Chapter 8

Chapter 8

The Cellular Basis of Reproduction and Inheritance

Introduction  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.

PowerPoint Lectures for

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

Lecture by Edward J. Zalisko

© 2012 Pearson Education, Inc.


Figure 8.0_1

Introduction  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. © 2012 Pearson Education, Inc.

Figure 8.0_2

Figure 8.0_3

Chapter 8: Big Ideas

Cell Division and Reproduction

Meiosis and Crossing Over

The Eukaryotic Cell Cycle and Mitosis

Alterations of Chromosome Number and Structure

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

CELL DIVISION AND REPRODUCTION

 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

 Living organisms reproduce by two methods.

– for reproduction of single-celled organisms,

– Asexual reproduction – produces offspring that are identical to the original cell or organism and

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

– involves inheritance of all genes from one parent.

– repair and replacement of cells, and – sperm and egg production.

– 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

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Figure 8.1C

Figure 8.1D

Figure 8.1E

Figure 8.1F

8.2 Prokaryotes reproduce by binary fission

8.2 Prokaryotes reproduce by binary fission

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

 Binary fission of a prokaryote occurs in three stages:

 The chromosome of a prokaryote is – a singular circular DNA molecule associated with proteins and – much smaller than those of eukaryotes.


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.


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Figure 8.2A_s1

Figure 8.2A_s2

Plasma membrane

Plasma membrane

Prokaryotic chromosome

Cell wall

1

Duplication of the chromosome and separation of the copies

Figure 8.2A_s3


Cell wall

1


2

Continued elongation of the cell and movement of the copies

Figure 8.2B

Plasma membrane


Cell wall

1


Prokaryotic chromosomes

2

3

Continued elongation of the cell and movement of the copies

Division into two daughter cells

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

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.



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Figure 8.3A

 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 8.3B

Chromosomes

DNA molecules

Figure 8.3B_2

Sister chromatids

Sister chromatids

Chromosome duplication

Centromere

Sister chromatids

Centromere

Chromosome distribution to the daughter cells


Figure 8.3B_1

Chromosomes

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

DNA molecules


– two copies called sister chromatids – joined together by a narrowed “waist” called the centromere.

Centromere

Sister chromatids

 When a cell divides, the sister chromatids – separate from each other, now called chromosomes, and – sort into separate daughter cells.

Chromosome distribution to the daughter cells


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8.4 The cell cycle multiplies cells

8.4 The cell cycle multiplies cells

 The cell cycle is an ordered sequence of events that extends

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

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

1. Interphase: duplication of cell contents – G1—growth, increase in cytoplasm

– until its own division.

– S—duplication of chromosomes – G2—growth, preparation for division

2. Mitotic phase: division – Mitosis—division of the nucleus – Cytokinesis—division of cytoplasm



Figure 8.4

8.5 Cell division is a continuum of dynamic changes  Mitosis progresses through a series of stages: – prophase, G1 (first gap)

S (DNA synthesis)

– prometaphase, – metaphase,

M G2 (second gap)

– anaphase, and – telophase.

 Cytokinesis often overlaps telophase.


8.5 Cell division is a continuum of dynamic changes

Figure 8.5_1

INTERPHASE

 A mitotic spindle is – required to divide the chromosomes,

Centrosomes (with centriole pairs) Centrioles

– composed of microtubules, and

Chromatin

MITOSIS Prophase

Prometaphase

Early mitotic spindle Centrosome

Fragments of the nuclear envelope Kinetochore

– produced by centrosomes, structures in the cytoplasm that – organize microtubule arrangement and – contain a pair of centrioles in animal cells.

Video: Animal Mitosis

Nuclear envelope

Plasma membrane

Centromere Chromosome, consisting of two sister chromatids

Spindle microtubules

Video: Sea Urchin (time lapse) © 2012 Pearson Education, Inc.

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Figure 8.5_2

8.5 Cell division is a continuum of dynamic changes  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.

INTERPHASE


Figure 8.5_left

8.5 Cell division is a continuum of dynamic changes

MITOSIS

INTERPHASE

Prophase

Prometaphase

 Prophase – In the cytoplasm microtubules begin to emerge from centrosomes, forming the spindle. Centrosomes (with centriole pairs) Centrioles

Early mitotic spindle

Chromatin

Centrosome

Fragments of the nuclear envelope Kinetochore

– In the nucleus – chromosomes coil and become compact and – nucleoli disappear.

Nuclear envelope

Plasma membrane

Centromere Chromosome, consisting of two sister chromatids

Spindle microtubules


Figure 8.5_3

8.5 Cell division is a continuum of dynamic changes  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. Prophase

– The nuclear envelope disappears.


