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The Great DivideThe Great Divide
“MITOSIS IN A NUTSHELL”
To Be To Be Answered…Answered…THINK:
How many cells are you composed of?
When an organism grows bigger do you get more cells or just bigger cells or both?
When do your cells divide the fastest? Slowest?
Do cells ever stop dividing?
Are all cells capable of division and replacement?
WHY DO CELLS DIVIDE?•To keep a workable ratio
of surface area to volumeV = l x w x h SA = l x w x 6
1 cm 2cm 3 cm V = 1 cm3 V = 8 cm3 V = 27 cm3
SA = 6 cm2 SA = 24 cm2 SA = 54 cm2
Ratio = 6/1 Ratio = 24/8 Ratio = 96/64 6:1 3:1 2:1-As cell gets larger, ratio decreases
Why Would a Cell Why Would a Cell Divide?Divide? As cells absorb nutrients and get larger,
the volume of the cell increases faster than the surface area
This means that a cell can no longer absorb nutrients and get rid of wastes fast enough to support its demands (volume) = STARVE TO DEATH
So what’s a cell to do?
Solution: divide in 2!
Surface area for exchange not great enough to support cell’s needs
When Would a Cell When Would a Cell Divide?Divide? Growth
Repair or Replacement
Cancer
Different cells divide at different rates:
Most mammalian cells = 12-24 hours
Some bacterial cells = 20-30 minutes
Getting Older…Getting Older… All cells are only allowed to complete a
certain number of divisions
Then they die (programmed cell death)
How does cell division change over a lifetime?
Childhood = cell division > cell death
Adulthood = cell division = cell death
The Later Years = cell division < cell death
The Cell CycleThe Cell Cycle
INTERPHASE
Stages of the Cell Stages of the Cell CycleCycle 2 stages = interphase (growth &
replication of DNA) & mitotic phase (division of cell into 2 daughter cells)
Cell spends about 90% of the time in interphase
InterphaseInterphase Divided into 3 phases:
G1 = Growth Phase – Cells grow and develop, undergo protein synthesis and rapid growth
S = Synthesis Phase - DNA replication occurs (i.e. chromosomes copied) in preparation for making new cells during mitosis
G2 =Second Growth Phase = centrioles replicate, cell prepares for cell division, makes new cell parts
Becomes too large = divide in 2
The Mitotic PhaseThe Mitotic Phase Equal distribution of 2 sets of chromosomes
(DNA) into 2 identical daughter cells
Divided into 4 stages of Mitosis:
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Cell Cycle TidbitsCell Cycle TidbitsHow long is one cell cycle?
Depends on the cell- skin cells = ~24 hours, nerve cells = never after maturity, cancer cells = very short
Remember: every cell only has a certain # of divisions it can undergo, then it dies = (programmed cell death)
Mitosis: A Closer LookMitosis: A Closer Look
Prior to entering the mitotic phase, the cell has just come out of Interphase
Replicated DNA during S (synthesis)
2 complete set of chromosomes that must be distributed equally between 2 cells = mitosis
Interphase
ProphaseProphase Chromatin
condenses visible chromosomes
Appear as sister chromatids held together by centromere
Nuclear membrane dissolves
The centrioles migrate to opposite poles & spindle fibers form between them
Duplicated Chromosome
Mitotic Appearance (Prophase)
CentrioleCentriole CentromereCentromere SpindleSpindle Chromosome Chromosome
(chromatids) (chromatids) AsterAster
MetaphaseMetaphase
Chromosomes line-up on the metaphase plate
Centromeres are attached to spindle fiber
AnaphaseAnaphaseCentromeres divide
Spindle fibers contract
Result = sister chromatids are pulled away from one another towards the poles
TelophaseTelophase
The chromosomes reach the poles
Nuclear membranes form around the 2 new nuclei
CytokinesisCytokinesis The cytoplam distrubted equally between the 2 new cells
In animals, the cell membrane pinches together
In plants, a cell plate forms from inside out
PlantAnimal
Cytokinesis in plants
A cell plate made up of cell-wall components gradually forms in the middle of the cell.
Cytokinesis in animals
A cleavage (or division) of the animal cell. The membrane pinches together forming 2 cells
Cleavage Furrow
ANIMAL VS. PLANT MITOSIS
ANIMAL CELLANIMAL CELLCentriole and Centriole and
aster presentaster presentDaughter cells Daughter cells
separated by separated by cleavage cleavage furrowfurrow
PLANT CELLPLANT CELLNo visible No visible
centriole or centriole or asteraster
Daughter cells Daughter cells separated by separated by cell platecell plate
What Mitosis Actually Looks What Mitosis Actually Looks LikeLike
InterphaseProphase Metaphase
Anaphase Telophase
Figure 9.8 – Part 2
Figure 9.8 – Part 2
Mitosis consists of several different phases.
Summary of MitosisSummary of Mitosis
What Happens After What Happens After Mitosis?Mitosis?
The cell returns to interphase
Chromosomes uncoil back into chromatin
The cycle repeats itself over & over…
At What Stage Are Our Cells At In The Cell At What Stage Are Our Cells At In The Cell Cycle?Cycle?
Different cells can be in different stages
Interphase
Mitosis:
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Can You Identify the Stages of Can You Identify the Stages of Mitosis?Mitosis?
Put the following mitosis stages in the correct sequence
MitosisCan you name the stages?
A
B
C
D
E
The GuaranteeThe Guarantee
The product of mitosis is 2 cells
The daughter cells are identical to each other & to the mother cell
Mother cell
Identical daughter
cellsWhy is this so important?
The Daughter CellsThe Daughter Cells
In humans, the 2 daughter cells will have 46 chromosomes (23 pairs)
1 chromosome originally from mom & 1 from dad
Each chromosome is said to have the same gene sequence
Mother cell
Identical daughter
cells