chapter 8 the cellular basis of reproduction · pdf filechapter 8: big ideas cell division and...

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© 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko

PowerPoint Lectures for

Campbell Biology: Concepts & Connections, Seventh EditionReece, Taylor, Simon, and Dickey

Chapter 8 The Cellular Basis of Reproduction and Inheritance

Cancer cells

– start out as normal body cells,

– undergo genetic mutations,

– lose the ability to control the tempo of their own division, and

– run amok, causing disease.

Introduction

© 2012 Pearson Education, Inc.

In a healthy body, cell division allows for

– growth,

– the replacement of damaged cells, and

– development from an embryo into an adult.

In sexually reproducing organisms, eggs and sperm result from

– mitosis and

– meiosis.

Introduction


Figure 8.0_1

Figure 8.0_2Chapter 8: Big Ideas

Cell Division andReproduction

The Eukaryotic CellCycle and Mitosis

Meiosis andCrossing Over

Alterations of ChromosomeNumber and Structure

Figure 8.0_3

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CELL DIVISION AND REPRODUCTION


8.1 Cell division plays many important roles in the lives of organisms

Organisms reproduce their own kind, a key characteristic of life.

Cell division

– is reproduction at the cellular level,

– requires the duplication of chromosomes, and

– sorts new sets of chromosomes into the resulting pair of daughter cells.


Cell division is used

– for reproduction of single-celled organisms,

– growth of multicellular organisms from a fertilized egg into an adult,

– repair and replacement of cells, and

– sperm and egg production.




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.

– 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.


Figure 8.1A Figure 8.1B

3

Figure 8.1C Figure 8.1D

Figure 8.1E Figure 8.1F

Prokaryotes (bacteria and archaea) reproduce by binary fission (“dividing in half”).

The chromosome of a prokaryote is

– a singular circular DNA molecule associated with proteins and

– much smaller than those of eukaryotes.

8.2 Prokaryotes reproduce by binary fission


Binary fission of a prokaryote occurs in three stages:

1. duplication of the chromosome and separation of the copies,

2. continued elongation of the cell and movement of the copies, and

3. division into two daughter cells.

8.2 Prokaryotes reproduce by binary fission


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Figure 8.2A_s1

Plasmamembrane

Cell wall

Duplication of the chromosomeand separation of the copies

Prokaryoticchromosome

1

Figure 8.2A_s2

Plasmamembrane

Cell wall


Continued elongation of thecell and movement of the copies


1

2

Figure 8.2A_s3

Plasmamembrane

Cell wall


Continued elongation of thecell and movement of the copies


1

2

3Division into

two daughter cells

Figure 8.2B

Prokaryotic chromosomes

THE EUKARYOTIC CELL CYCLE AND MITOSIS


Eukaryotic cells

– are more complex and larger than prokaryotic cells,

– have more genes, and

– store most of their genes on multiple chromosomes within the nucleus.

8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division


5

Eukaryotic chromosomes are composed of chromatin consisting of

– one long DNA molecule and

– proteins that help maintain the chromosome structure and control the activity of its genes.

To prepare for division, the chromatin becomes

– highly compact and

– visible with a microscope.



Figure 8.3A

Figure 8.3B

Sisterchromatids

Chromosomes

Centromere

Chromosomeduplication

Sisterchromatids

Chromosomedistribution

to thedaughter

cells

DNA moleculesFigure 8.3B_2

Sisterchromatids

Centromere

Before a eukaryotic cell begins to divide, it duplicates all of its chromosomes, resulting in

– two copies called sister chromatids

– joined together by a narrowed “waist” called the centromere.

When a cell divides, the sister chromatids

– separate from each other, now called chromosomes, and

– sort into separate daughter cells.



Figure 8.3B_1Chromosomes

Centromere


Sisterchromatids

Chromosomedistribution

to thedaughter

cells

DNA molecules

6

The cell cycle is an ordered sequence of events that extends

– from the time a cell is first formed from a dividing parent cell

– until its own division.

