1 How Cells Divide – Mitosis and Meiosis Chapters 11&12.

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How Cells Divide – Mitosis and Meiosis

Chapters 11&12

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Cell Division in Prokaryotes

• Prokaryotic cell division occurs as binary fission in which cell divides into two halves.

– Genetic information exists as a single, circular double-stranded DNA molecule.

Copying begins at replication origin, and proceeds bi-directionally.

One genome ends up in each daughter cell.

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Binary Fission

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Discovery of Chromosomes

• All eukaryotic cells store genetic information in chromosomes.

– Most eukaryotes have between 10 and 50 chromosomes in their body cells.

Human cells have 46 chromosomes.23 nearly-identical pairs

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Structure of Chromosomes

• Chromosomes are composed of a complex of DNA and protein, chromatin.

– heterochromatin - not expressed– euchromatin - expressed

• DNA exists as a single, long, double-stranded fiber extending chromosome’s entire length.

– forms nucleosome every 200 nucleotides DNA coiled around histone proteins

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Eukaryotic Chromosomal Organization

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Structure of Chromosomes

• Karyotype - Individual’s particular array of chromosomes.

– diploid - A cell possessing two copies of each chromosome (human body cells).

Homologous chromosomes are made up of sister chromatids joined at the centromere.

– haploid - A cell possessing a single copy of each chromosome (human sex cells).

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Karyotype & Chromosomes

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Phases of the Cell Cycle

• Five phases of cell division:– G1 - primary growth phase– S - genome replicated– G2 - secondary growth phase

collectively called interphase– M - mitosis– C - cytokinesis

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Cell Cycle

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Interphase

• G1 - cells undergo majority of growth• S - each chromosome replicates to produce

sister chromatids– attached at centromere

contains attachment site (kinetochore)• G2 - chromosomes condense

– assemble machinery such as centrioles

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Mitosis

• Prophase– spindle apparatus assembled

Microtubules connect kinetochores on each pair of sister chromatids to the spindle poles.

– nuclear envelope breaks

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Mitosis

• Metaphase– chromosomes align in cell’s center

metaphase platespindle

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Mitosis

• Anaphase– sister chromatids pulled toward poles

poles move apart centromeres move toward poles

microtubules shorten• Telophase

– spindle disassembles– nuclear envelope forms around each set

of sister chromatids

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Cytokinesis

• Cleavage of cell into two halves– animal cells

constriction belt of actin filaments– plant cells

cell plate– fungi and protists

mitosis occurs within the nucleus

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Cytokinesis

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Mitosis / Cytokinesis

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Cell Cycle Control

• Two irreversible points in cell cycle:– replication of genetic material– separation of sister chromatids

• Cell can be put on hold at specific checkpoints.

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Cell Control Cycle

• G1 / S - primary division decision point

• G2 / M - commitment to mitosis• Spindle checkpoint - all chromosomes are

attached to spindle

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Cell Cycle Control

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Growth Factors and the Cell Cycle

• Each growing cell binds minute amounts of positive regulatory signals (growth factors) that stimulate cell division.

– If neighboring cells use up too much growth factor, there is not enough left to trigger cell division.

Growth factors trigger intercellular signaling systems.

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Cell Proliferation

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Sexual Reproduction and Meiosis

Chapter 12

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Reduction Division

• In sexual reproduction, gametes fuse (fertilization) to produce a zygote.

– Gamete formation involves a mechanism (meiosis) that reduces the number of chromosomes to half that found in other cells.

Adult body cells are diploid. Gamete cells are haploid.

alternation of generations

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Sexual Life Cycle

• Diploid cells carry chromosomes from two parents

– 2 haploid cells join to form diploid cell

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Sexual Life Cycle

• Three types of sexual life cycles.– In sexual reproduction, haploid cells or

organisms alternate with diploid cells or organisms

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Sexual Life Cycle

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Meiosis

• Synapsis – Homologues pair along their length.

• Homologous recombination – Genetic exchange (crossing over) occurs

between homologous chromosomes.• Reduction division

– Meiosis involves two successive divisions, with no replication of genetic material between them.

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Unique Features of Meiosis

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Prophase I

• Homologous chromosomes become closely associated in synapsis, exchange segments via crossing over, and then separate.

– Presence of a chiasma indicates crossing over has occurred.

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Metaphase I

• Terminal chiasmata holds homologous pair together.

– Spindle microtubules attach to kinetochore proteins on the outside of each centromere.

• Joined pairs of homologues lines up on metaphase plate.

– orientation of each pair is random

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Completing Meiosis

• Anaphase I– Spindle fibers begin to shorten and pull

whole centromeres toward poles. Each pole receives a member of each

homologous pair. complete set of haploid

chromosomes random orientation results in

independent assortment

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Completing Meiosis

• Telophase I– Chromosomes are segregated into two

clusters; one at each pole. Nuclear membrane re-forms around each

daughter cell.Sister chromatids are no longer

identical due to crossing over.

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Second Meiotic Division

• Meiosis II resembles normal mitotic division.– prophase II - nuclear envelope breaks

down and second meiotic division begins– metaphase II - spindle fibers bind to both

sides of centromere– anaphase II - spindle fibers contract and

sister chromatids move to opposite poles– telophase II - nuclear envelope re-forms

• Final result - four haploid cells

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Sex

• Asexual reproduction - individual inherits all its chromosomes from a single parent

– parthenogenesis - development of an adult from an unfertilized egg

• Sexual reproduction - produces genetic variability.

– Segregation of chromosomes tends to disrupt advantageous combinations.

Only some progeny maintain advantages.

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Origin and Maintenance Of Sex

• Theories– DNA repair hypothesis

Only diploid cells can effectively repair certain kinds of chromosomal damage.

– Contagion hypothesis A secondary consequence of the

infection of eukaryotes by mobile genetic elements.

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Origin and Maintenance Of Sex

• Red Queen hypothesis– Current recessive alleles can be stored in

reserve for future use.• Miller’s Ratchet

– Sexual reproduction may be a method of keeping the mutational load low.

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Evolutionary Consequences of Sex

• Evolutionary process is revolutionary and conservative.

– pace of evolutionary change is accelerated by genetic recombination

– evolutionary change not always favored by selection

may act to preserve existing gene combinations

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Independent Assortment