The Cell Cycle

1. The Cell Cycle Chapter 5.3, 5.4

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 • Asexual Cell division requires Mitosis & Cytokinesis

4. Parts of the Cell Involved • Chromosome • Chromatid: Strand of DNA, chromosomes have one or two chromatids depending on the stage of the cell cycle • Centromere: Region of chromatid where spindle fibers attach • Nucleus • Nucleolus: Region of RNA often visible inside a nucleus • Nuclear membrane: Borders the nucleus • Centrosomes • Produce microtubules (spindle fibers), responsible for moving things within the cell

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 centromere • G2: chemical components stockpiled • Nucleolus present

7. Nuclear division without a change in chromosome number Each new cell (daughter cell) will have the same quantity of DNA as the parental cell Why is this important? The different stages of Mitosis are a reflection of observable changes in cell structure Stage include: Prophase Metaphase Anaphase Telophase Mitosis

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

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

10. Anaphase • Centromere splits into two • Spindle fibers shorten from centromere end separating sister chromatids and pulling them toward the poles

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

12. Mitosis in Plant Cells • The cell wall around plant cells requires that it be synthesized prior to completion of cytokinesis

13. Cell Cycle Regulation • Cell cycle events are regulated by molecules found in the cytoplasm affected by internal and external controls • Cells that do not respond to these controls are said to be in G0, and never divide again • examples include muscle and nerve cells

14. Mutations are mistakes on the DNA • Occasionally a mismatch between the otherwise complimentary bases occurs • If uncorrected, it is called a mutation • Some mutations may give rise to oncogenes or mutated Tumor Suppressor Genes

15. Internal and External Factors Affecting Division Rates • Internal • Anaphase does not proceed until signal is received that all centromeres are attached to a spindle fiber • 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 • Anchorage dependence: some cells must be attached to a substrate in order to replicate