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Cell Division
Chapter 9
2-
The Importance of Cell Division


The ability to grow and reproduce are two
fundamental qualities of life.
During cell division, one cell becomes two new
cells.
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•

9-2
Reproduction occurs as binary fission in prokaryotes.
Growth and some reproduction occurs as mitosis in
eukaryotes.
Reproduction often involves meiosis in eukaryotes.
All cell division is preceded by DNA replication.
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Prokaryotic DNA




Prokaryotes typically have only one circular
chromosome - a strand of DNA
This chromosome contains all of the genes
required for this cell to survive
Prokaryotes are ‘Haploid’ – They have only
one copy of each gene
Compare this to humans: Humans are
Diploid – They have 2 copies of each gene
arranged in 46 chromosome pairs (23 pairs
of chromosomes)
Prokaryotic DNA

The prokaryotic chromosome is located in a
region of the cytosol called the Nucleiod

Recall: The Nucleiod is not surrounded by a
membrane like the nucleus of eukaryotic cell
Prokaryotic DNA
Eukaryotic DNA




Multiple linear chromosomes
Every species has a different number of
chromosomes
Nucleus contains chromatin – a complex of
DNA and proteins
Not all of the cell’s DNA is used all at once
–
–
heterochromatin – regions that are not expressed
euchromatin – expressed regions
Eukaryotic DNA
Eukaryotic DNA
9
DNA Replication


Recall: DNA is molecule that
stores the genetic information
of a cell
The genetic information is
stored in the order of the
nitrogenous bases
DNA Replication



Before a cell can divide, the DNA must be
copied
James Watson and Francis Crick were British
scientists who discovered the structure of
DNA in 1953
Their publication ended with this statement:
“It has not escaped our notice that the
specific pairing we have postulated
immediately suggests a possible copying
mechanism for the genetic material”
DNA Replication

The complementary
property of double
stranded DNA
allows each strand
to serve as a
template for DNA
Replication
DNA Replication

This model for
replication is known
as Semi-Conservative
Replication because
in each copy, one old
strand is conserved
(saved) and paired
with a newly made
strand
Binary Fission
and Mitosis

In single-celled organisms
–
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In multi-cellular organisms mitosis:
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–
–
9-3
Mitosis and binary fission are means of asexual
reproduction.
Causes growth by increasing the number of cells
Replaces lost cells
Repairs injuries
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Meiosis

Sexual reproduction involves the donation of
genetic information from two parents.
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Meiosis occurs prior to sexual reproduction.
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9-4
Each parent can only donate half of the genome.
Generates gametes (egg and sperm) with half of
a genome
The egg and sperm then join during fertilization to
make a unique offspring with a full complement of
genetic information.
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The Cell Cycle

Eukaryotic cells
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Pass through different
stages between cell
divisions
A continuous process
Cell cycle:


9-5
Interphase
Mitosis
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The Cell Cycle

The eukaryotic cell cycle has 5 phases:
1. 1. G1 (gap phase 1)
2. S (synthesis)
interphase
3. G2 (gap phase 2)
4. M (mitosis) - cell replication
5. C (cytokinesis)

The length of a complete cell cycle varies
greatly among cell types.
Interphase

During interphase, cells
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9-6
Cells conduct life activities
Engage in metabolic activities
Prepare for the next cell division
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Phases of Interphase-G1

The cell gathers nutrients, carries out its regular
metabolic roles, and performs its normal
function.
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9-6
Commits to divide
Some cells never divide; they stay in G1, called Go..
Prepares for DNA replications
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Phases of Interphase-S
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9-7
S phase
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DNA replication occurs.
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The DNA in chromosomes
condense and are copied
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When S phase is complete
 The identical chromosome
copies are connected
together.
 Each is called a sister
chromatid.
– Connected at the
centromere
– Held
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21
Phases of Interphase-G2

During G2
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9-8
Final preparations
are made for mitosis.
Proteins are made
that will move and
separate the
chromosomes within
the cell nucleus.
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Mitosis-cell Replication

The two events of cell division
– Mitosis
 Separating
the chromosome copies (sister
chromatids) into two new nuclei
 Occurs in four phases that are continuous with
one another
–
Cytokinesis
 Dividing
the cytoplasm into two new cells that
will house the new nuclei
9-9
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Mitosis

