1. The Cell Cycle Chapter 11

2. Objectives • Understand the need for cell division • Understand the process of Mitosis and cytokinesis • Know the parts of each phase of mitosis as discussed in class • Understand what regulates the cell cycle and at what phases regulation may occur

3. Cell CycleThe Cell’s Time Clock • Smallest unit of life • all living things must reproduce • Cells replicate for growth, replacement, and repair • Most cell division requires Mitosis & Cytokinesis

4. Parts of the Cell Involved • Chromosome • Chromatid: Strand of DNA • Kinetochore: Region of chromatid where spindle fibers attach • Nucleus • Nucleolus: Region of RNA • Nuclear membrane: Borders the nucleus • Centrosomes • Centrioles in some organisms: produce microtubules (spindle fibers and asters) • Spindle fibers: microtubules that grow toward the chromatids

5. Interphase • Cell performs normal function • Three subphases: • G1: cell duplicates most organelles • S: quantity of DNA in the cell is doubled. Each chromosome has a pair of sister chromatids connected by a kinetochore • G2: chemical components stockpiled • Nucleolus present

6. Nuclear division without a reduction in chromosome number Each new cell (daughter cell) will have the same quantity of DNA as the parental cell Why is this important? Mitotic events can be categorized into discrete stages based on what is happening to structure of the cell Stage include: Prophase Prometaphase Metaphase Anaphase Telophase Mitosis

7. Prophase • Pro • Three things visibly occur • Chromosomes condense (shorten) • Centrosomes migrate to the poles while producing spindle fibers • Nuclear membrane fragments

8. Metaphase • Meta • Chromosomes are moved by growing spindle fibers to the equator of the cell (metaphase plate) • Centrosomes are at the poles, nuclear membrane is gone

9. Anaphase • Ana • Kinetochore splits into two • Spindle fibers shorten from kinetochore end separating sister chromatids and pulling them toward the poles

10. Telophase • Telo • Nuclear membrane reforms around each region of chromosomes • Nucleolus reforms • Cytokinesis (division of the cytoplasm) may occur

11. Cell Cycle Regulation* • Cell cycle events are triggered by the cell-cycle control system; a set of molecules found in the cytoplasm affected by internal and external controls • Checkpoints in G1, G2, and M phases of the cycle • G1 checkpoint is most critical. May throw cells out of cyclic phase into G0, never to divide again

12. Cyclins &Cylin-Dependent Proteins • Cyclin is a cytoplasmic protein that fluctuates in concentration • Cyclin-Dependent Proteins: activate other proteins in the cell by phosphorylation (protein kinases)

13. Internal and External Factors • Internal • M checkpoint does not proceed until signal is received that all kinetochores are attached to a spindle • External • Growth factors: cycle will not proceed if requirements are not met • Density-dependent inhibition: under crowded conditions chemical requirements are insufficient to allow cell growth (p205: Cell division run amok) • Anchorage dependence: some cells must be attached to a substrate in order to replicate