Mitosis & Meiosis
AHSGE Science Standards
6
Describe the roles of mitotic &
meiotic divisions during
reproduction, growth & repair of
cells.
6a Comparing sperm & egg
formation in terms of ploidy.
6b Comparing sexual & asexual
reproduction.
I. DNA Replication
A.
Cells must be able to divide in
order for the organism to grow,
reproduce & repair itself.
B. Multicellular organisms are
made of 2 kinds of cells:
–1. reproductive cells (sex cells)
–2. somatic cells (body cells)
I. DNA Replication

C. Both kinds of cells contain DNA, which is stored in the
nucleus in the form of chromatin.
– 1. Chromatin consists of long strands of DNA, jumbled
up with proteins, that together form a kind of
disorganized mass of genetic material in the nucleus.
– 2. When the cell is ready to divide, the chromatin coils
& condenses to form chromosomes.
I. DNA Replication
D. Reproductive cells (sex cells) have a single
set, or haploid number (n), of chromosomes.
 E. Somatic cells (body cells) have 2 sets, or a
diploid number (2n), of chromosomes.

I. DNA Replication

F. When the cell divides, the chromosomes
must be distributed between the newly
produced cells.
– 1. This means that the DNA must be able to
copy itself through the process of replication.
II. The Cell Cycle
A. The cell
cycle is the
sequence of
stages through
which a cell
passes
between 1 cell
division & the
next.
II. The Cell Cycle
B.
Most of the
cell cycle is
spent in
interphase.
See Figure 4.4
The Cell Cycle
II. The Cell Cycle
–1. Interphase
consists of 3
major parts:
a. G1
b. S
c. G2
II. The Cell Cycle
 1.
During the G1 phase of interphase,
the cell grows in size.
 2. In the S phase, replication of the
DNA containing the genetic material
occurs, which gives the cell a double
amount of DNA.
 3. In the G2 phase, the cell prepares
for mitosis by replicating organelles &
increasing the amount of cytoplasm.
III. Mitosis
 A.
Somatic cells (body cells) undergo a
process called mitosis.
– 1. Mitosis is a type of cell division that
generates 2 daughter cells with the
identical components of the mother cell
(parent cell).
– 2. Mitosis is the mechanism for asexual
reproduction, which only requires 1
parent.
– 3. Mitosis also allows multicellular
organisms to grow & replace cells.
III. Mitosis
B.
The stages of mitosis are:
–1. Prophase
–2. Metaphase
–3. Anaphase
–4. Telophase

III. Mitosis
1. Prophase
– a. The nucleus of the
cell organizes the
chromatin material into
thread-like structures
called chromosomes.
– b. The centriole, in
animal cells only,
divides & moves to
each end of the cell.
– c. Spindles form
between the centrioles.
III. Mitosis
 2.
Metaphase
–a. The
chromosomes
attached at the
center, or
centromeres, line
up on the spindle
at the center of
the cell.

3. Anaphase III.
– a. Chromosomes
separate at the
center, & the
spindles pull them
toward either end
of the cell
– b. A nuclear
membrane forms
around the
chromosomes as
they disorganize.
Mitosis
III. Mitosis
 4.
Telophase
– a. Chromatin
again forms from
the chromosomes,
& a cell
membrane begins
to grow across the
center between
the 2 new nuclei.
See Figure 4.5
Stages of Mitosis
IV. Cytokinesis

A. Cytokinesis, the
division of the cell
cytoplasm, usually follows
mitosis.
– 1. Cytokinesis generally
begins during the
telophase of mitosis.
– 2. It finalizes the
production of 2
daughter cells, each
with approximately half
of the cytoplasm &
organelles as well as 1
of the 2 nuclei formed
during mitosis.
IV. Cytokinesis
B. The
processes of
mitosis &
cytokinesis are
together called
cell division.
V. Meiosis
A. Meiosis is a
type of cell
division
necessary for
sexual
reproduction.
V. Meiosis
 B.
It is limited to the reproductive cells in
the testes, the sperm cells, & the
reproductive cells in the ovaries, the eggs.
 C. Meiosis produces 4 reproductive cells
or gametes.
–1. These 4 cells contain half the #
(haploid) of chromosomes of the mother
cell, & the chromosomes are not
identical.
V. Meiosis
 D.
There are 2
phases of cell
division:
– 1. Meiosis I
– 2. Meiosis II
 E. Before meiosis
begins, each pair of
chromosomes
replicates while the
cell in its resting
phase (interphase).
V. Meiosis

