CHAPTER 8 The Cellular Basis of REPRODUCTION. CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION Copyright © 2009 Pearson Education, Inc

CHAPTER 8

The Cellular Basis ofREPRODUCTION

CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION

Copyright © 2009 Pearson Education, Inc.

Like begets like, more or less

– Living organisms reproduce by two methods– Asexual reproduction

– Offspring are identical to the original cell or organism– Involves inheritance of all genes from one parent– Involves MITOSIS (Eukaryotic Organisms); BINARY FISSION

(Prokaryotic Organisms)– CLONING is an asexual process

– Sexual reproduction– Offspring are similar to parents, but show variations in traits– Involves inheritance of unique sets of genes from two parents– Involves MEIOSIS


Cells arise only from preexisting cells

– Cell division perpetuates life– Cell division is the reproduction of cells – Cells are composed of : Carbohydrates, Lipids, Proteins,

Nucleic Acids, so cell division requires the building of these molecules from their monomers

– Is cell division an ENDERGONIC or EXERGONIC process? What does it require?


– Roles of cell division– Asexual reproduction

– Reproduction of an entire single-celled organism (MITOSIS or BINARY FISSION)

– Growth of a multicellular organism (MITOSIS)– Growth from a fertilized egg into an adult (MITOSIS)– Repair and replacement of cells in an adult (MITOSIS)

– Sexual reproduction – Sperm and egg production (MEIOSIS)

Cells arise only from preexisting cells


– Binary fission means “dividing in half”– Occurs in prokaryotic cells– Two identical cells arise from one cell– Steps in the process

– A single circular chromosome duplicates, and the copies begin to separate from each other

– The cell elongates, and the chromosomal copies separate further

– The plasma membrane grows inward at the midpoint to divide the cells

Prokaryotes reproduce by binary fission


Prokaryoticchromosome

Duplication of chromosomeand separation of copies

Cell wall

Plasmamembrane

1



Cell wall

Plasmamembrane

1

Continued elongation of thecell and movement of copies

2



Cell wall

Plasmamembrane

1

Continued elongation of thecell and movement of copies

2

Division intotwo daughter cells

3

Prokaryotic chromosomes

THE EUKARYOTIC CELL CYCLE AND MITOSIS


– Eukaryotic chromosomes are composed of chromatin– Chromatin = DNA + proteins– To prepare for division, the chromatin becomes highly

compact, and the chromosomes are visible with a microscope

– Early in the division process, chromosomes duplicate– Each chromosome appears as two sister chromatids,

containing identical DNA molecules– Sister chromatids are joined at the centromere, a narrow

region

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


Centromere

Chromosomeduplication

Sister chromatids

Chromosomedistribution

todaughter

cells

Sister chromatids

Centromere

– The cell cycle is an ordered sequence of events for cell division

– It consists of two stages– Interphase: duplication of cell contents

– G1—growth, increase in cytoplasm– S—duplication of chromosomes (DNA REPLICATION)– G2—growth, preparation for division

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

The Cell Cycle


S(DNA synthesis)G1

G2

Cytokinesis

Mito

sis

INTERPHASE

MITOTICPHASE (M)

– Mitosis progresses through a series of stages– Prophase (Prometaphase)– Metaphase– Anaphase– Telophase

– Cytokinesis overlaps telophase

Cell division is a continuum of dynamic changes


– Before Mitosis begins:– Interphase

– In the cytoplasm– Cytoplasmic contents double– New organelles are formed

– In the nucleus– Chromosomes (DNA) duplicate during

the S phase



– Prophase and Metaprophase– In the cytoplasm

– Microtubules begin to emerge forming the spindle

– In the nucleus– Chromosomes coil and become compact– Nuclear Membrane disappears



Centrosomes(with centriole pairs) Kinetochore

Early mitoticspindle

Chromatin

INTERPHASE PROMETAPHASEPROPHASE

Centrosome Fragmentsof nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Nuclearenvelope

Spindlemicrotubules

Nucleolus

Centromere

– Metaphase– Chromosomes align at the cell

equator (middle of the cell)



– Anaphase – Sister chromatids separate at the

centromeres and move to opposite poles of the cell



– Telophase– Opposite of PROPHASE– The nuclear membrane forms– Chromatin uncoils – The spindle disappears– CYTOKINESIS occurs



Metaphaseplate

Nucleolusforming

METAPHASE TELOPHASE AND CYTOKINESISANAPHASE

Cleavagefurrow

Daughterchromosomes

NuclearenvelopeformingSpindle

– Cytokinesis– Cleavage in animal cells

– A cleavage furrow forms from a contracting ring of microfilaments, interacting with myosin

– The cleavage furrow deepens to separate the contents into two cells

– Cytokinesis in plant cells– A cell plate forms in the middle from vesicles containing cell

wall material– The cell plate grows outward to reach the edges, dividing the

contents into two cells– Each cell has a plasma membrane and cell wall

Cytokinesis differs for plant and animal cells


Cleavagefurrow

Contracting ring ofmicrofilaments

Daughter cells

Cleavage furrow

Cell plate Daughter cells

Cell wall

Vesicles containingcell wall material

Daughter nucleus

Cell plateforming

Wall ofparent cell

New cell wall

– Applying Your KnowledgeHuman cells have 46 chromosomes (the DIPLOID number or 2 sets)

– At the end of Mitosis, how many chromosomes are in each cell?

– Is the genetic material identical in each cell?



