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Cell Division
MITOSIS
When a living thing grows
are its cells getting bigger
or is the organism producing
more of them?
 It is producing more cells. Cells of an adult are
usually no larger than cells of a younger
animal.
Why don’t our cells just
grow larger?
Cell Growth
 There are many reasons cells just can’t grow to be
huge . . .
 DNA overload – DNA controls the cell’s functions, but if
the cell were much larger the nucleus could not direct
all of that activity
 Material Exchange – if our cells were larger it would
take materials a long time to reach their destinations
(cell membrane)
 Cell Death – if we only had a few large cells and one
died it would be very detrimental to the entire organism
So, when cells reach a
critical size they . . .
Cell Cycles
 Cells go through a
cycle that
determine when
they should grow
and when they
should divide.
 There are four stage to the cell cycle:




G1 (Gap 1)
S Phase (DNA Synthesis)
G2 (Gap 2)
M-Phase (Mitotic phase)
Gap 1
 The cell grows
 They make new proteins and organelles
S-Phase
 During this phase the DNA in the nucleus is
replicated
 That means each leg of the chromosome is
copied
Gap 2
 Organelles required for cell division are
created for example centroile
 The centrioles are organelles that play a very
important role in mitosis
M-Phase
 This is the phase the cell goes through to divide
 Cell Division – the process cells go through to
split into two new daughter cells
 There are two important parts to this:
 Mitosis - the process eukaryotes go through to
replicate and divide their nucleus
 Cytokinesis – the division of the cytoplasm during cell
division
DNA
 One of the most important parts of cell
division is splitting up the DNA so that all cell
functions can be performed
 There two different forms of DNA
 Chromatin – DNA unwound with no particular
shaoe
 Chromosomes – DNA that is tightly wound into an
X shape
Chromosomes
Sister
Chromatids
Centromere
Mitosis
 There are four
stages to mitosis
 Prophase
 Metaphase
 Anaphase
 Telophase
Prophase
 During prophase the chromatin starts to
condense (thicken and shorten) forming
chromosomes
 The nuclear membrane begins to break down
 The centrioles move to opposite end of the
cell
 The centrioles start to create the spindle
fibers
Prophase
Metaphase
 During metaphase the
chromosomes line up
along the middle of
the cell
 The spindle fibers
lengthen and attach
to the centromere on
each chromosome
Metaphase
Anaphase
 During anaphase the centromeres separate
splitting up the sister chromatids
 The spindle fibers shorten pulling the sister
chromatids towards the centrioles at the
opposite ends of the cell.
 Anaphase ends when the chromatids reach
the opposite ends of the cell and stop moving
Anaphase
Telophase
 During telophase the nuclear envelop reforms
around the chromatids
 As the this happens the chromatids relax
becoming chromatin again
 The nucleolus also reappears
Telophase
Cytokinesis
 This usually happens about the same time as
telophase
 The cytoplasm separates and all of the
organelles separate into the new cells
Cytokinesis in Animal Cells
 The cell membrane pinches inward until it
touches.
 The cells
break off
and become
separate cells
Cytokinesis in Plant Cells
 A cell plate forms
down the middle
of the cell
 The cell
membranes
separates at the cell plate
 The cell wall begins to form in the middle
of the cell plate
Mitosis Animation
 http://www.teachersdomain.org/asset/lsps07
_int_celldivision/