Download chromosomes

Cellular reproduction and
Genetics
Chapter.8
2
Basic terminology
•  All the DNA in a cell constitutes the cell s genome
•  A genome can consist of a single DNA molecule (common in prokaryotic
cells) or a number of DNA molecules (common in eukaryotic cells)
•  DNA molecules in a cell are packaged into chromosomes
•  Eukaryotic chromosomes consist of chromatin, a complex of DNA and
protein that condenses during cell division
•  Every eukaryotic species has a characteristic number of chromosomes in
each cell nucleus
•  Somatic cells (non-reproductive cells) have two sets of chromosomes
•  Gametes (reproductive cells: sperm and eggs) have half as many
chromosomes as somatic cells
3
DNA, Chromatin and Chromosome
4
Cells arise only from preexisting cells
•  Cell division perpetuates life
–  Cell division is the reproduction of cells
–  Virchow’s principle states “Every cell from a cell”
•  Roles of cell division
–  Reproduction of an entire single-celled organism
–  Growth of a multicellular organism
–  Growth from a fertilized egg into an adult
–  Repair and replacement of cells in an adult
–  Sexual reproduction
•  Sperm and egg production
100 µm
(a) Reproduction
200 µm
(b) Growth and
development
20 µm
(c) Tissue renewal
6
Cellular reproduction
•  Living organisms reproduce by two methods.
• 
Asexual reproduction
•  produces offspring that are identical to the original cell or
organism and
•  involves inheritance of all genes from one parent.
•  A clone is a group of genetically identical individuals from the
same parent
•  Sexual reproduction
•  produces offspring that are similar to the parents, but show
variations in traits and
•  involves inheritance of unique sets of genes from two parents.
7
Star Wars: Episode II- Attack of the Clones (2002)
8
Asexual reproduction
9
Prokaryotes reproduce by binary fission
•  Binary fission means “dividing in half”
–  Occurs in prokaryotic cells
–  Two identical cells arise from one cell
–  Steps in the process
•  A single circular chromosome duplicates, and the
copies begin to separate from each other
•  The cell elongates, and the chromosomal copies
separate further
•  The plasma membrane grows inward at the midpoint to
divide the cells
Copyright © 2009 Pearson Education, Inc.
Plasma
membrane
Prokaryotic
chromosome
Cell wall
3
1
Duplication of chromosome
and separation of copies
2
Continued elongation of the
cell and movement of copies
Division into
two daughter cells
Prokaryotic chromosomes
THE EUKARYOTIC CELL
CYCLE AND MITOSIS
Distributing of chromosomes
•  In preparation for cell division, DNA is
replicated and the chromosomes condense
•  Each duplicated chromosome has two sister
chromatids (joined copies of the original
chromosome), which separate during cell
division
•  The centromere is the narrow waist of the
duplicated chromosome, where the two
chromatids are most closely attached
14
Chromosomes
DNA molecules
Sister
chromatids
Chromosome
duplication
Centromere
Sister
chromatids
Chromosome
distribution
to the
daughter
cells
15
Chromosomes
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
Cell cycle
•  Interphase vs mitosis
–  Visualized different stage under light microscope
•  Interphase : the period between mitotic cell
divisions
–  Interphase (about 90% of the cell cycle) can be
divided into subphases
•  G1 phase ( first gap )
•  S phase ( synthesis )
•  G2 phase ( second gap )
–  The cell grows during all three phases, but
chromosomes are duplicated only during the S
phase
Dynamic change in cell culture
19
INTERPHASE
G1
S
(DNA synthesis)
G2
M
(M) ITOTIC
PHA
SE
The large, complex chromosomes of
eukaryotes duplicate with each cell division
•  Eukaryotic chromosomes are composed of chromatin
–  Chromatin = DNA + proteins
–  To prepare for division, the chromatin becomes highly compact, and
the chromosomes are visible with a microscope
–  Early in the division process, chromosomes duplicate
•  Each chromosome appears as two sister chromatids, containing identical
DNA molecules
•  Sister chromatids are joined at the centromere, a narrow region
Chromosome condensation
Sister chromatids
Chromosome
duplication
Centromere
Sister
chromatids
Chromosome
distribution
to
daughter
cells
Mitosis
•  Mitosis : eukaryotic cell separates the chromosomes
in its cell nucleus into two identical sets in two
nuclei.