8.4 The cell cycle multiplies cells


The cell cycle consists of two stages, characterized as follows:

1. Interphase: duplication of cell contents

– G1—growth, increase in cytoplasm

– S—duplication of chromosomes

– G2—growth, preparation for division

2. Mitotic phase: division

– Mitosis—division of the nucleus

– Cytokinesis—division of cytoplasm

8.4 The cell cycle multiplies cells


Figure 8.4

G1(first gap)

S(DNA synthesis)

G2

(second gap)

M

Mitosis progresses through a series of stages:

– prophase,

– prometaphase,

– metaphase,

– anaphase, and

– telophase.

Cytokinesis often overlaps telophase.

8.5 Cell division is a continuum of dynamic changes


A mitotic spindle is

– required to divide the chromosomes,

– composed of microtubules, and

– produced by centrosomes, structures in the cytoplasm that

– organize microtubule arrangement and

– contain a pair of centrioles in animal cells.



Video: Sea Urchin (time lapse)

Video: Animal Mitosis

Figure 8.5_1

INTERPHASEMITOSIS

Prophase Prometaphase

Centrosome

Early mitoticspindle

Chromatin

Fragments ofthe nuclearenvelope

Kinetochore

Centrosomes(with centriole pairs)

Centrioles

Nuclearenvelope

Plasmamembrane

Chromosome,consisting of twosister chromatids

CentromereSpindlemicrotubules

7

Interphase

– The cytoplasmic contents double,

– two centrosomes form,

– chromosomes duplicate in the nucleus during the S phase, and

– nucleoli, sites of ribosome assembly, are visible.



Figure 8.5_2

INTERPHASE

Figure 8.5_left

INTERPHASEMITOSIS

Prophase Prometaphase

CentrosomeEarly mitoticspindle

Chromatin

Fragments ofthe nuclear envelope

Kinetochore

Centrosomes(with centriole pairs)

Centrioles

Nuclearenvelope

Plasmamembrane

Chromosome,consisting of twosister chromatids

Centromere Spindlemicrotubules

Prophase

– In the cytoplasm microtubules begin to emerge from centrosomes, forming the spindle.

– In the nucleus

– chromosomes coil and become compact and

– nucleoli disappear.



Figure 8.5_3

Prophase

Prometaphase

– Spindle microtubules reach chromosomes, where they

– attach at kinetochores on the centromeres of sister chromatids and

– move chromosomes to the center of the cell through associated protein “motors.”

– Other microtubules meet those from the opposite poles.

– The nuclear envelope disappears.



8

Figure 8.5_4

Prometaphase

Figure 8.5_5

MITOSIS

AnaphaseMetaphase Telophase and Cytokinesis

Metaphaseplate Cleavage

furrow

NuclearenvelopeformingDaughter

chromosomesMitoticspindle

Figure 8.5_right

MITOSIS

AnaphaseMetaphase Telophase and Cytokinesis

Metaphaseplate

Cleavagefurrow

Nuclearenvelopeforming

Daughterchromosomes

Mitoticspindle

Metaphase

– The mitotic spindle is fully formed.

– Chromosomes align at the cell equator.

– Kinetochores of sister chromatids are facing the opposite poles of the spindle.



Figure 8.5_6

Metaphase

Anaphase – Sister chromatids separate at the centromeres.

– Daughter chromosomes are moved to opposite poles of the cell as

– motor proteins move the chromosomes along the spindle microtubules and

– kinetochore microtubules shorten.

– The cell elongates due to lengthening of nonkinetochore microtubules.



9

Figure 8.5_7

Anaphase

Telophase– The cell continues to elongate.

– The nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei.

– Chromatin uncoils and nucleoli reappear.

– The spindle disappears.



Figure 8.5_8

Telophase and Cytokinesis

During cytokinesis, the cytoplasm is divided into separate cells.

The process of cytokinesis differs in animal and plant cells.



In animal cells, cytokinesis occurs as

1. a cleavage furrow forms from a contracting ring of microfilaments, interacting with myosin, and

2. the cleavage furrow deepens to separate the contents into two cells.

