Chapter 12 The Cell Cycle

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Fig. 12-UN1 Telophase and Cytokinesis Anaphase Metaphase Prometaphase Prophase MITOTIC (M) PHASE Cytokinesis Mitosis S G1G1 G2G2

Fig EXPERIMENT Kinetochore RESULTS CONCLUSION Spindle pole Mark Chromosome movement Kinetochore Microtubule Motor protein Chromosome Tubulin subunits

Binary Fission Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart

Fig Origin of replication Two copies of origin E. coli cell Bacterial chromosome Plasma membrane Cell wall Origin

The Evolution of Mitosis Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis

Fig (a) Bacteria Bacterial chromosome Chromosomes Microtubules Intact nuclear envelope (b) Dinoflagellates Kinetochore microtubule Intact nuclear envelope (c) Diatoms and yeasts Kinetochore microtubule Fragments of nuclear envelope (d) Most eukaryotes

Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control system The frequency of cell division varies with the type of cell These cell cycle differences result from regulation at the molecular level

Evidence for Cytoplasmic Signals The cell cycle appears to be driven by specific chemical signals present in the cytoplasm Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei

Experiment 1 Experiment 2 EXPERIMENT RESULTS SG1G1 M G1G1 M M S S When a cell in the S phase was fused with a cell in G 1, the G 1 nucleus immediately entered the S phase—DNA was synthesized. When a cell in the M phase was fused with a cell in G 1, the G 1 nucleus immediately began mitosis—a spindle formed and chromatin condensed, even though the chromosome had not been duplicated.

12 Control of the Cell Cycle Current view integrates 2 concepts 1.Cell cycle has two irreversible points –Replication of genetic material –Separation of the sister chromatids 2.Cell cycle can be put on hold at specific points called checkpoints –Process is checked for accuracy and can be halted if there are errors –Allows cell to respond to internal and external signals

3 Checkpoints 1.G 1 /S checkpoint –Cell “decides” to divide –Primary point for external signal influence 2.G 2 /M checkpoint –Cell makes a commitment to mitosis –Assesses success of DNA replication 3.Late metaphase (spindle) checkpoint –Cell ensures that all chromosomes are attached to the spindle 13

Fig S G1G1 M checkpoint G2G2 M Control system G 1 checkpoint G 2 checkpoint

Fig G1G1 G0G0 G 1 checkpoint (a)Cell receives a go-ahead signal G1G1 (b) Cell does not receive a go-ahead signal

The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) The activity of cyclins and Cdks fluctuates during the cell cycle MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G 2 checkpoint into the M phase

Fig Protein kinase activity (– ) % of dividing cells (– ) Time (min) RESULTS Cdc2: Schizosaccaromycetes pombe, a fission yeast

Fig M G1G1 S G2G2 M G1G1 SG2G2 M G1G1 MPF activity Cyclin concentration Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle Degraded cyclin Cdk G1G1 S G2G2 M G2G2 checkpoint Cyclin is degraded Cyclin MPF (b) Molecular mechanisms that help regulate the cell cycle Cyclin accumulation

Fig b Cyclin is degraded Cdk MPF Cdk M S G1G1 G 2 checkpoint Degraded cyclin Cyclin (b) Molecular mechanisms that help regulate the cell cycle G2G2 Cyclin accumulation

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Cdk – cyclin complex –Also called mitosis-promoting factor (MPF) Activity of Cdk is also controlled by the pattern of phosphorylation –Phosphorylation at one site (red) inactivates Cdk –Phosphorylation at another site (green) activates Cdk 21

22 Anaphase-promoting complex (APC) Also called cyclosome (APC/C) At the spindle checkpoint, presence of all chromosomes at the metaphase plate and the tension on the microtubules between opposite poles are both important Function of the APC/C is to trigger anaphase itself Marks securin for destruction; no inhibition of separase; separase destroys cohesin

Act by triggering intracellular signaling systems Platelet-derived growth factor (PDGF) one of the first growth factors to be identified PDGF receptor is a receptor tyrosine kinase (RTK) that initiates a MAP kinase cascade to stimulate cell division Growth factors can override cellular controls that otherwise inhibit cell division 23 Stop and Go Signs: Growth factors

Fig a Receptor Growth factor G protein GTP Ras GTP Ras Protein kinases (phosphorylation cascade) Transcription factor (activator) DNA Hyperactive Ras protein (product of oncogene) issues signals on its own MUTATION NUCLEUS Gene expression Protein that stimulates the cell cycle (a) Cell cycle–stimulating pathway

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Fig b MUTATION Protein kinases DNA DNA damage in genome Defective or missing transcription factor, such as p53, cannot activate transcription Protein that inhibits the cell cycle Active form of p53 UV light (b) Cell cycle–inhibiting pathway 2 3 1

27 Cancer Unrestrained, uncontrolled growth of cells Failure of cell cycle control Two kinds of genes can disturb the cell cycle when they are mutated 1.Tumor-suppressor genes 2.Proto-oncogenes

28 Tumor-suppressor genes p53 plays a key role in G 1 checkpoint p53 protein monitors integrity of DNA –If DNA damaged, cell division halted and repair enzymes stimulated –If DNA damage is irreparable, p53 directs cell to kill itself Prevent the development of mutated cells containing mutations p53 is absent or damaged in many cancerous cells

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First tumor-suppressor identified was the retinoblastoma susceptibility gene (Rb) –Predisposes individuals for a rare form of cancer that affects the retina of the eye Inheriting a single mutant copy of Rb means the individual has only one “good” copy left –During the hundreds of thousands of divisions that occur to produce the retina, any error that damages the remaining good copy leads to a cancerous cell –Single cancerous cell in the retina then leads to the formation of a retinoblastoma tumor Rb protein integrates signals from growth factors –Role to bind important regulatory proteins and prevent stimulation of cyclin or Cdk production 30 Tumor-suppressor genes

31 Proto-oncogenes Normal cellular genes that become oncogenes when mutated –Oncogenes can cause cancer Some encode receptors for growth factors –If receptor is mutated in “on”, cell no longer depends on growth factors Some encode signal transduction proteins Only one copy of a proto-oncogene needs to undergo this mutation for uncontrolled division to take place

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Fig Petri plate Scalpels Cultured fibroblasts Without PDGF cells fail to divide With PDGF cells prolifer- ate 10 µm

Fig Anchorage dependence Density-dependent inhibition (a) Normal mammalian cells (b) Cancer cells 25 µm Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence

Fig Tumor A tumor grows from a single cancer cell. Glandular tissue Lymph vessel Blood vessel Metastatic tumor Cancer cell Cancer cells invade neigh- boring tissue. Cancer cells spread to other parts of the body. Cancer cells may survive and establish a new tumor in another part of the body transformation benign tumor metastasize Malignant tumors