Download phase: cell division

Chapter 8
Cellular Reproduction:
Cells from Cells
PowerPoint® Lectures for
Campbell Essential Biology, Fifth Edition, and
Campbell Essential Biology with Physiology,
Fourth Edition
– Eric J. Simon, Jean L. Dickey, and Jane B. Reece
Lectures by Edward J. Zalisko
© 2013 Pearson Education, Inc.
What We will learn
1) Cell reproduction – Asexual and Sexual
2) Cell cycle and mitosis
i.
ii.
iii.
iv.
Chromosomes and their organization
The different phases of cell cycle
Different phases of Mitosis and cytokinesis
Cancer cells – out of cell cycle control
3) Meiosis
i.
ii.
iii.
iv.
v.
Homologous chromosome
Life cycle of sexual organism
Different phases of Meiosis
Origin of genetic variation
Disorders/diseases - When meiosis goes wrong
Three Function of Mitotic Cell division
• Cell division plays important roles in the lives
of organisms.
Cell Replacement
Human kidney cell
Growth via Cell Division
Early human embryo
Colorized SEM
• Cell division is critical as it
– replaces damaged or lost cells
– permits growth
– allows for reproduction
– Unicellular organisms
• Cell division is called mitosis
LM
– assure the continuity of life
WHAT CELL REPRODUCTION ACOMPLISHES
• Reproduction:
–
may result in the birth of new organisms
–
more commonly involves the production of new cells
• When a cell undergoes reproduction, or cell division, two
“daughter” cells are produced that are genetically identical to
each other and to the “parent” cell.
• Before a parent cell splits into two, it duplicates its
chromosomes, the structures that contain most of the
organism’s DNA.
• During cell division, each daughter cell receives one set of
chromosomes.
WHAT CELL REPRODUCTION ACOMPLISHES
• Two type of reproduction, asexual and sexual reproduction.
• In asexual reproduction:
– single-celled organisms reproduces by simple cell division
– there is no fertilization of an egg by a sperm
– the lone parent and its offspring have identical genes (cloning)
• Some multicellular organisms, such as sea stars, can grow
new individuals from fragmented pieces
• Growing a new plant from a clipping is another example of
asexual reproduction.
Budding in Yeast
Simple multicellular eukaryote,
Hydra reproduces asexually by
budding
Binary fission in Amoeba
WHAT CELL REPRODUCTION ACOMPLISHES
• Mitosis is the type of cell division responsible for:
– Asexual reproduction
– Growth and maintenance of multicellular organisms
• Sexual reproduction requires fertilization of an egg by
a sperm using a special type of cell division called
meiosis (the process of gametes production).
– Two organisms exchange their genetic material to produce
new generation
• Thus, sexually reproducing organisms use:
– Meiosis for reproduction (production of gametes )
– Mitosis for growth and maintenance
THE CELL CYCLE AND MITOSIS
•
Each cell contains all of the genetic information that makes up
the organism. This information is known as the genome
In a eukaryotic cell:
– most genes are located on chromosomes in the cell nucleus
– a few genes are found in dna in mitochondria and chloroplasts
– each eukaryotic chromosome contains one very long dna
molecule, typically bearing thousands of genes.
•
Most cells does two things : conducts metabolic activities
(Interphase) and divides (Mitosis)
•
The number of chromosomes in a eukaryotic cell depends on
the species.
Species
Indian muntjac deer
Koala
Opossum
Giraffe
Mouse
Human
Duck-billed platypus
Buffalo
Dog
Red viscacha rat
Number of chromosomes
in body cells
6
16
22
30
40
46
54
60
78
102
Figure 8.2
Organization of Eukaryotic Chromosomes
• Chromosomes:
– are made of chromatin fibers composed of roughly
equal amounts of DNA and protein molecules and,
– are not visible in a cell until cell division occurs
LM
Chromosomes
Condensation of
chromosomes into
distinct units
Organization of Eukaryotic
Chromosomes
• Histones: associated proteins used
to package DNA
DNA double helix
Histones
“Beads
on a
string”
TEM
• DNA (up to 6ft ) is packed into an
elaborate, multilevel system of
coiling and folding.
Nucleosome
• Nucleosomes consist of DNA
wound around histone molecules.
