Download Chapter 5 – Cell Division

Cell Division
L3 Biology
Why do cells divide?

Growth

Repair/regeneration

Reproduction
 asexual
The Cell Cycle – life cycle of a cell
that will divide:
Cells go through a life cycle
Cell division – mitosis – 1 hr
– followed by division of the
cytoplasm
Second
growth
phase – 2 hr
First growth phase
interphase
9 hr
Synthesis phase – DNA is copied – 10 hr
Chromosomes
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

When cells divide by mitosis, each daughter cell
receives the same number of chromosomes as
its mother cell has.
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
Humans have 22 pairs of similar
chromosomes; one pair of sex chromosomes
Sex Chromosomes

Similar in females: XX

Not similar in males: XY
Mitosis
Cell division resulting in nuclei identical
to parent cell – asexual reproduction for
some organisms.
 Four phases: Prophase
Metaphase
Anaphase
Telophase

Mother cell
Chromosomes become visible,
nucleus disintegrates, spindle
fibers form, centrioles move to
poles
Prophase
Chromosomes line up
in the middle
Chromatids separate and move
to opposite poles
Chromosomes become invisible, nuclear
membranes form around new nuclei,
spindle fibers disappear.
Prophase
Chromosomes become visible
 Spindle fibers form
 Nuclear membrane disintegrates
 Nucleolus disintegrates
 In animal cells, centrioles move to opposite
ends of the cell (poles) and spindle fibers
attach to them.

Metaphase

Chromosomes line up single file at the
middle of the cell
Anaphase

Sister chromatids are pulled apart toward
opposite poles
Telophase





Cell plate
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 in an
animal cell
Occurs
after nucleus has
been duplicated
Begins in anaphaseby the
formation of a cleavage furrow
Begins
in telophase in plant cells
by the formation of a cell plate.
Find the different stages of mitosis in these onion root cells:
B
C
D
A
E
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.

Why is cell division important to
understand?

Cancer is uncontrolled cell division – cells then
spread to other parts of the body.
Gametes = sex cells
Eggs or sperm
 Have half the normal number of chromosomes
= 23 in humans
 Combine DNA from two different parents,
producing offspring that is different from each
parent

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 = Gametogenesis – the
formation of eggs and sperm

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
Sperm
formation
Egg
formation
4 sperm
One egg
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