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Cell Reproduction:
Mitosis and Meiosis
Chapters 12 and 13
Bio 1 Review
Cell Reproduction
Mitosis
Daughter 2 cloned Only result
cells are daughter in somatic
2n
cells
cells
(diploid)
Meiosis
Daughter
cells are n
(haploid)
4
genetically
shuffled
daughter
cells
Only result in
gametes
LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 12
The Cell Cycle
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
What is this?
Concept 12.1: Most cell division results in
genetically identical daughter cells
© 2011 Pearson Education, Inc.
Cellular Organization of the Genetic
Material
• All the DNA in a cell constitutes the cell’s genome
• DNA molecules in a cell are packaged into
chromosomes
© 2011 Pearson Education, Inc.
Figure 12.3
20 m
Cell Type Review
• Somatic cells
• Gametes
– germ cells give rise to gametes
© 2011 Pearson Education, Inc.
Figure 12.4
In preparation for cell division, DNA is
replicated and the chromosomes condense
Into a chromosome, from chromatin
Sister
chromatids
Centromere
0.5 m
Figure 12.5-1
Chromosomes
Mitosis
overview
1
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Figure 12.5-2
Chromosomes
Mitosis
overview
1
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Chromosome duplication
(including DNA replication)
and condensation
2
Sister
chromatids
Figure 12.5-3
Chromosomes
Replication
& Mitosis
1
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Chromosome duplication
(including DNA replication)
and condensation
2
Sister
chromatids
Separation of sister
chromatids into
two chromosomes
3
Phases of the Cell Cycle
• The cell cycle consists of:
– Mitotic (M) phase (mitosis and cytokinesis)
AND
– Interphase (cell growth and copying of
chromosomes in preparation for cell division)
© 2011 Pearson Education, Inc.
Figure 12.6
INTERPHASE
G1
S
(DNA synthesis)
G2
Figure 12.16
G0
G1 checkpoint
G1
(a) Cell receives a go-ahead
signal.
G1
(b) Cell does not receive a
go-ahead signal.
The Cell Cycle Clock: Cyclins and CyclinDependent Kinases
• Two types of regulatory proteins are involved in cell
cycle control: cyclins and cyclin-dependent
kinases (Cdks)
– Review: what do kinases do?
• Example: MPF (maturation-promoting factor) is a
cyclin-Cdk complex that triggers a cell’s passage
past the G2 checkpoint into the M phase
© 2011 Pearson Education, Inc.
• Mitosis is conventionally divided into five phases
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
• Cytokinesis overlaps the latter stages of mitosis
© 2011 Pearson Education, Inc.
BioFlix: Mitosis
© 2011 Pearson Education, Inc.
10 m
Figure 12.7
G2 of Interphase
Centrosomes
(with centriole pairs)
Nucleolus
Chromatin
(duplicated)
Nuclear
envelope
Plasma
membrane
Prophase
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids
Prometaphase
Fragments
of nuclear
envelope
Kinetochore
Metaphase
Nonkinetochore
microtubules
Kinetochore
microtubule
Anaphase
Metaphase
plate
Spindle
Centrosome at
one spindle pole
Telophase and Cytokinesis
Cleavage
furrow
Daughter
chromosomes
Nuclear
envelope
forming
Nucleolus
forming
Whitefish blastula
Figure 12.8: The microtubules shorten by depolymerizing at their kinetochore ends
Aster
Centrosome
Sister
chromatids
Metaphase
plate
(imaginary)
Microtubules
Chromosomes
Kinetochores
Centrosome
1 m
Overlapping
nonkinetochore
microtubules
Kinetochore
microtubules
0.5 m
The Mitotic Spindle: A Closer Look
skip to diagram
• The mitotic spindle is a structure made of
microtubules that controls chromosome movement
during mitosis
• Mitosis visualization video (2 min)
• In animal cells, assembly of spindle microtubules
begins in the centrosome, the microtubule
organizing center
© 2011 Pearson Education, Inc.
• An aster (a radial array of short microtubules)
extends from each centrosome
– The spindle includes the centrosomes, the spindle
microtubules, and the asters
© 2011 Pearson Education, Inc.
• During prometaphase, some spindle microtubules
attach to the kinetochores of chromosomes and
begin to move the chromosomes
• Kinetochores are protein complexes associated
with centromeres
• At metaphase, the chromosomes are all lined up
at the metaphase plate, an imaginary structure at
the midway point between the spindle’s two poles
© 2011 Pearson Education, Inc.
