Chapter 8 Cellular Basis Of Reproduction And Inheritance

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings

Chapter 8

Cellular Basis Of Reproduction

And Inheritance


Rain Forest Rescue

• Scientists in Hawaii

– Have attempted to “rescue” endangered species from extinction


• The goals of these scientists were

– To promote reproduction to produce more individuals of specific endangered plants

Cyanea kuhihewa

Kauai, Hawaii


• In sexual reproduction

– Fertilization of sperm and egg produces offspring

• In asexual reproduction

– Offspring are produced by a single parent, without the participation of sperm and egg


CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION

8.1 Like begets like, more or less

• Some organisms reproduce asexually

– And their offspring are genetic copies of the parent and of each other

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40

Figure 8.1A


• Other organisms reproduce sexually

– Creating a variety of offspring

Figure 8.1B


8.2 Cells arise only from preexisting cells

• Cell division is at the heart of the reproduction of cells and organisms

– Because cells come only from preexisting cells


8.3 Prokaryotes reproduce by binary fission

• Prokaryotic cells

– Reproduce asexually by cell division

Col

oriz

ed T

EM

32,

500

Prokaryotic chromosomes

Figure 8.3B


Prokaryoticchromosome

Plasmamembrane

Cell wall

Duplication of chromosomeand separation of copies

1

Continued elongation of thecell and movement of copies2

Division intotwo daughter cells

3

• As the cell replicates its single chromosome, the copies move apart

– And the growing membrane then divides the cells

Figure 8.3A


THE EUKARYOTIC CELL CYCLE AND MITOSIS

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

• A eukaryotic cell has many more genes than a prokaryotic cell

– And they are grouped into multiple chromosomes in the nucleus


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00

• Individual chromosomes contain a very long DNA molecule associated with proteins– And are visible only when the cell is in the

process of dividing• If a cell is not undergoing division

– Chromosomes occur in the form of thin, loosely packed chromatin fibers

Figure 8.4A


• Before a cell starts dividing, the chromosomes replicate

– Producing sister chromatids joined together at the centromere

TEM

36,

000

Centromere

Sister chromatids

Figure 8.4B


• Cell division involves the separation of sister chromatids

– And results in two daughter cells, each containing a complete and identical set of chromosomes

Centromere

Chromosomeduplication

Sisterchromatids

Chromosomedistribution

todaughter

cellsFigure 8.4C


8.5 The cell cycle multiplies cells

• The cell cycle consists of two major phasesINTERPHASE

S(DNA synthesis)

G1

G2

Cytokin

esis

Mitosis

MITOTICPHASE (M)

Figure 8.5


• During interphase

– Chromosomes duplicate and cell parts are made

• During the mitotic phase

– Duplicated chromosomes are evenly distributed into two daughter nuclei


8.6 Cell division is a continuum of dynamic changes

• In mitosis, after the chromosomes coil up

– A mitotic spindle moves them to the middle of the cell


• The sister chromatids then separate

– And move to opposite poles of the cell, where two nuclei form

• Cytokinesis, in which the cell divides in two

– Overlaps the end of mitosis


• The stages of cell division

INTERPHASE PROPHASE PROMETAPHASE

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50

Chromatin

Centrosomes(with centriole pairs)

Nucleolus

Nuclearenvelope

Plasmamembrane

Early mitoticspindle

Centrosome

CentromereChromosome, consistingot two sister chromatids

Spindlemicrotubules

Kinetochore

Fragmentsof nuclearenvelope

Figure 8.6 (Part 1)


METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate

Daughterchromosomes

Nuclearenvelopeforming

Cleavagefurrow

Nucleolusforming

Figure 8.6 (Part 2)


8.7 Cytokinesis differs for plant and animal cells

• In animals

– Cytokinesis occurs by a constriction of the cell (cleavage) Cleavage

furrow

SE

M 1

40

Daughter cells

Cleavage furrow Contracting ring ofmicrofilaments

Figure 8.7A


• In plants

– A membranous cell plate splits the cell in two

Figure 8.7B

TEM

7,5

00

Cell plateforming

Wall ofparent cell

Daughternucleus

Cell wall New cell wall

Vesicles containingcell wall material

Cell plate Daughter cells


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

• Most animal cells divide

– Only when stimulated, and some not at all


Cells anchor todish surfaceand divide.

When cells haveformed a completesingle layer, theystop dividing (density-dependent inhibition).

If some cells arescraped away, theremaining cells divideto fill the dish with asingle layer and thenstop (density-dependentinhibition).

• In laboratory cultures

– Most normal cells divide only when attached to a surface

• They continue dividing

– Until they touch one another

Figure 8.8A


• Growth factors

– Are proteins secreted by cells that stimulate other cells to divide

After forming asingle layer,cells havestopped dividing.

Providing anadditional supply ofgrowth factorsstimulatesfurther cell division.

