Prokaryotes divide by binary fission (not sexual), not mitosis or meiosis. (see Table 4.1)
1 How Cells Divide – Mitosis and Meiosis Chapters 11&12.
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Transcript of 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