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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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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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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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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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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
• 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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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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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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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