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Cell Division
You are made of
~37,200,000,000,000
Cells!
You make about
2,500,000 New Cells
Every Second!
In Prokaryotes (1 Circular Chromosome)
• Binary Fission
In Eukaryotes, cell division occurs in
two major stages.
The first stage, division of the cell
nucleus, is called mitosis.
The second stage, division of the cell
cytoplasm, is called cytokinesis.
DNA Structure Video
DNA Packaging Video
Chromosomes
Genetic information is passed from one
generation to the next on chromosomes.
Before cell division, each chromosome is
duplicated, or copied.
Alleles
• Same Gene but Different Form on the Two
Homologous Chromosomes
Somatic Cells / Sex Cells
• Sex Cells = Gametes (Haploid)
• Somatic Cells = Everything Else (Diploid)
Autosomes / Sex Chromosomes
• Sex Chromosomes = X and Y
• Autosomes = All Other Chromosomes
Each chromosome
consists of two identical
“sister” chromatids.
Each pair of chromatids
is attached at an area
called the centromere.
Go to SMART
“Mitosis”
Sister chromatids
Centromere
Cell Cycle
The cell cycle is the series of events that cells go
through as they grow and divide.
Interphase is the period of growth that occurs
between cell divisions.
Mitosis Video
During the cell cycle:
●a cell grows
●prepares for division
●divides to form two daughter
cells, each of which begins
the cycle again
The cell cycle consists of four phases:
• G1 (First Gap Phase)
• S Phase
• G2 (Second Gap Phase)
• M Phase
Events of the Cell Cycle
During G1, the cell
•increases in size
•synthesizes new proteins and
organelles
During the S phase,
• chromosomes are replicated
• DNA Synthesis takes place
Once a cell enters the S phase, it
usually completes the rest of the cell
cycle.
The G2 Phase (Second Gap Phase)
• organelles and molecules required
for cell division are produced
• Once G2 is complete, the cell is
ready to start the M phase—
Mitosis
Cell Cycle
M phase
Mitosis
Biologists divide the events of
mitosis into four phases: (PMAT)
● Prophase
● Metaphase
● Anaphase
● Telophase
Mitosis
Prophase
Spindle
forming
Prophase
Chromosomes
(paired
chromatids)
Centromere
Prophase
Spindle
forming
Prophase is the first and
longest phase of mitosis.
The centrioles separate
and take up positions on
opposite sides of the
nucleus.
Centromere
Chromosomes
(paired
chromatids)
The centrioles lie in a
region called the
centrosome.
The centrosome helps
to organize the spindle,
a fanlike microtubule
structure that helps
separate the
chromosomes.
Spindle
forming
Centromere
Chromosomes
(paired
chromatids)
Spindle
forming
Chromatin condenses
into chromosomes.
The centrioles separate
and a spindle begins to
form.
The nuclear envelope
breaks down.
Centromere
Chromosomes
(paired
chromatids)
Metaphase
Centriole
Spindle
Centriole
Metaphase
Centriole
Metaphase
The second phase of
mitosis is metaphase.
The chromosomes line
up across the center of
the cell.
Microtubules connect
the centromere of each
chromosome to the
poles of the spindle.
Spindle
Anaphase
Individual
chromosomes
Anaphase
Anaphase
Individual
chromosomes
Anaphase is the third
phase of mitosis.
The sister chromatids
separate into individual
chromosomes.
The chromosomes
continue to move until
they have separated into
two groups.
Telophase
Nuclear
envelope
reforming
Telophase
Telophase
Telophase is the
fourth and final phase
of mitosis.
Chromosomes gather
at opposite ends of the
cell and lose their
distinct shape.
A new nuclear envelope
forms around each cluster
of chromosomes.
Cytokinesis
Cytokinesis
During cytokinesis, the
cytoplasm pinches in
half.
Each daughter cell has
an identical set of
duplicate chromosomes
In plants, a structure known as the cell plate forms
midway between the divided nuclei.
Cell plate
Cell wall
Quiz 3-1
The series of events that cells go through as
they grow and divide is called
a. mitosis.
b. the cell cycle.
c. interphase.
d. cytokinesis.
