© Amy Brown Science
There are two main reasons why cells divide
rather than continuing to grow larger and larger:
The larger a cell
becomes, the …….
If the cell grows too large, it
will have trouble moving
enough nutrients and wastes
across the cell membrane.
…. more demands the
cell places on its
DNA.
• All of the information that a cell needs to
function is stored in the DNA of the cell.
• DNA is packaged into chromosomes. A
chromosome consists of one very long linear
DNA molecule consisting of 1000's of genes.
• Each gene is the instructions for making a
particular protein that the cell needs.
• The cell is constantly making copies of these
genes and sending the copies (in the form of
RNA) out to the ribosomes.
• When the cell is small, the information stored
in the cell’s DNA is adequate to meet the
needs of the cell.
• As the cell grows too large, there is an
“information crisis”. The DNA cannot keep
up with the demands of running a larger cell.
A cell must take in a constant inflow
of food, oxygen and water across
the membrane.
Waste products must constantly
be crossing the membrane in
order to leave the cell.
A larger cell will require much more food, oxygen and water. A
larger cell will generate much more waste.
As the cell grows, the volume of the cell increases much more rapidly than
the surface area of the cell membrane.
When the cell gets too large, the membrane surface area is not adequate
enough to transport the large quantities of food and water in and waste
products out.
Cell division is the process
by which cellular material is
divided between two new
daughter cells.
1 Mother Cell  2 Daughter cells.
The two daughter cells will be….
…identical to each other and to
the mother cell.
Each daughter is half
the size of the parent
cell, but immediately
begins growing.
A typical human cell has about 2 meters of DNA. Before the
cell can divide, all of this DNA must be copied and then the
two copies separated so that each daughter cell ends up with
a complete set of DNA.
Each species has a characteristic
number of chromosomes in each
cell nucleus; humans have 23
pairs or 46.
copy its chromosomes
Each cell must first __________________
before cell division occurs.
Each daughter cell gets a complete
copy of that information.
Cell division occurs in two main stages:
Mitosis – The division of the nucleus
Cytokinesis – The division of the cytoplasm
The chromosomes are not visible except during cell division. At
the beginning of cell division, the chromosomes condense into
compact, visible structures that are easily seen with a
microscope.
Replicated
chromosome
consisting of 2 sister
chromatids.
centromere
Well before cell
division takes
place, each
chromosome is
replicated or
copied.
At the beginning of cell division, each chromosome
sister chromatids
consists of two identical “_______________”.
These
centromere
chromatids are connected at an area called a __________.
The Cell Cycle
The cell cycle is:
The series of events that cells go through
as they grow and divide.
The cell cycle is the life of the cell from the
time it is first formed from a dividing
parent cell until its own division into two
cells.
During the cell cycle:
1. A cell grows.
2. The cell prepares for division.
3. The cell divides to form two daughter cells.
The cell cycle consists of five major phases:
 G1 (first gap)
 S (synthesis)
 G2 (second gap)
Mitosis
Cytokinesis
Mitosis is the division
of the nucleus.
Cytokinesis is the division
of the cytoplasm.
Two new cells
are produced.
G1
S
G2
Before a cell can
begin mitosis and
actually divide, it
must do two things:
a) It must form duplicates
of its chromosomes.
b) It must produce a supply
of organelles for the two
daughter cells.
interphase
These preparations occur during the G1, S, and G2
phases of the cell cycle.
These three (G1, S, and G2) are collectively known
as interphase.
G1 phase
S phase
a) The cell doubles in size.
b) The enzymes, cytoplasmic
organelles and other molecules
double in number.
Replication of DNA occurs.
G2 Phase
The cell assembles the special
structures needed for cell division
When interphase (G1, S, G2) is complete, the cell
is ready to begin the process of cell division.
“Let’s break
mitosis down
into its
individual
stages and see
what is going
on in each
stage.”
centrioles
nuclear membrane
nucleolus
chromosomes
membrane
well defined and bounded by the nuclear
Nucleus is ___________
________________.
centrioles Their function is to:
Outside of the nucleus are two __________.
organize the microtubules into a spindle.
They will begin to move apart as spindle microtubules grow out of them.
centrioles
nuclear membrane
nucleolus
chromosomes
G1 phase is a period of intense
biochemical activity:
The cell doubles in size and the enzymes, cytoplasmic
organelles and other molecules double in number.
The chromosomes have duplicated during the S phase and they appear as
a jumbled mass of fibers. They have not yet condensed.
G2 Phase: The cell assembles the special structures needed
for cell division
Remember!
Interphase includes the
G1 phase, the S phase,
and the G2 phase. It is
the period of time
preceding mitosis.
Mitosis has 4 stages:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
Prophase
Early prophase
The chromosomes coil and
thicken and become distinct
from one another. The
chromosomes are now
visible.
disappears
The nucleolus __________.
The chromosomes are
doubled throughout their
length.
centrioles
Spindle made
of microtubules
Chromatids
connected by a
centromere.
chromatid
Each half of the double chromosome is a __________.
centromere
The chromatids are connected by a ___________.
centrioles separate and start moving to opposite ends of the cell. A
The __________
spindle made of ____________
microtubules begins to form.
