Transcript Why do Cells Divide?

Why do Cells
Divide?
Cell Size Limits
1. Replacement of Cells
 Humans need to replace old, worn
out cells from normal wear and tear
2. Rate of Diffusion
 Remember, the cell membrane allows
nutrients (ex. Glucose, oxygen, etc.)
in and wastes out (diffusion)
 Diffusion is fast over short distances
but slow over long distances
 So, b/c of slow rate of diffusion, cells
can’t be giants b/c they would starve
to death or be poisoned from the build
up of wastes!!
Diffusion Animation
 Remember diffusion works best over
short distances!!
 diffusion animation
3. Need enough DNA to
support protein needs of cell
 Since DNA codes for making protein
there needs to be enough to support
the protein needs of the cell
ex. Think how much protein is in the
cell membrane and if the cell
size increases than you would
need more protein to build it
4. Surface Area to Volume
Ratio
 Need a proper SA:Vol ratio to
support the needs of the cell
 You need the cell to remain small!!
 For example, if the cell size doubles
it would need 8x more nutrients to
survive and it would create 8x the
waste to excrete!!
 The vol. increases faster than the
surface area of the cell membrane!!
Reminder from cell theory
 All cells come from pre-existing
cells!!
-Cell Division follows this b/c it’s the
process where new cells are made
from 1 cell
 Cell Division makes 2 new cells that
are identical to the original
Cell Division
Requirements
 I) Cell Division
 A. Mitosis- nuclear division *
 B. Cytokinesis- Division of the
cytoplasm
Cell Cycle- Interphase
Interphase (Nucleus enclosed in
nuclear membrane, nucleolus
present, chromosomes appear as
chromatin)
 a) G1 phase- intense cellular
activity and growth
 b) S phase- DNA synthesischromosomes replicate
Interphase continued
 c) G2 phase- spindle fibers are
assembled/ centrioles
replicate
- cell prepares for cell division
Interphase Diagram
MITOSIS PHASES
Prophase (1st stage)
Prophase- 1st stage of actual cell division
 a.) Chromosomes condense and shorten,
become distinguishable
b.) Chromatids attached near center by
centromere
 c.) Centrioles separate, migrate to the poles
 d.) Spindle fibers form between centrioles,
asters form
 e.) Nuclear membrane disintegrates,
nucleolus disappears (in order to
separate contents of nucleus, it must
disappear)
Prophase
Early prophase
Late prophase
Metaphase(shortest)
nd
2
stage
 a.) chromosomes line up in pairs
at equator (center of cell)
 b.) centromere of each
chromosome is attached to a
separate spindle fiber
Metaphase diagram
Metaphase
Anaphase- 3rd stage
 a.) Centromere of each chromosome
separates
 b.) Separation of the chromatids in each
pair
 c.) Spindle fibers appear to shorten,
pulling the chromatids apart at the
centromere (now called
chromosomes)
 d.) migration of the chromosomes ends
with the arrival at the poles and the
formation of clusters
Anaphase diagram
Anaphase
Telophase –
th
4
stage
 a.) Cleavage furrow forms b/w 2 cells
and spindle fibers disappear
 b.) Nuclear membrane forms around
each set of chromosomes (forms 2
new nuclei)
 c.) Chromosomes uncoil to form
chromatin net (getting ready for
interphase)
 d.) Nucleoli reorganized
Telophase Diagram
Telophase
Cytokinesis
 Cytokinesis- division of the
cytoplasm
 In animal cells- cell membrane
pinches together, furrow forms along
the equator.
 -In plant cells- cell plate forms in the
middle of the dividing cell