Mitosis is divided into 4 phases:
1.Prophase
2.Metaphase
3.Anaphase
4.Telophase
1. Prophase
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The thin, tangled
chromatin gradually
coils and thickens.
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
9-10
Becomes visible as
separate chromosomes,
each with two sister
chromatids
Nucleus disassembles.
Nucleolus is no longer
visible.
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Late Prophase

Spindle fibers attach
to chromosomes at
their centromeres.
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–
9-11
Spindles are made of
microtubules.
Will move chromosomes
around within the cell.
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2. Metaphase

The spindle fibers move the chromosomes
so that they are all arranged at the middle
of the cell.
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–
9-12
This is called the equatorial plate.
Chromosomes complete this process at
metaphase.
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Metaphase
9-13
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3. Anaphase

Sister chromatids separate and move toward
opposite poles.
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Spindle fibers are used to pull the sister
chromatids to opposite poles of the cell
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–
9-14
Once the sister chromatids are separated, they
are known as daughter chromosomes.
The poles begin to move farther apart.
The kinetochore (proteins attached at the
centromere) pulls the chromatid along the spindle
fiber.
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Kinetochores
9-15
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Anaphase
9-16
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4. Telophase
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9-17
Spindle fibers
disassemble.
Nuclear membranes
form around the two
new sets of
chromosomes.
Chromatin uncoils.
Nucleolus reforms.
The daughter cells
enter interphase again.
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Cytokinesis

Separates the two new nuclei
into new cells

Roughly divides the cytoplasm
and its contents in half

Animal cells
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9-18
–
Membrane forms a
cleavage furrow
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Cell pinches into two
Plant cells
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Cell plate is formed.
–
A new cell wall is built,
separating the nuclei.
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34
http://www.youtube.com/watch?v=DD3IQkn
CEdc&feature=related
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Controlling Cell Division
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Cell division must be tightly regulated.
Cells gather information about themselves and their
environment in order to decide whether or not to
divide.
A cell decides whether to proceed through the cell
cycle at checkpoints during interphase.
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9-19
Cells evaluate their genetic health, their location in the body
and the body’s need for more cells.
 Poor genetic health, wrong location in the body or overcrowding will cause the cell to wait before dividing.
 The opposite signals will trigger the cell to proceed with
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division.
Genes Regulate the Cell Cycle
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Cells use checkpoint proteins make the decision to
proceed through the cell cycle or to stop.
Two classes of genes that code for checkpoint
proteins
1. Proto-oncogenes
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Code for proteins that encourage cell division
2. Tumor-suppressor genes
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9-20
Code for proteins that discourage cell division
The balance of these two types of proteins tells the
cell whether or not to proceed with cell division.
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p53, a Tumor-suppressor Gene
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Near the end of G1, the p53 protein identifies if the
cell’s DNA is damaged.
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If the damage is too severe, p53 will trigger the events of apoptosis
(cell suicide).
Mutations in the p53 gene
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9-21
If the DNA is healthy, the p53 allows the cell to divide.
If the DNA is damaged, p53 activates other proteins that will
repair the DNA.
Lead to cells that will proceed through the cell cycle with
damaged DNA
Lead to an accumulation of mutations
If the mutations occur in proto-oncogenes or tumor-suppressor
genes, then cancer will result.
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p53, a Tumor-suppressor Gene
9-22
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Cancer

Cancer is caused by a failure to control cell
division.
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Leads to cells that divide too frequently
Cell masses (tumors) interfere with normal body
functions.
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p53 is mutated in 40% of all cancers.
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9-23
Benign tumors
Malignant (metastatic) tumors
Leads to other mutations that result in cancer
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Causes of Cancer

Mutagens are agents that damage DNA.
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Chemicals
Radiation
Free radicals
Carcinogens are mutagens that cause
mutations that lead to cancer.
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Cigarette smoke

9-25
Has been linked directly to p53 mutations
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Malignant Tumors Metastasize
9-24
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Treatment Strategies ̶ Surgery
 Surgical removal
– Once tumors are identified they can be
surgically removed.
– Skin cancers and breast cancers are
frequently treated this way.
– If the cancer is spread diffusely,
surgery is not an option.
9-26
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Treatment Options ̶ Chemotherapy
and Radiation Therapy

Chemotherapy
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Some drugs will target rapidly dividing cells.
Normal cells that divide rapidly will suffer as well.
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Weakens the immune system
Causes hair loss
Radiation therapy
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Uses x-rays or gamma rays directed at the tumor
to kill the cancerous cells
Whole-body radiation is used to treat leukemia.

