The Mito,c Spindle: A Closer Look. The Mito,c Spindle. Aster. a radial array of short microtubules. extends from each centrosome

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1 The Mito,c Spindle: A Closer Look Aster a radial array of short microtubules extends from each centrosome anchors centrosome to rest of cytoskeleton The spindle includes the centrosomes, the spindle microtubules, and the asters The Mito,c Spindle A:achment point for microtubules of spindle Protein structure bound to DNA at centromere Fig Aster Centrosome Sister chromatids Microtubules Chromosomes Metaphase plate s Centrosome 1 µm Overlapping nonkinetochore microtubules microtubules 0.5 µm 1

2 The Mito,c Spindle In anaphase sister chromafds separate and move along the kinetochore microtubules toward opposite ends of the cell Spindle microtubules shorten by depolymerizing at their kinetochore ends Fig EXPERIMENT Spindle pole Mark RESULTS CONCLUSION Microtubule Chromosome movement Motor protein Chromosome Tubulin subunits The Mito,c Spindle Nonkinetochore microtubules from opposite poles overlap and push against each other elongafng the cell In telophase genefcally idenfcal daughter nuclei form at opposite ends of the cell 2

3 Cytokinesis: A Closer Look Animal cells cytokinesis occurs by cleavage, forming a cleavage furrow Plant cells a cell plate forms during cytokinesis Cleavage furrow 100 µm Vesicles forming cell plate Wall of parent cell Cell plate 1 µm New cell wall Contractile ring of microfilaments Daughter cells Daughter cells (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM) PLAY Animal Mitosis PLAY Sea Urchin (Time Lapse) Binary Fission Prokaryotes (Eubacteria and archaebacteria) Reproduce by a type of cell division called binary fission Single replicates (beginning at the origin of replica,on) two daughter s acfvely move apart 3

4 Fig of replication E. coli cell Two copies of origin Cell wall Plasma membrane Fig of replication E. coli cell Two copies of origin Cell wall Plasma membrane Fig of replication E. coli cell Two copies of origin Cell wall Plasma membrane 4

5 Fig of replication E. coli cell Two copies of origin Cell wall Plasma membrane The Evolu,on of Mitosis Binary fission predates mitosis Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission Some profsts are intermediate between binary fission and mitosis Dinoflagellates Diatoms/Yeasts (a) Bacteria Chromosomes microtubule Microtubules (b) Dinoflagellates Intact nuclear envelope Intact nuclear envelope (c) Diatoms and yeasts microtubule (d) Most eukaryotes Fragments of nuclear envelope Regula,on of the Cell Cycle The frequency of cell division varies with the type of cell Muscle cells Neurons Hepatocytes These cell cycle differences result from regulafon at the molecular level Signaling molecules 5

6 The Cell Cycle Control System Cell cycle control system Regulated by both internal and external controls Has specific checkpoints where the cell cycle stops unfl a go- ahead signal is received G 1 checkpoint G 1 Control system S M G 2 M checkpoint G 2 checkpoint G 1 checkpoint Appears most important cell receives a go- ahead signal at the G 1 checkpoint, it will usually complete the S, G 2, and M phases and divide No go- ahead signal The Cell Cycle Control System Cells exits cycle, switching into G 0 phase - a non- dividing state G 0 G 1 checkpoint G 1 G 1 (a) Cell receives a go-ahead signal (b) Cell does not receive a go-ahead signal Cyclins & Cyclin- Dependent Kinases Two types of regulatory proteins involved in cell cycle control Cyclins Cyclin- dependent kinases (Cdks) AcFvity fluctuates during the cell cycle MPF (maturafon- promofng factor) Cyclin- Cdk complex that triggers a cell s passage past the G 2 checkpoint into the M phase 6

7 Stop/Go Signs: Signals at the Checkpoints Internal signal M checkpoint kinetochores not a:ached to spindle microtubules send a molecular signal that delays anaphase External signals Growth factors proteins released by certain cells that sfmulate other cells to divide platelet- derived growth factor (PDGF) sfmulates the division of human fibroblast cells in culture Stop/Go Signs: Signals at the Checkpoints Other external signals density- dependent inhibi,on crowded cells stop dividing anchorage dependence Most animal cells must be a:ached to a substratum in order to divide Anchorage dependence Density-dependent inhibition Density-dependent inhibition 25 µm (a) Normal mammalian cells (b) Cancer cells 25 µm Transforma,on A normal cell is converted to a cancerous cell which forms tumors Benign tumor Loss of Cell Cycle Controls in Cancer Cells masses of abnormal cells within otherwise normal Fssue Abnormal cells remain at the original site Malignant tumors Invade surrounding Fssues Can metastasize exporfng cancer cells to other parts of the body, possibly form secondary tumors Tumor Lymph vessel Blood vessel Glandular tissue Cancer cell Metastatic tumor 1 A tumor grows 2 Cancer cells 3 Cancer cells spread 4 from a single invade neigh- to other parts of cancer cell. boring tissue. the body. Cancer cells may survive and establish a new tumor in another part of the body. 7

8 You should now be able to: 1. Describe the structural organizafon of the prokaryofc genome and the eukaryofc genome 2. List the phases of the cell cycle; describe the sequence of events during each phase 3. List the phases of mitosis and describe the events characterisfc of each phase 4. Draw or describe the mitofc spindle, including centrosomes, kinetochore microtubules, nonkinetochore microtubules, and asters 5. Compare cytokinesis in animals and plants 6. Describe the process of binary fission in bacteria and explain how eukaryofc mitosis may have evolved from binary fission 7. Explain how the abnormal cell division of cancerous cells escapes normal cell cycle controls 8. DisFnguish between benign, malignant, and metastafc tumors 8