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WHAT CELL REPRODUCTION ACCOMPLISHES
• Reproduction:
– May result in the birth of new organisms
– More commonly involves the production of new cells
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• 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.
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• 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.
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• Cell division plays important roles in the lives of organisms.
• Cell division:
– Replaces damaged or lost cells
– Permits growth
– Allows for reproduction
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• In asexual reproduction:
– Single-celled organisms reproduce by simple cell division
– There is no fertilization of an egg by a sperm
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• Some multicellular organisms, such as sea stars, can grow new individuals from fragmented pieces.
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• Growing a new plant from a clipping is another example of asexual reproduction.
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LM
Asexual Reproduction
Figure 8.1ba
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Asexual Reproduction
Figure 8.1bb
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Asexual Reproduction
Figure 8.1bc
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• In asexual reproduction, the lone parent and its offspring have identical genes.
• Mitosis is the type of cell division responsible for:
– Asexual reproduction
– Growth and maintenance of multicellular organisms
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• Sexual reproduction requires fertilization of an egg by a sperm using a special type of cell division called meiosis.
• Thus, sexually reproducing organisms use:
– Meiosis for reproduction
– Mitosis for growth and maintenance
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• In a eukaryotic cell:
– Most genes are located on chromosomes in the cell nucleus
THE CELL CYCLE AND MITOSIS
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Eukaryotic Chromosomes• Each eukaryotic chromosome contains one very long DNA
molecule, typically bearing thousands of genes.
• The number of chromosomes in a eukaryotic cell depends on the species.
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Number of chromosomesin body cells
Indian muntjac deer
Species
Opossum
Koala
Human
Mouse
Giraffe
Buffalo
Dog
Red viscacha rat
Duck-billed platypus
102
78
60
54
46
40
30
22
16
6
Figure 8.2
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• Chromosomes:
– Are made of chromatin, a combination of DNA and protein molecules
– Are not visible in a cell until cell division occurs
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Chromosomes
LM
Figure 8.3
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• Before a cell divides, it duplicates all of its chromosomes, resulting in two copies called sister chromatids.
• Sister chromatids are joined together at a narrow “waist” called the centromere.
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• 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
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Chromosomeduplication
Sisterchromatids
Chromosomedistribution todaughter cells
Figure 8.5
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The Cell Cycle• A cell cycle is the orderly sequence of events that extend from the
time a cell is first formed from a dividing parent cell to its own division into two cells.
• The cell cycle consists of two distinct phases:
– Interphase
– The mitotic phase
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• Most of a cell cycle is spent in interphase.
• During interphase, a cell:
– Performs its normal functions
– Doubles everything in its cytoplasm
– Grows in size
Video: Animal Mitosis
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Nuclearenvelope
LM
Plasmamembrane
Chromosome, consistingof two sister chromatids
Spindle microtubules
Fragments of nuclear envelopeCentrosome
Centromere
Early mitotic spindle
Centrosomes (with centriole pairs)
Chromatin
PROPHASEINTERPHASE
Figure 8.7.a
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• The mitotic (M) phase includes two overlapping processes:
– Mitosis, in which the nucleus and its contents divide evenly into two daughter nuclei
– Cytokinesis, in which the cytoplasm is divided in two
Video: Sea Urchin (time lapse)
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Cytokinesis(division ofcytoplasm)
Mitosis(division of nucleus)
Mitotic (M) phase:cell division(10% of time)
Interphase: metabolism andgrowth (90% of time)
S phase (DNA synthesis;chromosome duplication)
G1 G2
Figure 8.6
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Mitosis and Cytokinesis• During mitosis the mitotic spindle, a football-shaped structure of
microtubules, 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.