Cytokinesis: A Closer Look
• In animal cells, cytokinesis occurs by a process
known as cleavage, forming a cleavage furrow
• In plant cells, a cell plate forms during cytokinesis
© 2011 Pearson Education, Inc.
Animation: Cytokinesis
Right-click slide / select ”Play”
© 2011 Pearson Education, Inc.
Animation: Animal Mitosis
Right-click slide / select ”Play”
© 2011 Pearson Education, Inc.
Animation: Sea Urchin (Time Lapse)
Right-click slide / select ”Play”
© 2011 Pearson Education, Inc.
Figure 12.10
(a) Cleavage of an animal cell (SEM)
100 m
Cleavage furrow
Contractile ring of
microfilaments
(b) Cell plate formation in a plant cell (TEM)
Vesicles
forming
cell plate
Wall of parent cell
Cell plate
1 m
New cell wall
Daughter cells
Daughter cells
Figure 12.11
Nucleus
Chromatin
condensing
Nucleolus
1 Prophase
Chromosomes
2 Prometaphase 3 Metaphase
Cell plate
4 Anaphase
10 m
5 Telophase
Binary Fission in Bacteria
• Prokaryotes (bacteria and archaea) reproduce by
a type of cell division called binary fission
– In binary fission, the chromosome replicates
(beginning at the origin of replication), and the
two daughter chromosomes actively move apart
© 2011 Pearson Education, Inc.
Figure 12.12-1
Origin of
replication
E. coli cell
1 Chromosome
replication
begins.
Two copies
of origin
Cell wall
Plasma membrane
Bacterial chromosome
Figure 12.12-2
Origin of
replication
E. coli cell
1 Chromosome
replication
begins.
2 Replication
continues.
Cell wall
Plasma membrane
Bacterial chromosome
Two copies
of origin
Origin
Origin
Figure 12.12-3
Origin of
replication
E. coli cell
1 Chromosome
replication
begins.
2 Replication
continues.
3 Replication
finishes.
Cell wall
Plasma membrane
Bacterial chromosome
Two copies
of origin
Origin
Origin
Figure 12.12-4
Origin of
replication
E. coli cell
1 Chromosome
replication
begins.
2 Replication
continues.
3 Replication
finishes.
4 Two daughter
cells result.
Cell wall
Plasma membrane
Bacterial chromosome
Two copies
of origin
Origin
Origin
Stop and Go Signs: Internal and External
Signals at the Checkpoints
• An example of an internal signal is that
kinetochores not attached to spindle microtubules
send a molecular signal that delays anaphase
• Some external signals are growth factors,
proteins released by certain cells that stimulate
other cells to divide
© 2011 Pearson Education, Inc.
Figure 12.18: Figure 12.18 The effect of platelet-derived growth factor (PDGF) on cell division.
1 A sample of human
connective tissue is
cut up into small
pieces.
Scalpels
Petri
dish
2 Enzymes digest
the extracellular
matrix, resulting in
a suspension of
free fibroblasts.
3 Cells are transferred to
culture vessels.
Without PDGF
4 PDGF is added
to half the
vessels.
With PDGF
10 m
External signals associated with cell cycle
• A clear example of external signals is densitydependent inhibition, in which crowded cells
stop dividing
• Most animal cells also exhibit anchorage
dependence, in which they must be attached to a
substratum in order to divide
****Cancer cells exhibit neither density-dependent
inhibition nor anchorage dependence
© 2011 Pearson Education, Inc.
Figure 12.19
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
20 m
20 m
(a) Normal mammalian cells
(b) Cancer cells
Loss of Cell Cycle Controls in Cancer Cells
• Cancer cells do not respond normally to the body’s
control mechanisms
• Why cancer cells make tumors:
– They may make their own growth factor
– They may convey a growth factor’s signal without
the presence of the growth factor
– They may have an abnormal cell cycle control
system
© 2011 Pearson Education, Inc.
Growth Factors and Cancer
• Growth factors can create cancers
– proto-oncogenes
• normally activates cell division
– growth factor genes
– become oncogenes (cancer-causing) when mutated
• if switched “ON” can cause cancer
• example: RAS (activates cyclins)
– tumor-suppressor genes
• normally inhibits cell division
• if switched “OFF” can cause cancer
• example: p53
Cancer & Cell Growth
• Cancer is essentially a failure
of cell division control
– unrestrained, uncontrolled cell growth
• What control is lost?
