The Cellular Basis of Reproduction and Inheritancepbaluch/courses/bio100/cell_division.pdfTight helical fiber Supercoil Sister chromatids ... Copyright © 2004 Pearson Education, Inc

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Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings

Chapter 8

The Cellular Basis of Reproduction and Inheritance


• You began life as a single cell, but there are now more cells in your body than stars in the Milky Way

• Just in the past second, millions of your cells have divided in two


• The dance of the chromosomes in a dividing cell is so precise that only one error occurs in 100,000 cell divisions

• Each sperm or egg produced in your reproductive organs carries one of over 8 million possible combinations of parental chromosomes

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• A few years ago a sterile couple was willing to pay $50,000 to a woman willing to donate her eggs

BIOLOGY AND SOCIETY:

A $50,000 EGG!


• Infertility

– Affects one in ten American couples

• In vitro fertilization (IVF)

– A sperm and an egg are joined in a petri dish

– The embryo is implanted into the mother’s uterus

– IVF is one of many reproductive technologies

Figure 8.1


• Reproduction

WHAT CELL REPRODUCTION ACCOMPLISHES

– Is the birth of new organisms

– Occurs much more often at the cellular level.

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• Cell division plays a role in

– The replacement of lost or damaged cells

– Cell reproduction and growth


• Before a parent cell divides, it duplicates its chromosomes

Passing On the Genes from Cell to Cell

• The two resulting “daughter” cells are genetically identical.


• In asexual reproduction, single-celled organisms reproduce by simple cell division

The Reproduction of Organisms

Figure 8.2a(a) Amoeba

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– Some multicellular organisms can divide into pieces that then grow into new individuals

Figure 8.2b(b) Sea star


• Sexual reproduction is different

– It requires fertilization of an egg by a sperm

– Production of egg and sperm is called meiosis.


• A genome

THE CELL CYCLE AND MITOSIS

– Is the complete set of an organism’s genes

– Is located mainly on chromosomes in the cell’s nucleus.

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• Chromosomes

Eukaryotic Chromosomes

– Are made of chromatin, a combination of DNA and protein molecules

– Are not visible in a cell until cell division.

Figure 8.3


• The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding

Figure 8.4

DNA double helixHistones

“Beadson astring”

Nucleosome

Tight helical fiberSupercoil

Sisterchromatids

Centromere


• Before a cell divides, it duplicates all of its chromosomes, resulting in two copies called sister chromatids.

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• When the cell divides, the sister chromatidsseparate from each other.

Figure 8.5

Chromosomeduplication

Sisterchromatids

Chromosomedistribution

todaughter

cells


• Eukaryotic cells that divide undergo an orderly sequence of events called the cell cycle.

The Cell Cycle


• The cell cycle consists of two distinct phases

– Interphase

– Mitotic phase.

Figure 8.6

S phase(DNA synthesis;

chromosome duplication)

Interphase(90% of time)

G1

Mitoticphase (M)

(10% of time)

Cytokinesis Mitosis

G2

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• Mitosis

Mitosis and Cytokinesis

– Is the division of the chromosomes

– Is preceded by interphase.

Interphase

Centrosomes(withcentriolepairs)

NucleolusNuclearenvelope

Plasmamembrane

Chromatin

Figure 8.7.1


• Mitosis consists of four distinct phases

– Prophase

– Metaphase.


Figure 8.7.2

Earlymitoticspindle

Centrosome Centromere

Chromosome, consistingof two sister chromatids

Spindlemicrotubules

Fragments ofnuclear envelope

Metaphase

Spindle

Prophase

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– Anaphase

– Telophase

Figure 8.7.3

Anaphase Telophase and Cytokinesis

Daughterchromosomes

Cleavagefurrow

Nuclearenvelopeforming

Nucleolusforming


• Cytokinesis

– Typically occurs during telophase

– Is the division of the cytoplasm.


• Cytokinesis is different in plant and animal cells.

Figure 8.8a

Cleavagefurrow

Cleavage furrowContracting ring ofmicrofilaments

Daughter cells(a) Animal cell cytokinesis

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Figure 8.8b

Wall of parent cell

Cell plateforming

Daughternucleus

Cell wall

Vesicles containingcell wall material

Cell plate

New cell wall

Daughter cells(b) Plant cell cytokinesis


• Normal plant and animal cells have a cell cycle control system

Cancer Cells: Growing Out of Control

• When the cell cycle control system malfunctions

– Cells may reproduce at the wrong time or place

– A benign tumor may form.