•  Sub-phases : The sequence of events is divided into
stages corresponding to the completion of one set of
activities and the start of the next.
•  interphase, prophase, prometaphase, metaphase,
anaphase and telophase.
Cell division is a continuum of dynamic changes
•  Mitosis progresses through a series of stages
– 
– 
– 
– 
– 
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
•  Cytokinesis often overlaps telophase
•  A mitotic spindle is required to divide the chromosomes
–  The mitotic spindle is composed of microtubules
–  It is produced by centrosomes, structures in the cytoplasm that
–  Organize microtubule arrangement
–  Contain a pair of centrioles in animal cells
– 
The role of centrioles in cell division is unclear
Multipolar spindle formation
26
Cell division
•  Interphase
–  In the cytoplasm
•  Cytoplasmic contents double
•  Two centrosomes form
–  In the nucleus
•  Chromosomes duplicate during the S phase
•  Nucleoli, sites of ribosome assembly, are visible
•  Prophase
–  In the cytoplasm
–  Microtubules begin to emerge from centrosomes, forming the spindle
–  In the nucleus
–  Chromosomes coil and become compact
–  Nucleoli disappear
Cell division
• 
Prometaphase
–  Spindle microtubules reach chromosomes, where they
•  Attach at kinetochores on the centromeres of sister chromatids
•  Move chromosomes to the center of the cell through associated protein “motors”
–  Other microtubules meet those from the opposite poles
–  The nuclear envelope disappears (NEBD: Nuclear envelope break down)
• 
Metaphase
–  Spindle is fully formed
–  Chromosomes align at the cell equator
• 
–  Kinetochores of sister chromatids are facing the opposite poles of the spindle
Anaphase
–  Sister chromatids separate at the centromeres
–  Daughter chromosomes are moved to opposite poles of the cell
–  Motor proteins move the chromosomes along the spindle microtubules
–  Kinetochore microtubules shorten
• 
Cytokinesis
–  Cytoplasm is divided into separate cells
INTERPHASE
Chromatin
Centrosomes
(with centriole pairs)
PROPHASE
Early mitotic Centrosome
spindle
PROMETAPHASE
Fragments
of nuclear
envelope
Centromere
Plasma
Nuclear
envelope membrane Chromosome, consisting
of two sister chromatids
Nucleolus
Kinetochore
Spindle
microtubules
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Daughter
chromosomes
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nuclear
envelope
forming
Nucleolus
forming
Aster
Centrosome
Sister
chromatids
Metaphase
plate
(imaginary)
Microtubules
Chromosomes
Kinetochores
Centrosome
1 µm
Overlapping
nonkinetochore
microtubules
Kinetochore
microtubules
0.5 µm
Spindle and kinetochore
32
Cytokinesis differs for plant and animal cells
•  Cleavage in animal cells
–  A cleavage furrow forms from a contracting ring of
microfilaments, interacting with myosin
–  The cleavage furrow deepens to separate the contents into
two cells
•  Cytokinesis in plant cells
–  A cell plate forms in the middle from vesicles containing
cell wall material
–  The cell plate grows outward to reach the edges, dividing
the contents into two cells
–  Each cell has a plasma membrane and cell wall
(a) Cleavage of an animal cell (SEM)
Cleavage furrow
Contractile ring of
microfilaments
100 µm
(b) Cell plate formation in a plant cell (TEM)
Vesicles
forming
cell plate
Wall of parent cell
Cell plate
1 µm
New cell wall
Daughter cells
Daughter cells
Factors affecting cell division
•  Factors that control cell division
–  Presence of essential nutrients
–  Growth factors, proteins that stimulate division
–  Presence of other cells causes density-dependent
inhibition
–  Contact with a solid surface; most cells show
anchorage dependence
Scalpels
1 A sample of human
connective tissue is
cut up into small
pieces.
