12 cellcycle text
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Cell Replication
• The Cell Cycle, Mitosis, Meiosis, and Basic Inheritance
Figure 12.1
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• Unicellular organisms
– Reproduce by cell division
100 µm
(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).Figure 12.2 A
Multicellular organisms depend on cell division for
a. Development from a fertilized cellb. Growthc. Repair
20 µm200 µm
(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).
(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).
Figure 12.2 B, C
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• Cell division results in genetically identical daughter cells
• The DNA molecules in a cell
– Are packaged into chromosomes
• A cell’s endowment of DNA, its genetic information
– Is called its genome
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• Eukaryotic chromosomes
– Consist of chromatin, a complex of DNA and protein that condenses during cell division
• In animals
– Somatic cells have two sets of chromosomes (diploid)
– Gametes have one set of chromosomes (haploid)
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• In preparation for cell division
– DNA is replicated and the chromosomes condense
• Each duplicated chromosome
– Has two sister chromatids, which separate during cell division0.5 µm
Chromosomeduplication(including DNA synthesis)
Centromere
Separation of sister
chromatids
Sisterchromatids
Centromeres Sister chromatids
A eukaryotic cell has multiplechromosomes, one of which is
represented here. Before duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosomeconsists of two sister chromatids
connected at the centromere. Eachchromatid contains a copy of the
DNA molecule.
Mechanical processes separate the sister chromatids into two chromosomes and distribute
them to two daughter cells.
Figure 12.4
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• Eukaryotic cell division consists of
– Mitosis, the division of the nucleus
– Cytokinesis, the division of the cytoplasm
• In meiosis
– Sex cells are produced after a reduction in chromosome number
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Phases of the Cell Cycle
• The cell cycle consists of
– The mitotic phase
– InterphaseINTERPHASE
G1
S(DNA synthesis)
G2Cyto
kines
is
Mito
sis
MITOTIC(M) PHASE
Figure 12.5
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• The mitotic phase
– Is made up of mitosis and cytokinesis
• Interphase can be divided into subphases
– G1 phase
– S phase
– G2 phase
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• Mitosis consists of five distinct phases
– Prophase
– Prometaphase
G2 OF INTERPHASE
PROPHASE PROMETAPHASE
Centrosomes(with centriole pairs) Chromatin
(duplicated)
Early mitoticspindle
Aster
CentromereFragmentsof nuclearenvelope
Kinetochore
Nucleolus Nuclearenvelope
Plasmamembrane
Chromosome, consistingof two sister chromatids
Kinetochore microtubule Figure 12.6
Nonkinetochoremicrotubules
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– Metaphase
– Anaphase
– Telophase
Centrosome at one spindle pole
Daughter chromosomes
METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS
Spindle
Metaphaseplate Nucleolus
forming
Cleavagefurrow
Nuclear envelopeforming
Figure 12.6
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The Mitotic Spindle: A Closer Look
• The mitotic spindle
– Is an apparatus of microtubules that controls chromosome movement during mitosis
• The spindle arises from the centrosomes
– And includes spindle microtubules and asters
CentrosomeAster
Sisterchromatids
MetaphasePlate
Kinetochores
Overlappingnonkinetochoremicrotubules
Kinetochores microtubules
Centrosome
ChromosomesMicrotubules0.5 µm
1 µm
Figure 12.7
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• In anaphase, sister chromatids separate
– And move along the kinetochore microtubules toward opposite ends of the cell
EXPERIMENT
1 The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow).
Spindlepole
Kinetochore
Figure 12.8
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• Nonkinetechore microtubules from opposite poles
– Overlap and push against each other, elongating the cell
• In telophase
– Genetically identical daughter nuclei form at opposite ends of the cell
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Cytokinesis: A Closer Look
• In animal cells
– Cytokinesis occurs by a process known as cleavage, forming a cleavage furrow
Cleavage furrow
Contractile ring of microfilaments
Daughter cells
100 µm
(a) Cleavage of an animal cell (SEM)Figure 12.9 A
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• In plant cells, during cytokinesis
– A cell plate forms
Daughter cells
1 µmVesiclesforming cell plate
Wall of patent cell Cell plateNew cell wall
(b) Cell plate formation in a plant cell (SEM)Figure 12.9 B
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• Mitosis in a plant cell
1 Prophase. The chromatinis condensing. The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is staring to from.
Prometaphase.We now see discretechromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelop will fragment.