• What is cancer?

What Is Cancer?

– Cancer is caused by a breakdown in control of the cell cycle.

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– Cancer cells divide excessively

– Cancer cells spread from a malignant tumor

– Metastasis is the spread of cancer.

Tumor

Glandulartissue

A tumor grows from asingle cancer cell.

Lymphvessels

Cancer cells invadeneighboring tissue.

MetastasisCancer cells spread through lymph and blood vessels to other parts of the body

Figure 8.9


• Cancer treatment

Cancer Treatment

– Radiation therapy disrupts cell division

– Chemotherapy involves drugs that disrupt cell division.


• Cancer cells are often grown in culture for study.

Figure 8.10

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• Cancer prevention includes changes in lifestyle

Cancer Prevention and Survival

– Not smoking

– Avoiding exposure to the sun

– Eating a high-fiber, low-fat diet

– Visiting the doctor regularly

– Performing regular self-examinations.


• Sexual reproduction depends on

MEIOSIS, THE BASIS OF SEXUAL REPRODUCTION

– Meiosis

– Fertilization.

Figure 8.11


• Different organisms of the same species have the same number and types of chromosomes.

Homologous Chromosomes

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• A somatic cell

– Is a typical body cell

– Has 46 chromosomes in a human.


• A karyotype is an orderly arrangement of chromosomes

• Homologous chromosomes are matching pairs of chromosomes.

Pair of homologouschromosomes

Centromere

Sister chromatids

Figure 8.12


• Humans have

– Two different sex chromosomes, X and Y

– 22 pairs of matching chromosomes, called autosomes.

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• The life cycle of a multicellularorganism is the sequence of stages leading from the adults of one generation to the adults of the next.

Gametes and the Life Cycle of a Sexual Organism

Figure 8.13

Haploid gametes (n = 23)

Diploidzygote

(2n = 46)

Multicellulardiploid adults

(2n = 46)

Mitosis anddevelopment

Egg cell

Sperm cell

Meiosis Fertilization


• Humans are diploid organisms

– Their cells contain two sets of chromosomes

– Their gametes are haploid, having only one set of chromosomes.


• Fertilization

– Is the fusion of sperm and egg

– Creates a zygote, or fertilized egg.

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• Sexual life cycles involve an alternation of diploid and haploid stages.

Figure 8.14

Homologouspair ofchromosomesin diploidparent cell

Homologouspair ofduplicatedchromosomes

Meiosis II

Sisterchromatids

Interphase before meiosis Meiosis I

Chromosomesduplicate

1 2

3

Homologouschromosomesseparate

Sisterchromatidsseparate


• In meiosis

The Process of Meiosis

– Haploid gametes are produced in diploid organisms

– Two consecutive divisions occur, meiosis I and meiosis II, preceded by interphase

– Crossing over occurs.

Figure 8.15.1

Interphase

Nuclearenvelope Chromatin

Chromosomes duplicate

Centrosomes(withcentriolepairs)


• Meiosis I

Figure 8.15.2

Meiosis I: Homologous chromosomes separate

Microtubules attached to Chromosomes

Sister chromatidsremain attached

Cleavagefurrow

Sisterchromatids

Tetrad Centromere

Anaphase I Telophase Iand Cytokinesis

Sites of crossing over

Spindle

Prophase I Metaphase I

Homologous chromosomespair and exchange segments

Two haploid cells form: chromosomes are still double

Tetrads line up Pairs of homologouschromosomessplit up

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• Meiosis II

Figure 8.15.3

Meiosis II:Sister chromatids separate

Anaphase II Telophase IIand Cytokinesis

Sister chromatidsseparate

Haploid daughter cellsforming

Prophase II Metaphase II

During another round of cell division, the sister c hromatids finally separate; four haploid daughter cells result, containing sing le chromosomes


Review: Comparing Mitosis and Meiosis

Figure 8.16

Mitosis

Duplicated chromosome(two sisterchromatids)

Prophase


2n = 4

MeiosisParent cell

(before chromosome duplication)