CLONING

• A somatic cell from one parent is used• The nucleus of a somatic cell in put into an egg

cell (ovum) that has had its nucleus removed• The ovum with the somatic cell nucleus

behaves like a Zygote• A new eukaryotic organism is produced with

the DNA of only one parent (it is a clone of the parent)

Removenucleusfrom eggcell

Implant blastocyst insurrogate mother

Add somatic cellfrom adult donor

Donorcell

Remove embryonicstem cells fromblastocyst andgrow in culture

Reproductivecloning

Nucleus fromdonor cell

Grow in cultureto produce anearly embryo(blastocyst)

Therapeuticcloning

Clone ofdonor is born

Induce stemcells to formspecialized cells

– Cancer cells escape controls on the cell cycle– Cancer cells divide rapidly– They spread to other tissues through the

circulatory system– Growth is not inhibited by other cells, and tumors

form– Benign tumors remain at the original site– Malignant tumors spread to other locations

by metastasis

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


• Mitosis produces genetically identical cells for – Growth– Replacement– Asexual reproduction

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


MEIOSIS AND CROSSING OVER


– Meiosis is a process that converts diploid cells into haploid cells

– Diploid cells have two homologous sets (2n) of chromosomes

– Haploid cells have one set (1n) of chromosomes– Meiosis occurs in the sex organs, producing gametes—

sperm and eggs

– Fertilization is the union of sperm and egg– The zygote formed by fertilization has a diploid

chromosome number (2n), one set from each parent

Gametes have a single set of chromosomes


– Somatic cells have pairs of homologous chromosomes, receiving one member of each pair from each parent

– Homologous chromosomes are matched in– Length– 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

Chromosomes are matched in homologous pairs


Sister chromatids One duplicatedchromosome

Centromere

Homologous pair ofChromosomes: One from Mother; One from Father

– Which characteristics are similar for mitosis and meiosis?

– One duplication of chromosomes– Which characteristics are unique to meiosis?

– Two divisions of the cells (stages I and II): 4 new cells formed instead of 2

– Pairing of homologous chromosomes during PROPHASE I and exchange of genetic material by CROSSING OVER

– Homologous pairs of chromosomes line up a the cell equator during METAPHASE I

– Cells formed are NOT GENETICALLY IDENTICAL

Mitosis and meiosis have important similarities and differences


Centrosomes(with centriolepairs)

PROPHASE I

Microtubulesattached tokinetochore

INTERPHASE

Sites of crossing over Metaphaseplate

Spindle

MEIOSIS I: Homologous chromosomes separate

METAPHASE I

Sister chromatidsremain attached

ANAPHASE I

Nuclearenvelope

Sisterchromatids

Centromere(with kinetochore)

Homologouschromosomes separateChromatin

Tetrad

Separation of homologous chromosomes at anaphase I

C E

c e

Chiasma

Separation of chromatids at anaphase II andcompletion of meiosis

C E

c e

c E

C e

c e

c E

C E

C e

Parental type of chromosome

Gametes of four genetic types

Recombinant chromosome

Parental type of chromosome

Recombinant chromosome

4

3

PROPHASE I

MEIOSIS II: Sister chromatids separate

METAPHASE II ANAPHASE II

Cleavagefurrow

TELOPHASE IIAND CYTOKINESIS

Sister chromatidsseparate

Haploid daughtercells forming

TELOPHASE IIAND CYTOKINESIS

– What is the outcome of each process?– Mitosis: two genetically identical cells,

with the same chromosome number as the original cell

– Meiosis: four genetically different cells, with half the chromosome number of the original cell

Mitosis and meiosis have important similarities and differences


Prophase

Metaphase IMetaphase

2n = 4

Tetradsalign at themetaphase plate

Duplicatedchromosome(two sisterchromatids)

Parent cell(before chromosome duplication)


Chromosomesalign at themetaphase plate

AnaphaseTelophase Sister chromatids

separate duringanaphase

Daughter cellsof mitosis

2n 2n

n


Site ofcrossing over

Tetrad formedby synapsis ofhomologouschromosomes

MEIOSIS

Prophase I

Anaphase ITelophase I

MITOSIS

MEIOSIS I

Haploidn = 2

Daughtercells of

meiosis I

MEIOSIS II

n n n

Daughter cells of meiosis II

Homologouschromosomesseparate(anaphase I);sister chroma-tids remaintogether

No furtherchromosomalduplication;sisterchromatidsseparate(anaphase II)

Independent orientation of chromosomes and crossing over 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 (How many combinations for human?)

Crossing over in Prophase I (How many combinations in humans?)

Random Fertilization– The combination of each unique sperm with each unique

egg increases genetic variability

Two equally probablearrangements ofchromosomes at

metaphase I

Possibility 1 Possibility 2

Metaphase II

Combination 1

Gametes

Combination 2 Combination 3 Combination 4

– Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis

– During Meiosis I – Both members of a homologous pair go to one pole

– During Meiosis II– Both sister chromatids go to one pole

– Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes

Accidents during meiosis can alter chromosome number


Centromere

Sisterchromatids

Pair of homologouschromosomes

5

Nondisjunctionin meiosis I

Normalmeiosis II

n + 1

Gametes

Number of chromosomes

n + 1 n – 1 n – 1

Nondisjunctionin meiosis II

Normalmeiosis I

Gametes

Number of chromosomes

n + 1 n – 1 n n

– Trisomy 21 involves the inheritance of three copies of chromosome 21

– Trisomy 21 is the most common human chromosome abnormality

– An imbalance in chromosome number causes Down syndrome, which is characterized by

– Characteristic facial features– Susceptibility to disease– Shortened life span– Mental retardation– Variation in characteristics

– The incidence increases with the age of the mother

An extra copy of chromosome 21 causes Down syndrome


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CHAPTER 8 The Cellular Basis of REPRODUCTION. CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION Copyright © 2009 Pearson Education, Inc