Learning Objectives – Chapter 8
• Brief overview of prokaryotic cell replication
• The three main phases of eukaryotic cell division:
Interphase, M phase, C phase
• Interphase is broken down into three sub-phases (know
what is happening during these phases)
• M phase (mitosis) is broken down into phases (know what
happens during mitosis)
• Cells divide a finite number of times
• Uncontrolled cell division = cancer
• Meiosis allows organisms to reproduce sexually while
maintaining the same number of chromosomes
Learning Objectives – Chapter 9
• Meiosis allows organisms to reproduce sexually while
maintaining the same number of chromosomes
 Become familiar with gametes and how they package hereditary
information
• What is reduction division
• Understand key differences between meiosis I and mitosis
• Understand key differences between meiosis II and
meiosis I
• Learn the sequence of events that occurs from the
beginning of meiosis I to the end of meiosis II
• What are the mechanisms by which meiosis introduces
genetic diversity?
Prokaryotes Have a Simple Cell Cycle
• Cell division in prokaryotes takes place in two
stages, which together make up a simple cell
cycle
1. copy the DNA

this process is called replication
2. split the cell in two to form daughter cells

this process is called binary fission
Does this process introduce
genetic diversity?
DNA Replication in Prokaryotes
 the prokaryotic chromosome is
a single circle of DNA
 DNA replication begins with the
unzipping of the doublestranded DNA at the origin of
replication
 a new double helix is formed by
adding complementary
nucleotides to the exposed DNA
strands
 At the end the cell possesses
two complete copies of the
chromosome
Prokaryotic Cell
Division
• After replication, the cell
grows in order to partition the
replicated DNA molecules
 when the cell reaches an
appropriate size, the cell splits
into two equal halves
 new plasma membrane and cell
wall are added at a point
between the partitioned DNA
 eventually the cell constricts in
two to form two daughter cells
• each daughter cell is a
complete, living cell with its own
DNA
The Eukaryotic Cell Cycle
Eukaryotic cells contain
more DNA than
prokaryotic cells and the
DNA is also packaged
differently
 DNA in eukaryotic cells
is linear and packaged
into a compact
chromosome
• there is more than one
chromosome in a
eukaryotic cell
Mitosis vs. Meiosis
Eukaryotic cells have two different
mechanisms to divide up the DNA
 mitosis is cell division that occurs in
nonreproductive cells
• these cells are called somatic cells
 meiosis is cell division that occurs in cells
of sexual reproduction
• these cells are called germ line cells
Eukaryotic Cell Cycle: 3 Phases
The eukaryotic cell cycle is divided into distinct
phases (each further broken down into subphases)
 Interphase (G1,S, and G2 phases)
 Mitosis (M phase)
 Cytokinesis (C phase)
Interphase
Interphase is comprised of three phases
• G1 phase (“Gap 1”)
– the primary growth phase of the cell following division
– most cells spend the majority of their lifespan in this
phase
• S phase (Synthesis)
– DNA replication occurs in preparation for cell division
• G2 phase (“Gap 2”)
– further preparation for cell division, including replication
of mitochondria and synthesis of microtubules
M Phase and C Phase
• Mitosis (M phase)
 a microtubular apparatus binds to the
chromosomes and moves them apart
• Cytokinesis (C phase)
 the cytoplasm divides, creating two daughter
cells
Essential Biological Process 8A:
The Cell Cycle
Animation: How the Cell Cycle Works
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Chromosomes
• Chromosome number varies among organisms
 most eukaryotes have between 10 and 50
chromosomes in their somatic cells
• Chromosomes exist as pairs in somatic cells
 these pairs are called homologous chromosomes,
or homologues
 homologues contain information about the same traits
but the information may vary
 cells that have two of each type of chromosome are
called diploid cells
• one chromosome of each pair is inherited from the mother
and the other is inherited from the father
Chromatids
• Prior to cell division, each of the
homologous chromosomes replicates,
forming two identical copies called sister
chromatids
 the sister chromatids are joined together by a
structure called a centromere
 humans have 23 pairs of homologous
chromosomes
• when each chromosome in the pair is replicated,
this makes for a total of 92 chromatids
Figure 8.2 The difference between homologous
chromosomes and sister chromatids
How many chromosomes?
