Cell Cycle and Mitosis 8.1 to 8.11 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

Genetic information is stored and transmitted through DNA 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

Fig. 12-3 Figure 12.3 Eukaryotic chromosomes 20 µm

Genetic Information is stored and transmitted through DNA 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

0.5 µm Chromosomes DNA molecules Chromo- some arm Chromosome Fig. 12-4 0.5 µm Chromosomes DNA molecules Chromo- some arm Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Figure 12.4 Chromosome duplication and distribution during cell division Separation of sister chromatids Centromere Sister chromatids

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

Interphase 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

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

The cell cycle is directed by internal controls or checkpoints.

Cell Cycle Checkpoints 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

Cell Cycle Checkpoints 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 G2 checkpoint into the M phase Cell Cycle Checkpoints

M G1 S G2 M G1 S G2 M G1 Fig. 12-17 MPF activity Cyclin concentration Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle G1 S Cdk Figure 12.17 Molecular control of the cell cycle at the G2 checkpoint Cyclin accumulation M Degraded cyclin G2 G2 Cdk Cyclin is degraded checkpoint Cyclin MPF (b) Molecular mechanisms that help regulate the cell cycle

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

Scalpels Petri plate Without PDGF cells fail to divide With PDGF Fig. 12-18 Scalpels Petri plate Without PDGF cells fail to divide Figure 12.18 The effect of a growth factor on cell division With PDGF cells prolifer- ate Cultured fibroblasts 10 µm

Density-dependent inhibition Fig. 12-19 Anchorage dependence Density-dependent inhibition Density-dependent inhibition Figure 12.19 Density-dependent inhibition and anchorage dependence of cell division 25 µm 25 µm (a) Normal mammalian cells (b) Cancer cells

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

G2 of Interphase Prophase Prometaphase Centrosomes (with centriole Fig. 12-6b G2 of Interphase Prophase Prometaphase Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Nonkinetochore microtubules Figure 12.6 The mitotic division of an animal cell Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore Kinetochore microtubule

Fig. 12-7 Aster Centrosome Sister chromatids Microtubules Chromosomes Metaphase plate Kineto- chores Centrosome 1 µm Figure 12.7 The mitotic spindle at metaphase Overlapping nonkinetochore microtubules Kinetochore microtubules 0.5 µm

Telophase and Cytokinesis Fig. 12-6d Metaphase Anaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Nucleolus forming Figure 12.6 The mitotic division of an animal cell Daughter chromosomes Nuclear envelope forming Spindle Centrosome at one spindle pole

Chromosome movement Kinetochore Tubulin Subunits Motor protein Fig. 12-8b Chromosome movement Kinetochore Tubulin Subunits Motor protein Microtubule Figure 12.8 At which end do kinetochore microtubules shorten during anaphase? Chromosome

Figure 12.9 Cytokinesis in animal and plant cells Vesicles forming cell plate Wall of parent cell 1 µm 100 µm Cleavage furrow Cell plate New cell wall Figure 12.9 Cytokinesis in animal and plant cells Contractile ring of microfilaments Daughter cells Daughter cells (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM)

Summary of Mitosis 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