________
Prophase
Late prophase
centrioles
nuclear membrane
The _________________
fragments and the
microtubules invade the
nuclear area. The spindle is
completely formed.
The spindle is a structure
that will help to
separate
the chromosomes
_______________________.
During prophase the pairs of
chromatids
__________ become
attached to the fibers of the
spindle.
Microtubules
form a complete
spindle.
chromatids
centrioles
The centrioles have
moved to the opposite
poles, forming the
spindle as they go.
Metaphase
 The centrioles are
now at opposite
sides of the cell.
centrioles
chromatids
 The spindle fibers
will push and pull the
centrioles
chromosomes.
 The chromosomes
line up at the center
of the cell.
Spindle
composed of
microtubules
Each chromosome is
connected to a spindle
fiber at its centromere.
Anaphase
The centromeres
divide and the
chromatids move
to opposite sides
of the cell.
The microtubules begin
to shorten and this pulls
the chromatids apart to
opposite sides of the cell.
Chromatids
are being
pulled to
opposite
sides of the
cell.
Shortening
of the
microtubules
By the end of anaphase,
the two ends of the cell
have equivalent and
complete sets of
chromosomes.
Telophase
Nuclear
membrane begins
to form.
Nuclear membrane
is returning.
cleavage furrow
Nucleolus returns.
The cell begins to
pinch in. This is
called a cleavage
furrow.
The end result is two
cells that are exact
copies of each other.
Can you name these phases?
1 - Prophase
4 - Telophase
2 - Metaphase
3 - Anaphase
5 - Interphase
metaphase
anaphase
anaphase
Metaphase
Telophase
Prophase
two nuclei have been
At the end of mitosis, __________
formed. Each nucleus has an identical set of
chromosomes
_____________.
Cytokinesis is: the division of the cytoplasm.
Cytokinesis usually
occurs at the same
time as telophase.
In animal cells, a
cleavage furrow
______________
pinches the cell
membrane inward until
the cell is pinched into
two separate cells. Each
new cell contains its
own nucleus,
cytoplasm, and
organelles.
not possible
In plants, it is __________
for the cell to pinch
inward because of the
cell wall
rigid _______.
cell plate forms
In plants, a ________
midway between the two nuclei.
The cell plate continues to form
across the cell until two separate
cells have been formed.
M
I
T
O
S
I
S
Takes between 30
minutes and 2
hours.
One Mother Cell =
Two Daughter
cells.
The two daughter
cells are identical
to the mother cell.
Results of
Mitosis
Importance
of Mitosis
• In unicellular plants and animals,
it results in new offspring by
asexual reproduction.
• In multicellular organisms, it
results in the growth and repair
of the organism.
• The two new cells are
exact duplicates.
• Insures that the new cells
will be able to carry on the
same functions as the
mother cell.
The frequency of cell division
varies with the type of cell.
Skin
cells divide frequently
_________
throughout our lives.
Liver cells maintain the ability to
_________
divide but only do so on rare
occasion - say to repair a wound.
The most specialized cells, such as muscle cells
and nerve cells, do not divide at all.
When cells come into contact with other
cells, they respond by not growing.
When an injury, like a cut in
the skin occurs, the cells at
the edge of the injury begin
to divide rapidly.
When the healing process nears
completion, the rate of cell
division slows down.
There are many
proteins found on the
inside and the outside
of the cell that
regulate cell division.
Some of these
proteins are
responsible for
starting and stopping
cell division.
Cell Cycle
Regulators
Other proteins seem
to speed up or slow
down the cell division
process.
These proteins send out
signals that prevent
excessive cell growth.
This keeps the tissues of
the body from disrupting
one another.
Uncontrolled Cell Growth
Cancer cells do not respond to the
signals that regulate the growth of cells.
Cancer cells divide uncontrollably and
form masses of cells called tumors.
If the cells in a tissue
grow uncontrollably,
the consequences may
be severe.
All cancers have one thing in common:
The protein regulators that control the
cell cycle have failed to do their job.
These tumors can
damage the surrounding
healthy tissues.
Asexual Reproduction
Offspring are
produced by only
one parent.
Each parent
passes ALL of
its genes to the
offspring.
Sexual reproduction requires two parents. Each
parent passes on HALF its genes to its offspring.
Must have male and female: male to produce sperm
and female to produce eggs.
Sexual Reproduction involves:
Gametes: Sex cells (egg and
sperm)
Fertilization: The union of sperm
and egg.
Zygote: A fertilized egg.
If an organism is the result of
sexual reproduction, it will have
____ sets of chromosomes.
two
One set comes from the
mother and one set comes
from the father.
These two sets are called homologous chromosomes.
Homologous chromosomes are the two copies of each chromosome,
one coming from the mother and one coming from the father.
Homologous chromosomes carry the same genes, but
they may have different expressions of that gene.
Diploid means that …
…there are two of each kind
of chromosome in each cell.
The symbol for diploid is 2N.
You are a
“diploid”
organism.