F. During meiosis I, each
set of replicated
chromosomes lines up with
its homologous pair.
– 1. Homologous
chromosomes are
matched pairs of
chromosomes.
 a) They are similar in
size & shape & carry
the same kinds of
genes.
 b) They are not
identical because each
set usually comes
from a different
parent.
V. Meiosis
G. The homo pairs of
chromo can break &
exchange segments
during the crossing
over process, a source
of genetic variation.
 H. They then
separate.
 I. The cell splits into 2
daughter cells, each
containing 1 pair of the
homo chromo. See
Figures 4.6 & 4.7

V. Meiosis
J. Interkinesis is the resting
period before meiosis II begins.
K. During meiosis II, 2 daughter
cells divide again without
replication of the chromosomes.
L. The result is 4 gametes, each
having half the # of chromosomes
of the mother cell.
V. Meiosis
1. In males,
all 4
gametes
produce a
long whiplike tail.
V. Meiosis
 2. In females,
1 gamete forms
an egg cell with
a large supply
of stored
nutrients.
–a. The other
3, called polar
bodies,
disintegrate.
V. Meiosis
 M.
In humans, the
body cells have 23
different pairs or a
diploid (2n) # of 46
chromosomes total.
– 1. Each egg &
each sperm have
23 single or
haploid (n) # of
chromosomes.
See Figure 4.8
Complete Section
Review 1:
Reproduction of Cells
VI. Asexual VS Sexual Reproduction
 A.
Asexual reproduction by mitosis is
a careful copying mechanism.
–1. Some unicellular organisms, like
the amoeba, & many plants
reproduce asexually.
–2. The offspring produced are
always genetically identical to the
parent.
VI. Asexual VS Sexual Reproduction
 B.
Sexual reproduction by meiosis
brings with it the enormous potential
for genetic variability.
–1. The # of possible chromosome
combinations in the gametes is 2n,
where n is the haploid chromosome
# & 2 is the # of chromosomes in a
homologous pair.
VI. Asexual VS Sexual Reproduction
–2. See Figure 4.9
a. When n=2, 4 distinct distributions
are possible.
b. When n=3, 8 distinct distributions
are possible.
–3. If humans have a haploid # of n=23,
then 223, or 8,388,608 distinct
distributions are possible.
–4. This is only the genetic variation that
occurs before fertilization.
VII. Fertilization & Cell Differentiation
A. The haploid gametes
produced during meiosis
are spermatozoa in
males & ova in females.
 B. During fertilization,
these gametes fuse to
form a new diploid
parent cell called the
zygote.
– 1. The zygote is 1 cell
with a set of 2n
chromosomes.

VII. Fertilization & Cell Differentiation
 C.
Each parent
contributes 1
homolog to each
homologous pair of
chromosomes.
 D. It then begins
the process of
mitosis to grow in
size becoming an
embryo.
VII. Fertilization &
Cell Differentiation
 E.
The group of
cells produced in
the very early
stages of the
embryo’s growth
are similar to the
original zygote.
–1. They are
called embryonic
stem cells.
VII. Fertilization & Cell Differentiation
F.
When the embryo reaches 20150 cells in size, this group begins
to produce cells that are different
from themselves
–1. This process is called cell
differentiation.
VII. Fertilization & Cell Differentiation
 G.
The cells become specialized &
later become tissues.
 H. As each cell differentiates, it
produces proteins characteristic to its
specific function.
 I. Stem cells have the capability to
become any type of cell.
Complete Section Review 2:
Reproduction, Fertilization,
& Cell Differentiation
Complete Review