Metaphase. The spindle is complete,and the chromosomes,attached to microtubulesat their kinetochores, are all at the metaphase plate.
Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomesare moving to the ends of cell as their kinetochoremicrotubles shorten.
Telophase. Daughternuclei are forming. Meanwhile, cytokinesishas started: The cellplate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell.
2 3 4 5
NucleusNucleolus
ChromosomeChromatinecondensing
Figure 12.10
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Binary Fission
• Prokaryotes (bacteria)
– Reproduce by a type of cell division called binary fission Origin of
replication
E. coli cell BacterialChromosome
Cell wall
Plasma Membrane
Two copiesof origin
OriginOrigin
Chromosome replication begins.Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.
1
Replication continues. One copy ofthe origin is now at each end of the cell.
2
Replication finishes. The plasma membrane grows inward, andnew cell wall is deposited.
3
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• The cell cycle is regulated by a molecular control system
• The frequency of cell division
– Varies with the type of cell
• These cell cycle differences
– Result from regulation at the molecular level
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The Cell Cycle Control System
• The sequential events of the cell cycle
– Are directed by a distinct cell cycle control system, which is similar to a clock
Figure 12.14
Control system
G2 checkpoint
M checkpoint
G1 checkpoint
G1
S
G2M
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• The clock has specific checkpoints
– Where the cell cycle stops until a go-ahead signal is received
G1 checkpoint
G1G1
G0
(a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.
(b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.
Figure 12.15 A, B
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The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
• Two types of regulatory proteins are involved in cell cycle control
• Cyclins and cyclin-dependent kinases (Cdks)
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• The activity of cyclins and Cdks– Fluctuates during the cell cycle
During G1, conditions in the cell favor degradation of cyclin, and the Cdk component of MPF is recycled.
5
During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G1 phase.
4
Accumulated cyclin moleculescombine with recycled Cdk mol-ecules, producing enough molecules of MPF to pass the G2 checkpoint and initiate the events of mitosis.
2
Synthesis of cyclin begins in late S phase and continues through G2. Because cyclin is protected from degradation during this stage, it accumulates.
1
Cdk
CdkG2
checkpoint
CyclinMPF
Cyclin is degraded
DegradedCyclin
G 1
G 2
S
M
G1G1 S G2 G2SM MMPF activity
Cyclin
Time
(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle
(b) Molecular mechanisms that help regulate the cell cycle
MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase.
3
Figure 12.16 A, B
M
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Stop and Go Signs: Internal and External Signals at the Checkpoints
• Both internal and external signals
– Control the cell cycle checkpoints
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• Cancer cells
– Exhibit neither density-dependent inhibition nor anchorage dependence
25 µm
Cancer cells do not exhibitanchorage dependence or density-dependent inhibition.
Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells.
(b)
Figure 12.18 B
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Loss of Cell Cycle Controls in Cancer Cells
• Cancer cells
– Do not respond normally to the body’s control mechanisms
– Form tumors
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• Malignant tumors invade surrounding tissues and can metastasize
– Exporting cancer cells to other parts of the body where they may form secondary tumors
Cancer cells invade neighboring tissue.
2 A small percentage of cancer cells may survive and establish a new tumor in another part of the body.
4Cancer cells spread through lymph and blood vessels to other parts of the body.
3A tumor grows from a single cancer cell.