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• Mitosis consists of four distinct phases:
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– (A) Prophase
– (B) Metaphase
– (C) Anaphase
– (D) Telophase
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ANAPHASEMETAPHASE TELOPHASE AND CYTOKINESIS
Spindle Daughterchromosomes
Cleavagefurrow
Nuclearenvelopeforming
Figure 8.7b
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ANAPHASEMETAPHASE TELOPHASE AND CYTOKINESIS
Spindle Daughterchromosomes
Cleavagefurrow
Nuclearenvelopeforming
Figure 8.7ba
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• Cytokinesis typically:
– Occurs during telophase
– Divides the cytoplasm
– Is different in plant and animal cells
Animation: Cytokinesis
Blast Animation: Cytokinesis in Plant Cells
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Cleavage furrow
Contracting ring ofmicrofilaments
Daughter cells
Figure 8.8ab
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Daughter cells
New cell wallVesicles containingcell wall material Cell plateCell wall
Wall ofparent cell
Cell plateforming
Daughternucleus
LM
Figure 8.8b
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Wall ofparent cell
Cell plateforming
Daughternucleus
LM
Figure 8.8ba
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Daughter cells
New cell wall
Vesicles containingcell wall material Cell plate
Cell wall
Figure 8.8bb
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Cancer Cells: Growing Out of Control• Normal plant and animal cells 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.
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What Is Cancer?
• Cancer is a disease of the cell cycle.
• Cancer cells do not respond normally to the cell cycle control system.
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• Cancer cells can form tumors, abnormally growing masses of body cells.
• The spread of cancer cells beyond their original 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.
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A tumor growsfrom a singlecancer cell.
Cancer cells invadeneighboring tissue.
Metastasis: Cancercells spread throughlymph and bloodvessels to other partsof the body.
Glandulartissue
Bloodvessel
Tumor
Lymphvessels
Figure 8.9
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Cancer Treatment
• Cancer treatment can involve:
– Radiation therapy, which damages DNA and disrupts cell division
– Chemotherapy, which uses drugs that disrupt cell division
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Cancer Prevention and Survival
• 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
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• Sexual reproduction:
– Uses meiosis
– Uses fertilization
– Produces offspring that contain a unique combination of genes from the parents
Meiosis, the Basis of Sexual Reproduction
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Figure 8.10
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Homologous Chromosomes• Different individuals of a single species have the same number
and types of chromosomes.
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• A human somatic cell:
– Is a typical body cell
– Has 46 chromosomes
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• A karyotype is an image that reveals an orderly arrangement of chromosomes.
• Homologous chromosomes are matching pairs of chromosomes that can possess different versions of the same genes.
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Pair of homologouschromosomes
LM
One duplicatedchromosome
Centromere
Sisterchromatids
Figure 8.11
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LM
Figure 8.11a
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• Humans have:
– Two different sex chromosomes, X and Y
– Twenty-two pairs of matching chromosomes, called autosomes
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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.
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Multicellulardiploid adults(2n 46)
MEIOSIS FERTILIZATION
MITOSIS
2n
and development Key
Sperm cell
n
n
Diploidzygote(2n 46)
Diploid (2n)
Haploid (n)
Egg cell
Haploid gametes (n 23)
Figure 8.12
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• Humans are diploid organisms in which:
– Their somatic cells contain two sets of chromosomes
– Their gametes are haploid, having only one set of chromosomes
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• 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.
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• Meiosis produces haploid gametes.
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MEIOSIS I
Sisterchromatidsseparate.
MEIOSIS II
Homologouschromosomesseparate.
INTERPHASE BEFORE MEIOSIS
Sisterchromatids
Duplicated pair ofhomologouschromosomes
Chromosomesduplicate.
Pair of homologouschromosomes indiploid parent cell
Figure 8.13-3
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The Process of Meiosis• In meiosis:
– Haploid daughter cells are produced in diploid organisms
– Interphase is followed by two consecutive divisions, meiosis I and meiosis II
– Crossing over occurs
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MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE
Sister chromatidsremain attached
Pair ofhomologouschromosomes
INTERPHASE
Sisterchromatids
Homologouschromosomespair up andexchangesegments.
Chromosomesduplicate.
Pairs of homologouschromosomesline up.
Pairs of homologouschromosomessplit up.
Nuclearenvelope
Chromatin
Centromere
Microtubulesattachedto chromosome
Sites of crossing over
Spindle
Centrosomes (with centriolepairs)
PROPHASE I METAPHASE I ANAPHASE I
Figure 8.14a
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TELOPHASE II AND
CYTOKINESIS
Sister chromatidsseparate
ANAPHASE II
Cleavagefurrow
TELOPHASE I AND
CYTOKINESIS
Two haploidcells form;chromosomesare stilldoubled.