8.6 Cytokinesis differs for plant and animal cells


Figure 8.6A

Cytokinesis

Cleavagefurrow Contracting ring of

microfilaments

Daughtercells

Cleavagefurrow

10

Figure 8.6A_1

Cleavagefurrow

Figure 8.6A_2

Cleavagefurrow Contracting ring of

microfilaments

Daughtercells

In plant cells, cytokinesis occurs as1. a cell plate forms in the middle, from vesicles

containing cell wall material,

2. the cell plate grows outward to reach the edges, dividing the contents into two cells,

3. each cell now possesses a plasma membrane and cell wall.

8.6 Cytokinesis differs for plant and animal cells


Animation: Cytokinesis

Figure 8.6B

Cytokinesis

Cell wallof theparent cell

Daughternucleus

Cell wall

Plasmamembrane

Vesiclescontainingcell wallmaterial

Cell plateforming

Newcell wall

Cell plate Daughtercells

Figure 8.6B_1

Cell wallof theparent cell

Daughternucleus

Cell plateforming

Figure 8.6B_2

Cell wall

Plasmamembrane

Vesiclescontainingcell wallmaterial

Newcell wall

Cell plate Daughtercells

11

The cells within an organism’s body divide and develop at different rates.

Cell division is controlled by

– the presence of essential nutrients,

– growth factors, proteins that stimulate division,

– density-dependent inhibition, in which crowded cells stop dividing, and

– anchorage dependence, the need for cells to be in contact with a solid surface to divide.

8.7 Anchorage, cell density, and chemical growthfactors affect cell division


Figure 8.7A

Cultured cellssuspended in liquid

The addition ofgrowthfactor

Figure 8.7B

Anchorage

Single layerof cells

Removalof cells

Restorationof singlelayer by celldivision

The cell cycle control system is a cycling set of molecules in the cell that

– triggers and

– coordinates key events in the cell cycle.

Checkpoints in the cell cycle can

– stop an event or

– signal an event to proceed.

8.8 Growth factors signal the cell cycle control system


There are three major checkpoints in the cell cycle.

1. G1 checkpoint

– allows entry into the S phase or

– causes the cell to leave the cycle, entering a nondividing G0

phase.

2. G2 checkpoint, and

3. M checkpoint.

Research on the control of the cell cycle is one of the hottest areas in biology today.

8.8 Growth factors signal the cell cycle control system


Figure 8.8A

G0

G1 checkpoint

G1S

M

G2

Controlsystem

M checkpoint

G2 checkpoint

12

Figure 8.8B

Receptorprotein

Signaltransductionpathway

Growthfactor

Relay proteins

Plasma membraneEXTRACELLULAR FLUID

CYTOPLASM

G1checkpoint

G1S

M

G2

Controlsystem

Cancer currently claims the lives of 20% of the people in the United States and other industrialized nations.

Cancer cells escape controls on the cell cycle.

Cancer cells

– divide rapidly, often in the absence of growth factors,

– spread to other tissues through the circulatory system, and

– grow without being inhibited by other cells.

8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors


A tumor is an abnormally growing mass of body cells.

– Benign tumors remain at the original site.

– Malignant tumors spread to other locations, called metastasis.



Figure 8.9

Tumor

Glandulartissue

Growth Invasion Metastasis

Lymphvessels

Bloodvessel

Tumor inanotherpart of the body

Cancers are named according to the organ or tissue in which they originate.

– Carcinomas arise in external or internal body coverings.

– Sarcomas arise in supportive and connective tissue.

– Leukemias and lymphomas arise from blood-forming tissues.



Cancer treatments

– Localized tumors can be

– removed surgically and/or

– treated with concentrated beams of high-energy radiation.

– Chemotherapy is used for metastatic tumors.



13

When the cell cycle operates normally, mitosis produces genetically identical cells for

– growth,

– replacement of damaged and lost cells, and

– asexual reproduction.