• After DNA duplication chromatin
condenses
Tight helical fiber
– coiling & folding to make a smaller
package
– from DNA to chromatin to highly
condensed mitotic chromosome
Duplicated
chromosomes
(sister
chromatids)
TEM
Thick supercoil
Centromere
Figure 8.4
Organization of Eukaryotic Chromosomes
1. At first level of packing, histones attach to the DNA (the
combination looks like bead on a string and each bead is
called nucleosome)
2. At second level, the beaded string is wrapped into tight
helical fiber
3. At third level, the fiber coils further into supercoils
4. At the last level, looping and folding can further compact the
DNA
When the cells are not dividing, DNA appears lightly packed,
with much of the DNA in ‘bead on string’ nucleosome
arrangement
When prepares to divide, they pack further
Chromosome duplication and distribution
• Before a cell divides, it duplicates all of
its chromosomes, resulting in two copies
called sister chromatids (S phase ).
• Sister chromatids are joined together at a
narrow “waist” of the chromosome called
the centromere.
This is still one
Chromosome
• When the cell divides, the sister
chromatids separate from each other.
• Once separated, each chromatid is:
 considered a full-fledged chromosome
 identical to the original chromosome
 Contain identical copies of the
chromosome’s DNA
This is one
Chromosome
Practice
A duplicated chromosome consists of two ______.
 A) centromeres
 B) centrosomes
 C) genomes
 D) sister chromatids
 E) genes
Sister chromatids are joined at the ______.
 A) chromatin
 B) centromere
 C) spindle
 D) centrosome
 E) centriole
The Cell Cycle
• A cell cycle is the orderly sequence of events that extend
– from the time a cell is formed from a dividing parent cell
– to its own division into two cells.
The cell cycle consists of two distinct phases:
1. Interphase - most of a cell cycle is spent here
2. Mitotic phase
During interphase, a cell:
90% of cell life cycle
• performs its normal functions
- produce RNA, synthesize proteins
• grows in size & doubles everything in its cytoplasm (organelles)
 Endoplasmic reticulum get busy
 Ribosome get busy to produce protein
Characteristics:
nucleus well-defined
DNA loosely, packed in long chromatin fibers
Divided into 3 phases:
Interphase
• G1 = 1st Gap
– cell doing its “everyday job”
– cell grows
• S = DNA Synthesis
– copies chromosomes
• G2 = 2nd Gap
– prepares for division
– cell grows
– produces organelles,
proteins, membranes
2005-2006
Interphase G2
• Nucleus well-defined
 chromosome duplication complete
 DNA loosely packed in long
chromatin fibers
• Prepares for mitosis
 produces proteins & organelles
2005-2006
The eukaryotic cell cycle
S phase
(DNA synthesis; chromosome duplication)
Interphase: metabolism and
growth (90% of time)
G1
G2
Mitotic (M) phase:
cell division
(10% of time)
New cell
Cytokinesis
(division of
cytoplasm)
Mitosis
(division
of nucleus)
Mitosis and Cytokinesis
• Mitotic (M) phase includes two overlapping processes:
1. Mitosis, in which the nucleus and its contents divide evenly
into two daughter nuclei
 exact copy of genetic material (DNA) is passed to
daughter cells
2. Cytokinesis, in which the cytoplasm is divided in two units
 assortment of organelles
• During mitosis the mitotic spindle guides
the separation of two sets of daughter chromosomes.
• Spindle microtubules grow from two centrosomes
 clouds of cytoplasmic material that in
animal cells contain centrioles
Prophase
Which of the following occurs during interphase?
 A) Chromatin becomes tightly coiled.
 B) Chromosome duplication.
 C) Sister chromatids separate.
 D) The mitotic spindle forms.
 E) Cytokinesis occurs.
Mitosis and Cytokinesis
• Mitosis means copying cell’s DNA & dividing it
between 2 daughter nuclei
• Mitosis consists of four distinct phases:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase.
Practice
The cell cycle results in the production of ______.
