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How Cells Divide

Chapter 11

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Outline

• Cell Division in Prokaryotes• Discovery of Chromosomes• Structure of Chromosomes• Phases of the Cell Cycle• Interphase• Mitosis• Cytokinesis• Cell Cycle Control • Cancer

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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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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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Cancer and Cell Proliferation

• Cancer is essentially a failure of cell division control (unrestrained, uncontrolled cellular growth).

– gene p53 plays a role in G1 checkpoint halts cell division if detects damaged

DNA and stimulates activity of repair enzymes

appears to be absent in many cancerous cells

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Growth Factors and Cancer

• Growth factors influence cell cycle– proto-oncogenes - normal cellular genes

that become oncogenes when mutated positive approach that stimulates cell

growth– tumor-suppressor genes - negative

approach that inhibits cell division

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Summary

• Cell Division in Prokaryotes• Discovery of Chromosomes• Structure of Chromosomes• Phases of the Cell Cycle• Interphase• Mitosis• Cytokinesis• Cell Cycle Control • Cancer

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