Chapter 10 – Cell Division
L2 Biology
Size of most eukaryotic cells:

10-100 micrometers (um)
Diameter of high power field of view in
our scopes is 500 um (0.5 mm)
How long is this
paramecium cell?
500 um
Paramecium
400x
Is this cell
average, or
smaller or larger
than average?
A crocodile and an earthworm
have cells the same size.Why is
that?
Read pp 242-243 in your
textbook.
Surface area to volume ratio determines
how long cells can grow before they either
die or divide:
 As cells grow larger, their surface area to
volume ratio (increases or decreases?).
 If the volume is so big that stuff can’t
diffuse into or out of the entire cell in a
reasonable amount of time, the cell will die
from waste buildup or starvation.

If these were cells, which would be most
efficient in taking in and
getting rid of materials in the same amount of
time?
Which square would take longest to get stuff to its middle?
Why do cells divide?

Growth
Repair/regeneration
Reproduction

asexual
Cells go through a life cycle
Cell division – mitosis – 1 hr
Second
growth
phase – 2 hr
First growth phase
interphase
9 hr
Synthesis phase – DNA is copied – 10 hr
Cell cycle phases: These only
occur if a cell is going to divide
G1 = cell grows after being formed
 S – DNA makes a copy of itself in
preparation for cell division
 G2 – short growth phase
 M – mitosis – nucleus is copied


If a cell is not going to divide, it stays in G0
phase – doesn’t prepare for cell division.
Chromatin
Invisible most of the time - Only visible
during cell division (mitosis or meiosis)
 During S-phase – the DNA replicates
(makes an exact copy of itself)
 This means the cell has twice as much DNA
in it after replication
 Once a chromosome has replicated, it
shortens and thickens and can now be seen
in our microscopes.

One Chromosome
Sister
Chromatids

Each strand is an identical
copy of the other one
Centromere
Where the two chromatids
Are attached to each other –
This is different for each
chromosome
DNA
Chromosome Number

Each species has the same number of chromosomes in
all their cells that are made by mitosis. This is the
diploid number (2n). In humans this number is 46. So
cells of your skin and muscle and liver each have 46
chromosomes in them. Look how many chromosomes
are in the cells of these creatures:
2n = 42
2n = 78
2n = 38
2n = 94

These 46 chromosomes are actually pairs, one came
from your father, one from your mother. So each of
your cells has 23 pairs of chromosomes. Each
member of the pair is homologous to the other of that
pair (same length, centromere in same place, genes for
same traits in same location).

When cells divide by mitosis, each daughter cell
receives the same number of chromosomes as its
mother cell has – 2n.
46
46
46

In order to do this, the chromosomes must be
copied first, then one of each copy is placed in the
new cells.
46
46
92
46
22 Homologous pairs of chromosomes;
one pair of sex chromosomes
Sex Chromosomes

Homologous in females: XX

Not homologous in males: XY
Mitosis
Nuclear division resulting in nuclei identical
to parent cell – asexual reproduction for
some organisms.
 Begins after interphase
 Ends before cytokinesis
 Four phases: Prophase
Metaphase
Anaphase
Telophase

Mother cell
Prophase
Prophase
Chromosomes become visible
 Spindle forms from protein microtubules
 Nuclear envelope disintegrates
 Nucleolus disintegrates
 In animal cells, centrioles migrate to
opposite ends of the cell (poles) and spindle
fibers attach to them

Metaphase

Chromosomes line up single file at the
equator of the cell
Anaphase

Sister chromatids are pulled apart toward
opposite poles
Telophase





Nuclear membrane forms around each group of
chromosomes
Nucleolus reappears in each nucleus
Spindle fibers disappear
Chromosomes become invisible again as
chromatin
Cytokinesis begins in plant cell by formation of
cell plate; cleavage furrow in animal cell
completely separates the two nuclei into two
different cells.
Cytokinesis
Occurs
after nucleus has
been duplicated
Begins in anaphase in
animal cells by the
formation of a cleavage furrow
Begins
in telophase in plant cells
by the formation of a cell plate.
Zone of
specialization
Zone of
elongation
Zone of cell
division
Zone
of cell
division
Root cap
Onion Root Tip – where the actively dividing cells are.
Find the different stages of mitosis in these onion cells:
Why is cell division important to
understand?

Cancer is uncontrolled cell division – cells then
spread to other parts of the body.
This is how skin cancer looks – a change in a mole is
the first symptom. If you know how your moles
Skin cancer cells
usually look, you can identify any changes.
This is the worst kind of skin cancer –
Malignant melanoma – it has often
metastesized by the time it is
diagnosed.
Meiosis
Cell division producing cells that have half the
number of chromosomes of the mother cell
 Produces gametes – eggs and sperm
 Occurs so that fertilization doesn’t increase the
number of chromosomes in each generation.

Gametes = sex cells
Eggs or sperm
 Have half the normal number of chromosomes
= haploid (n) = 23 in humans
 Sexual reproduction needs these to combine
DNA from two different parents, producing
offspring that is different from each parent

Meiosis = Reduction Division
Two complete cell divisions
 First cell division – separates homologous
chromosomes (reduction of chromosome
number)
 Second cell division – separates sister
chromatids (like mitosis) - Division
 Produces 4 haploid cells

M
M
E
E
I
I
O
O
S
S
I
I
S
S
I
Crossing over can occur
Homologous pairs are separated
Sister chromatids are separated
II
Gametogenesis

Oogenesis
Production of an egg

Spermatogenesis
Production of sperm
One mother cell
produces one egg cell
and three polar bodies
that die
One mother cell
produces 4 equally
sized sperm cells
Fertilization
The sperm unites with the egg forming a zygote (fertilized
egg).
The zygote then divides by mitosis to produce the trillions of
cells that make up a multicellular body like yours.
The End