– lose checkpoint stops
– gene p53 plays a key role in G1/S restriction point
p53 is the
Cell Cycle
Enforcer
• p53 protein halts cell division if it detects damaged DNA
– options:
» stimulates repair enzymes to fix DNA
» forces cell into G0 resting stage
» keeps cell in G1 arrest
» causes apoptosis of damaged cell
• ALL cancers have to shut down p53 activity
p53 discovered at Stony Brook by Dr. Arnold Levine
p53 — master regulator gene
NORMAL p53
p53 allows cells
with repaired
DNA to divide.
p53
protein
DNA repair enzyme
p53
protein
Step 1
Step 2
Step 3
DNA damage is caused
by heat, radiation, or
chemicals.
Cell division stops, and
p53 triggers enzymes to
repair damaged region.
p53 triggers the destruction
of cells damaged beyond repair.
ABNORMAL p53
abnormal
p53 protein
Step 1
Step 2
DNA damage is
caused by heat,
radiation, or
chemicals.
The p53 protein fails to stop
cell division and repair DNA.
Cell divides without repair to
damaged DNA.
cancer
cell
Step 3
Damaged cells continue to divide.
If other damage accumulates, the
cell can turn cancerous.
Key mutations causing Cancer
• Cancer develops only after a cell experiences ~6
key mutations (“hits”)
1. unlimited growth
turn on growth promoter genes
2. ignore checkpoints
turn off tumor suppressor genes (p53)
3. escape apoptosis
turn off suicide genes
4. immortality = unlimited divisions
turn on chromosome maintenance genes
5. promotes blood vessel growth
turn on blood vessel growth genes
6. overcome anchor & density dependence
turn off touch-sensor gene
What causes these “hits”?
• Mutations in cells can be triggered by




UV radiation
chemical exposure
radiation exposure
heat




cigarette smoke
pollution
age
genetics
Tumors
• Mass of abnormal cells
– Benign tumor
• abnormal cells remain at original site as a lump
– p53 has halted cell divisions
• most do not cause serious problems &
can be removed by surgery
– Malignant tumor
• cells leave original site
– lose attachment to nearby cells
– carried by blood & lymph system to other tissues
– start more tumors = metastasis
• impair functions of organs throughout body
Traditional treatments for cancers
• Treatments target rapidly dividing cells
– high-energy radiation
• kills rapidly dividing cells
– chemotherapy
• stop DNA replication
• stop mitosis & cytokinesis
• stop blood vessel growth
New “miracle drugs”
• Drugs targeting proteins (enzymes) found only in
cancer cells
– Gleevec
• treatment for adult leukemia (CML)
& stomach cancer (GIST)
• 1st successful drug targeting only cancer cells
without
Gleevec
Novartes
with
Gleevec
Figure 12.21
Figure 12.UN02
REVIEW: Can you identify the phases of the cell cycle?
Figure 12.UN05
Figure 12.UN05
Figure 12.UN04
Why do humans have a sex drive?
MEIOSIS
BIG THEME:
• Meiosis is the key to genetic
diversity of offspring, thus the
key to evolution, and by
extension, the key to sex drive
LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 13
Meiosis and Sexual
Life Cycles
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
Figure 13.1: What accounts for family resemblance?
Mitosis vs. Meiosis animation - NOVA
• The difference is really simple. Just one step.
Concept 13.1: Offspring acquire genes from
parents by inheriting chromosomes
• See the title
© 2011 Pearson Education, Inc.
Inheritance of Genes: review
• Terms: Gametes,
genes, locus, (pl.
loci)
© 2011 Pearson Education, Inc.
Asexual vs. Sexual Reproduction
• Asexual reproduction
• sexual reproduction
© 2011 Pearson Education, Inc.
Video: Hydra Budding
© 2011 Pearson Education, Inc.
Reproduction without sex can work 
0.5 mm
Parent
Bud
Concept 13.2: Fertilization and meiosis
alternate in sexual life cycles
© 2011 Pearson Education, Inc.
Lets review chromosome basics
Lets review chromosome basics
terms: somatic cells, homologues (or
homologous pairs), diploid (vs. haploid), 2n,
n, sex chromosomes, autosomes
Figure 13.3
APPLICATION
A karyotype is an
ordered display of the
pairs of chromosomes
from a cell
TECHNIQUE
Pair of homologous
duplicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
5 m
Describing chromosomes
Key
2n  6
Maternal set of
chromosomes (n  3)
Paternal set of
chromosomes (n  3)
Sister chromatids
of one duplicated
chromosome
Two nonsister
chromatids in
a homologous pair
Centromere
Pair of homologous
chromosomes
(one from each set)
• A gamete (sperm or egg) contains a single set
of chromosomes, and is haploid (n)
• For humans, the haploid number is 23 (n = 23)
– Each set of 23 consists of 22 autosomes and a
single sex chromosome
– In an unfertilized egg (ovum), the sex
chromosome is X
– In a sperm cell, the sex chromosome may be
either X or Y
© 2011 Pearson Education, Inc.