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Figure 8.5_4

Figure 8.5_5

MITOSIS Anaphase

Metaphase Metaphase plate

Prometaphase

Figure 8.5_right

MITOSIS Anaphase

Metaphase

Daughter chromosomes

Mitotic spindle

Telophase and Cytokinesis

Telophase and Cytokinesis Cleavage furrow

Nuclear envelope forming

8.5 Cell division is a continuum of dynamic changes  Metaphase – The mitotic spindle is fully formed. – Chromosomes align at the cell equator.

Metaphase plate Cleavage furrow

Mitotic spindle

Daughter chromosomes

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

Nuclear envelope forming © 2012 Pearson Education, Inc.

Figure 8.5_6

8.5 Cell division is a continuum of dynamic changes  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. Metaphase


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Figure 8.5_7

8.5 Cell division is a continuum of dynamic changes  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.

Anaphase


Figure 8.5_8

8.5 Cell division is a continuum of dynamic changes  During cytokinesis, the cytoplasm is divided into separate cells.  The process of cytokinesis differs in animal and plant cells.

Telophase and Cytokinesis


8.6 Cytokinesis differs for plant and animal cells  In animal cells, cytokinesis occurs as

Figure 8.6A

Cytokinesis Cleavage furrow

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

Contracting ring of microfilaments

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


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Figure 8.6A_1

Figure 8.6A_2

Cleavage furrow

Contracting ring of microfilaments

Daughter cells

Cleavage furrow

8.6 Cytokinesis differs for plant and animal cells

Figure 8.6B

 In plant cells, cytokinesis occurs as 1. 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.

New cell wall

Cytokinesis Cell wall

Cell wall of the parent cell

Plasma membrane

Daughter nucleus Vesicles containing cell wall material

Cell plate forming

Cell plate

Daughter cells

Animation: Cytokinesis © 2012 Pearson Education, Inc.

Figure 8.6B_1

Figure 8.6B_2

Cell wall of the parent cell Daughter nucleus Cell plate forming

New cell wall Cell wall Plasma membrane

Vesicles containing cell wall material

Cell plate

Daughter cells

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8.7 Anchorage, cell density, and chemical growth factors affect cell division

Figure 8.7A

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

Cultured cells suspended in liquid

 Cell division is controlled by

The addition of growth factor

– 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. © 2012 Pearson Education, Inc.

Figure 8.7B

Anchorage

8.8 Growth factors signal the cell cycle control system  The cell cycle control system is a cycling set of molecules in the cell that

Single layer of cells

– triggers and – coordinates key events in the cell cycle.

 Checkpoints in the cell cycle can Removal of cells

– stop an event or – signal an event to proceed.

Restoration of single layer by cell division © 2012 Pearson Education, Inc.

8.8 Growth factors signal the cell cycle control system

Figure 8.8A

G1 checkpoint G0

 There are three major checkpoints in the cell cycle. 1. G1 checkpoint

G1

– allows entry into the S phase or

S

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

Control system

2. G2 checkpoint, and

M G2

3. M checkpoint.

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

M checkpoint G2 checkpoint


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Figure 8.8B

Growth factor

EXTRACELLULAR FLUID

Plasma membrane

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

Relay proteins

Receptor protein Signal transduction pathway

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

G1 checkpoint

 Cancer cells escape controls on the cell cycle. G1

S Control system

M G2

 Cancer cells – divide rapidly, often in the absence of growth factors, – spread to other tissues through the circulatory system, and

CYTOPLASM

– grow without being inhibited by other cells. © 2012 Pearson Education, Inc.


Figure 8.9

 A tumor is an abnormally growing mass of body cells.

Lymph vessels Blood vessel

– Benign tumors remain at the original site. Tumor

– Malignant tumors spread to other locations, called metastasis.

Tumor in another part of the body

Glandular tissue Growth

Invasion

Metastasis




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

 Cancer treatments

– Carcinomas arise in external or internal body coverings. – Sarcomas arise in supportive and connective tissue.

– Localized tumors can be – removed surgically and/or – treated with concentrated beams of high-energy radiation.

– Chemotherapy is used for metastatic tumors.

– Leukemias and lymphomas arise from blood-forming tissues.



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

Figure 8.10A

 When the cell cycle operates normally, mitosis produces genetically identical cells for – growth, – replacement of damaged and lost cells, and – asexual reproduction.

Video: Hydra Budding © 2012 Pearson Education, Inc.