2 Enzymes digest
the extracellular
matrix, resulting in
a suspension of
free fibroblasts.
Petri
dish
3 Cells are transferred to
culture vessels.
Without PDGF
4 PDGF is added
to half the
vessels.
10 µm
With PDGF
36
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
20 µm
20 µm
(a) Normal mammalian cells
(b) Cancer cells
37
Growth factors signal the cell cycle control system
•  Cell cycle control system
–  A set of molecules, including growth factors, that triggers
and coordinates events of the cell cycle
•  Checkpoints
–  Control points where signals regulate the cell cycle
•  G1 checkpoint allows entry into the S phase or causes the cell to
leave the cycle, entering a nondividing G0 phase
•  G2 checkpoint
•  M checkpoint
Quiescence (cell cycle exit)
G1 checkpoint (or Restriction point)
G0
Control
system
G1
M
M checkpoint
G2 checkpoint
G2
S
Growth factors signal the cell cycle control system
•  Effects of a growth factor at the Restriction point
–  A growth factor binds to a receptor (Growth factor
Receptor) in the plasma membrane
–  Within the cell, a signal transduction pathway propagates
the signal through a series of relay molecules
–  The signal reaches the cell cycle control system to trigger
entry into the S phase
G0
G1 checkpoint
G1
(a) Cell receives a go-ahead
signal.
G1
(b) Cell does not receive a
go-ahead signal.
41
Growth factor
Plasma membrane
Receptor
protein
Signal
transduction
pathway
Relay
proteins
Restriction point
Control
system
G1
M
G2
S
Cyclins and Cyclin-Dependent Kinases
•  Two types of regulatory proteins are involved in cell
cycle control: cyclins and cyclin-dependent
kinases (Cdks)
•  Cdks activity fluctuates during the cell cycle because
it is controled by cyclins, so named because their
concentrations vary with the cell cycle
•  MPF (maturation-promoting factor) is a cyclin-Cdk
complex that triggers a cell s passage past the G2
checkpoint into the M phase
43
M
G1
S G2
M
G1 S
G2
M
G1
MPF activity
Cyclin
concentration
Time
(a) Fluctuation of MPF activity and cyclin concentration
during the cell cycle
Cdk
Degraded
cyclin
Cyclin is
degraded
G2
Cdk
checkpoint
MPF
Cyclin
(b) Molecular mechanisms that help regulate the cell cycle
44
Growing out of control and cancer cells
•  Cancer cells escape controls on the cell cycle
–  Cancer cells divide rapidly, often in the absence of growth factors
–  They spread to other tissues through the circulatory system
–  Growth is not inhibited by other cells, and tumors form
•  Benign tumors remain at the original site
•  Malignant tumors spread to other locations by metastasis
•  Cancer treatments
–  Localized tumors can be treated with surgery or radiation
–  Chemotherapy is used for metastatic tumors
Cancer
•  Classification of cancer by origin
–  Carcinomas arise in external or internal body coverings
–  Sarcomas arise in supportive and connective tissue
–  Leukemias and lymphomas arise from blood-forming tissues
•  Cancer stage (ex in lung cancer)
– 
– 
– 
– 
Stage I : Cancer in nodule
Stage II : Cancer in lung
Stage III : Cancer in Chest
Stage IV : Cancer in Body
Lymph
vessels
Tumor
Blood
vessel
Glandular
tissue
A tumor grows from a
single cancer cell.
Cancer cells invade
neighboring tissue.
Cancer cells spread
through lymph and
blood vessels to
other parts of the body.
