The Great Divide
05/04
Why Would a Cell 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)
▪ So what’s a cell to do? DIVIDE !!!!
Why Would a Cell Divide?
Besides growth a cell would also divide for:
▪ Repair or Replacement
▪ Cancer
Different cells divide at different rates:
▪ Most mammalian cells = 12-24 hours
▪ Some bacterial cells = 20-30 minutes
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
Cell Cycle Tidbits
How 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 = apoptosis (programmed
cell death)
The Cell Cycle
Stages of the Cell Cycle
There are two stages to a cells life.
interphase (growth & replication of DNA)
mitotic phase (division of cell into 2 daughter cells)
▪ Cell spends
about 90% of
the time in
interphase
Interphase
▪ Divided into 3 phases:
▪ G1 (1st gap) = small cell is absorbing nutrients,
growing & doing its job.
▪ S (synthesis) = cell is continuing to grow &
duplicates its DNA.
▪ G2 (2nd gap) = cell keeps growing & doing its
job.
Mitosis: A Closer Look
DNA is all twisted up into a
Chromosome.
▪ During S phase the
Chromosome is copied.
▪ 2 complete identical
sets of chromosomes.
▪ They are connected in
the middle by a
centromere.
▪ A single copied
chromosome is called a
Chromatid.
The Mitotic Phase
▪ Divided into 4 stages of Mitosis:
▪ Prophase
▪ Metaphase
▪ Anaphase
▪ Telophase
(+) PLUS
▪ Cytokinesis
Prophase
▪ Chromatids condense
becoming visible.
▪ Nuclear membrane
dissolves
▪ The centrioles (an organelle
that makes microtubules)
appears and migrate to
opposite sides.
▪ spindle fibers start to form
between them
▪ http://www.biostudio.com/demo_freeman_dna_coiling.htm
Metaphase
▪ Chromosomes
line-up on the
metaphase
plate
▪ Centromeres
are attached to
spindle fibers
Anaphase
▪ Spindle fibers contract
▪ Centromeres divide
▪ Sister chromatids are
pulled away from each
other towards the poles
Telophase
▪ The chromosomes
reach the poles
▪ Nuclear
membranes form
around the 2 new
nuclei
Cytokinesis
▪ The cytoplasm
distributed equally Animal Plant
between the 2 new
cells
▪ In animals, a
cleavage furrow
forms from outside in
▪ In plants, a cell plate
forms from inside out
What Mitosis Actually Looks Like
Interphase
Prophase Metaphase
Telophase
Anaphase
http://www.sci.sdsu.edu/multimedia/mitosis/mitosis_gif2.html
http://science.nhmccd.edu/biol/bio1int.htm
What Happens After 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 Cycle?
▪ Different cells can
be in different
stages
▪ Interphase
▪ Mitosis:
▪ Prophase
▪ Metaphase
▪ Anaphase
▪ Telophase
▪ Cytokinesis
The Guarentee
▪ The product of
mitosis is 2 cells Mother
cell
▪ The daughter cells
are identical to
each other & to the
mother cell
Identical
daughter
cells
Why is this so
important?
The Daughter Cells
▪ In humans, the 2
daughter cells will have
46 chromosomes (23
pairs)
▪ Each chromosome is
said to have the same
gene sequence Identical
daughter
cells
The Beauty of Asexual Reproduction
▪ Mitosis is a
form of Mother
cell
asexual
reproduction
Runners produces by
▪ New strawberries
individuals are
produced by 1 Identical
Budding by hydra & yeast
daughter
parent & thus, cells
are identical to
their parent Cuttings from plants