Behavior of Chromosome Sets in the
Human Life Cycle
• Fertilization is the union of gametes (the sperm
and the egg)
– The fertilized egg is called a zygote and has
one set of chromosomes from each parent
– The zygote produces somatic cells by mitosis
and develops into an adult
© 2011 Pearson Education, Inc.
Figure 13.5
Haploid gametes (n  23)
Key
Haploid (n)
Diploid (2n)
Egg (n)
Sperm (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n  46)
Mitosis and
development
Multicellular diploid
adults (2n  46)
Can you find difference?
3 types of organismal life cycles
Key
Haploid (n)
Diploid (2n)
n
Gametes
n
Mitosis
n
n
MEIOSIS
FERTILIZATION
n
Diploid
multicellular
organism
(a) Animals
Zygote 2n
Mitosis
n
Mitosis
n
Mitosis
n
Spores
Gametes
MEIOSIS
2n
Haploid unicellular or
multicellular organism
Haploid multicellular organism
(gametophyte)
n
n
n
n
Gametes
FERTILIZATION
2n Zygote
Mitosis
(b) Plants and some algae
n
FERTILIZATION
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
Mitosis
2n
Zygote
(c) Most fungi and some protists
Figure 13.6
3 types of organismal life cycles
Key
Haploid (n)
Diploid (2n)
n
Gametes
n
Mitosis
n
n
MEIOSIS
FERTILIZATION
n
Diploid
multicellular
organism
(a) Animals
Zygote 2n
Mitosis
n
Mitosis
n
Mitosis
n
Spores
Gametes
MEIOSIS
2n
Haploid unicellular or
multicellular organism
Haploid multicellular organism
(gametophyte)
n
n
n
n
Gametes
FERTILIZATION
2n Zygote
Mitosis
(b) Plants and some algae
n
FERTILIZATION
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
Mitosis
2n
Zygote
(c) Most fungi and some protists
Plants and some algae exhibit an alternation of generations
• This life cycle includes both a diploid and haploid multicellular stage
• The diploid organism, called the sporophyte, makes haploid spores
by meiosis
Concept 13.3: Meiosis reduces the number of
chromosome sets from diploid to haploid
• What does this mean?
© 2011 Pearson Education, Inc.
Meiosis overview
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Figure 13.7-2
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Meiosis I
1 Homologous
chromosomes separate
Haploid cells with
duplicated chromosomes
Figure 13.7-3
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Meiosis I
1 Homologous
chromosomes separate
Haploid cells with
duplicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unduplicated chromosomes
BioFlix: Meiosis
© 2011 Pearson Education, Inc.
Meiosis: Step by step
Figure 13.8
MEIOSIS I: Separates sister chromatids
MEIOSIS I: Separates homologous chromosomes
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Telophase I and
Cytokinesis
Anaphase I
Duplicated homologous
chromosomes (red and blue)
pair and exchange segments;
2n  6 in this example.
Anaphase II
Telophase II and
Cytokinesis
Centromere
(with kinetochore)
Metaphase
plate
Cleavage
furrow
Fragments
of nuclear
envelope
Metaphase II
Sister chromatids
remain attached
Spindle
Homologous
chromosomes
Prophase II
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Chromosomes line up
by homologous pairs.
Each pair of homologous
chromosomes separates.
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
Two haploid cells
form; each chromosome
still consists of two
sister chromatids.
Haploid daughter
cells forming
Figure 13.8a
Prophase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Telophase I and
Cytokinesis
Anaphase I
Metaphase I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Metaphase
plate
Fragments
Homologous
chromosomes of nuclear
envelope
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Cleavage
furrow
Each pair of homologous
chromosomes separates.
Chromosomes line up
Duplicated homologous
chromosomes (red and blue) by homologous pairs.
pair and exchange segments;
2n  6 in this example.
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.
Prophase I –where
meiosis differs from
mitosis
• Prophase I typically
occupies more than 90%
of the time required for
meiosis, chromosomes
begin to condense
• In synapsis,
homologous
chromosomes loosely
pair up, aligned gene by
gene, forming a tetrad
© 2011 Pearson Education, Inc.
• In crossing over, nonsister chromatids
exchange DNA segments
– Each pair of chromosomes forms a tetrad, a
group of four chromatids
• Each tetrad usually has one or more
chiasmata, X-shaped regions where crossing
over occurred
© 2011 Pearson Education, Inc.