Can lead to radiation sickness
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9-27
Nausea, hair loss, etc.
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Meiosis

Meiosis is a form of cell division that leads to
the production of gametes.
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gametes are sex cells: egg cells and sperm cells
Gametes contain half the number of
chromosomes of an adult body cell
Adult body cells (somatic cells) are diploid,
containing 2 sets (pairs) of each chromosome.
Gametes are haploid, containing only 1 of
every chromosomes.
Cell Division and Sexual
Reproduction

Meiosis makes haploid gametes.
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9-30
Eggs are made in ovaries (animals) and pistils (plants).
Sperm are made in testes (animals) and anthers (plants).

Egg and sperm only have half of the individual’s
genetic information (haploid).

Fertilization - when egg and sperm join during
sexual reproduction - restores the diploid genome
– the zygote receives half of its chromosomes from
the egg and half from the sperm.
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47
Cell Division and Sexual
Reproduction
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Sexual reproduction includes the fusion of
gametes (fertilization) to produce a diploid
zygote.
Life cycles of sexually reproducing organisms
involve the alternation of haploid and diploid
stages.
Some life cycles include longer diploid
phases, some include longer haploid phases.
Haploid and Diploid Cells
9-31
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Life Cycles Involving Meiosis
and Mitosis
9-32
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51
Pairs of Chromosomes

Diploid cells have two sets of chromosomes.
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Homologous chromosomes
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Have the same order of genes along their DNA
Have different alleles of the same genes
One chromosome in the pair came from mom; the
other came from dad.
Non-homologous chromosomes
–
9-33
One set from each parent
Have different genes on their DNA
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A Pair of Homologous
Chromosomes
9-34
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Different Species Have Different
Numbers of Chromosomes
9-35
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Meiosis
9-36

The production of gametes

Meiosis involves two successive cell
divisions with no replication of genetic
material between them

Results in a reduction of the chromosome
number
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Meiosis

Meiosis includes two rounds of division –
Meiosis I and Meiosis II.

Meiosis I resembles mitosis
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Meiosis II reduces the chromosome number from
diploid to haploid.
Meiosis II resembles mitosis, but without
DNA replication
Meiosis
9-37
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Meiosis I: Prophase I

Prophase I
–
9-38
Synapsis occurs
 Homologous chromosomes move toward one another
and associate with one another.
 While associated homologs experience crossing over
– Homologs trade equivalent sections of DNA.
– Shuffles the genes that are passed to the next
generation
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59
Meiosis I: Metaphase I

Metaphase I:
–
9-39
The synapsed pairs
of homologous
chromosomes are
moved into position
at the equatorial
plate.
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Meiosis I: Anaphase I

Homologous pairs segregate to opposite poles.
–
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Chromosome number is reduced from
diploid to haploid.

Homologous pairs line up randomly at the
equatorial plate,
–
9-40
Sister chromatids do not separate at this point.
Each pair separates independently of the others.
 This is called independent assortment.
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Anaphase I
9-41
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Meiosis I: Telophase I


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
Chromatin uncoils.
Nuclear membrane
reforms.
Nucleoli reappear.
Cytokinesis divides the
two haploid nuclei into
two daughter cells.
–
9-42
Each chromosome still
contains two sister
chromatids.
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Meiosis I
9-43
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Meiosis II: Prophase II



9-44
Similar to prophase in
mitosis
Nuclear membrane is
disassembled.
Spindle begins to form.
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Meiosis II: Metaphase II


9-45
Similar to metaphase in
mitosis
Chromosomes are lined
up at the equatorial
plate.
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Meiosis II: Anaphase II


Centromeres divide.
Sister chromatids
separate.
–
9-46
Now called daughter
chromosomes
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Meiosis II: Telophase II

9-47
Similar to telophase and cytokinesis in mitosis
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Summary of Meiosis II
http://www.youtube.com/watch?v=D
1_-mQS_FZ0&feature=related
9-48
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Meiosis vs. Mitosis