1
Tumor
Glandulartissue
Cancer cell
Bloodvessel
Lymphvessel
MetastaticTumor
Figure 12.19
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Meiosis
• Overview: Hereditary Similarity and Variation
• Living organisms
– Are distinguished by their ability to reproduce their own kind
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• Heredity
– Is the transmission of traits from one generation to the next
• Variation
– Shows that offspring differ somewhat in appearance from parents and siblings
• Genetics
– Is the scientific study of heredity and hereditary variation
Figure 13.1
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• Offspring acquire genes from parents by inheriting chromosomes
• Genes
– Are the units of heredity
– Are segments of DNA
• Each gene in an organism’s DNA
– Has a specific locus on a certain chromosome
• We inherit
– One set of chromosomes from our mother and one set from our father
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Comparison of Asexual and Sexual Reproduction
• In asexual reproduction
– One parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm
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• In sexual reproduction
– Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
• Fertilization and meiosis alternate in sexual life cycles
• A life cycle
– Is the generation-to-generation sequence of stages in the reproductive history of an organism
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Sets of Chromosomes in Human Cells
• In humans
– Each somatic cell has 46 chromosomes, made up of two sets (diploid 2n = 46)
– One set of chromosomes comes from each parent (haploid)
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5 µmPair of homologous
chromosomes
Centromere
Sisterchromatids
Figure 13.3
• A karyotype
– Is an ordered, visual representation of the chromosomes in a cell
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• Homologous chromosomes
– Are the two chromosomes composing a pair
– Have the same characteristics
– May also be called autosomes
• Sex chromosomes
– Are distinct from each other in their characteristics
– Are represented as X and Y
– Determine the sex of the individual, XX being female, XY being male
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• In a cell in which DNA synthesis has occurred
– All the chromosomes are duplicated and thus each consists of two identical sister chromatids
Figure 13.4
Key
Maternal set ofchromosomes (n = 3)
Paternal set ofchromosomes (n = 3)
2n = 6
Two sister chromatidsof one replicated
chromosome
Two nonsisterchromatids in
a homologous pair
Pair of homologouschromosomes
(one from each set)
Centromere
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• Unlike somatic cells
– Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes
• At sexual maturity
– The ovaries and testes produce haploid gametes by meiosis
• During fertilization
– These gametes, sperm and ovum, fuse, forming a diploid zygote
• The zygote
– Develops into an adult organism
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Figure 13.5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
SpermCell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploidzygote
(2n = 46)
Mitosis anddevelopment
Multicellular diploidadults (2n = 46)
• The human life cycle
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• In animals
– Meiosis occurs during gamete formation
– Gametes are the only haploid cells
Gametes
Figure 13.6 A
Diploidmulticellular
organism
Key
MEIOSIS FERTILIZATION
n
n
n
2n2nZygote
Haploid
Diploid
Mitosis
(a) Animals
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MEIOSIS FERTILIZATION
nn
n
nn
2n2n
Haploid multicellularorganism (gametophyte)
Mitosis Mitosis
SporesGametes
Mitosis
Zygote
Diploidmulticellular
organism(sporophyte)
(b) Plants and some algaeFigure 13.6 B
• Plants and some algae
– Exhibit an alternation of generations
– The life cycle includes both diploid and haploid multicellular stages
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MEIOSIS FERTILIZATION
nn
n
n
n
2n
Haploid multicellularorganism
Mitosis Mitosis
Gametes
Zygote(c) Most fungi and some protistsFigure 13.6 C
• In most fungi and some protists
– Meiosis produces haploid cells that give rise to a haploid multicellular adult organism
– The haploid adult carries out mitosis, producing cells that will become gametes
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• Meiosis reduces the number of chromosome sets from diploid to haploid
• Meiosis
– Takes place in two sets of divisions, meiosis I and meiosis II
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The Stages of Meiosis
• An overview of meiosis
Figure 13.7
Interphase
Homologous pairof chromosomes
in diploid parent cell
Chromosomesreplicate
Homologous pair of replicated chromosomes
Sisterchromatids Diploid cell with
replicatedchromosomes
1
2
Homologous chromosomes
separate
Haploid cells withreplicated chromosomes
Sister chromatids separate
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II
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• Meiosis I
– Reduces the number of chromosomes from diploid to haploid
• Meiosis II
– Produces four haploid daughter cells
