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The Great DivideThe Great Divide
05/04
DO NOW…DO NOW…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?
Why Would a Cell Why Would a Cell Divide?Divide? As cells absorb nutrients and get larger,
the volume of the cell increases, and a cell can no longer absorb nutrients and get rid of wastes fast enough.
So what’s a cell to do?
Solution: divide in 2!
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
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 (1st gap) = small cell is absorbing nutrients, growing & making proteins
S (synthesis) = cell is continuing
to grow & duplicates its DNA (i.e. chromosomes) in preparation for making duplicate cells during mitosis
G2 (2nd gap) = cell keeps grow- ing & making proteins; it
grows too big…solution = divide in 2
The Mitotic PhaseThe Mitotic Phase Equal distribution of chromosomes (DNA)
into 2 identical daughter cells
Divided into 4 stages of Mitosis:
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
ProphaseProphase
DNA condenses (gets shorter & thicker) so they are now visible
Appear as sister chromatids
Nuclear membrane dissolves
The centrioles move to opposite poles & spindle fibers form between them http://www.biostudio.com/demo_freeman_dna_coiling.htm
MetaphaseMetaphase
Chromosomes line-up along the center and attach to the spindle fibers
AnaphaseAnaphase
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 cytoplasm
distributed equally between the 2 new cells
In animals, a cleavage furrow forms from outside in
In plants, a cell plate forms from inside out
PlantAnimal
What Mitosis Actually Looks What Mitosis Actually Looks LikeLike
InterphaseProphase Metaphase
Anaphase Telophasehttp://www.sci.sdsu.edu/multimedia/mitosis/mitosis_gif2.html
http://science.nhmccd.edu/biol/bio1int.htm
What Happens After What Happens After Mitosis?Mitosis? The cell
returns to interphase
Chromosomes uncoil back into thin strands of DNA
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
Identify the phase in the following 18 cells: