CELL DIVISION
CH 9
Process by which a cell splits in 2
2 types: mitosis and meiosis
I. Why Do Cells Divide?
A. Growth
Organisms grow in size by
producing more cells
B. Repair
Many damaged cells are
replaced by new ones thru
mitosis
C. Reproduction
Organisms reproduce by cell
division
II. When Do Cells Divide?
Cells divide when they get too big
A. Surface – to – volume theory
 Explains why cells are so
small
 As cells increase in size,
volume (the cytoplasm)
increases at faster rate than
surface area (cell membrane)
 At a certain size, the cell
membrane can’t transport
enuf stuff to satisfy the
cytoplasm
 At this size, the cell divides
III. Cell Division By Mitosis
Cell division that produces 2
identical daughter cells
https://www.youtue.com/watch?v=mx8itzrdV7I
A. The Cell Cycle
 Period from the beginning of one
cell division to the beginning of
the next
 Three parts of the cell cycle:
o Interphase: cell prepares to
divide
o Mitosis: division of the
nucleus
o Cytokinesis: division of the
cell
B. Chromosomes and Cell Division
1. Structure of Chromosomes
 Made of protein and DNA
o DNA: has genetic info
o Protein: helps condense
DNA so it fits in the nucleus
2. Two forms of chromosomes:
Chromatin
chromosomes
Loosely coiled
highly coiled
In interphase
And cytokinesis
In mitosis
C. The Process of Cell Division by
Mitosis
1. Interphase
 Cell grows and prepares for cell
division
 3 parts:
i. G1 phase: cell grows in size
ii. S phase: DNA replicates
making 2 exact copies. The 2
copies are called sister
chromatids and they are held
together at the centromere
iii. G2 phase: makes spindle
proteins needed for cell
division
2. Mitosis
 Division of the nucleus
 Occurs in 4 stages:
i. Prophase:
o Chromatin condenses to
chromosomes
o
Centrioles move to
opposite sides of the cell
o Nucleus disappears
o Spindle fibers attach to
centromere on both sides
ii. metaphase
o chromosomes line up in
center of cell at the
metaphase plate
iii. anaphase
o sister chromatids
separate and move to
opposite sides of the cell
iv. telophase and cytokinesis
o chromosomes
decondense to chromatin
o nuclei reform
o spindles disappear
Cell splits in 2 in cytokinesis
Cytokinesis in animal cells: membrane
pinches in forming cleavage furrow
and cell splits
Cytokinesis in plant cells: cell wall
and cell membrane are built from the
center outwards
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.html
D. Mitosis and Cancer
1. What is cancer?
Uncontrolled cell division
2. How does this occur?
Mutations in genes that control
the cell cycle result in
uncontrolled cell division
3. How is cancer treated?
With drugs that inhibit cell
division. Some drugs inhibit
DNA replication, some drugs
inhibit spindle formation.
4. What effect do these drugs
have on noncancerous cells?
IV. Cell Division by Meiosis
 Cell division that reduces the
chromosome # by ½
 ONLY occurs in cells that
produce egg and sperm
(gametes)
A. Homologous Chromosomes
 Pairs of chromosomes that
have the same genetic
information on them but
maybe in slightly different
form
 One of each pair came from
mom and other from dad
 All cells EXCEPT EGG AND
SPERM have homologous pairs
of chromosomes
B. Diploid Cells and Haploid Cells
1. Diploid cell
 A cell that has homologous
pairs of chromosomes
 Ex: skin cells, nerve cells…
2. Haploid cell
 A cell that has one of each
pair of homologous
chromosomes
 Ex: only egg and sperm
Diploid cells are 2n and haploid cells
are n where n = number of pairs
C. The Process of Meiosis
 Chromosome number is
reduced by ½
 Only occurs in cells that make
egg and sperm
 Occurs in 3 stages:
interphase, meiosis I, and
meiosis II
1. Interphase
 Same as mitosis
 G1, S, G2: cell grows,
replicates DNA, and prepares
to divide
2. Meiosis I
 Separates homologous
chromosomes
a. prophase I
o chromatin condenses to
chromosomes
o homologous chromosomes
pair
o nucleus disappears
o centrioles move to opposite
sides of cell
o spindles attach to one side of
centromere
o homologous chromosomes
exchange DNA
b. Metaphase I
o homologous pairs line up in
the center of the cell
c. anaphase I
o homologous pairs separate
d. telophase I and cytokinesis
o nuclei reform and spindles
break apart
o cell splits
3. Meiosis II
 identical to mitosis
 separates sister chromatids
a. prophase II
o nuclei disappear
o spindles reform and attach
to BOTH sides of centromere
o centrioles move to opposite
sides of cell
b. metaphase II
o chromosomes line up in
center of cell
c. anaphase II
o sister chromatids separate
d. telophase II and cytokinesis
o nuclei reform
o spindles disappear
o chromosomes decondense
to chromatin
o cell splits
MEIOSIS I
MEIOSIS II
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter28/animation__unique_features_of_meiosis.html
D. Variation and Meiosis
 Meiosis is used in sexual
reproduction
 Purpose of sexual reproduction
is to provide genetic variation
for evolution
2 ways genetic variation is
achieved thru meiosis:
o Random separation of
homologous chromosomes
 The way the homologous
chromosomes pair up
during prophase I is
random
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter28/animation__random_orientation_of_chromosome
s_during_meiosis.html
o
Genetic recombination due
to crossover
 Homologous pairs
exchange DNA during
prophase I
V. Compare and Contrast Meiosis
and Mitosis
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter28/animation__comparison_of_meiosis_and_mitosis
__quiz_1_.html