8.10 Review: Mitosis provides for growth, cell replacement, and asexual reproduction


Video: Hydra Budding

Figure 8.10A

Figure 8.10A_1 Figure 8.10B

Figure 8.10C

MEIOSIS AND CROSSING OVER


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In humans, somatic cells have

– 23 pairs of homologous chromosomes and

– one member of each pair from each parent.

The human sex chromosomes X and Y differ in size and genetic composition.

The other 22 pairs of chromosomes are autosomes with the same size and genetic composition.

8.11 Chromosomes are matched in homologous pairs


Homologous chromosomes are matched in

– length,

– centromere position, and

– gene locations.

A locus (plural, loci) is the position of a gene.

Different versions of a gene may be found at the same locus on maternal and paternal chromosomes.

8.11 Chromosomes are matched in homologous pairs


Figure 8.11

Pair of homologouschromosomes

Locus

Centromere

Sisterchromatids

One duplicatedchromosome

An organism’s life cycle is the sequence of stages leading

– from the adults of one generation

– to the adults of the next.

Humans and many animals and plants are diploid, with body cells that have

– two sets of chromosomes,

– one from each parent.

8.12 Gametes have a single set of chromosomes


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.

Fertilization is the union of sperm and egg.

The zygote has a diploid chromosome number, one set from each parent.



Figure 8.12AHaploid gametes (n 23)

Egg cell

Sperm cell

Fertilization

n

n

Meiosis

Ovary Testis

Diploidzygote(2n 46)

2n

MitosisKey

Haploid stage (n)Diploid stage (2n)

Multicellular diploidadults (2n 46)

15

All sexual life cycles include an alternation between

– a diploid stage and

– a haploid stage.

Producing haploid gametes prevents the chromosome number from doubling in every generation.



Figure 8.12B

A pair ofhomologouschromosomesin a diploidparent cell

A pair ofduplicatedhomologouschromosomes

Sisterchromatids

1 2 3

INTERPHASE MEIOSIS I MEIOSIS II

Meiosis is a type of cell division that produces haploid gametes in diploid organisms.

Two haploid gametes combine in fertilization to restore the diploid state in the zygote.

8.13 Meiosis reduces the chromosome number from diploid to haploid


Meiosis and mitosis are preceded by the duplication of chromosomes. However,

– meiosis is followed by two consecutive cell divisions and

– mitosis is followed by only one cell division.

Because in meiosis, one duplication of chromosomes is followed by two divisions, each of the four daughter cells produced has a haploid set of chromosomes.



Meiosis I – Prophase I – events occurring in the nucleus.

– 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.



Figure 8.13_left

Centrosomes(with centriolepairs) Centrioles

Sites of crossing over

Spindle

Tetrad

Nuclearenvelope

Chromatin Sisterchromatids Fragments

of thenuclearenvelope

Centromere(with akinetochore)

Spindle microtubulesattached to a kinetochore

Metaphaseplate Homologous

chromosomesseparate

Sister chromatidsremain attached

Chromosomes duplicate Prophase I Metaphase I Anaphase IINTERPHASE:

MEIOSIS I: Homologous chromosomes separate

16

Figure 8.13_1

Centrosomes(with centriolepairs) Centrioles

Sites of crossing over

Spindle

Tetrad

Nuclearenvelope

Chromatin Sisterchromatids Fragments

of thenuclearenvelope

Chromosomes duplicate Prophase IINTERPHASE:

MEIOSIS I

Figure 8.13_2

Centromere(with akinetochore)

Spindle microtubulesattached to a kinetochore

Metaphaseplate Homologous

chromosomesseparate

Sister chromatidsremain attached

Metaphase I Anaphase I

MEIOSIS I

Meiosis I – Metaphase I – Tetrads align at the cell equator.

Meiosis I – Anaphase I – Homologous pairs separate and move toward opposite poles of the cell.



Meiosis I – Telophase I

– Duplicated chromosomes have reached the poles.

– A nuclear envelope re-forms around chromosomes in some species.

– Each nucleus has the haploid number of chromosomes.



Meiosis II follows meiosis I without chromosome duplication.

Each of the two haploid products enters meiosis II.