Loss of cell cycle controls in cancer cells
• 
Cancer cells do not respond normally to the body s control mechanisms
• 
Cancer cells may not need growth factors to grow and divide
–  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
• 
A normal cell is converted to a cancerous cell by a process called transformation
• 
Cancer cells that are not eliminated by the immune system, form tumors, masses
of abnormal cells within otherwise normal tissue
• 
If abnormal cells remain at the original site, the lump is called a benign tumor
• 
Malignant tumors invade surrounding tissues and can metastasize, exporting
cancer cells to other parts of the body, where they may form additional tumors
• 
Recent advances in understanding the cell cycle and cell cycle signaling have led
to advances in cancer treatment
48
49
50
MEIOSIS AND
CROSSING OVER
Chromosomes are matched in homologous pairs
•  Somatic cells have pairs of homologous chromosomes, receiving one
member of each pair from each parent
•  Homologous chromosomes are matched in
–  Length
–  Centromere position
–  Gene locations
•  A locus (plural, loci) is the position of a gene
–  Each gene has a specific location called a locus on a
certain chromosome
•  Different versions of a gene may be found at the same locus on
maternal and paternal chromosomes
•  The human sex chromosomes X and Y differ in size and genetic
composition (XY for male, XX for female)
•  The remaining 22 pairs of chromosomes are called autosomes
Karyotyping
53
Human chromosomes
•  A diploid cell (2n) has two sets of chromosomes
•  For humans, the diploid number is 46 (2n = 46)
•  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
54
Homologous pair of
chromosomes
Centromere
Sister chromatids One duplicated
chromosome
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)
56
Gametes have a single set of chromosome
•  Meiosis is a process that converts diploid
nuclei to haploid nuclei
–  Diploid cells have two homologous sets of
chromosomes
–  Haploid cells have one set of chromosomes
–  Meiosis occurs in the sex organs, producing
gametes—sperm and eggs
–  Each gametes : a single set of chromosomes
•  N=22+X (Y)
•  Fertilization is the union of sperm and egg
–  The zygote (fertilized egg) has a diploid
chromosome number, one set from each parent
–  The zygote produces somatic cells by mitosis and
develops into an adult
Haploid gametes (n = 23)
n
Egg cell
n
Sperm cell
Meiosis
Fertilization
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
2n
Meiosis
•  Meiosis : cell division to produce haploid gametes in diploid
organisms
•  Like mitosis, meiosis is preceded by interphase
–  Chromosomes duplicate during the S phase
•  Unlike mitosis, meiosis has two divisions
–  During meiosis I, homologous chromosomes separate
•  The chromosome number is reduced by half
–  During meiosis II, sister chromatids separate
•  The chromosome number remains the same
•  Meiosis results in four daughter cells
•  Each daughter cell has only half as many chromosomes as
the parent cell
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
60
Meiosis
•  Events in the nucleus during meiosis I
–  Prophase I
•  Chromosomes coil and become compact
•  Homologous chromosomes come together as pairs by
synapsis
•  Each pair, with four chromatids, is called a tetrad
•  Nonsister chromatids exchange genetic material by
crossing over
–  Important step for genetic variability
Meiosis
•  Events in the nucleus during meiosis I
–  Metaphase I
•  Tetrads align at the cell equator
–  Anaphase I
•  Homologous pairs separate and move toward
•  Events in the nucleus during meiosis I
–  Telophase I
–  Duplicated chromosomes have reached the poles
–  A nuclear envelope forms around chromosomes in some
species
–  Each nucleus has the haploid number of chromosomes
Copyright © 2009 Pearson Education, Inc.
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
PROPHASE I
Sites of crossing over
Spindle
Sister
Chromatin chromatids
Tetrad
METAPHASE I
ANAPHASE I
Microtubules Metaphase Sister chromatids
attached to
remain attached
plate
kinetochore
Centromere
(with kinetochore)
Homologous
chromosomes separate
Meiosis
• 
Meiosis II follows meiosis I without chromosome duplication
• 
Each of the two haploid products enters meiosis II
• 
Events in the nucleus during meiosis II
–  Prophase II
•  Chromosomes coil and become compact
• 
Events in the nucleus during meiosis II
–  Metaphase II
–  Duplicated chromosomes align at the cell equator
–  Anaphase II
–  Sister chromatids separate and chromosomes move
–  Events in the nucleus during meiosis II
–  Telophase II
–  Chromosomes have reached the poles of the cell
–  A nuclear envelope forms around each set of chromosomes
–  With cytokinesis, four haploid cells are produced
MEIOSIS II: Sister chromatids separate
TELOPHASE II
AND CYTOKINESIS
PROPHASE I
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister chromatids
separate
Haploid daughter
cells forming
Mitosis and meiosis
•  Which characteristics are similar for mitosis and
meiosis?
