Today: Mitosis & Meiosis

Thinking About Mitosis When/where do you want your cells to divide? What will the first step be?

DNA must first be replicated, producing identical sister chromatids

How will the DNA get replicated?

Homologous Chromosomes DIPLOID organisms have two of each chromosome. These pairs are called HOMOLOGOUS Chromosomes.

Modeling Mitosis With a neighbor or two, build some chromosomes! If you are building sister chromatids, make them the same color and length. If building homologous chromosomes, make them different colors but the same length. With your group, walk through each stage, making sure you can explain what’s happening at each!

Thinking About Mitosis Let’s Watch: http://www.microscopyu.com/moviegallery/c1si/mitosiseb3/index.html

The Mitotic Cell Cycle: An Overview ~90% of the cell cycle

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??

Cell Cycle Checkpoints: Other Factors More than half of all human cancers are associated with the disruption of the tumor suppressor protein, p53!

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 “density-dependent inhibition”?

Other Key Ideas: Attachment of the microtubules to the kinetochores creates the “tug of war” that generates the metaphase plate.

Other Key Ideas: CYTOKINESIS

Mitosis Allows for Asexual Reproduction, but Not Sexual Reproduction!

Asexual vs Sexual Reproduction Advantages and Disadvantages?

Asexual Reproduction occurs through MITOSIS and CYTOKINESIS The resulting offspring are CLONES (genetically identical to the parent)

Genes: discrete units of coded information (DNA) 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!

Homologous Chromosomes DIPLOID organisms have two of each chromosome. These pairs are called HOMOLOGOUS Chromosomes.

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: Most human females have a homologous pair of X chromosomes (XX) in their somatic cells; most males have one X and one Y chromosome (XY)

Our GAMETES (reproductive cells) are HAPLOID. 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Telo-phase Unlike Mitosis, in Meiosis, cells divide twice, creating four, haploid daughter cells

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

Homologous Chromosomes Modeling Meiosis As before, walk through the stages of meiosis, making sure everyone in your group can clearly identify and demonstrate the following key ideas and structures: Homologous Chromosomes Sister Chromatids Tetrads Crossover

MEIOSIS I in real cells:

MEIOSIS II in real cells: What happens to cell size with each division??

Challenge Question!?