Meiosis II – Prophase II

– Chromosomes coil and become compact (if uncoiled after telophase I).

– Nuclear envelope, if re-formed, breaks up again.



Figure 8.13_right

Cleavagefurrow

Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II

MEIOSIS II: Sister chromatids separate

Sister chromatidsseparate

Haploid daughtercells forming

Telophase IIand Cytokinesis

17

Figure 8.13_3

Cleavagefurrow

Telophase I and Cytokinesis

Figure 8.13_4

Prophase II Metaphase II Anaphase II

MEIOSIS II: Sister chromatids separate

Sister chromatidsseparate

Haploid daughtercells forming

Telophase IIand Cytokinesis

Figure 8.13_5

Two lily cellsundergo meiosis II

Meiosis II – Metaphase II – Duplicated chromosomes align at the cell equator.

Meiosis II – Anaphase II

– Sister chromatids separate and

– chromosomes move toward opposite poles.



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.



Mitosis and meiosis both

– begin with diploid parent cells that

– have chromosomes duplicated during the previous interphase.

However the end products differ.

– Mitosis produces two genetically identical diploid somatic daughter cells.

– Meiosis produces four genetically unique haploid gametes.

8.14 Mitosis and meiosis have important similarities and differences


18

Figure 8.14

Prophase

Metaphase

Duplicatedchromosome(two sisterchromatids)

MITOSIS

Parent cell(before chromosome duplication)



Site ofcrossingover

2n 4

Chromosomesalign at themetaphase plate

Tetrads (homologouspairs) align at themetaphase plate

Tetrad formedby synapsis ofhomologouschromosomes

Metaphase I

Prophase I

MEIOSIS I

AnaphaseTelophase

Sister chromatidsseparate during

anaphase

2n 2n

Daughter cells of mitosis

No furtherchromosomalduplication;sisterchromatidsseparate duringanaphase II

n n n n

Daughter cells of meiosis II

Daughtercells of

meiosis I

Haploidn 2

Anaphase ITelophase I

Homologouschromosomesseparate duringanaphase I;sisterchromatidsremain together

MEIOSIS II

Figure 8.14_1

Prophase

Metaphase

MITOSIS

Parent cell(before chromosome duplication)



Site ofcrossingover

2n 4



Tetrad

Metaphase I

Prophase I

MEIOSIS I

Figure 8.14_2

Metaphase

MITOSIS


AnaphaseTelophase

Sister chromatidsseparate during

anaphase

2n 2nDaughter cells of mitosis

Figure 8.14_3


Metaphase I

No furtherchromosomalduplication;sisterchromatidsseparate duringanaphase II

n

Daughter cells of meiosis II

Daughtercells of

meiosis I

Haploidn 2

Anaphase ITelophase I

Homologouschromosomesseparate duringanaphase I;sisterchromatidsremain together

MEIOSIS II

MEIOSIS I

n n n

Genetic variation in gametes results from

– independent orientation at metaphase I and

– random fertilization.

8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring


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.



19

Random fertilization – The combination of each unique sperm with each unique egg increases genetic variability.



Animation: Genetic Variation

Figure 8.15_s1

Possibility A

Two equally probablearrangements ofchromosomes at

metaphase I

Possibility B

Figure 8.15_s2

Possibility A


metaphase I

Possibility B

Metaphase II

Figure 8.15_s3

Possibility A


metaphase I

Possibility B

Metaphase II

Gametes

Combination 3 Combination 4Combination 2Combination 1

8.16 Homologous chromosomes may carry different versions of genes

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.


Figure 8.16

Coat-colorgenes

Eye-colorgenes

Brown Black

Meiosis

White Pink

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

Chromosomes ofthe four gametes White coat (c);

pink eyes (e)

Brown coat (C);black eyes (E)

EC

ec

e

E

E

ec

c

C

C

20

Figure 8.16_2

Brown coat (C);black eyes (E)

Figure 8.16_3

White coat (c);pink eyes (e)

Figure 8.16_1

Coat-colorgenes

Eye-colorgenes

Brown Black

Meiosis

White Pink

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

EC

ec

Chromosomes ofthe four gametes

e

E

E

ec

c

C

C

Figure 8.16Q

Sister chromatidsSister chromatids

Pair of homologouschromosomes

Genetic recombination is the production of new combinations of genes due to crossing over.