The phase of mitosis during which the
chromosomes line up across the center of the
cell is
a. prophase.
b. metaphase.
c. anaphase.
d. telophase.
Cytokinesis usually occurs
a. after telophase.
b. at the same time as telophase.
c. during interphase.
d. during anaphase
DNA replication takes place during the
a. S phase of the cell cycle.
b. G1 phase of the cell cycle.
c. G2 phase of the cell cycle.
d. M phase of the cell cycle.
During mitosis, “sister” chromatids separate from
one another during
a. metaphase.
b. anaphase.
c. telophase.
d. interphase.
Regulating the Cell Cycle
Controls on Cell Division
Experiments show that normal cells will reproduce
until they come into contact with other cells.
When cells come into contact with other cells, they
respond by not growing.
This demonstrates that controls on cell growth and
division can be turned on and off.
Contact Inhibition
Cell Cycle Regulators
• The cell cycle is regulated by proteins
which amounts rise and fall with the
cell cycle.
• Cyclins regulate the timing of the cell
cycle in eukaryotic cells by activating
Kineases (Cyclin Dependent Kianeases,
CDKs)
Cyclins were discovered during a similar
experiment to this one.
A sample of cytoplasm
is removed from a cell
in mitosis.
The sample is injected
into a second cell in
G2 of interphase.
As result, the second
cell enters mitosis.
Checkpoints
Some proteins allow the cell cycle to
proceed only when certain processes
have happened inside the cell.
Regulatory Proteins
Cell Cycle Control Video
P53
• Gene (and protein product) that
regulates the passage into
mitosis
Apoptosis
• programmed cell death
p53 Video
Uncontrolled Cell Growth
• Cancer is a disorder in which some of
the body's own cells lose the ability to
control growth.
Cancer cells do not respond to
the signals that regulate the
growth of most cells.
Oncogenes
• a gene that has the potential to cause
cancer
Cancer cells divide uncontrollably and form
masses of cells called tumors that can damage
the surrounding tissues.
Cancer cells may break loose from tumors and
spread throughout the body (metastasis),
disrupting normal activities and causing serious
medical problems or even death.
Quiz 3-2
The cell cycle is believed to be controlled by
proteins called
a. spindles.
b. cyclins.
c. regulators.
d. centrosomes.
Proteins (enzymes) that add phosphate groups
to other molecules
a. kineases.
b. polymerases.
c. cyclins.
d. growth factors.
Once a multicellular organism reaches adult
size, the cells in its body
a. stop dividing.
b. grow and divide at different rates, depending
on the type.
c. have the same life span between cell
divisions.
d. undergo cell division randomly.
One effect of an internal regulator is that a cell
will not begin mitosis until
a. it becomes too large.
b. the cell’s growth is stimulated.
c. it is in physical contact with other cells.
d. all its chromosomes have been replicated.
One factor common to almost all cancer cells is
a. a lack of cyclin.
b. a defect in gene p53.
c. exposure to tobacco smoke.
d. exposure to radiation.
Asexual Reproduction
• In sexual reproduction,
haploid gametes fuse to
form a diploid zygote
Asexual Reproduction
• In asexual reproduction,
diploid cells produce more
identical diploid cells
(Cloning)
Asexual Reproduction
•Fission
•
parent cell splits into two
identical daughter cells of the
same size.
•Fragmentation
•
parent organism breaks into
fragments and each fragment
develops into a new organism.
Asexual Reproduction
•Budding
•
parent cell forms a bubble-like
bud.
Sexual Reproduction
Meiosis
Each organism must inherit a
single copy of every gene from
each of its “parents.”
Gametes are formed by a process
that separates the two alleles on
each homologous chromosome so
that each gamete ends up with just
one allele – MEIOSIS
Chromosome
Number
Organisms have different
numbers of chromosomes.
An adult fruit fly has 8
chromosomes:
4 from the male parent
4 from its female parent.
A cell that contains both sets of
homologous chromosomes is said to be
diploid.