Pairing ofhomologouschromosomes


Site of crossing over Meiosis I

Tetrad formed by pairing of homologouschromosomes

Prophase I

MetaphaseTetradsalign at themiddle ofthe cell

Metaphase I

AnaphaseTelophase

2n

Chromosomesalign at the middle of the cell

Sisterchromatidsseparateduring anaphase 2n

Daughter cellsof mitosis

Daughtercells of

meiosis I

HomologouschromosomesseparateduringanaphaseI; sisterchromatidsremaintogether

n n n n

Meiosis II

Daughter cells of meiosis II

Anaphase ITelophase I

Haploidn = 2

No furtherchromosomalduplication;sisterchromatidsseparateduringanaphase II


• Offspring of sexual reproduction are genetically different from their parents and from one another.

The Origins of Genetic Variation

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• In independent assortment every chromosome pair orients independently of the others during meiosis.

Independent Assortment of Chromosomes

Figure 8.17

Possibility 1

Metaphase of

meiosis II

Combination a Combination b Combination c Combination d

Gametes

Possibility 2

Metaphase ofmeiosis I


• The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote.

Random Fertilization


• In crossing over

Crossing Over

– Homologous chromosomes exchange genetic information

– Genetic recombination occurs.

Figure 8.18

Prophase Iof meiosis

Metaphase I

Metaphase II

Gametes

Recombinant chromosomes

Spindlemicrotubules

Tetrad

Chiasma,site ofcrossingover

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• What happens when errors occur in meiosis?

When Meiosis Goes Awry.


• Down Syndrome

Down Syndrome: An Extra Chromosome 21

– Is a condition where an individual has an extra chromosome 21

– Is also called trisomy 21.

Figure 8.19


• The incidence of Down Syndrome increases with the age of the mother.

Figure 8.20

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• In nondisjunction

How Accidents During Meiosis Can Alter Chromosome Number

– The members of a chromosome pair fail to separate during anaphase

– Gametes with an incorrect number of chromosomes are produced.


Figure 8.21

Meiosis I

Nondisjunction

n + 1 n + 1 n - 1 n - 1 n + 1 n - 1 n n

Number of chromosomes

Gametes

Nondisjunction

Meiosis II

(a) Nondisjunction in meiosis I (b) Nondisjunction in meiosis II


• The result of nondisjunction

Figure 8.22

Eggcell

n (normal)

Zygote2n + 1

n + 1

Spermcell

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• Nondisjunction

Abnormal Numbers of Sex Chromosomes

– Also affects the sex chromosomes

Table 8.1


• Two kinds of sex chromosome abnormalities

Figure 8.23

Breastdevelopment

Constrictionof aorta

(b) A woman with Turnersyndrome (XO)

Poor breast development

Poor beardgrowth

Web ofskin

Under-developedtestes

(a) A man with Klinefeltersyndrome (XXY)

Under-developedovaries


• Errors in meiosis may have been instrumental in the evolution of many species

NEW SPECIES FROM ERRORS IN CELL DIVISION

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• Polyploids

– Are new species

– Have more than two sets of homologous chromosomes in each somatic cell.

Figure 8.24


• The Cell Cycle.

SUMMARY OF KEY CONCEPTS

Visual Summary 8.1

S phaseDNA synthesis; chromosome duplication

G2

Mitosis(division ofnucleus)

Mitoticphase (M)

Interphasecell growth and

chromosome duplication

G1

Geneticallyidentical“daughter”cells

Cytokinesis(division ofcytoplasm)


• Gametes and the Life Cycle of a Sexual Organism.

Visual Summary 8.2

Haploid gametes (n = 23)

n

n

Male and femaleAdults (2 n = 46)

Diploidzygote

(2n = 46)2n

Mitosis and development

Egg cell

Sperm cell

Meiosis Fertilization

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• Review: Comparing Mitosis and Meiosis.

Visual Summary

8.3

Mitosis Meiosis

Parentcell (2 n)

Crossingover

2n

Daughtercells

2n

n n n

Parentcell (2 n)


Chromosomeduplication Tetrad

n

Meiosis II

Daughter cells

Meiosis I

Documents

The Cellular Basis of Reproduction and Inheritancepbaluch/courses/bio100/cell_division.pdfTight helical fiber Supercoil Sister chromatids ... Copyright © 2004 Pearson Education, Inc