Review
 What is the purpose of mitosis?
 What is the cell cycle?
 What are the phases of mitosis?
 Describe at least one event from each phase.
Chapter 12: The Cell Cycle
 Rudolf Virchow-1855 “Omnis cellula e cellula”
Every cell from a cell.
 Cells reproduce to form genetically equivalent
daughter cells.

Roles of Cell Division




Reproduction
Growth
Repair
In all cases, cell
division must
distribute
identical genetic
material to two
daughter cells.
Genome
 The cell's hereditary
endowment of DNA.
 Usually packaged into
chromosomes for
manageability.

Chromosomes

Made of a DNA and protein
complex called Chromatin.
 During cell division, the
chromatin becomes highly
condensed into the
chromosomes.
Chromosomes - Structure
 At cell
division, each
chromosome
has been
duplicated.
 The
duplicated
chromosome
consists of
two sister
chromatids.
Cell Cycle - parts
1. Interphase - (90% of cycle) - when the cell
grows and duplicates the chromosomes.
2. Mitotic Phase (M) - when the chromosomes
are split into separate cells.

Mitosis – equal division of replicated chromosomes,
by separation of the sister chromatids into separate
cells.

Cytokinesis - division of the cell’s cytoplasm.
Cell grows and carries out
regular biochemical functions.
When the DNA is replicated
or synthesized.
Cell completes preparations for division.
Centrosomes and organelles are replicated.
Interphase
Centrosome, made
of two centrioles is
replicated.
Mitosis Steps
 Prophase
 Prometaphase
 Metaphase
 Anaphase
 Telophase
Prophase
Prophase
1. Nucleoli disappear.
2. Chromatin condenses into the
chromosomes.
3. Centrosomes separate to opposite ends of
the cell.
4. Mitotic spindle begins to form.
Prophase
Prometaphase
Prometaphase
1. Nuclear envelope dissolves.
2. Spindle fibers attach to the kinetochore (a
structure of proteins and specific sections of
chromosomal DNA) of the centromeres.
Prometaphase
Metaphase - movie
Metaphase
1. Centrosomes now at opposite ends of the
cell.
2. Chromosomes line up on the metaphase
plate.
3. Spindle apparatus fully developed.
Metaphase
Anaphase
Anaphase
1. Centromeres break and the duplicate
chromosomes are pulled away from each
other toward opposite ends of the cell.


Kinetochore appear to “ratchet” the
chromosome down the spindle fiber
microtubule with a motor protein.
Microtubules dissolve behind the kinetochore
(shown with laser microbeam wxperiment)
2. Cell elongates; poles move slightly further
apart.
Microtubules
shorten only on
the kinetochore
side.
Anaphase
Telophase
Telophase
1. Chromosomes uncoil back to chromatin.
2. Nuclear envelope reforms.
3. Nucleoli reappear.
4. Spindle fibers disappear.
5. Cytokinesis usually starts.
Telophase
Cyotkinesis
Cytokinesis
Cytokinesis
 Animal


Cleavage furrow forms.
Microfilaments contracts and divides the
cytoplasm into two parts.
 Plants


Cell plate develops from Golgi vesicles.
New cell wall developed around the cell plate.
Cell Plate
Cell Division
Animal Cell - Mitosis
Plant Cell - Mitosis
Evolution of Mitosis
Regulation of Cell Division
 Must be controlled.
 Rate of cell division depends on the cell type.



Ex - skin: frequently
liver - as needed
brain - rarely or never
Checkpoints
 A critical control point in the cell cycle.


Several are known.
Cells must receive a “go-ahead” signal before
proceeding to the next phase.
 G1 Checkpoint


Also called the “restriction point” in
mammalian cells.
Places cells in a non-dividing phase called the
Go phase.
Go Phase
 Non-dividing state.
 Most cells are in this state.
 Some cells can be reactivated back into M
phase from the Go phase.
Protein Kinase Checkpoint - G2
 Uses protein kinases to signal “go-ahead” for
the G2 phase.
 Activated by a protein complex whose
concentration changes over the cell cycle.

M-phase Promoting Factor (MPF).


Protein complex required for a cell to progress
from G2 to Mitosis.
Role of MPF - to trigger a chain of protein kinase
activations.
Active M-phase Promoting Factor (MPF) has:
1.
Cdk (Cyclin dependent kinase)


2.
Cyclin


is a Protein Kinases (activates and deactivates other
proteins by phosphoryalating them.

Amount remains constant during cycle.

Inactive unless bound with cyclin.
When enough cyclin is present, active MPF is
formed.
Protein whose concentration builds up over G1, S and
G2, then falls abruptly during Mitosis..
The activity of Cdk rises and falls with changes in
the concentration of its cyclin partner.
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120082/bio34a.swf::Control%20of%20the%20Cell%20Cycle
Active M-phase Promoting Factor (MPF)
 Triggers Mitosis.
 Activates a cyclin-degrading enzyme, which
lowers the amount of cyclin in the cell.
 Result - no active MPF to trigger another
mitosis until the cycle is repeated.
http://www.cellsalive.com/cell_cycle.htm
http://bcs.whfreeman.com/lodish5e/content/cat_010/21010-01.htm?v=chapter&i=21010.01&s=21000&n=00010&o=00PRS
Growth Factors
 External signals that affect mitosis.

Examples:
1. Platelet-Derived Growth Factor (PDGF)
 Stimulates cell division to heal injuries.
2. Density-dependent inhibition
 The number of cells in an area force competition
for nutrients, space, and growth factors .
 When density is high - no cell division.
 When density is low - cells divide.
3. Anchorage dependence
PDGF
Density-Dependent Inhibition
Growth Factors (Continued)
 External signals that affect mitosis.
 Examples:

3. Anchorage dependence


Inhibition of cell division unless the cell is
attached to a substratum.
Prevents cells from dividing and floating off in the
body.
Cancer Cells
 Do not stop dividing. The control
mechanisms for cell division have failed.
Comment
 Regulation of cell division is a balance
between:
Mitosis - making new cells.
Apoptosis - cell suicide.
 Cancer can result if either process doesn’t
work.
Summary
 Know the phases and steps of the cell cycle.
 Be able to discuss the “regulation” of the cell
mitosis tutorial
http://www.sci.sdsu.edu/multimedia/mitosis/
onion cell mitosis activity
http://www.biology.arizona.edu/Cell_BIO/activities/cell_cycle/cell_cycle.html
cycle.
Cell cycle animation
http://www.cellsalive.com/cell_cycle.htm
mitosis tutorial (U of Arizona)
http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html
animal cell mitosis
http://www.cellsalive.com/mitosis.htm