 A) four cells, each with the same amount of genetic material and
the same genetic information
 B) two cells, each with the same amount of genetic material but
with different genetic information
 C) two cells, each with the same amount of genetic material and
the same genetic information
 D) four cells, each with the same amount of genetic material but
with different genetic information
 E) two cells with different amounts of genetic material
PROPHASE
 Each sister chromatids start to condense into visible discrete
chromosomes
– Fibers extent from centromeres
 The nucleolus disappears
 The nuclear membrane breaks down
 The centrosomes move away from each other,
 The mitotic spindle (made of microtubules ) begins to form
and extends from each centrosome and invades the nuclear area
INTERPHASE
Centrosomes (with
centriole pairs)
Chromatin
Plasma
membrane
Centromere
Chromosome, consisting
of two sister chromatids
Spindle
microtubules
LM
Nuclear
envelope
PROPHASE
Fragments of
Early mitotic
Centrosome
nuclear envelope
spindle
Figure 8.7.a
METAPHASE AND ANAPHASE
Metaphase
Chromosomes are fully condensed and most visible at this stage
Spindle fibers align chromosomes along the middle of the cell
forming the metaphase plate, helps to ensure chromosomes
separate properly
Anaphase
Sister chromatids separate and move apart and become fullfledged chromosomes.
Each freed chromatid (now referred to as a chromosome or
daughter chromosome) is pulled at centromeres toward the
opposite pole of the cell
Poles move farther apart
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Nuclear
envelope
forming
Spindle
Cleavage
furrow
Daughter
chromosomes
Figure 8.7b
TELOPHASE
 Each set of chromosomes have reached the opposite
pole of the cell
 The chromosomes decondense or disperse as they are
no longer visible under light microscope
 The mitotic spindle disappears
 Two daughter nuclei begin to form, one at each pole
 The nuclear envelope and nucleolus reappear
 Mitosis, the division of one nucleus into two
genetically identical nuclei, is now complete
 Cytokinesis begins
CYTOKINESIS
• Cytokinesis typically:
– occurs during telophase
– divides the cytoplasm
– is different in plant and animal cells
 In animal cells, cytokinesis involves the formation of a
cleavage furrow, which contracts to pinch the cell in
two.
 In plant cells, small vesicles containing cell wall
materials move to equator line up & fuse to form a cell
plate, which grows outwards to complete the formation
of two daughter cells.
Cytokinesis in plant cells
Cell plate
forming
Daughter
nucleus
Cell wall
Vesicles containing
Cell plate New cell wall
cell wall material
LM
Wall of
parent cell
Daughter cells
Cytokinesis in animal cells
Cleavage furrow
Contracting ring of
microfilaments
Daughter cells
Practice
Which of the following occurs during prophase?
 A) Chromosomes line up on the midline of the cell.
 B) The nuclear envelope forms.
 C) Sister chromatids separate.
 D) The mitotic spindle begins to form.
 E) Cytokinesis occurs.
Practice
Which of the following is a stage of mitosis?
 A) cytokinesis
 B) telophase
 C) meiosis
 D) interphase
 E) DNA synthesis
Practice
The correct sequence of stages of mitosis is ______.
 A) prophase, metaphase, anaphase, telophase
 B) telophase, prophase, interphase, anaphase, metaphase
 C) anaphase, interphase, prophase, metaphase, telophase
 D) metaphase, prophase, anaphase, telophase
 E) interphase, prophase, metaphase, anaphase, telophase
Practice
During metaphase ______.
 A) the nuclear envelope breaks up
 B) cytokinesis occurs
 C) sister chromatids separate
 D) centromeres divide
 E) chromosomes line up in the middle of the cell
Practice
Which of these events occurs during anaphase?
 A) Sister chromatids separate.
 B) Chromosomes line up in the middle of the cell.
 C) Cytokinesis occurs.
 D) The nuclear envelope reappears.
 E) The nuclear envelope breaks up.
Practice
A cell that completed the cell cycle without undergoing
cytokinesis would ______.
 A) have less genetic material than it started with
 B) not have completed anaphase
 C) have its chromosomes lined up in the middle of the
cell
 D) be a prokaryotic cell
 E) have two nuclei
Cancer Cells: Growing Out of Control
• Normal cells (plant, animal) have a cell cycle control system that
consists of specialized proteins, which send “stop” and “go-ahead”
signals at certain key points during the cell cycle.
• Cancer is a disease of the cell cycle, that do not respond normally to
the cell cycle control system.