Figure 8.10A_1

Figure 8.10B

Figure 8.10C

MEIOSIS AND CROSSING OVER


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8.11 Chromosomes are matched in homologous pairs

8.11 Chromosomes are matched in homologous pairs

 In humans, somatic cells have

 Homologous chromosomes are matched in

– 23 pairs of homologous chromosomes and

– length,

– one member of each pair from each parent.

– centromere position, and

 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.


– 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. © 2012 Pearson Education, Inc.

Figure 8.11

8.12 Gametes have a single set of chromosomes

Pair of homologous chromosomes

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

Locus

– from the adults of one generation

Centromere

– to the adults of the next.

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

Sister chromatids One duplicated chromosome

– two sets of chromosomes, – one from each parent.



Figure 8.12A

Haploid gametes (n  23) n

 Meiosis is a process that converts diploid nuclei to haploid nuclei.

Egg cell n Sperm cell

– Diploid cells have two homologous sets of chromosomes.

Meiosis

Fertilization

– Haploid cells have one set of chromosomes. – Meiosis occurs in the sex organs, producing gametes—sperm and eggs.

Ovary

Testis Diploid zygote (2n  46)

 Fertilization is the union of sperm and egg.  The zygote has a diploid chromosome number, one set from each parent.

2n

Key Multicellular diploid adults (2n  46)

Mitosis

Haploid stage (n) Diploid stage (2n)


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Figure 8.12B

MEIOSIS II

Sister chromatids

– a diploid stage and – a haploid stage.

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

MEIOSIS I

INTERPHASE

 All sexual life cycles include an alternation between

2

1

A pair of homologous chromosomes in a diploid parent cell

3

A pair of duplicated homologous chromosomes


8.13 Meiosis reduces the chromosome number from diploid to haploid


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

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

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

– 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.


8.13 Meiosis reduces the chromosome number from diploid to haploid  Meiosis I – Prophase I – events occurring in the nucleus. – Chromosomes coil and become compact.


Figure 8.13_left

MEIOSIS I: Homologous chromosomes separate

INTERPHASE:

Chromosomes duplicate Centrosomes (with centriole pairs)

Prophase I

Metaphase I

Sites of crossing over

Spindle microtubules attached to a kinetochore

Centrioles

Anaphase I Sister chromatids remain attached

Spindle

– 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.

Tetrad Nuclear envelope

Chromatin

Sister chromatids

Fragments of the nuclear envelope

Centromere (with a kinetochore)

Metaphase plate Homologous chromosomes separate


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Figure 8.13_1

Figure 8.13_2

MEIOSIS I

INTERPHASE: Chromosomes duplicate Centrosomes (with centriole pairs)

MEIOSIS I

Prophase I

Metaphase I

Sites of crossing over

Spindle microtubules attached to a kinetochore

Centrioles

Anaphase I Sister chromatids remain attached

Spindle

Tetrad Nuclear envelope

Chromatin

Sister chromatids

Centromere (with a kinetochore)

Fragments of the nuclear envelope

Metaphase plate Homologous chromosomes separate



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

 Meiosis I – Telophase I

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

– Duplicated chromosomes have reached the poles. – A nuclear envelope re-forms around chromosomes in some species. – Each nucleus has the haploid number of chromosomes.


8.13 Meiosis reduces the chromosome number from diploid to haploid  Meiosis II follows meiosis I without chromosome duplication.  Each of the two haploid products enters meiosis II.


Figure 8.13_right

MEIOSIS II: Sister chromatids separate Telophase I and Cytokinesis

Prophase II

Metaphase II

Anaphase II

Telophase II and Cytokinesis

Cleavage furrow

 Meiosis II – Prophase II Sister chromatids separate

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

Haploid daughter cells forming

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


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Figure 8.13_3

Figure 8.13_4

MEIOSIS II: Sister chromatids separate

Telophase I and Cytokinesis

Prophase II

Metaphase II

Anaphase II

Telophase II and Cytokinesis

Cleavage furrow Sister chromatids separate

Figure 8.13_5

Haploid daughter cells forming

8.13 Meiosis reduces the chromosome number from diploid to haploid  Meiosis II – Metaphase II – Duplicated chromosomes align at the cell equator.  Meiosis II – Anaphase II – Sister chromatids separate and – chromosomes move toward opposite poles.

Two lily cells undergo meiosis II



8.14 Mitosis and meiosis have important similarities and differences

 Meiosis II – Telophase II

 Mitosis and meiosis both

– Chromosomes have reached the poles of the cell.

– begin with diploid parent cells that

– A nuclear envelope forms around each set of chromosomes.

– have chromosomes duplicated during the previous interphase.

– With cytokinesis, four haploid cells are produced.

 However the end products differ. – Mitosis produces two genetically identical diploid somatic daughter cells. – Meiosis produces four genetically unique haploid gametes.