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Centrosomes(with centriole pairs)
Sisterchromatids
Chiasmata
Spindle
Tetrad
Nuclearenvelope
Chromatin
Centromere(with kinetochore)
Microtubuleattached tokinetochore
Tertads line up
Metaphaseplate
Homologouschromosomes
separate
Sister chromatidsremain attached
Pairs of homologouschromosomes split up
Chromosomes duplicateHomologous chromosomes
(red and blue) pair and exchangesegments; 2n = 6 in this example
INTERPHASE MEIOSIS I: Separates homologous chromosomes
PROPHASE I METAPHASE I ANAPHASE I
• Interphase and meiosis I
Figure 13.8
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TELOPHASE I ANDCYTOKINESIS
PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS
MEIOSIS II: Separates sister chromatids
Cleavagefurrow Sister chromatids
separate
Haploid daughter cellsforming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes
Two haploid cellsform; chromosomes
are still doubleFigure 13.8
• Telophase I, cytokinesis, and meiosis II
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A Comparison of Mitosis and Meiosis
• Meiosis and mitosis can be distinguished from mitosis
– By three events in Meiosis l
a. Synapsis and crossing over Homologous chromosomes physically connect
and exchange genetic information
b. Tetrads on the metaphase plate
At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the
metaphase plates
c. Separation of homologues
At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell
In anaphase II of meiosis, the sister chromatids separate
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Figure 13.9
MITOSIS MEIOSIS
Prophase
Duplicated chromosome(two sister chromatids)
Chromosomereplication
Chromosomereplication
Parent cell(before chromosome replication)
Chiasma (site ofcrossing over)
MEIOSIS I
Prophase I
Tetrad formed bysynapsis of homologous
chromosomes
Metaphase
Chromosomespositioned at themetaphase plate
Tetradspositioned at themetaphase plate
Metaphase I
Anaphase ITelophase I
Haploidn = 3
MEIOSIS II
Daughtercells of
meiosis I
Homologuesseparate
duringanaphase I;
sisterchromatids
remain together
Daughter cells of meiosis II
n n n n
Sister chromatids separate during anaphase II
AnaphaseTelophase
Sister chromatidsseparate during
anaphase
2n 2nDaughter cells
of mitosis
2n = 6
• A comparison of mitosis and meiosis
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• Genetic variation produced in sexual life cycles contributes to greater variation
• Reshuffling of genetic material in meiosis
– Produces genetic variation
• In species that produce sexually
– The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation
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Independent Assortment of Chromosomes
• Homologous pairs of chromosomes
– Orient randomly at metaphase I of meiosis
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• In independent assortment
– Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs
Figure 13.10
Key
Maternal set ofchromosomesPaternal set ofchromosomes
Possibility 1
Two equally probable arrangements ofchromosomes at
metaphase I
Possibility 2
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
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Crossing Over
• Crossing over
– Produces recombinant chromosomes that carry genes derived from two different parents
Figure 13.11
Prophase Iof meiosis
Nonsisterchromatids
Tetrad
Chiasma,site of
crossingover
Metaphase I
Metaphase II
Daughtercells
Recombinantchromosomes
Variation
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Random Fertilization
• The fusion of gametes
– Will produce a zygote with any of about 64 trillion diploid combinations
Number of
children from one couple without
two exactly
the same: 102017
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• Mutations
– Are the original source of genetic variation
• Sexual reproduction
– Produces new combinations of variant genes, adding more genetic diversity
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Wild dogs 1Aardwolf
African wild dog
Arctic fox
Argentine gray fox
Black-backed jackal
Blanford’s fox
Bat-eared fox
Bush dog
Wild dogs
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Wild dogs 2
Wild dogs
Arctic wolf
Cape fox
Corsac fox
Coyote
Crab-eating fox
Culpeo fox
DholeFennec fox
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Wild dogs
Arctic fox
Dingo
Dingo
Ethiopian wolf
Falkland Island’s fox
Golden jackal
Tibetan sand fox
Gray wolf
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Wild dogs 4
Wild dogs
Gray fox
Hoary zorroKit fox
Maned wolfMexican gray wolf
Raccoon dog
Sand fox
Small-eared dog
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Wild dogs 5
Wild dogs
Pale fox
Pampas fox
Red fox
Red wolfSechuan zorro
Timber wolf
Iberian wolf
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Q 910 Savolainen, et.alThe origin of the domestic dog from wolves has been established … we examined the mitochondrial DNA
(mtDNA) sequence variation among 654 domestic dogs representing all major dog populations worldwide … suggesting a common origin from a
single gene pool for all dog populations.
Q 910
Savolainen, et.al., ‘Genetic Evidence for an East Asian origin of Domestic Dogs,’ Science, Vol 298:5598, 22 Nov 2002,
pp 1610-1613.