– Tumors are abnormally growing masses of body cells.
– If the abnormal cells remain at the original site, the lump is a benign tumor.
– The spread of cancer beyond their site of origin is metastasis.
• Malignant tumors can:
– spread to other parts of the body
– interrupt normal body functions
• A person with a malignant tumor is said to have cancer.
Growth and metastasis of a malignant tumor of
the breast
Lymph
vessels
Tumor
Blood
vessel
Glandular
tissue
A tumor grows
from a single
cancer cell.
Cancer cells invade
neighboring tissue.
Metastasis: Cancer
cells spread through
lymph and blood
vessels to other parts
of the body.
Cancer Treatment and Prevention
Cancer treatment can involve:
• Radiation therapy, which damages DNA and disrupts cell division
• Chemotherapy, which uses drugs that disrupt cell division by
interfering with mitotic spindle
• Certain behaviors can decrease the risk of cancer:
– Not smoking
– Exercising adequately
– Avoiding exposure to the sun
– Eating a high-fiber, low-fat diet
– Performing self-exams
– Regularly visiting a doctor to identify tumors early
Practice
What is the difference between a benign and a malignant
tumor?
A) Benign tumors are composed of cancer cells; malignant tumors
are not.
B) Benign tumors cannot kill you; malignant tumors can.
C) Benign tumors are not the result of a failure of a cell cycle
control system; malignant tumors are.
D) Benign tumors do not metastasize; malignant tumors do.
E) Benign tumors do not form lumps; malignant tumors do form
lumps.
Practice
A chemical that disrupts microfilament formation would
interfere with
A) DNA replication.
B) Formation of the mitotic spindle.
C) Cleavage.
D) Crossing over
Meiosis &
Sexual Reproduction
Meiosis, the Basis of Sexual Reproduction
Sexual reproduction (is important because it introduce
such unique combination and also variation ):
– uses meiosis
– uses fertilization
– produces offspring that contain a unique
combination of genes from the parents
© 2010 Pearson Education, Inc.
Figure 8.10
Homologous Chromosomes
• Different individuals of a single species have the same
 number and
 types of chromosomes.
• Human somatic cell is a typical body cell
– All body cells except for the gametes (egg and sperm)
 has 46 chromosomes (23 pairs)
 22 pairs of matching chromosomes, called autosomes
– Humans have two different sex chromosomes, X and Y
• Homologous chromosomes are matching pairs of chromosomes
 has same gene in the same position on the chromosome
 but possess different versions of the same genes.
Pair of homologous
chromosomes
Centromere
Sister
chromatids
One duplicated
chromosome
LM
A karyotype is an image that reveals an orderly
arrangement of chromosomes.
Practice
Homologous chromosomes __________________.
 A) carry the same genes
 B) include only the autosomes
 C) are a set of chromosomes that the cell received from
one parent
 D) include only the sex chromosomes
 E) separate during interphase
Practice
What sex chromosomes belong to a normal human male?
 A) YO
 B) XX
 C) XXY
 D) XY
 E) XO
Gametes and the Life Cycle of a Sexual Organism
The life cycle of a multicellular organism is the sequence of stages
leading from the adults of one generation to the adults of the next.
Haploid gametes (n  23)
Egg cell
n
n
Sperm cell
FERTILIZATION
MEIOSIS
Multicellular
diploid adults
(2n  46)
2n
MITOSIS
and development
Diploid
zygote
(2n  46)
Key
Haploid (n)
Diploid (2n)
Gametes and the Life Cycle of a Sexual Organism
• Humans are diploid organisms in which:
– their somatic cells contain two sets of chromosomes
– their gametes are haploid, having only one set of
chromosomes
• In humans, a haploid sperm fuses with a haploid egg
during fertilization to form a diploid zygote.
• Sexual life cycles involve an alternation of diploid and
haploid stages.
• Meiosis produces haploid gametes, which keeps the
chromosome number from doubling every generation.
How do we make sperm & eggs?
 reduce 46 chromosomes  23 chromosomes
• half the number of chromosomes
23
46
meiosis
23
46
egg
23
46
23
fertilization
sperm
How meiosis halves chromosome number
1
Chromosomes
duplicate.
