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Chapter 12
The Cell Cycle
Why do cells divide?
• The continuity of life is based on the
reproduction of cells: cell division
• Cells divide to:
– Reproduce
– Renewal
– Repair
– Replacement
– Make new cells
Cell Cycle
• The cell cycle is the entire life of the cell,
including growing and reproducing.
Organization of Genetic Material
• DNA: our genetic
material, our genes
• Chromatin: DNA and
proteins
• Chromosomes: threadlike structures in the
nucleus that are made
of chromatin
• Genome: all of our DNA
Chromosome Duplication
• Before a cell can divide,
chromosomes must
duplicate.
• Each duplicated
chromosome has two
identical sister chromatids,
attached at a centromere.
Phases of the Cell Cycle
• Interphase: 90% of the cell’s life, during which
growth, protein synthesis, and chromosome
duplication occurs. Has 3 sub-phases:
• G1 phase: “first gap” the cell grows
• S phase: “synthesis” chromosomes duplicate
• G2 phase: “second gap” the cell grows some
more and prepares for division
Phases of the Cell Cycle
• The other phase is the Mitotic Phase (M)
during which the cell divides. It has 2 subphases:
• Mitosis: when the nucleus divides
• Cytokinesis: when the cytoplasm and the rest
of the cell divides
Cell Cycle
The Mitotic Spindle
• The mitotic spindle is made of microtubules and proteins
which help move the chromosomes around.
• It is made in the centrosome (an organelle)
• Asters are short microtubules that extend out from the
centrosome
• Kinetochores are where the asters attach to chromosomes
The Phases of Mitosis
•
•
•
•
•
•
Prophase
(Prometaphase)
Metaphase
Anaphase
Telophase
Cytokinesis
Prophase
• Chromatin condenses and can be seen in the
microscope
Prometaphase
• The nuclear membrane breaks up
• Microtubules begin to get into place around
the chromosomes
Metaphase
• Longest phase (20 minutes!)
• Chromosomes line up in the middle of the cell
(metaphase plate)
• Kinetochores connect centromeres to spindle poles
Anaphase
• Shortest phase (2-3 minutes)
• Sister chromatids pull apart and begin to
move to opposite sides of the cell
Telophase
• Two new nuclei (daughter nuclei) begin to
form
• New nuclear membranes begin to form
Cytokinesis
• The cytoplasm of the parent cells divide to
form 2 new daughter cells
Cytokinesis in Animals
• In animals, a cleavage furrow forms to pinch
the cell in two
Cytokinesis in Plants
• In plant cells, a cell plate made of vesicles
filled with cellulose, forms in between the two
new cells to make a new cell wall.
Binary Fission
• Prokaryotes (bacteria and
Archea) reproduce by binary
fission, meaning “division in
half.”
• Bacteria have one chromosome,
which is a a big circle, which
reproduce starting at the origin
of replication.
• After DNA is duplicated, the
plasma membrane pinches
inward and a new cell wall grows
between the daughter cells.
Regulation of the Cell Cycle
• The cell cycle is driven by molecular signals
present in the cytoplasm, called the cell cycle
control system.
• Checkpoints in the cell cycle are critical point
where the cell is told to “stop” or “go-ahead”
• G1 checkpoint: “restriction point” if the cell is
told to stop it goes to G0 non-dividing phase
(like muscle and nerve cells)
Kinases and Cyclins
• Kinases are enzymes that
active or inactive proteins
of the cell cycle
• Cyclins are proteins that
must be attached to
kinases to be active; they
are cyclin dependent
kinases (Cdks)
• MPF “maturationpromotion factor”
triggers G2
How cells grow
• Cells don’t grow if they are over-crowded,
which is called density-dependent inhibition
• Most cells are anchorage dependent which
means that they must be attached to
something to grow
• Cancer cells are NEITHER of these things, they
are cells growing out of control wherever they
want.
Cancer
• Cancer cells are cells that divide excessively
and invade other tissues.
• They do not heed the normal signals that
regulate the cell cycle.
Cancer
• When a normal cell becomes cancerous, this is
called transformation.
• If it stays at the original site, this is a benign
tumor.
• If it metastasizes, this means it moves to
another tissue, which is what malignant
tumors do.
Cancer Treatment
• Radiation: damages cancer cell DNA
• Chemotherapy: toxic drugs interfere with the
cell cycle of rapidly dividing cancer cells
• Surgery: physically cut out the cancer cells