Figure 8.8B


8.9 Growth factors signal the cell cycle control system

• A set of proteins within the cell

– Controls the cell cycle


• Signals affecting critical checkpoints in the cell cycle

– Determine whether a cell will go through the complete cycle and divide

Controlsystem

G1S

G2M

G1 checkpoint

G2 checkpoint

M checkpoint

G0

Figure 8.9A


• The binding of growth factors to specific receptors on the plasma membrane

– Is usually necessary for cell division.

ControlsystemG1 S

G2M

G1 checkpoint

Plasma membrane

Growth factor

Receptorprotein

Relayproteins

Signaltransductionpathway

Figure 8.9B


CONNECTION

8.10 Growing out of control, cancer cells produces malignant tumors

• Cancer cells

– divide excessively to form masses called tumors


• Malignant tumors

– Can invade other tissues

Tumor

Glandulartissue

A tumor grows from asingle cancer cell.

Cancer cells invadeneighboring tissue.

Cancer cells spread throughlymph and blood vessels toother parts of the body.

Lymphvessels

Bloodvessel

Figure 8.10


• Radiation and chemotherapy

– Are effective as cancer treatments because they interfere with cell division


8.11 Review of the functions of mitosis: Growth, cell replacement, and asexual reproduction

• When the cell cycle operates normally, mitotic cell division functions in

– Growth

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Figure 8.11A


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• Replacement of damaged or lost cells

Figure 8.11B


• Asexual reproduction

LM 1

0Figure 8.11C


MEIOSIS AND CROSSING OVER

8.12 Chromosomes are matched in homologous pairs

• The somatic (body) cells of each species

– Contain a specific number of chromosomes

• For example human cells have 46

– Making up 23 pairs of homologous chromosomes


• The chromosomes of a homologous pair

– Carry genes for the same characteristics at the same place, or locus

Chromosomes

Centromere

Sister chromatidsFigure 8.12


8.13 Gametes have a single set of chromosomes

• Cells with two sets of chromosomes

– Are said to be diploid

• Gametes, eggs and sperm, are haploid

– With a single set of chromosomes


• Sexual life cycles

– Involve the alternation ofhaploid and diploid stages

Mitosis and development

Multicellulardiploid adults

(2n = 46)

Diploidzygote

(2n = 46) 2n

Meiosis Fertilization

Egg cell

Sperm cell

n

Haploid gametes (n = 23)

n

Figure 8.13


8.14 Meiosis reduces the chromosome number from diploid to haploid

• Meiosis, like mitosis

– Is preceded by chromosome duplication

• But in meiosis

– The cell divides twice to form four daughter cells


• The first division, meiosis I

– Starts with synapsis, the pairing of homologous chromosomes

• In crossing over

– Homologous chromosomes exchange corresponding segments

• Meiosis I separates each homologous pair

– And produce two daughter cells, each with one set of chromosomes


• Meiosis II is essentially the same as mitosis

– The sister chromatids of each chromosome separate

– The result is a total of four haploid cells


• The stages of meiosis

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Centrosomes (with centriole pairs)

Sites of crossing over

Spindle

Microtubulesattached to kinetochore

Metaphaseplate

Sister chromatids remain attached

Nuclearenvelope Chromatin

Sisterchromatids Tetrad

Centromere(with kinetochore)

Homologouschromosomes separate

Figure 8.14 (Part 1)


PROPHASE II METAPHASE II ANAPHASE II

TELOPHASE IAND CYTOKINESIS

TELOPHASE IIAND CYTOKINESIS

Cleavagefurrow

Haploid daughter cellsforming

Sister chromatidsseparate

MEIOSIS II: Sister chromatids separate

Figure 8.14 (Part 2)


Mitosis Meiosis

Parent cell(before chromosome replication)

Chromosome replication

Chromosome replication

Chromosomes align at themetaphase plate

Tetradsalign at themetaphase plate

Sister chromatidsseparate during anaphase

Homologous chromosomesseparate duringanaphase I;sister chromatidsremain together

No furtherchromosomalreplication; sisterchromatidsseparateduringanaphase II

Prophase

Metaphase

AnaphaseTelophase

Duplicated chromosome(two sister chromatids)

Daughter cellsof mitosis

2n 2n

Daughtercells of

meiosis I

n n nn

2n = 4

Tetrad formedby synapsis ofhomologouschromosomes

Meiosis i

Meiosis ii

Prophase I

Metaphase I

Anaphase ITelophase I

Haploidn = 2

Daughter cells of meiosis II

8.15 Review: A comparison of mitosis and meiosis

Figure 8.15


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

• Each chromosome of a homologous pair

– Differs at many points from the other member of the pair


• Random arrangements of chromosome pairs at metaphase I of meiosis

– Lead to many different combinations of chromosomes in eggs and sperm

Combination 1 Combination 2 Combination 3 Combination 4

Gametes

Metaphase II

Two equally probablearrangements of chromosomes at

metaphase I

Possibility 1 Possibility 2

Figure 8.16


• Random fertilization of eggs by sperm

– Greatly increases this variation


8.17 Homologous chromosomes carry different versions of genes• The differences between homologous chromosomes

– Are based on the fact that they can bear different versions of a gene at corresponding loci