MEIOSIS II: SISTER CHROMATIDS SEPARATE
PROPHASE II METAPHASE II
During another round of cell division, the sisterchromatids finally separate; four haploiddaughter cells result, containing single
chromosomes.
Haploid daughtercells forming
Figure 8.14b
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TELOPHASE II AND
CYTOKINESIS
Sister chromatidsseparate
ANAPHASE IIPROPHASE II METAPHASE II
Haploid daughter cells forming
Figure 8.14ba
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PROPHASE II METAPHASE II
Figure 8.14bb
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Review: Comparing Mitosis and Meiosis• In mitosis and meiosis, the chromosomes duplicate only once,
during the preceding interphase.
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• The number of cell divisions varies:
– Mitosis uses one division and produces two diploid cells
– Meiosis uses two divisions and produces four haploid cells
• All the events unique to meiosis occur during meiosis I.
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Duplicated chromosome(two sister chromatids)
MITOSIS
Prophase
Chromosome duplication
Chromosomes align at the middle of thecell.
Metaphase
Sister chromatidsseparateduringanaphase.
AnaphaseTelophase
Daughter cellsof mitosis
2n2n
Prophase I
Metaphase I
Anaphase ITelophase I
MEIOSIS
Chromosome duplication
Homologous chromosomes come together in pairs.
MEIOSIS I
Site of crossing overbetween homologous(nonsister) chromatids
Homologous pairsalign at the middle of the cell.
Chromosome with twosister chromatids
Homologous chromosomes separate duringanaphase I;sister chromatidsremain together.
Daughtercells of meiosis I
Sister chromatidsseparate duringanaphase II.
Haploidn 2
MEIOSIS II
Parent cell(before chromosome duplication)
2n 4
Daughter cells of meiosis II n n n n
Figure 8.15
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Duplicated chromosome(two sister chromatids)
MITOSIS
Prophase
Chromosome duplication
Chromosomes align at the middle of thecell.
Metaphase
Prophase I
Metaphase I
MEIOSIS
Chromosome duplication
Homologous chromosomes come together in pairs.
MEIOSIS I
Site of crossing over between homologous(nonsister) chromatids
Homologous pairs align at the middle of the cell.
Parent cell(before chromosome duplication)
2n 4
Figure 8.15a
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Sister chromatidsseparateduringanaphase.
AnaphaseTelophase
Daughter cellsof mitosis
2n2n
Anaphase ITelophase I
Chromosome with two sister chromatids
Homologous chromosomes separate duringanaphase I;sister chromatidsremain together.
Daughtercells of meiosis I
Sister chromatidsseparate duringanaphase II.
Haploidn 2
MEIOSIS II
Daughter cells of meiosis II n n n n
Figure 8.15b
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The Origins of Genetic Variation• Offspring of sexual reproduction are genetically different from
their parents and one another.
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Independent Assortment of 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.
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• 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
Blast Animation: Genetic Variation: Independent Assortment
Animation: Genetic Variation
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Metaphase ofmeiosis I
POSSIBILITY 1 POSSIBILITY 2
Figure 8.16-1
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Metaphase ofmeiosis I
Metaphase of meiosis II
POSSIBILITY 1 POSSIBILITY 2
Figure 8.16-2
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Metaphase ofmeiosis I
Metaphase of meiosis II
Combination a
POSSIBILITY 1 POSSIBILITY 2
Combination b Combination c Combination d
Gametes
Figure 8.16-3
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Crossing Over
• In crossing over:
– Homologous chromosomes exchange genetic information
– Genetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs
Blast Animation: Genetic Variation: Fusion of Gametes
Animation: Crossing Over
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Prophase Iof meiosis
Duplicated pair ofhomologouschromosomes
Figure 8.18-1
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Homologous chromatidsexchange correspondingsegments.
Prophase Iof meiosis
Duplicated pair ofhomologouschromosomes
Chiasma, site ofcrossing over
Figure 8.18-2
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Metaphase I
Homologous chromatidsexchange correspondingsegments.
Sister chromatids remain joined at theircentromeres.
Prophase Iof meiosis
Duplicated pair ofhomologouschromosomes
Chiasma, site ofcrossing over
Spindlemicrotubule
Figure 8.18-3
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Metaphase I
Metaphase II
Homologous chromatidsexchange correspondingsegments.