Crossing over is an exchange of corresponding segments between separate (nonsister) chromatids on 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.

8.17 Crossing over further increases genetic variability


Animation: Crossing Over

Figure 8.17A

Chiasma

Tetrad

21

Figure 8.17A_1

Chiasma

Figure 8.17BTetrad

(pair of homologouschromosomes in synapsis)

Breakage of homologous chromatids

Joining of homologous chromatids

Chiasma

Separation of homologouschromosomes at anaphase I

Separation of chromatids atanaphase II andcompletion of meiosis

Parental type of chromosome

Recombinant chromosome


Parental type of chromosomeGametes of four genetic types

1

2

3

4

C

c e

E

C

c e

E

c e

C E

C e

e

C E

c

c E

C E

C e

c E

ec

Figure 8.17B_1

Tetrad(pair of homologouschromosomes in synapsis)

Breakage of homologous chromatids

Joining of homologous chromatids

1

2

C

c e

E

C

c e

E

C

c e

E

Chiasma

Figure 8.17B_2

Separation of homologouschromosomes at anaphase I

3

C E

C e

Chiasma

c

c

E

e

C E

c e

Figure 8.17B_3

Separation of chromatids atanaphase II andcompletion of meiosis





Gametes of four genetic types

C E

eC

4

Ec

c e

EC

eC

c E

c e

ALTERATIONS OF CHROMOSOME NUMBER

AND STRUCTURE


22

A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs.

Karyotypes

– are often produced from dividing cells arrested at metaphase of mitosis and

– allow for the observation of

– homologous chromosome pairs,

– chromosome number, and

– chromosome structure.

8.18 A karyotype is a photographic inventory of an individual’s chromosomes


Figure 8.18_s1

Bloodculture

Packed redand whiteblood cells

Centrifuge

Fluid1

Figure 8.18_s2

Bloodculture


Centrifuge

Fluid

Hypotonicsolution

2

1

Figure 8.18_s3

Bloodculture


Centrifuge

Fluid

Hypotonicsolution Fixative

Whitebloodcells

Stain

3

2

1

Figure 8.18_s4

4

Figure 8.18_s5

Centromere

Sisterchromatids

Pair ofhomologouschromosomes

5

Sex chromosomes

23

Trisomy 21

– involves the inheritance of three copies of chromosome 21 and

– is the most common human chromosome abnormality.

8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome


Trisomy 21, called Down syndrome, produces a characteristic set of symptoms, which include:

– mental retardation,

– characteristic facial features,

– short stature,

– heart defects,

– susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and

– shortened life span.

The incidence increases with the age of the mother.

8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome


Figure 8.19A

Trisomy 21

Figure 8.19A_1

Trisomy 21

Figure 8.19A_2 Figure 8.19B

Age of mother

504540353025200

10

20

30

40

50

60

70

80

90

Infa

nts

wit

h D

ow

n s

ynd

rom

e(p

er 1

,000

bir

ths)

24

Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during

– meiosis I, if both members of a homologous pair go to one pole or

– meiosis II if both sister chromatids go to one pole.

Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.

8.20 Accidents during meiosis can alter chromosome number


Figure 8.20A_s1

Nondisjunction

MEIOSIS I

Figure 8.20A_s2

Nondisjunction

MEIOSIS I

MEIOSIS II

Normalmeiosis II

Figure 8.20A_s3

Nondisjunction

MEIOSIS I

MEIOSIS II

Normalmeiosis II

Gametes

Number ofchromosomes

Abnormal gametes

n 1 n 1 n 1 n 1

Figure 8.20B_s1

Normalmeiosis I

MEIOSIS IFigure 8.20B_s2

Normalmeiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

25

Figure 8.20B_s3

Normalmeiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

Abnormal gametes Normal gametes

n 1 n 1 n n

Sex chromosome abnormalities tend to be less severe, perhaps because of

– the small size of the Y chromosome or

– X-chromosome inactivation.

