WHY DO CELLS DIVIDE?
The
larger a cell becomes the
harder it is to move enough
nutrients and waste across the
cell membrane
As the cell grows and get bigger
and bigger the DNA can no
longer meet the needs of the
bigger cell.
CELLS DIVIDE BEFORE THEY
BECOME TOO BIG.
The
cell will divide forming two
identical daughter cells.
Before the cell divides the DNA
is replicated so there is an exact
duplicate DNA in each daughter
cell at the end.
WHY IS CELL DIVISION NECESSARY
Need for growth – a multicellular organism
needs more cells as it grows and develops.
 Needed for repair – worn out or damaged cells
need to be replaced. Some cells like your never
cells and brain cells are not replaced often and
take a long time to regenerate. Other cells like
your skin cells are replaced all the time since
they are more apt to be injured and exposed to a
harsh environment.

CHROMOSOMES
 The
DNA that is passed on to new cells is
carried on the chromosomes. So
chromosomes carry the genetic
information
 Every organism has a specific number of
chromosomes – 46 in humans (23 pairs), 8
(4 pairs) in fruit flies, 48 in werewolves
(24 pairs) and 50 in vampires (25 pairs) 
for those of you reading the Twilight
series
DNA REPLICATION
DNA
must replicate before
mitosis (cell division)
Each daughter cell needs
an exact copy of the DNA
Takes place in the nucleus
DNA
helicase (a group of
enzymes)unzips the two
DNA strands at one end (by
breaking the hydrogen
bonds between the bases)
Free nucleotides bond with
each strand of the DNA –
until two new strands are
made
DNA REPLICATION
 After
DNA is
replicated; two
identical sister
chromatids exist.
 They will split
apart later so each
new cell has
identical genetic
material
Short arm
centromere
Long arm
cromatid
MITOSIS AND CYTOKINESIS
Mitosis
is the dividing of
the nucleus after DNA is
replicated.
Cytokinesis is the dividing
of the cytoplasm at the
very end
INTERPHASE
 Consists
of 3 phases
G1 – period of cell growth
 S phase – key proteins and DNA
molecules are synthesized and DNA is
replicated
 G2 – once DNA replication is complete
the cell enters G2 phase – many of the
organelles and molecules need for cell
division are made

M PHASE
mitosis
(division of the nucleus)
– 4 phases prophase, metaphase,
anaphase and telophase (and
cytokenesis – splitting of the
cytoplasm)takes place
CELL CYCLE

http://www.cellsalive.com/cell_cycle.htm