Figure 13.8b
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
Haploid daughter
cells forming
Figure 13.9a
MEIOSIS
MITOSIS
Parent cell
MEIOSIS I
Chiasma
Prophase I
Prophase
Duplicated
chromosome
Chromosome
duplication
2n  6
Chromosome
duplication
Homologous
chromosome pair
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Daughter
cells of
meiosis I
2n
Daughter cells
of mitosis
2n
Haploid
n3
MEIOSIS II
n
n
n
n
Daughter cells of meiosis II
Figure 13.9b
SUMMARY
Property
Mitosis
Meiosis
DNA
replication
Occurs during interphase before
mitosis begins
Occurs during interphase before meiosis I begins
Number of
divisions
One, including prophase, metaphase,
anaphase, and telophase
Two, each including prophase, metaphase, anaphase,
and telophase
Synapsis of
homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Number of
daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Four, each haploid (n), containing half as many
chromosomes as the parent cell; genetically different
from the parent cell and from each other
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes
by half and introduces genetic variability among the
gametes
comparison
MITOSIS
MEIOSIS
Parent cell
MEIOSIS I
Chiasma
Prophase I
Prophase
Duplicated
chromosome
Chromosome
duplication
2n  6
Chromosome
duplication
Homologous
chromosome pair
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Daughter
cells of
meiosis I
2n
Haploid
n3
MEIOSIS II
2n
Daughter cells
of mitosis
n
n
n
n
Daughter cells of meiosis II
SUMMARY
Property
Mitosis
Meiosis
DNA
replication
Occurs during interphase before
mitosis begins
Occurs during interphase before meiosis I begins
Number of
divisions
One, including prophase, metaphase,
anaphase, and telophase
Two, each including prophase, metaphase, anaphase,
and telophase
Synapsis of
Does not occur
homologous
chromosomes
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Two, each diploid (2n) and genetically
Number of
daughter cells identical to the parent cell
and genetic
composition
Four, each haploid (n), containing half as many
chromosomes as the parent cell; genetically different
from the parent cell and from each other
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes
by half and introduces genetic variability among the
gametes
Concept 13.4: Genetic variation produced in
sexual life cycles contributes to evolution
• Three mechanisms contribute to genetic
variation from sexual reproduction:
• Shuffling of alleles:
1. Independent assortment of chromosomes
2. Crossing over
• Mating process
3. Random fertilization of sperm
4. Also, mutations occur in all organisms
© 2011 Pearson Education, Inc.
Math test:
• How many chromosome combinations are
possible in a given human gamete, not
including crossing over or mutations?
Statistics of meiotically-derived diversity
• The number of combinations possible when
chromosomes assort independently into
gametes is 2n, where n is the haploid number
– For humans (n = 23), there are more than 8
million (223) possible combinations of
chromosomes
© 2011 Pearson Education, Inc.
Independent assortment visualized
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Independent assortment visualized
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Independent assortment visualized
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1 Combination 2
Combination 3 Combination 4
Crossing Over
• Crossing over produces recombinant
chromosomes, which combine DNA inherited
from each parent into a single cromosome
– Crossing over begins very early in prophase I,
as homologous chromosomes pair up gene by
gene
– homologous portions of two nonsister chromatids
trade places
© 2011 Pearson Education, Inc.
• In crossing over, homologous portions of two
nonsister chromatids trade places
• Crossing over contributes to genetic variation
by combining DNA from two parents into a
single chromosome
© 2011 Pearson Education, Inc.
Figure 13.11-1
Prophase I
of meiosis
Pair of homologs
Nonsister chromatids
held together
during synapsis
Figure 13.11-2
Prophase I
of meiosis
Pair of homologs
Chiasma
Centromere
TEM
Nonsister chromatids
held together
during synapsis
Figure 13.11-3
Prophase I
of meiosis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Nonsister chromatids
held together
during synapsis
Figure 13.11-4
Prophase I
of meiosis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Nonsister chromatids
held together
during synapsis
Figure 13.11-5
Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
Random Fertilization and variation
• Random fertilization adds to genetic variation
because any sperm can fuse with any ovum
(unfertilized egg)
• Fun Tidbit: The fusion of two gametes (each
with 8.4 million possible chromosome
combinations from independent assortment)
produces a zygote with any of about 70 trillion
diploid combinations
– Crossing over and mutations add even more
variation
© 2011 Pearson Education, Inc.
Animation: Genetic Variation
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Figure 13.12
A bdelloid rotifer,
an animal that
reproduces only
asexually
How do they
survive
evolutionarily?
200 m