8.21 CONNECTION: Abnormal numbers of sexchromosomes do not usually affect survival


The following table lists the most common human sex chromosome abnormalities. In general,

– a single Y chromosome is enough to produce “maleness,” even in combination with several X chromosomes, and

– the absence of a Y chromosome yields “femaleness.”

8.21 CONNECTION: Abnormal numbers of sexchromosomes do not usually affect survival


Table 8.21

Errors in mitosis or meiosis may produce polyploid species, with more than two chromosome sets.

The formation of polyploid species is

– widely observed in many plant species but

– less frequently found in animals.

8.22 EVOLUTION CONNECTION: New species can arise from errors in cell division


Figure 8.22

26

Chromosome breakage can lead to rearrangements that can produce

– genetic disorders or,

– if changes occur in somatic cells, cancer.

8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer


These rearrangements may include

– a deletion, the loss of a chromosome segment,

– a duplication, the repeat of a chromosome segment,

– an inversion, the reversal of a chromosome segment, or

– a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal.



Chronic myelogenous leukemia (CML)

– is one of the most common leukemias,

– affects cells that give rise to white blood cells (leukocytes), and

– results from part of chromosome 22 switching places with a small fragment from a tip of chromosome 9.



Figure 8.23A

Deletion

Duplication

Inversion

Reciprocal translocation

Homologouschromosomes Nonhomologous

chromosomes

Figure 8.23A_1

Deletion

Duplication

Homologouschromosomes

Figure 8.23A_2

Inversion

Reciprocal translocation

Nonhomologouschromosomes

27

Figure 8.23B

Chromosome 9

Chromosome 22 Reciprocaltranslocation

“Philadelphia chromosome”

Activated cancer-causing gene

1. Compare the parent-offspring relationship in asexual and sexual reproduction.

2. Explain why cell division is essential for prokaryotic and eukaryotic life.

3. Explain how daughter prokaryotic chromosomes are separated from each other during binary fission.

4. Compare the structure of prokaryotic and eukaryotic chromosomes.

5. Describe the stages of the cell cycle.

You should now be able to


6. List the phases of mitosis and describe the events characteristic of each phase.

7. Compare cytokinesis in animal and plant cells.

8. Explain how anchorage, cell density, and chemical growth factors control cell division.

9. Explain how cancerous cells are different from healthy cells.

10. Describe the functions of mitosis.

11. Explain how chromosomes are paired.

12. Distinguish between somatic cells and gametes and between diploid cells and haploid cells.



13. Explain why sexual reproduction requires meiosis.

14. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase.

15. Compare mitosis and meiosis noting similarities and differences.

16. Explain how genetic variation is produced in sexually reproducing organisms.

17. Explain how and why karyotyping is performed.

18. Describe the causes and symptoms of Down syndrome.



19. Describe the consequences of abnormal numbers of sex chromosomes.

20. Define nondisjunction, explain how it can occur, and describe what can result.

21. Explain how new species form from errors in cell division.

22. Describe the main types of chromosomal changes. Explain why cancer is not usually inherited.



Figure 8.UN01

Geneticallyidenticaldaughtercells

SG1

G2

M

Mitosis(division ofthe nucleus)

Cytokinesis(division of thecytoplasm)

(DNA synthesis)

28

Figure 8.UN02

Haploid gametes (n 23)

Egg cell

Sperm cell

Fertilization

n

n

Meiosis

Human life cycle

2nMulticellulardiploid adults(2n 46)

Mitosis

Diploidzygote(2n 46)

Figure 8.UN03

Number of chromosomalduplications

Number of cell divisions

Number of daughter cellsproduced

Number of chromosomes inthe daughter cells

How the chromosomes lineup during metaphase

Genetic relationship of thedaughter cells to the parent cell

Functions performed in thehuman body

Mitosis Meiosis

Figure 8.UN04

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