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

ec

EC

White Pink ec

ec

EC

EC

Meiosis

BlackBrown

Chromosomes of the four gametes

Eye-colorgenes

Coat-colorgenes

Brown coat (C); black eyes (E) White coat (C); pink eyes (e)

Figure 8.17AFigure 8.17B


8.18 Crossing over further increases genetic variability

• Genetic recombination

– Which results from crossing over during prophase I of meiosis, increases variation still further

Figure 8.18A

ChiasmaTetrad

Centromere

TEM

2,2

00


Coat-colorgenes

Eye-colorgenes

Tetrad (homologous pair of chromosomes

in synapsis)

C E

c e

C E

c e

C E

c e

Chiasma

C E

C e

c E

c e

C E

C e

c E

c e

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

Gametes of four genetic types

• How crossing over leads to genetic variation Breakage of homologous chromatids1

Joining of homologous chromatids2

3Separation of homologous chromosomes at anaphase I

4Separation of chromatids at anaphase II and completion of meiosis

Figure 8.18B


ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE

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

• A karyotype

– Is an ordered arrangement of a cell’s chromosomes


Blood culture

Fluid

Centrifuge

Packed red andwhite blood cells

Hypotonicsolution

Fixative

White blood cells

Stain

Centromere

Pair of homologouschromosomes

Sisterchromosomes

2,60

0X

A bloodculture is centrifuged to separate the blood cells from the culture fluid.

1 The fluid is discarded, and a hypotonic solution is mixed with the cells. This makes the red blood cells burst. The white blood cells swell but do not burst, and their chromosomes spread out.

2 Another centrifugation step separates the swollen whiteblood cells. The fluid containing the remnants of the red blood cells is poured off. A fixative (preservative) is mixedwith the white blood cells. A drop of the cell suspension is spread on a microscope slide, dried, and stained.

3

The slide is viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is entered into a computer, which electronically arranges them by size and shape.

4 The resulting display is the karyotype. The 46 chromosomes here include 22 pair of autosomes and 2 sex chromosomes, X and Y. Although difficult to discern in the karyotype, each of the chromosomes consists of two sister chromatids lying very close together (see diagram).

5

• Preparation of a karyotype from a blood sample

Figure 8.19


CONNECTION

8.20 An extra copy of chromosome 21 causes Down syndrome

• A person may have an abnormal number of chromosomes

– Which causes problems


• Down syndrome is caused by trisomy 21

– An extra copy of chromosome 21

5,00

0

Figure 8.20A Figure 8.20B


• The chance of having a Down syndrome child

– Goes up with maternal age

Age of mother45 50353025 4020

90

0

10

20

30

40

50

60

70

80

Infa

nts

with

Dow

n sy

ndro

me

(per

1,0

00 b

irths

)

Figure 8.20C


8.21 Accidents during meiosis can alter chromosome number• Abnormal chromosome count is a result of

nondisjunction– The failure of homologous pairs to separate

during meiosis I– The failure of sister chromatids to separate

during meiosis II


Nondisjunction in meiosis I

Normal meiosis II

Gametes

n 1 n 1 n 1 n 1

Number of chromosomes

Nondisjunction in meiosis II

Normal meiosis I

Gametes

n 1 n 1 n n

Number of chromosomes

Figure 8.21A

Figure 8.21B


• Fertilization after nondisjunction in the mother

Sperm cell

Egg cell

n (normal)

n + 1

Zygote2n + 1

Figure 8.21C


CONNECTION

8.22 Abnormal numbers of sex chromosomes do not usually affect survival

• Nondisjunction can also produce gametes with extra or missing sex chromosomes

– Leading to varying degrees of malfunction in humans but not usually affecting survival

Figure 8.22A

Poor beard growth

BreastDevelopment

Under-developedtestes

Figure 8.22B

Characteristic facialfeaturesWeb of skinConstrictionof aorta

Poor breastdevelopment

Under developedovaries


• Human sex chromosome abnormalities


CONNECTION

8.23 Alterations of chromosome structure can cause birth defects and cancer

• Chromosome breakage can lead to rearrangements

– That can produce genetic disorders or, if the changes occur in somatic cells, cancer


• Deletions, duplications, inversions, and translocations

Deletion

Duplication

Inversion

Homologouschromosomes

Reciprocaltranslocation

Nonhomologouschromosomes

“Philadelphia chromosome”

Chromosome 9

Chromosome 22Reciprocaltranslocation

Activated cancer-causing geneFigure 8.23A Figure 8.23C

Figure 8.23B

Chapter 8 Cellular Basis Of Reproduction And Inheritance

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