Karyotype
• A karyotype is an
arrangement of
chromosomes
Figure 8.4 The 46
chromosomes of a human
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Homologous pair
• Chromosomes can be
compared based on size,
shape, and centromere
location
• The karyotype at right
shows the 23 pairs of
human chromosomes
© Andrew S. Bajer
Chromatin
Chromosomes are comprised of chromatin, a complex of
DNA and protein
Why?
 there is also some RNA associated with chromosomes
 the DNA in a chromosome is one very long double-stranded fiber
that extends unbroken for the length of the chromosome
 the DNA is coiled in order to allow it to fit into a small space
despite being very long
Figure 8.5 Levels of eukaryotic
chromosomal organization
DNA is coiled around histones
Cell Division - Interphase
Interphase sets the stage for cell
division


chromosomes are first duplicated
although not visible, chromosomes
begin to wind up tightly in a
process called condensation
Mitosis
The cell division that follows
interphase is a division of
the nuclear contents, known
as mitosis
 mitosis is a continuous
process but it is divided, for
ease of study, into four
distinct stages
1.
2.
3.
4.
prophase
metaphase
anaphase
telophase
Mitosis: 1st Phase
• Prophase
 in prophase, the condensed
chromosomes first become visible with
a light microscope
 the nuclear envelope begins to
disintegrate
 centrosomes (centrioles in animal
cells) begin to assemble a network of
protein cables called the spindle
• each cable in the spindle is made of
microtubules
• some of the microtubules attach to the
chromosomes
• when the process is complete, the sister
chromatids of a chromosome are attached
by microtubules to opposite poles of the cell
Mitosis: 2nd Phase
Metaphase
 the chromosomes attached to
microtubules of the spindle are
aligned in the center of the cell
• the centromeres are aligned
along an imaginary plane that
divides the cell in half, known as
the equatorial plane
Mitosis: 3rd Phase
Anaphase
 centromeres split
 sister chromatids separate
 the microtubules of the spindle
are dismantled starting at the
poles
• this pulls the chromatids
toward the poles
Mitosis: 4th Phase
Telophase
 the spindle is dismantled
 a nuclear envelope forms
around the set of
chromosomes at each pole
 the chromosomes begin to
decondense
 the nucleolus reappears
Cytokinesis
 End of mitosis – division of
the cytoplasm into halves
 In animals, cytokinesis
occurs by actin filaments
contracting and pinching the
cell in two
• this action is evident as a
cleavage furrow that appears
between the daughter cells
 In plants, a new cell wall is
laid down to divide the two
daughter cells
• the cell wall grows at right
angles to the mitotic spindle and
is called the cell plate
Essential Biological Process 8B: Cell Division
Animation: Mitosis
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Animation: Mitosis and Cytokinesis
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What Is Cancer?
• Cells are programmed undergo only so many times and
then die
 human cells divide about 50 times
• Cancer is a growth disorder of cells
 begins when apparently normal cells grow uncontrollably
 the result is a growing cluster of cells called a tumor
• malignant tumors are invasive
– cells from malignant tumors can metastasize, spreading to
different areas of the body to form new tumors
Lung Cancer
Figure 8.9 Lung cancer cells (300X)
Figure 8.10 Portrait of a tumor
Mutation and Cancer
• Cell division is regulated by proteins called
growth factors
• Cancer is caused by damage to genes the
encode growth factors
 mutation causes damage to genes
• may result from chemical or environmental
exposure, such as UV rays
 viral exposure may also alter DNA
Cancer Genetics
• There are two general classes of growth factor
genes that are usually involved in cancer
 proto-oncogenes
• these genes encode proteins that stimulate cell division
• mutations to these genes can cause cells to divide
excessively
– when mutated, these genes become oncogenes
 tumor-suppressor genes
• these genes normally turn off cell division in healthy cells
• when mutated, these genes allow uncontrolled cell division
Animation: How Tumor Suppressor
Genes Block Cell Division
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Inquiry & Analysis
Why Do Human Cells Age?
Animation: Telomerase Function
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Sexual Reproduction
Gametes are reproductive cells (eggs and
sperm) that contain half the complement of
chromosomes found in somatic cells
(haploid or diploid?)
 The gametes fuse to form a new cell called a
zygote, which contains two complete copies
of each chromosome (haploid or diploid?)
• the fusion of gametes is called fertilization, or
syngamy
Meiosis
The formation of gametes must involve
some mechanism to halve the number of
chromosomes found in somatic cells
 If not the number of chromosomes would
double with each fertilization
 Meiosis is a process of reduction division in
forming gametes
• this ensures a consistent chromosome number
across generations
Haploid Gametes
• Meiosis and fertilization
constitute a cycle of
sexual reproduction
• Somatic cells have two
sets of chromosomes
making them diploid
• Gametes have only one
set of chromosomes,
making them haploid
What is the diploid number in humans?
