Cell Division Mitosis and Meiosis Mitosis • Asexual reproduction • Purpose: Growth, maintenance, and repair Cell Cycle 1. Interphase Cells are in interphase 90% of the time. – – – G1- growth S - chromosome replication G2- Prepare for mitosis 2. Mitosis (nuclear division) 3. Cytokinesis (cytoplasmic division) Interphase (pre game show) • Chromosomes are copied (# doubles) • Chromosomes appear as threadlike coils (chromatin) at the start, but each chromosome and its copy (sister chromosome) change to sister chromatids at end of this phase Synthesis Phase (S phase) • S phase during interphase of the cell cycle • Nucleus of eukaryotes S DNA replication takes place in the S phase. phase G1 interphase G2 Mitosis -prophase -metaphase -anaphase -telophase 6 DNA Replication Facts • DNA has to be copied before a cell divides in order to make new cells • New cells will need identical DNA strands 7 How do cells replicate their DNA? 1. The DNA double helix is unwound by an enzyme. 2. Another enzyme moves down the 2 original DNA strands and adds complementary nucleotides. Base Pairing Rule 3. Two separate DNA molecules are formed, each with one new strand and one old strand. Practice DNA Replication Replication Mistakes • 6 billion base pairs per cell. • All are replicated in a few hours. • Mistake happens one per billion nucleotides. Mistakes = Mutations • Mutations are changes in the base sequence of the DNA molecules. • This can have serious affects on genes and the functions of cells. • Cancer is caused by mutations in DNA. Mutagens • Physical or chemical factor that increases the frequency of mutations. • Examples: UV rays, X-rays, smoking Overview of Mitosis Parent Cell=2n (diploid) Daughter cell= 2n Daughter cell= 2n (diploid) (diploid) PROPHASE CENTRIOLES APPEAR AND MOVE TO OPPOSITE ENDS OF THE CELL Spindle fibers form between centrioles Centriole Spindle Fibers METAPHASE • Chromosomes “meet” in the middle. • Spindle fibers attach to the centromere of each chromosome. ANAPHASE • Sister chromatids separate from each other and are “pulled” to opposite ends of the cell. Telophase • Cell plate forms and cell membrane begins pinching in= “cleavage.” • Chromosomes look like threads (chromatin) again. Nuclear envelope forms again. Cytokinesis • Division of the cytoplasm • Cell membrane completely pinches off. • Cycle starts again. Results: 2 diploid (2n) daughter cells, genetically identical to parent cell Meiosis Meiosis Type: Sexual Reproduction Purpose: To create haploid gametes Meiosis • Types of cells that undergo meiosis: Gametes (sex cells) – Egg and sperm Meiosis 1 First division of meiosis • Prophase 1: Each chromosome dupicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage. • Metaphase 1: Homologous chromosomes align at the equatorial plate. • Anaphase 1: Homologous pairs separate with sister chromatids remaining together. • Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair. Meiosis II Second division of meiosis: Gamete formation • Prophase 2: DNA does not replicate. • Metaphase 2: Chromosomes align at the equatorial plate. • Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. • Telophase 2: Cell division is complete. Four haploid daughter cells are obtained. Meiosis: Overview Parent Cell: diploid= 2n After Meiosis I After Meiosis II Daughter Cells: Haploid= n ; and all genetically different from each other and parent cell ??? Why would it be important for the gametes to contain half as many chromosomes as a somatic cell? Comparing Mitosis and Meiosis Mitosis vs. meiosis Meiosis – key differences from mitosis • • • • Meiosis reduces the number of chromosomes by half. Daughter cells differ from parent, and each other. Meiosis involves two divisions, Mitosis only one. Meiosis I involves: – Synapsis – homologous chromosomes pair up. – Crossing over of non-sister chromatids occur. – In Metaphase I, homologous pairs line up at metaphase plate. – In Anaphase I, sister chromatids do NOT separate. – Overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome. Exit Ticket • Name 3 key differences between mitosis and meiosis.