Sister chromatids remain joined at theircentromeres.
Prophase Iof meiosis
Duplicated pair ofhomologouschromosomes
Chiasma, site ofcrossing over
Spindlemicrotubule
Figure 8.18-4
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Metaphase I
Metaphase II
Recombinant chromosomes
Gametes Recombinant chromosomescombine geneticinformation fromdifferent parents.
Homologous chromatidsexchange correspondingsegments.
Sister chromatids remain joined at theircentromeres.
Prophase Iof meiosis
Duplicated pair ofhomologouschromosomes
Chiasma, site ofcrossing over
Spindlemicrotubule
Figure 8.18-5
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• What happens when errors occur in meiosis?
• Such mistakes can result in genetic abnormalities that range from mild to fatal.
When Meiosis Goes Awry
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How Accidents during Meiosis Can Alter Chromosome Number
• 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.
© 2010 Pearson Education, Inc.
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Meiosis I
Nondisjunction:Pair of homologouschromosomes failsto separate.
NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II
Figure 8.20-1
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Meiosis I
Nondisjunction:Pair of homologouschromosomes failsto separate.
NONDISJUNCTION IN MEIOSIS I
Meiosis II
Nondisjunction:Pair of sisterchromatidsfails to separate.
NONDISJUNCTION IN MEIOSIS II
Figure 8.20-2
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Meiosis I
Abnormal gametes
Gametes
Nondisjunction:Pair of homologouschromosomes failsto separate.
NONDISJUNCTION IN MEIOSIS I
Number of chromosomes
Meiosis II
Nondisjunction:Pair of sisterchromatidsfails to separate.
Abnormal gametes Normal gametes
n n n 1n 1 n – 1n 1
NONDISJUNCTION IN MEIOSIS II
n – 1 n – 1
Figure 8.20-3
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© 2010 Pearson Education, Inc.
• 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 number of genes.
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Abnormal eggcell with extrachromosome
Normalsperm cell
n 1
n (normal)
Abnormal zygotewith extrachromosome 2n 1
Figure 8.21
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© 2010 Pearson Education, Inc.
Down Syndrome: An Extra Chromosome 21
• Down Syndrome:
– 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
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Chromosome 21Figure 8.22a
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Figure 8.22b
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• The incidence of Down Syndrome increases with the age of the mother.
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Age of mother
25 35 4520 30 40 50
10
0
20
30
40
50
60
70
80
90In
fan
ts w
ith
Do
wn
sy
nd
rom
e(p
er
1,0
00 b
irth
s)
Figure 8.23
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Abnormal Numbers of Sex Chromosomes
• Nondisjunction can also affect the sex chromosomes.
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Table 8.1
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Figure 8.24
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• Sexual reproduction may convey an evolutionary advantage by:
– Speeding adaptation to a changing environment
– Allowing a population to more easily rid itself of harmful genes
• Asexual reproduction conveys an evolutionary advantage when plants are:
– Sparsely distributed
– Superbly suited to a stable environment
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Duplicationof all
chromosomes
Geneticallyidenticaldaughter
cells
Distribution viamitosis
Figure 8.UN1
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Duplicated chromosome
Chromosome (onelong piece of DNA)
Centromere
Sisterchromatids
Figure 8.UN2
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Interphase Cell growth and
chromosome duplication
G2
Mitotic(M) phase
S phase DNA synthesis; chromosome duplication
G1
Genetically identical“daughter”cells
Cytokinesis (division ofcytoplasm)
Mitosis (division ofnucleus)
Figure 8.UN3
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MITOSIS
Male and femalediploid adults(2n 46)
MEIOSIS
Sperm cell
Human Life Cycle
KeyHaploid (n)
Diploid (2n)
Haploid gametes (n 23)
Egg cell
Diploid zygote(2n 46)
and development
FERTILIZATION
2n
n
n
Figure 8.UN4
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Daughtercells
Parentcell (2n)
MITOSIS
Chromosomeduplication
2n 2n
MEIOSIS
MEIOSIS I Parentcell (2n)
Chromosomeduplication
Daughter cellsn
MEIOSIS II
Pairing of homologouschromosome
Crossing over
n nn
Figure 8.UN5