–  One duplication of chromosomes
•  Which characteristics are unique to meiosis?
–  Two divisions of chromosomes
–  Pairing of homologous chromosomes
–  Exchange of genetic material by crossing over
•  What is the outcome of each process?
–  Mitosis: two genetically identical cells, with the same
chromosome number as the original cell
–  Meiosis: four genetically different cells, with half the
chromosome number of the original cell
67
MITOSIS
MEIOSIS
Parent cell
(before chromosome duplication)
Site of
crossing over
MEIOSIS I
Prophase I
Prophase
Duplicated
chromosome
(two sister
chromatids)
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
duplication
Chromosome
duplication
2n = 4
Chromosomes
align at the
metaphase plate
Metaphase
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
2n
Daughter cells
of mitosis
Tetrads
align at the
metaphase plate
Homologous
chromosomes
separate
(anaphase I);
sister chromatids remain
together
No further
chromosomal
duplication;
sister
chromatids
separate
(anaphase II)
Metaphase I
Anaphase I
Telophase I
Haploid
n=2
Daughter
cells of
meiosis I
MEIOSIS II
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
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
69
70
Independent orientation of chromosomes in meiosis
•  Reshuffling of the gene during sexual reproduction ! genetic
variation
•  Independent orientation at metaphase I
–  Each pair of chromosomes independently aligns at the cell equator
–  There is an equal probability of the maternal or paternal chromosome
facing a given pole
–  The number of combinations for chromosomes packaged into gametes
is 2n where n = haploid number of chromosomes
•  For humans (n = 23), there are more than 8 million (223) possible
combinations of chromosomes
•  Random fertilization
–  The combination of each unique sperm with each unique egg
increases genetic variability
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1 Combination 2
Combination 3 Combination 4
Homologous chromosomes
•  Separation of homologous chromosomes during meiosis can
lead to genetic differences between gametes
–  Homologous chromosomes may have different versions of a gene at
the same locus
–  One version was inherited from the maternal parent, and the other
came from the paternal parent
–  Since homologues move to opposite poles during anaphase I,
gametes will receive either the maternal or paternal version of the
gene
–  Total number of possible combination of chromosomes = 2^23 = 8.4
million
–  Random fusion of a sperm with a oocyte = 8.4 million X 8.4 million =
70 trillion
Brown coat (C); black eyes (E)
White coat (c); pink eyes (e)
Coat-color
genes
Eye-color
genes
Brown
Black
C
E
C
E
C
E
c
e
c
e
c
White
e
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Meiosis
Chromosomes of
the four gametes
Crossing over
•  Genetic recombination is the production of new
combinations of genes due to crossing over
•  Crossing over involves exchange of genetic material
between homologous chromosomes
–  Nonsister chromatids join at a chiasma (plural, chiasmata),
the site of attachment and crossing over
–  Corresponding amounts of genetic material are exchanged
between maternal and paternal (nonsister) chromatids
Tetrad
Chiasma
Centromere
Coat-color
genes
C
Eye-color
genes
E
c
e
1
Breakage of homologous chromatids
C
E
c
e
2
C
Tetrad
(homologous pair of
chromosomes in synapsis)
Joining of homologous chromatids
E
Chiasma
c
e
C
E
Chiasma
e
c
3
Separation of homologous
chromosomes at anaphase I
C
E
C
e
c
E
c
4
C
e
Separation of chromatids at
anaphase II and
completion of meiosis
E
Parental type of chromosome
C
e
c
E
c
e
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
ALTERATIONS OF
CHROMOSOME NUMBER
AND STRUCTURE
Karyotype
•  A karyotype shows stained and magnified versions
of chromosomes
–  Karyotypes are produced from dividing white blood cells,
stopped at metaphase by colchicine
–  Colchicine : mitotic arrest (due to spindle damage)
–  Karyotypes allow observation of
–  Homologous chromosome pairs
–  Chromosome number
–  Chromosome structure
Hypotonic
solution
Packed red
and white blood
cells
Fixative
Stain
Centrifuge
Blood
culture
2
White
blood
cells
3
1
Fluid
4
Centromere
Sister
chromatids
Pair of homologous
chromosomes
5
Classic karyotype: a dye specific for
phosphate group of DNA
- Each chromosome has a characteristic
binding patterns
Spectral karyotype (SKY technique) : Fluorescently labeled
probes for each chromosome
Example of karyotype in biological research
Nat.Biotech. 2007
Lee JS. Stem Cells 2009
85
An extra copy of chromosome 21 in Down syndrome
•  Trisomy 21 : three copies of chromosome 21
–  Trisomy 21 is the most common human chromosome abnormality
–  An imbalance in chromosome number causes Down syndrome,
which is characterized by
–  Characteristic facial features, Susceptibility to disease,
Shortened life span, Mental retardation, Variation in
characteristics
–  The incidence increases with the age of the mother
Copyright © 2009 Pearson Education, Inc.