Pair of
homologous
chromosomes
in diploid
parent cell
2
Duplicated pair
of homologous
chromosomes
INTERPHASE BEFORE MEIOSIS
Homologous
chromosomes
separate.
3
Sister chromatids
separate.
Sister
chromatids
MEIOSIS I
MEIOSIS II
Figure 8.13-3
The Process of Meiosis
In meiosis, haploid daughter cells are produced from diploid cells
Two special features
– The number of chromosome is reduced to half
 diploid  haploid (2n  n)
 humans: 46  23
–
Involves two consecutive divisions, meiosis I and meiosis II that
occur after interphase
– There is an exchange of genetic material – pieces of chromosomesbetween homologous chromosome. This exchange of chromosome
is called crossing over
– Fertilization restores chromosome number
 haploid  diploid (n  2n)
MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
Chromatin
Chromosomes
duplicate.
PROPHASE I
Sites of
crossing over
Spindle
Sister
chromatids
METAPHASE I
Microtubules
attached
to chromosome
ANAPHASE I
Sister chromatids
remain attached
Centromere
Pair of
homologous
chromosomes
Homologous
chromosomes
pair up and
exchange
segments.
Pairs of
homologous
chromosomes
line up.
Pairs of
homologous
chromosomes
split up.
MEIOSIS II: SISTER CHROMATIDS SEPARATE
TELOPHASE I
AND
CYTOKINESIS
PROPHASE II METAPHASE II
ANAPHASE II TELOPHASE II
AND
CYTOKINESIS
Cleavage
furrow
Sister
chromatids
separate
Two haploid
cells form;
chromosomes
are still
doubled.
Haploid
daughter
cells forming
During another round of cell division, the sister
chromatids finally separate; four haploid
daughter cells result, containing single chromosomes.
Mitosis vs. Meiosis
In mitosis and meiosis, the chromosomes duplicate only once,
during the preceding interphase.
Mitosis
Meiosis
o 1 cell division
o 2 cell divisions
o daughter cells genetically
identical to parent cell
o daughter cells genetically
different from parent
o produces 2 cells
o produces 4 non identical cells
o Diploids: 2n  2n
o Haploid: 2n  1n
o produces cells for growth &
repair
o produces gametes
o No crossing over
o crossing over
MITOSIS
MEIOSIS
Prophase
Prophase I
Duplicated chromosome
(two sister chromatids)
Chromosome
duplication
MEIOSIS I
Chromosome
duplication
Parent cell
(before chromosome duplication)
2n  4
Homologous
Site of crossing over
between homologous
(nonsister) chromatids
chromosomes come
together in pairs.
Metaphase
Metaphase I
Homologous pairs
align at the middle
of the cell.
Chromosomes
align at the
middle of the cell.
Anaphase
Telophase
2n
Anaphase I & Telophase I
Sister
chromatids
separate during
Anaphase.
Daughter cells
of mitosis
2n
MEIOSIS I
Homologous
Chromosomes
separate during
anaphase I; sister
chromatids
remain together.
Chromosome with two
sister chromatids
Haploid
n2
MEIOSIS II
Sister
chromatids
separate during
Anaphase II.
n
n
n
Daughter cells of meiosis II
n
Figure 8.15
Steps of meiosis
Meiosis 1
 interphase
 prophase 1
 metaphase 1
 anaphase 1
1st division of
meiosis separates
homologous pairs
(2n  1n)
 telophase 1
Meiosis 2
 prophase 2
 metaphase 2
 anaphase 2
 telophase 2
2nd division of meiosis separates
sister chromatids
(1n  1n)
* just like mitosis *
Practice
During metaphase I, ______.
 A) crossing over occurs
 B) homologous chromosomes line up in the middle of
the cell
 C) the nuclear envelope breaks up
 D) centrosomes migrate to opposite poles
 E) sister chromatids separate and migrate to opposite
poles
Practice
Upon completion of telophase I and cytokinesis,
there is(are) ______ cell(s).