“N” is the number of different chromosomes an
organism has. Humans are 2N because we have
2 of each kind of chromosome.
Diploid cells contain two complete
sets of chromosomes.
So in mitosis:
1 (2N) cell ------->
2 (2N) cells
Egg
and sperm cells must have _____
half
_____________
the number of chromosomes so that
zygotewill
when added together, the ______
have the proper number.
Human egg cell
Example: Gametes of the Human Body
Egg (23) + sperm (23)  zygote (46)
1N
+
1N

2N
Gametes are said to be haploid or
1N because they contain only one
of each kind of chromosome.
Human sperm cells
The cells which produce eggs and the cells which
produce sperm are diploid or 2N. So how do the egg
and sperm cells get to be 1N?
Meiosis is a process of reduction
division in which the number of
chromosomes per cell is cut in half
through the separation of
homologous chromosomes.
Phases of Meiosis
Purpose is to
Occurs in
reduce the
the sex chromosome
cells only: number of
the egg and
the egg
and sperm. sperm by
half.
Meiosis, like
mitosis, is
preceded by the
replication of
chromosomes.
Unlike mitosis, this
replication is then
followed by two
divisions: meiosis I
and meiosis II.
The stages of meiosis I and II
Interphase
Prophase I
Prophase II
Telophase I
(and cytokinesis)
Metaphase I Anaphase I
Metaphase II
Anaphase II
Telophase II
(and cytokinesis)
The Stages of Meiosis I
Interphase
Prophase I
Metaphase I
Anaphase I
The chromosomes
replicate. It is
similar to
chromosome
replication of
mitosis. Two
identical sister
chromatids are held
together by a
centromere.
Chromosomes
shorten and thicken.
Each chromosome
pairs with its
corresponding
homologous
chromosome to form
a tetrad. There are 4
chromatids in a
tetrad.
Tetrads line up
at the center
of the cell.
The tetrads break
apart and the pairs
move to opposite
sides of the cell.
Sister chromatids
remain attached at
their centromeres.
The Stages of Meiosis II
Telophase I
(and cytokinesis)
The cell
separates into
two cells.
Meiosis I results
in 2 haploid (1N)
daughter cells
Each daughter cell has
half the number of
chromosomes as the
original cell.
Prophase II
Metaphase II
Anaphase II
Telophase II
The pairs of
sister
chromatids
start
toward the
center.
Pairs of
sister
chromatids
line up at
the center.
The pairs of
sister
chromatids
separate
and move
to opposite
sides of the
cell.
(and cytokinesis)
Results in 4
new cells
that are 1N.
The Importance of Meiosis
1 (2N) cell -------> 4 (1N) cells
The chromosome number of the egg and
sperm is cut in half to insure that the zygote
will have the proper number of
chromosomes.
“Crossing Over” During Meiosis
tetrad
During prophase
________,I
each pair of
chromatids lines
up next to its
homologue
_________.
homologous
This process is
called
crossing over
“___________”
chromosomes
This pairing of
homologous
chromosomes
produces
tetrads
_______.
A tetrad
consists of
4 chromatids
___________.
It is possible for the chromatids within a
homologous pair to twist around one another.
Portions of the chromatids may break off and
attach to adjacent chromatids.
“Crossing over” is the exchange of genetic
information (genes) between segments of
homologous chromosomes during meiosis.
The result is that the offspring will receive a new
combination of genetic information. This leads
to variation
________ in the offspring.
adaptation and change These variations will cause
Variation leads to ____________________.
better suited for their particular
some of the offspring to be ___________
environment. If they are better suited for their environment, it is
survive to reproductive age and pass these
more
likely that they will _______
_________
favorable variations on to their offspring.
If the result of crossing over causes the offspring
to be less suited for its environment, it may not
survive. Or, if the offspring does survive, it may
not be reproductively competitive. This means
that it may not be able to secure a mate. These
“unfavorable” genes are not likely to be passed
on to the offspring.
Crossing over leads to
Evolution!
______________.
Meiosis produces four haploid
cells that are different.
In males, meiosis results in 4
sperm cells.
In females, 4 cells are produced,
but only one will become an egg
cell. All of the cytoplasm and all
of the organelles are put into one
egg cell. The other three cells will
never be functional.
Mitosis occurs in all cells of the
egg and sperm
_____
body except _____________.
Meiosis only occurs in the
egg and sperm
formation of _____________.
diploid cell
In meiosis, each _______
divides ______
twice to produce a total of
4 cells.
___
diploid cell divides
In mitosis each _______
2 cells.
once to form ___
_____
In meiosis, each of the four cells contains
half the number of chromosomes as the
_____
parent cell.
In mitosis, each new cell contains the
same number of chromosomes as the
______
original cell.
In mitosis, the homologous pairs do not
tetrads
come together to form _______.
In meiosis, the homologous pairs do come
tetrads While the
together to form _______.
crossing over
chromosomes are in tetrads, _____________
may occur. There will be no crossing over in
mitosis
_______.
In meiosis, the four haploid cells
different combinations of
contain ________
chromosomes from each other.
In mitosis, the new cells contain
identical copies.
________
Created by Amy Brown
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