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
25
40
30
35
Age of mother
45
50
Accidents during meiosis
•  Nondisjunction is the failure of chromosomes or
chromatids to separate during meiosis
–  During Meiosis I
– Both members of a homologous pair go to one pole
–  During Meiosis II
– Both sister chromatids go to one pole
•  Fertilization after nondisjunction yields zygotes with
altered numbers of chromosomes
Copyright © 2009 Pearson Education, Inc.
Nondisjunction
in meiosis I
Normal
meiosis II
Gametes
n+1
n+1
n–1
n–1
Number of chromosomes
Normal
meiosis I
Nondisjunction
in meiosis II
Gametes
n+1
n–1
n
n
Number of chromosomes
92
94
Abnormal numbers of sex chromosomes
•  Sex chromosome abnormalities tend to be less
severe as a result of
–  Small size of the Y chromosome
–  X-chromosome inactivation
–  In each cell of a human female, one of the two X
chromosomes becomes tightly coiled and inactive
–  This is a random process that inactivates either
the maternal or paternal chromosome
–  Inactivation promotes a balance between the
number of X chromosomes and autosomes
Calico Cat
“Tortoiseshell”, exclusively found in female cats
96
New species can arise from errors in cell division
•  Polyploid species have more than two chromosome
sets
–  Observed in many plant species
–  Seen less frequently in animals
•  Example
–  Diploid gametes are produced by failures in meiosis
–  Diploid gamete + Diploid gamete → Tetraploid offspring
–  The tetraploid offspring have four chromosome sets
Copyright © 2009 Pearson Education, Inc.
Polyploid: multiple sets of chromosome
• 
Polyploidy is pervasive in plants and some estimates suggest that 30–80% of living plant
species are polyploid,
• 
Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or
mitotic failures, and fusion of unreduced (2n) gametes.
• 
Polyploid plants tend to be larger and better at flourishing in early succession habitats such as
farm fields
• 
In the breeding of crops, the tallest and best thriving plants are selected for. Thus, many crops
(and agricultural weeds) may have unintentionally been bred to a higher level of ploidy.
• 
In some situations polyploid crops are preferred because they are sterile. For example many
seedless fruit varieties are seedless as a result of polyploidy.
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Alterations of chromosome for birth defects and cancer
•  Structure changes result from breakage and rejoining of chromosome
segments
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Deletion is the loss of a chromosome segment
Duplication is the repeat of a chromosome segment
Inversion is the reversal of a chromosome segment
Translocation is the attachment of a segment to a nonhomologous
chromosome; can be reciprocal
•  Altered chromosomes carried by gametes cause birth defects
•  Chromosomal alterations in somatic cells can cause cancer
Copyright © 2009 Pearson Education, Inc.
Deletion
Duplication
Homologous
chromosomes
Inversion
Reciprocal
translocation
Nonhomologous
chromosomes
Chromosome 9
Chromosome 22
Reciprocal
translocation
“Philadelphia chromosome”
Activated cancer-causing gene
The result of the translocation is the oncogenic BCR-ABL gene fusion, located on
the shorter derivative 22 chromosome. This gene encodes the
Bcr-abl fusion protein.
- Cause of 90% of CML
Imatinib (Glivec)
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