Figure 9.1 Diploid cells carry
chromosomes from two parents
Sexual vs. Asexual Reproduction
• Some organisms reproduce by mitotic division and do
not involve gametes
 this is called asexual reproduction
 an example is binary fission in prokaryotes
• Other organisms are
able to reproduce
both sexually and
asexually
• For example,
strawberry plants
flower (sexual
reproduction) and
send out runners
(asexual reproduction)
• Many fungi have
sexual and asexual
forms
The Sexual Life Cycle
• In sexual reproduction, haploid cells or
organisms alternate with diploid cells or
organisms
Figure 9.4 Three types of sexual life cycles
Germ-Line Cells
In animals, the cells that will eventually
undergo meiosis are reserved early on for
the purpose of reproduction
 these cells are referred to as germ-line cells
and are diploid like somatic cells
 but only the germ-line cells will undergo
meiosis to produce haploid gametes
Figure 9.3 The sexual life cycle in animals
Animation: How Meiosis Works
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The Stages of Meiosis
Meiosis involves two divisions, meiosis I and meiosis II




DNA is replicated only before meiosis I
meiosis I separates the homologous chromosomes
meiosis II separates the replicate sister chromatids
when meiosis is complete, the result is that one diploid cell
has become four haploid cells
How many chromosomes are present in a human cell after meiosis I?
How many chromatids?
After meiosis II?
Meiosis I
Meiosis I is traditionally divided into four stages
1.
Prophase I
•
2.
Metaphase I
•
3.
the paired homologous chromosomes align on a central plane
Anaphase I
•
4.
homologues pair up and exchange segments
homologues separate and move to opposite poles
Telophase I
•
chromosomes gather at each of the two poles
What’s the big difference between meiosis I and mitosis?
Animation: Meiosis I
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Meiosis I: Prophase
During prophase I, homologous
chromosomes line up together as
a pair
 crossing over occurs between
nonsister chromatids of
homologous chromosomes
• the chromatids break in the same
place and sections of
chromosomes are swapped
• the result is a hybrid
chromosome
 the pairing is held together by the
cohesion between sister
chromatids and the crossovers
What is the difference between mitosis and meiosis
in terms of how chromosomes are held together?
Figure 9.5 Crossing over
Animation: Meiosis with Crossing Over
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Meiosis I: Metaphase
During metaphase I, the orientation of
the homologous chromosomes on the
metaphase plate is random
 each possible orientation of which
homologue faces which pole results in
gametes with different combinations of
parental chromosomes
 this process is called independent
assortment
• In humans, produces over 8 million different
chromosome combinations!
How is this different from metaphase in
mitosis?
Figure 9.6 Independent assortment
Figure Independent assortment
increases genetic variability
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Paternal gamete
Maternal gamete
Diploid offspring
Homologous pairs
Potential gametes
Meiosis I: Anaphase
• In anaphase I, the
chromosome pairs separate
and individual homologues
move to each pole
Meiosis I: Telophase
• In telophase I, the
chromosomes gather at their
respective poles to form two
chromosome clusters
How many copies of each
chromosome is present after
meiosis I?
Meiosis II
• After meiosis I, a brief interphase occurs
where there is no replication of DNA
• Meiosis II follows and is basically a mitotic
division of the products of meiosis I
 except that the sister chromatids are nonidentical because of crossing over in meiosis I
The Stages of Meiosis II
Meiosis II is also divided into four stages
1. Prophase II
• new spindle forms to attach to chromosome clusters
2. Metaphase II
• spindle fibers bind to both sides of the centromere and individual
chromosomes align along a central plane
3. Anaphase II
• sister chromatids are pulled to opposite poles
• Non-disjunction of chromatids at this stage can result in trisomy
4. Telophase II
• the nuclear envelope is reformed around each of the four sets of
daughter chromosomes
Animation: Meiosis II
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Essential Biological Process 9A:
Meiosis I
Essential Biological Process 9A:
Meiosis II
Animation: Stages of Meiosis
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How Meiosis Differs from Mitosis
Meiosis has two unique features not found in
mitosis
 synapsis
• homologous chromosomes pair along their entire
lengths and are held together by cohesin proteins;
this close association permits crossing over
 reduction division
• because meiosis involves two nuclear divisions but
only one replication of DNA, the final amount of
genetic material passed to the gametes is halved
Animation: Comparison of
Meiosis and Mitosis
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Figure 9.8 Unique features of meiosis
How do sister
chromatids entering
meiosis II differ from
each other?
Animation: The Function of
Cohesin
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Animation: Unique Features of
Meiosis
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Figure 9.9 A comparison of meiosis and mitosis