Cell Cycle & Mitosis Ms. Levensailor
Plan for understanding the cell cycle • Establish important vocabulary. • Develop an understanding of the overall process. • Dive into the details of the mitotic cell cycle. • Know what is happening at each step. • Identify significant features of the cell during each step.
Cell Division Functions • Reproduction: asexual and sexual • Growth & Development: fertilized egg • Tissue renewal: repair and replacing cells that die • Involves the distribution of identical genetic material (DNA) to 2 daughter cells.
Cell Division • Genome: a cells DNA (genetic information). • Eukaryotic genomes are enormous. • DNA is packaged into chromosomes. • This packaging makes replication manageable within the cell. • Eukaryotes have a set number of chromosomes in each cell nucleus. • Human somatic cells (body cells not including reproductive cells) contain 46 chromosomes.
Cell Division • Each chromosome is one very long, linear DNA molecule. • Represents thousands of genes (an organisms inherited traits). • DNA is associated with proteins that maintain the structure of the chromosomes. • Chromatin: DNA-protein complex. • Organized into a long thin fiber.
Chromatin • After DNA is duplicated for division, chromatin is condensed! • It becomes densely folded and coiled. • We can now see it using a light microscope.
Sister Chromatids • Each duplicated chromosome has 2 sister chromatids. • Contain identical copies of the chromosome’s DNA molecule. • Centromere: narrow waist of the chromosome. • Pulled apart and repackaged as complete chromosome sets. • In 2 new nuclei, one at each end of the cell.
Mitosis • Mitosis: division of the nucleus. • Followed by cytokinesis. • Cytokinesis: division of the cytoplasm.
Details of the Cell Cycle • 2 main phases: • Interphase: • Accounts for 90% of the cycle. • Cell grows and copies chromosomes in prep for division. • Mitotic phase: • Includes mitosis and cytokinesis. • Shortest part of cell cycle.
Interphase • Divided into subphases: • G1 phase (first gap): • S phase (synthesis of DNA): Chromosomes are duplicated! • G2 phase (second gap): • During all 3 phases cell is growing by producing proteins and cytoplasmic organelles. *NOTE: Fill in graphic organizer!
Interphase • Nucleus is well defined and bound by the nuclear envelope. • 2 centrosomes are outside the nucleus. • Features a pair of centrioles (animal cells only). • Asters: microtubules extend from the centrosomes in radial arrays.
Interphase Onion roottip
Mitosis • Broken up into 5 main subphases: • Prophase • Prometaphase • Metaphase • Anaphase • Telophase
Prophase • Changes in the nucleus: • Chromatin fibers become more tightly coiled. • Condense into discrete chromosomes. • Chromosomes appear as 2 sister chromatids. • Nucleoli disappear. • Changes in the cytoplasm: • Mitotic spindle forms (made of microtubules). • Centrosomes move away from each other.
Prometaphase • Nuclear envelope fragments. • Microtubules interact with the chromosomes: • Bundles of microtubules extend from each pole and toward the middle of the cell. • Each of the chromatids has a kinetochore. • Located at the centromere. • Microtubules attach at the kinetochore.
Prophase Onion roottip
Metaphase • Centrosomes are at opposite poles of the cell. • Chromosomes line up on the metaphase plate. • Imaginary plane that is equidistant between the 2 poles. • Kinetochores are attached to microtubules of the opposite pole. • Spindle is formed. • Entire apparatus of microtubules.
Anaphase • Paired centromeres of each chromosome separate. • Frees sister chromatids from each other. • Now considered chromosomes. • Chromosomes move toward opposite poles. • Result of kinetochore microtubules.
Telophase • Daughter nuclei form at the two poles of the cell. • Nuclear envelope forms (from parent cell). • Chromatin fiber of each chromosome becomes less tightly coiled. • Mitosis is complete! • The division of one nucleus into 2 genetically identical nuclei.
Cytokinesis • Division of the cytoplasm. • 2 daughter cells appear at the end of mitosis. • In animal cells cytokinesis involves: • Formation of a cleavage furrow. • Pinching the cell in 2. • In plant cells: • No cleavage furrow. • Produce a cell plate.
Cytokinesis in Plants • During telophase: • Vesicles from the golgi apparatus move along microtubules to the middle of the cell. • This produces a cell plate. • Cell wall materials carried in the vesicles collect in the cell plate. • Cell plate enlarges and fuses with the plasma membrane. • Results in 2 daughter cells.
Mitotic Spindle • Mitosis is dependent on the mitotic spindle! • Consists of fibers made of microtubules and associated proteins. • Interesting note: while mitotic spindle is forming the microtubules of the cytoskeleton disassemble. • Microtubules elongate by subunits of the protein tubulin (does this sound familiar?).
Regulation of the Cell Cycle • Brainstorm with a neighbor: • What controls/regulates the cell cycle? • Possibilities if there are errors in this system.
Molecular Control System • Cell cycle is driven by specific chemical signals present in the cytoplasm. • Cell Cycle Control System • Cell cycle is regulated at 3 checkpoints by both internal and external controls. • G1 • G2 • M
Checkpoints • Stop and go signals regulate cell cycle. • Signals report whether crucial cellular processes have been completed correctly. • G1 Checkpoint- “Restriction Point” • Once past this point, cell will complete the cell cycle. • If not, exit cell cycle to non-dividing state- “G0 Phase”
Cell Cycle Clock • Regulatory molecules are proteins • Kinase: enzymes that activate or inactivate other proteins by phosphorylation. • Present at a constant concentration in a growing cell. • Mostly in an inactive form. • Activated by cyclin. (Cyclin Dependent Kinases) • Cyclin: protein that fluctuates in concentration in the cell.
Abnormal Cell Division • Cancer cells do not respond to the body’s control mechanisms. • They ignore: • Density-dependent inhibition: crowded cells stop dividing. • Anchorage dependence: cells must be attached to a substratum. • Cell cycle checkpoints. • Can divide indefinitely with a continual supply of nutrients. • “immortal”
Impacts on the Body • Cancer cells are “transformed cells”- conversion from normal to cancer cell. • Normally the immune system would destroy these cells. • If a transformed cell survives it will form a tumor: a mass of transformed cells. • Benign if it stays in its original site. • Malignant if it spreads to other parts of the body.
Spreading Mechanism • Cancer cells lose their attachment to neighboring cells. • Can enter the blood and lymph vessel of the circulatory system. • They can now invade other parts of the body and form more tumors (metastasis).
Treatment • Benign tumors: Removed by surgery. • Radiation: therapy using high-energy radiation to shrink tumors and kill cancer cells. • Kills cancer cells by damaging their DNA • Cancer cells whose DNA is damaged beyond repair stop dividing or die
Treatment • Chemotherapy: drugs that interfere with the ability of cancer cells to divide and reproduce themselves. • Delivered by the bloodstream to reach cancer cells all over the body. • How is treatment chosen? • Based on type of cancer and location in the body.