Today: - Lab 4 Debrief - Mitosis - Lunch -Meiosis Other: Blood Drive Today! TIME: 11:00am 1:00pm + 2:00pm 5:00pm PLACE: Baxter Events Center Thinking About Mitosis When and where do you want your cells to divide? The Role of Mitosis 1. Growth & Development 2. Repair 3. Asexual Reproduction A Few Terms: A single CHROMOSOME duplicates to form a pair of SISTER CHROMATIDS, with a visible CENTROMERE. Remembering How Chromosomes are Formed DIPLOID organisms have two of each chromosome. These pairs are called HOMOLOGOUS Chromosomes. Homologous Chromosomes 1
The Mitotic Cell Cycle: An Overview The Mitotic Cell Cycle: An Overview ~90% of the cell cycle Cell Cycle Checkpoints Three parts: 1. A signal specific to a particular event (like DNA replication) 2. A signal transduction pathway 3. A target to receive the signal, typically linked to key enzymes needed for the next step What happens to a cell if its checkpoints don t work correctly?? Cyclin-Dependent Kinases Help Regulate the Cell Cycle External Factors Also Regulate Cell Division What do you notice about these two cultures? Do either of them demonstrate densitydependent inhibition? 2
The Mitotic Cell Cycle: Focus on Mitosis Mitosis Occurs in 4-5 Steps: Prophase Prometaphase Metaphase Anaphase Telophase Step 1: Prophase Chromatin condenses to form visible chromosomes Microtubules are reassembled, spindle begins to form Centrosomes migrate away from one another Step 2: Prometaphase Nuclear envelope fragments Microtubules extend from each pole towards the middle of the cell Spindle microtubules attach to the kinetochores and begin to move the chromosomes (Nature: 408. 423, 2000 ) Step 3: Metaphase Step 4: Anaphase Spindle is complete Push-pull aligns chromosomes at the midpoint to form the METAPHASE PLATE Sister Chromatids separate (released at centromere) and migrate towards opposite spindle poles Spindle elongates 3
Step 5: Telophase Chromosomes arrive at poles and relax Nuclear membrane reforms Spindle is disassembled Thinking About How Chromatids Move Let s watch: http://www.cellsalive.com/mitosis.htm http://www.molbiolcell.org/misc/video1.shtml Attachment of the microtubules to the kinetochores creates the tug of war that generates the metaphase plate. During Anaphase, the proteins holding the sister chromatids together are inactivated, allowing the chromosomes to separate and move toward opposite poles. Finally! CYTOKINESIS! Testing the Hypothesis: 4
House Rules: Homologous Chromosomes should be the same length Sister Chromatids should be the same color and length You Try! The resulting offspring are CLONES (genetically identical to the parent)! Asexual Reproduction occurs through MITOSIS and CYTOKINESIS Mitosis Allows for Asexual Reproduction, but Not Sexual Reproduction! Asexual vs Sexual Reproduction Advantages and Disadvantages? Inheriting Genes: Genes: discrete units of coded information (DNA) Genes are packaged on CHROMOSOMES. A gene s location along the length of a chromosome is its LOCUS Sexual Reproduction Genetic Variation! Through sexual reproduction, offspring inherit new combinations of alleles. An ALLELE is a version of a particular gene Genetic Diversity is Essential for Evolution! 5
Sexual Reproduction requires MEIOSIS- (reduces the chromosome number to half) In humans, each SOMATIC CELL, has 23 pairs of HOMOLOGOUS CHROMOSOMES, or a total of 46 CHROMOSOMES. Important Exception! One pair of Chromosomes are NOT Necessarily Homologous SEX CHROMOSOMES: Human females have a homologous pair of X chromosomes (XX) in their somatic cells; males have one X and one Y chromosome (XY) Humans do have HAPLOID cells, or cells containing a single set of the 22 autosomes plus a single sex chromosomes. (n=23) Our GAMETES (reproductive cells) are HAPLOID. The Human Life Cycle Note that Meiosis and Fertilization are opposing processes. Meiosis reduces the chromosome number to half (n=23) Fertilization restores the chromosome number (2n=46) Meiosis is the Process that Produces Haploid Gametes Meiosis is similar to Mitosis: Cells copy their DNA in S-Phase Cells divide in a series of steps, Prophase Metaphase Anaphase Telophase Unlike Mitosis, in Meiosis, cells divide twice, creating four, haploid daughter cells 6
Meiosis occurs in two stages: Meiosis I- Homologous chromosomes separate Meiosis II- Sister chromatids separate Overview of Meiosis I: Pairs of duplicated, homologous chromosomes (tetrads) align at the metaphase plate. Crossover occurs between Homologous Chromosomes Duplicated Homologous Chromosomes are separated in Anaphase The Cytoplasm Divides, producing two haploid cells Overview of Meiosis II: Sister chromatids in the haploid cells line up at the metaphase plate and are separated. The cell divides into 2 haploid daughter cells. Thus Meiosis of a single, diploid cell produces 4 haploid daughter cells. Comparing Mitosis and Meiosis 7
MEIOSIS I in real cells: MEIOSIS II in real cells: Let s Watch! https://darchive.mblwhoilibrary. org/handle/1912/2642 House Rules: Homologous Chromosomes oso should oud be the same length Sister Chromatids should be the same color and length You Try! Challenge Question Compare the amount and arrangement of genetic material in each cell following telophase I of meiosis and telophase of mitosis. 8