The number of chromosomes in a diploid
cell is sometimes represented by the symbol
2n.
For Drosophila, the diploid number is 8,
which can be written as 2n=8.
The gametes of sexually reproducing
organisms contain only a single set of
chromosomes, and therefore only a single
set of genes.
These cells are haploid. Haploid cells
are represented by the symbol n.
For Drosophila, the haploid number is 4,
which can be written as n=4.
Phases of Meiosis
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 in a
diploid cell.
Meiosis Video
Meiosis involves two divisions,
meiosis I and meiosis II.
By the end of meiosis II, the
diploid cell that entered meiosis
has become 4 haploid cells.
Another Meiosis Video
Meiosis I
Interphase I
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
Cells undergo a
round of DNA
replication, forming
duplicate
chromosomes.
Interphase
Each chromosome
pairs with its
corresponding
homologous
chromosome to
form a tetrad.
There are 4
chromatids in a
tetrad.
Prophase I
When homologous chromosomes form tetrads in
meiosis I, they exchange portions of their
chromatids in a process called crossing over.
Crossing-over produces new combinations of
alleles.
Spindle fibers attach
to the
chromosomes.
Metaphase I
The fibers pull the
homologous
chromosomes
toward opposite
ends of the cell.
Anaphase I
Nuclear membranes
form.
The cell separates into
two cells.
The two cells
produced by meiosis I
have chromosomes
and alleles that are
different from each
other and from the
diploid cell that
entered meiosis I.
Telophase I and
Cytokinesis
Meiosis II
The two cells produced by meiosis I
now enter a second meiotic division.
Unlike meiosis I, neither cell goes
through chromosome replication.
Each of the cell’s chromosomes has
2 chromatids.
Meiosis II
Telophase I and
Cytokinesis I
Meiosis II
Prophase II
Metaphase II
Anaphase II Telophase II
and
Cytokinesis
Meiosis I results in
two haploid (N)
daughter cells, each
with half the number
of chromosomes as
the original cell.
Prophase II
The chromosomes
line up in the center
of cell.
Metaphase II
The sister
chromatids separate
and move toward
opposite ends of the
cell.
Anaphase II
Meiosis II
results in four
haploid (N)
daughter cells.
Yet Another
Meiosis Video
Telophase II and Cytokinesis
Gamete Formation
In male animals, meiosis results in four equal-sized
gametes called sperm.
In many female animals, only one egg
results from meiosis. The other three cells,
called polar bodies, are usually not
involved in reproduction.
Comparing Mitosis and Meiosis
Mitosis results in the
production of two
genetically identical diploid
cells.
Meiosis produces four
genetically different haploid
cells.
Mitosis
Meiosis
• Cells produced by mitosis • Cells produced by
meiosis have half the
have the same number of
number of
chromosomes and alleles
chromosomes as the
as the original cell.
parent cell.
• Mitosis allows an
• These cells are
organism to grow and
genetically different
replace cells.
from the diploid cell
and from each other.
• Some organisms
reproduce asexually by
mitosis.
• Meiosis is how
sexually-reproducing
organisms produce
gametes.
Quiz 3-3
If the body cells of humans contain 46
chromosomes, a single sperm cell should have
a. 46 chromosomes.
b. 23 chromosomes.
c. 92 chromosomes.
d. between 23 and 46 chromosomes.
During meiosis, the number of chromosomes
per cell is cut in half through the separation of
a. daughter cells.
b. homologous chromosomes.
c. gametes.
d. centrioles.
The formation of a tetrad occurs during
a. anaphase I.
b. metaphase II.
c. prophase I.
d. prophase II.
In many female animals, meiosis results in the
production of
a. only 1 egg.
b. 1 egg and 3 polar bodies.
c. 4 eggs.
d. 1 egg and 2 polar bodies.
Compared to egg cells formed during meiosis,
daughter cells formed during mitosis are
a. genetically different, while eggs are
genetically identical.
b. genetically different, just as egg cells are.
c. genetically identical, just as egg cells are.
d. genetically identical, while egg cells are
genetically different.