 A) four haploid
 B) two diploid
 C) two haploid
 D) one diploid
 E) four diploid
SUMMARY
Property
DNA
replication
Number of
divisions
Synapsis of
homologous
chromosomes
Number of
daughter cells
and genetic
composition
Role in the
animal body
Mitosis
Meiosis
SUMMARY
Property
DNA
replication
Number of
divisions
Mitosis
Occurs during interphase before
mitosis begins
One, including prophase, metaphase,
anaphase, and telophase
Does not occur
Synapsis of
homologous
chromosomes
Number of
Two, each diploid (2n) and genetically
daughter cells identical to the parent cell
and genetic
composition
Role in the Enables multicellular adult to arise from
animal body zygote; produces cells for growth, repair,
&, in some species, asexual reproduction
Meiosis
Occurs during interphase before meiosis I begins
Two, each including prophase, metaphase, anaphase,
and telophase
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Four, each haploid (n), containing half as many
chromosomes as the parent cell; genetically different
from the parent cell and from each other
Produces gametes; reduces number of chromosomes
by half and introduces genetic variability among the
gametes
The Origins of Genetic Variation
• Offspring of sexual reproduction are genetically
different from their parents and one another
• This is due to natural selection and 3 events that
happens in meiosis accounts for these variation
1. Independent Assortment of Chromosomes
2. Random Fertilization
3. Crossing over.
Independent Assortment of Chromosomes
• Also known as Random distribution of homologous chromosomes
– When aligned during metaphase I of meiosis, the side-by-side
orientation of each homologous pair of chromosomes is a matter of
chance.
– Every chromosome pair orients independently of the others during
meiosis.
• For any species the total number of chromosome combinations that
can appear in the gametes due to independent assortment is:
– 2n where n is the haploid number.
• For a human: n = 23
– 223 = 8,388,608 different chromosome combinations possible in a
gamete
POSSIBILITY 2
POSSIBILITY 1
Two equally probable
arrangements of
chromosomes
at metaphase of
meiosis I
Metaphase
of
meiosis II
Gametes
Combination a Combination b
Combination c Combination d
Because possibilities 1 and 2 are equally likely, the four possible types of
gametes will be made in approximately equal numbers.
Figure 8.16-3
2. Random Fertilization
• A human egg cell is fertilized randomly by one sperm, leading
to genetic variety in the zygote.
• If each gamete represents one of 8,388,608 different
chromosome combinations, at fertilization, humans would have
8,388,608 × 8,388,608, or more than 70 trillion, different
possible chromosome combinations.
• So we see that the random nature of fertilization adds a huge
amount of potential variability to the offspring of sexual
reproduction.
3. Crossing over
During Prophase 1, non-sister chromatids of
homologous chromosomes exchange genetic
information
 homologous pairs swap pieces of chromosome sister
chromatids intertwine
Genetic recombination, the production of gene
combinations different from those carried by
parental chromosomes, occurs
tetrad
synapsis
Prophase I
of meiosis
Homologous chromatids
exchange corresponding
segments.
Duplicated pair of
homologous
chromosomes
Chiasma, site of
crossing over
Metaphase I
Sister chromatids
remain joined at their
centromeres.
Spindle
microtubule
Metaphase II
Gametes
Recombinant
chromosomes
combine genetic
information from
different parents. Recombinant chromosomes
Practice
Which of the following is a characteristic seen in prophase
I that does not occur in prophase II?
 A) Chromosomes move to the middle of the cell.
 B) Spindle formation occurs.
 C) Chromosomes have been duplicated.
 D) Crossing over occurs.
 E) Cytokinesis occurs.
When Meiosis Goes Awry
• What happens when errors occur in meiosis?
– Such mistakes can result in genetic abnormalities that range from
mild to fatal.
• In nondisjunction,
 the members of a chromosome pair fail to separate during anaphase,
 producing gametes with an incorrect number of chromosomes.
• Nondisjunction can occur during meiosis I or II.
• If nondisjunction occurs, and a normal sperm fertilizes an egg
with an extra chromosome, the result is a zygote with a total
of 2n + 1 chromosomes.
• If the organism survives, it will have
– an abnormal karyotype and
– probably a syndrome of disorders caused by the abnormal
number of genes
Fertilization after nondisjunction in the mother
Abnormal egg
cell with extra
chromosome
n1
Normal
sperm cell
n (normal)
Abnormal zygote
with extra
chromosome 2n  1
Down Syndrome: An Extra Chromosome 21
• Is also called trisomy 21
• Is a condition in which an individual has an extra chromosome
21
• Affects about one out of every 700 children
• The incidence of Down Syndrome increases with the age of the
mother.