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Figure 8.14

Figure 8.14_1 MEIOSIS I

MITOSIS Parent cell (before chromosome duplication)

Prophase

Duplicated chromosome (two sister chromatids)


Site of crossing over

Prophase I

MEIOSIS I

MITOSIS Tetrad formed by synapsis of homologous chromosomes

Chromosome duplication 2n  4

Prophase

Parent cell (before chromosome duplication)

Prophase I Site of crossing over

Metaphase I

Metaphase Chromosomes align at the metaphase plate


Tetrads (homologous pairs) align at the metaphase plate

Chromosome duplication Tetrad

2n  4 Anaphase I Telophase I

Anaphase Telophase Homologous chromosomes separate during anaphase I; sister chromatids remain together

Sister chromatids separate during anaphase

Daughter cells of meiosis I

Metaphase

Metaphase I

Chromosomes align at the metaphase plate

Haploid n2

Tetrads (homologous pairs) align at the metaphase plate

MEIOSIS II 2n

2n Daughter cells of mitosis

No further chromosomal duplication; sister chromatids separate during anaphase II

n

n n n Daughter cells of meiosis II

Figure 8.14_2

Figure 8.14_3

MEIOSIS I

MITOSIS

Metaphase I

Metaphase Tetrads (homologous pairs) align at the metaphase plate

Chromosomes align at the metaphase plate

Anaphase I Telophase I Homologous chromosomes separate during anaphase I; sister chromatids remain together

Anaphase Telophase

Haploid n2

MEIOSIS II

Sister chromatids separate during anaphase

No further chromosomal duplication; sister chromatids separate during anaphase II

2n

2n

Daughter cells of meiosis I

Daughter cells of mitosis

n

n n n Daughter cells of meiosis II

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


 Genetic variation in gametes results from

 Independent orientation at metaphase I

– independent orientation at metaphase I and – random fertilization.

– 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.



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Figure 8.15_s1

Possibility A

Possibility B Two equally probable arrangements of chromosomes at metaphase I

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

Animation: Genetic Variation © 2012 Pearson Education, Inc.

Figure 8.15_s2

Figure 8.15_s3

Possibility A

Possibility A

Possibility B

Possibility B

Two equally probable arrangements of chromosomes at metaphase I

Two equally probable arrangements of chromosomes at metaphase I

Metaphase II

Metaphase II

Gametes

Combination 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.

Combination 2

– Since homologues move to opposite poles during anaphase I, gametes will receive either the maternal or paternal version of the gene.

Combination 4

Figure 8.16

Coat-color genes

Eye-color genes

Brown C

Black E Meiosis

– One version was inherited from the maternal parent and the other came from the paternal parent.

Combination 3

c White

e Pink

Tetrad in parent cell (homologous pair of duplicated chromosomes)

C

E

C

E

c

e

c

e

Chromosomes of the four gametes

Brown coat (C); black eyes (E)

White coat (c); pink eyes (e)


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Figure 8.16_2

Figure 8.16_3

White coat (c); pink eyes (e)

Brown coat (C); black eyes (E)

Figure 8.16_1

Figure 8.16Q

Coat-color genes

Eye-color genes

Brown C

Black E Meiosis

c White

e Pink

Tetrad in parent cell (homologous pair of duplicated chromosomes)

C

E

C

E

c

e

c

e

Sister chromatids Sister chromatids

Pair of homologous chromosomes

Chromosomes of the four gametes

8.17 Crossing over further increases genetic variability

Figure 8.17A

 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.

Chiasma Tetrad

– Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids. Animation: Crossing Over © 2012 Pearson Education, Inc.

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Figure 8.17A_1

Figure 8.17B

C

E

c

e 1

Breakage of homologous chromatids

C

E

c

e 2

Tetrad (pair of homologous chromosomes in synapsis)

Joining of homologous chromatids

C

E

c

e

Chiasma

3

Chiasma

Separation of homologous chromosomes at anaphase I

C

E

C c

e E

c

e 4

Separation of chromatids at anaphase II and completion of meiosis

C

E

C

e

c

E

Parental type of chromosome Recombinant chromosome Recombinant chromosome

e c Parental type of chromosome Gametes of four genetic types

Figure 8.17B_1

C

E

c

e

Figure 8.17B_2

Tetrad (pair of homologous chromosomes in synapsis)

C

E

c

e

C

E

c

e

Chiasma

Figure 8.17B_3

C

E

C c

e

c

e

E

4

c

e 3

Joining of homologous chromatids

2

E Chiasma

Breakage of homologous chromatids

1

C

Separation of chromatids at anaphase II and completion of meiosis

C

E

C

e

c

E

c

e

Separation of homologous chromosomes at anaphase I

C

E

C

e

c

E

c

e

ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE

Parental type of chromosome Recombinant chromosome Recombinant chromosome

Parental type of chromosome Gametes of four genetic types © 2012 Pearson Education, Inc.

