Cell Cycle and Mitosis 8.1 to 8.11

1. In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization Cell Cycle and Mitosis 8.1 to 8.11

2. All the DNA in a cell constitutes the cell’s genome • A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells) • DNA molecules in a cell are packaged into chromosomes Genetic information is stored and transmitted through DNA

3. Fig. 12-3 20 µm

4. Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus • Somatic cells (nonreproductive cells) have two sets of chromosomes - DIPLOID • Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells - HAPLOID • Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division Genetic Information is stored and transmitted through DNA

5. Fig. 12-4 0.5 µm Chromosomes DNA molecules Chromo- some arm Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Separation of sister chromatids Centromere Sister chromatids

6. The cell cycle is a complex set of stages that is highly regulated with checkpoints, which determine the ultimate fate of the cell

7. Majority of the cell cycle • Time when a cell’s metabolic activity is very high and the cell performs various functions • 3 stages • G1 – cell growth • S – DNA replication (Synthesis of DNA) • G2 – prepare for mitosis Interphase

8. About 10% of the cell cycle • 2 stages • Mitosis – nuclear division • Cytokinesis – cytoplasm division • Results in two genetically identical cells M Phase (mitotic phase)

9. The cell cycle is directed by internal controls or checkpoints.

10. For many cells, the G1 checkpoint seems to be the most important one • If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide • If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase Cell Cycle Checkpoints

11. Two types of regulatory proteins are involved in cell cycle control: cyclinsand 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 G2 checkpoint into the M phase Cell Cycle Checkpoints

12. Fig. 12-17 M M S G1 G1 M G1 S G2 G2 MPF activity Cyclin concentration Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle S G1 Cdk Cyclin accumulation M Degraded cyclin G2 G2 Cdk checkpoint Cyclin is degraded Cyclin MPF (b) Molecular mechanisms that help regulate the cell cycle

13. An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase • Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide • For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture Internal and External signals provide stop-and-go signs at the checkpoints

14. Fig. 12-18 Scalpels Petri plate Without PDGF cells fail to divide With PDGF cells prolifer- ate Cultured fibroblasts 10 µm

15. Fig. 12-19 Anchorage dependence Density-dependent inhibition Density-dependent inhibition 25 µm 25 µm (a) Normal mammalian cells (b) Cancer cells

16. Mitosis is conventionally divided into five phases: • Prophase • Prometaphase • Metaphase • Anaphase • Telophase • Cytokinesis is well underway by late telophase Mitosis passes a complete genome from the parent cell to the daughter cell

17. Fig. 12-6b G2 of Interphase Prophase Prometaphase Chromatin (duplicated) Centrosomes (with centriole pairs) Early mitotic spindle Fragments of nuclear envelope Centromere Aster Nonkinetochore microtubules Kinetochore Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule Nucleolus

18. Fig. 12-7 Aster Centrosome Sister chromatids Microtubules Chromosomes Metaphase plate Kineto- chores Centrosome 1 µm Overlapping nonkinetochore microtubules Kinetochore microtubules 0.5 µm

19. Fig. 12-6d Telophase and Cytokinesis Metaphase Anaphase Nucleolus forming Metaphase plate Cleavage furrow Daughter chromosomes Nuclear envelope forming Centrosome at one spindle pole Spindle

20. Fig. 12-8b Chromosome movement Kinetochore Tubulin Subunits Motor protein Microtubule Chromosome

21. Fig. 12-9 Vesicles forming cell plate Wall of parent cell 1 µm 100 µm Cleavage furrow Cell plate New cell wall Daughter cells Contractile ring of microfilaments Daughter cells (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM)

22. What must occur before mitosis? • DNA replication • Pass G2 checkpoint • What is “checked” at the ‘M’ checkpoint? • Chromosome attachment to microtubule • What follows mitosis? • Cytokinesis • What are the products of the mitosis? • 2 genetically identical cells • Why do we need mitosis? • Growth, repair, asexual reproduction Summary of Mitosis