Chromosome 21
NONDISJUNCTION IN MEIOSIS I
NONDISJUNCTION IN MEIOSIS II
Meiosis I
Nondisjunction:
Pair of homologous
chromosomes fails
to separate.
Figure 8.20-1
NONDISJUNCTION IN MEIOSIS II
NONDISJUNCTION IN MEIOSIS I
Meiosis I
Nondisjunction:
Pair of homologous
chromosomes fails
to separate.
Meiosis II
Nondisjunction:
Pair of sister
chromatids
fails to separate.
Figure 8.20-2
Two types of nondisjunction
NONDISJUNCTION IN MEIOSIS II
NONDISJUNCTION IN MEIOSIS I
Meiosis I
Nondisjunction:
Pair of homologous
chromosomes fails
to separate.
Meiosis II
Nondisjunction:
Pair of sister
chromatids
fails to separate.
Gametes
n1
n1
n–1
Abnormal gametes
Number of
n – 1 chromosomes
n1
n–1
Abnormal gametes
n
n
Normal gametes
Abnormal Numbers of Sex Chromosomes
• Nondisjunction in meiosis can also affect the sex
chromosomes.
– can lead to abnormal numbers of sex chromosomes but
– seems to upset the genetic balance less than unusual numbers of
autosomes, perhaps because the Y chromosome is very small
and carries relatively few genes.
© 2013 Pearson Education, Inc.
Practice
One difference between mitosis and meiosis is ______.
 A) mitosis produces cells genetically identical to the parent cell,
but meiosis does not
 B) mitosis produces more daughter cells than meiosis
 C) meiosis is needed for growth and tissue repair, but mitosis is
not
 D) mitosis produces haploid cells, but meiosis produces diploid
cells
 E) mitosis requires only one parent cell, but meiosis requires two
parent cells
Practice
Crossing over during prophase I results in ______.
 A) nondisjunction
 B) translocation
 C) reciprocal translocation
 D) duplication
 E) genetic recombination
Challenge Question
1. A cell biologist carefully measured the quantity of DNA in
grasshopper cells growing in cell culture. Cells examined during
the G2 phase of the cell cycle contained 200 units of DNA. What
would be the amount of DNA at G1 of the cell cycle in one of the
grasshopper.
2. Cytokinesis usually, but not always, follows mitosis. If a cell
completed mitosis but not cytokinesis, what would be the result?
Challenge Question#3
Evolution Connection: The Advantages of Sex
• Asexual reproduction conveys an evolutionary
advantage when plants are
– sparsely distributed and unlikely to be able to exchange
pollen or
– superbly suited to a stable environment.
• Asexual reproduction also eliminates the need to expend
energy
– forming gametes and
– copulating with a partner.
© 2013 Pearson Education, Inc.
Evolution Connection: The Advantages of Sex
• Sexual reproduction may convey an evolutionary
advantage by
– speeding adaptation to a changing environment or
– allowing a population to more easily rid itself of harmful
genes.
© 2013 Pearson Education, Inc.
Runner
Figure 8.24
The Process of Science:
Do All Animals Have Sex?
• Observation: No scientists have ever found male
bdelloid rotifers, a microscopic freshwater
invertebrate.
• Question: Does this entire class of animals reproduce
solely by asexual means?
• Hypothesis: Bdelloid rotifers have thrived for millions
of years despite a lack of sexual reproduction.
• Prediction: Bdelloid rotifers would display much more
variation in their pairs of homologous genes than most
organisms.
• Experiment: Researchers compared sequences of a
particular gene in bdelloid and non-bdelloid rotifers.
The Process of Science:
Do All Animals Have Sex?
• Results:
– Non-bdelloid sexually reproducing rotifers had a nearly
identical homologous gene, differing by only 0.5% on
average.
– The two versions of the same gene in asexually
reproducing bdelloid rotifers differed by 3.5–54%.
• Conclusion: Bdelloid rotifers have evolved for
millions of years without any sexual reproduction.
LM
Figure 8.19
Figure 8.7.aa