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

Figure 8.18_s1

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

Packed red and white blood cells Blood culture

 Karyotypes

Centrifuge

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

– allow for the observation of

1

– homologous chromosome pairs, – chromosome number, and – chromosome structure. © 2012 Pearson Education, Inc.

Figure 8.18_s2

Figure 8.18_s3

Packed red and white blood cells Blood culture

Packed red and white blood cells

Hypotonic solution Blood culture

Centrifuge

Hypotonic solution

Centrifuge

2

2

Fixative Stain

White blood cells 3

Fluid 1

Fluid 1

Figure 8.18_s4

Figure 8.18_s5

Centromere

Sister chromatids Pair of homologous chromosomes

4

5

Sex chromosomes

22

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

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

 Trisomy 21

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

– involves the inheritance of three copies of chromosome 21 and – is the most common human chromosome abnormality.

– 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. © 2012 Pearson Education, Inc.


Figure 8.19A

Figure 8.19A_1

Trisomy 21

Trisomy 21

Figure 8.19B

90 Infants with Down syndrome (per 1,000 births)

Figure 8.19A_2

80 70 60 50 40 30 20 10 0 20

25

30 35 40 Age of mother

45

50

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8.20 Accidents during meiosis can alter chromosome number

Figure 8.20A_s1

MEIOSIS I

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

Nondisjunction

– 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.


Figure 8.20A_s2

Figure 8.20A_s3

MEIOSIS I

MEIOSIS I

Nondisjunction

MEIOSIS II

Nondisjunction

MEIOSIS II

Normal meiosis II

Normal meiosis II

Gametes Number of chromosomes

n1

n1

n1

n1

Abnormal gametes

Figure 8.20B_s1

Figure 8.20B_s2

MEIOSIS I

MEIOSIS I

Normal meiosis I

Normal meiosis I

MEIOSIS II

Nondisjunction

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Figure 8.20B_s3

8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival

MEIOSIS I

Normal meiosis I

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

MEIOSIS II

– the small size of the Y chromosome or – X-chromosome inactivation.

Nondisjunction

n1

n1

Abnormal gametes

n

n

Normal gametes © 2012 Pearson Education, Inc.

8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival

Table 8.21

 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.”


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

Figure 8.22

 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.


25

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


 Chromosome breakage can lead to rearrangements that can produce

 These rearrangements may include – a deletion, the loss of a chromosome segment,

– genetic disorders or,

– a duplication, the repeat of a chromosome segment,

– if changes occur in somatic cells, cancer.

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




Figure 8.23A

 Chronic myelogenous leukemia (CML) – is one of the most common leukemias,

Deletion

Inversion

Duplication

Reciprocal translocation

– 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.

Homologous chromosomes

Nonhomologous chromosomes


Figure 8.23A_1

Figure 8.23A_2

Deletion

Inversion

Duplication


Homologous chromosomes

Nonhomologous chromosomes

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Figure 8.23B

You should now be able to 1. Compare the parent-offspring relationship in asexual and sexual reproduction.

Chromosome 9

Chromosome 22


Activated cancer-causing gene “Philadelphia chromosome”

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.

13. Explain why sexual reproduction requires meiosis.

7. Compare cytokinesis in animal and plant cells.

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

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

15. Compare mitosis and meiosis noting similarities and differences.

9. Explain how cancerous cells are different from healthy cells. 10. Describe the functions of mitosis.

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

11. Explain how chromosomes are paired.

17. Explain how and why karyotyping is performed.

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

18. Describe the causes and symptoms of Down syndrome.




Figure 8.UN01

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.

G1 Genetically identical daughter cells

S (DNA synthesis)

M G2

Cytokinesis (division of the cytoplasm) Mitosis (division of the nucleus) © 2012 Pearson Education, Inc.

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Figure 8.UN02

Figure 8.UN03

Haploid gametes (n  23)

Mitosis

Egg cell

n

Number of cell divisions

n

Number of daughter cells produced

Sperm cell

Meiosis

Meiosis

Number of chromosomal duplications

Fertilization Human life cycle

Number of chromosomes in the daughter cells How the chromosomes line up during metaphase

2n

Multicellular diploid adults (2n  46) Mitosis

Diploid zygote (2n  46)

Genetic relationship of the daughter cells to the parent cell Functions performed in the human body

Figure 8.UN04

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