nonkinetochore microtubules
communicate only with poles
Metaphase
 Centrosomes
at
opposite poles
 Centromeres are
aligned
 Kinetochores of sister
chromatids attached to
microtubules (spindle)
Anaphase
 Paired
centromeres
separate; sister
chromatids liberated
 Chromosomes move to
opposite poles
 Each pole now has a
complete set of
chromosomes
Telophase
 Daughter
nuclei form
 Nuclear envelopes arise
 Chromatin becomes
less coiled
 Two new nuclei
complete mitosis
Cytokinesis
 Cytoplasmic
division
 Animals:
cleavage furrow
 Plants:
cell plate
Mitosis animation
http://www.johnkyrk.c
om/mitosis.html
Binary Fission
 Cell
division of prokaryotes
 DNA replication begins at a specific sites on
chromosome called origin of replication
 As DNA replicates one origin moves rapidly toward
the opposite end of the cell while the cell elongates
 When replication is complete its plasma membrane
grows inward producing two daugther cells
Cell Cycle Control System
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Molecules present in the cytoplasm regulate progress
through the cell cycle
Cyclic changes in regulatory proteins works as a cell cycle
clock through signal transduction pathways
• Cyclin-Dependent Kinases, Cdks
• Cyclins
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External factors
• Density-dependent inhibition of cell division
• Crowded cells stop dividing
• Anchorage Dependence
• Cell must be attached to a substrate in order to divide
Checkpoints
G1
•Most important restriction point
•If cell does not receive a go-ahead signal at this
checkpoint, the cell exits the cell cycle and goes into
G0
G2
M
G0
•Non-dividing state
Cancer ~ Loss of Cell Cycle Controls
 Transformation
• Process that converts a normal cell into a cancer cell
• If left unchecked by the immune system a tumor may
develop
• Benign
• Malignant
 Metastasis
• Spread of cancer cells to new locations