Meiosis is characterized by 4 features:
–
–
–
–
1. Synapsis and crossing over
2. Sister chromatids remain joined at their
centromeres throughout meiosis I
3. Kinetochores of sister chromatids attach to the
same pole in meiosis I
4. DNA replication is suppressed between meiosis
I and meiosis II.
Meiosis vs. Mitosis


Meiosis produces haploid cells that are not
identical to each other.
Genetic differences in these cells arise from:
–
–

crossing over
random alignment of homologues in metaphase I
(independent assortment)
Mitosis produces 2 cells identical to each
other.
Mitosis vs. Meiosis
9-49
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Genetic Diversity ̶
The Advantage of Sex

Sexually reproducing organisms
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–
Need two individuals to reproduce
Reproduce more slowly than asexually
reproducing organisms
Have large genetic diversity


9-50
When environmental conditions change, they are more
adaptable and likely to survive
Genetic diversity is due to different alleles
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Genetic Diversity ̶
The Advantage of Sex

Five factors create genetic diversity by
creating new alleles, or new combinations of
alleles.
1. Mutation
2. Crossing-over
3. Segregation
4. Independent assortment
5. Fertilization
9-51
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Mutations


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9-52
Mutations are changes in the nucleotide
sequence of DNA.
This creates new alleles.
New alleles lead to new forms of proteins.
Increases genetic diversity
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Crossing-over

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The exchange of equivalent portions of DNA
between homologous chromosomes
Occurs during prophase I when
chromosomes are synapsed
Allows new combinations of genetic
information to occur
–
9-53
Each gamete then receives some of your
mother’s and some of your father’s genes on
each chromosome.
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Synapsis and Crossing-over
9-54
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The Results of Crossing-over
9-55
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Crossing-over Separates Linked
Genes

The closer genes are
together on a
chromosome, the less
likely they will be
separated by crossingover.
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9-56
These genes will be
inherited together.
The farther genes are
apart on a
chromosome, the more
likely they will be
separated by crossingCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Segregation

Alleles on homologous chromosomes
separate during anaphase I.
–
Consider a person who has two alleles for insulin,
one allele produces functional insulin, the other is
a mutation


Half of that person’s gametes would get the gene for
functional insulin (I).
Half of the gametes would get the gene for nonfunctional
gametes (i).
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9-57
If this gamete were used during fertilization, and was
joined with another mutant insulin gamete, the offspring
would not be able to make functional insulin.
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Independent Assortment

The segregation of homologous
chromosomes is independent of how other
homologous pairs segregate.

Consider two pairs of chromosomes.
–
9-58
Given the two ways these pairs can line up on the
equatorial plate, there are four possible
combinations of chromosomes in gametes.
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Independent Assortment
9-59
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Fertilization

Due to the large number of possible gametes
resulting from independent assortment,
segregation, mutation and crossing-over,
–

Since gametes join randomly
–
9-60
A large number of different offspring can be
generated from two parents.
The combinations of alleles is nearly infinite.
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Nondisjunction and Chromosome
Abnormalities

Nondisjunctions occur when homologous
chromosomes do not separate during cell
division.
–
–
Frequently results in the death of the cells
Some abnormal gametes live.

When these gametes participate in fertilization, the
offspring will have an abnormal number of chromosomes.
Monosomy describes a cell that has just one of a given pair
of chromosomes.
– Trisomy describes a cell that has three copies of a given
chromosome.
–
9-61
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Nondisjunction During
Gametogenesis
9-62
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Karyotypes are a Picture
of a Person’s Chromosomes
9-63
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A Karyotype can Reveal
Trisomy 21

Down syndrome
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–
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Three copies of
chromosome #21
Results in 47 chromosomes
instead of 46
Symptoms include:



9-64
Thickened eyelids
Mental impairment
Faulty speech
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Determination
and Differentiation



During sexual reproduction, fertilization of an egg by
a sperm results in a single-celled zygote.
The zygote undergoes mitosis to develop into an
adult.
As mitosis occurs, cells must become specific cell
types.
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–
–
–
9-28
All body cells are genetically identical.
Cells only differ in the genes they express.
Determination is the process a cell goes through to select which
genes it will express, committing itself to becoming a certain cell
type.
When a cell is fully developed into a specific type of cell, it is said
to be differentiated.
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Determination and
Differentiation of Skin Cells
9-29
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