Steps of Meiosis. Meiosis I: Prophase I Homologous pairs align with each other.Homologous pairs align with each other. Synapsis occurs: pairing of homologues.Synapsis.

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Steps of Meiosis

Meiosis I: Prophase I Homologous pairs align with each other.Homologous pairs align with each other. Synapsis occurs: pairing of homologues.Synapsis occurs: pairing of homologues. Each pair of homologues chromosomes is called a tetrad.Each pair of homologues chromosomes is called a tetrad. –Crossing over occurs.

Crossing over occurs during tetrad positioning of chromosomes.Crossing over occurs during tetrad positioning of chromosomes. Results in genetic recombinationResults in genetic recombination –Alleles are swapped Crossing Over Pair share: why is it called a tetrad?

Actual picture of chromosomes crossing over.Actual picture of chromosomes crossing over. This creates variation among offspring of a species.This creates variation among offspring of a species. Crossing Over

Meiosis I : Metaphase I Microtubules attach only to outside kinetochores.Microtubules attach only to outside kinetochores. Homologous pairs move to equator of cell due to kinetochore fibers.Homologous pairs move to equator of cell due to kinetochore fibers. Pair share: why do you think it is called the equator?

Meiosis I: Anaphase I Independent assortmentIndependent assortment –Homologues are pulled apart by kinetochore fibers. –Homologous pairs are separated and moved to opposite poles. Note: still two chromotidsNote: still two chromotids

Meiosis I: Telophase I Cell goes through cytokinesis.Cell goes through cytokinesis. Cells are now 1n or haploid.Cells are now 1n or haploid.

Meiosis II: Prophase II Spindle fibers form and begin to move the chromosomes toward the middle of the dividing cellSpindle fibers form and begin to move the chromosomes toward the middle of the dividing cell

Meiosis II: metaphase II Chromosomes are lined up in the centerChromosomes are lined up in the center No replicationNo replication No synapsisNo synapsis No crossing over.No crossing over.

Meiosis II: anaphase II Chromotids are split apart and moved to opposite poles of cell.Chromotids are split apart and moved to opposite poles of cell. A second division occurs.A second division occurs. Pair share: what is a key difference between anaphase 1 & 2?

Comparing Mitosis and Meiosis Cells “want” identical genesCells “want” genetic diversity

White board Review 1.What happens during crossing over? 2.What happens during independent assortment? 3.What results from meiosis II?