Cellular Division

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Tetrads Form in Prophase I

Homologous chromosomes (each with sister chromatids)

Join to form a TETRAD

Called Synapsis

Slide 79

Crossing-Over

Homologous chromosomes in a tetrad cross over each other

Pieces of chromosomes or genes are exchanged

Produces Genetic recombination in the offspring

Slide 80

Homologous Chromosomes During Crossing-Over

Slide 81

Crossing-over multiplies the already huge number of different gamete types produced by independent assortment

Crossing-Over

Slide 82

Metaphase I

Homologous pairs of chromosomes align along the equator of the cell

Slide 83

Anaphase I

Homologs separate and move to opposite poles.

Sister chromatids remain

attached at their centromeres.

Slide 84

Telophase I

Nuclear envelopes reassemble.

Spindle disappears.

Cytokinesis divides cell into two.

Slide 85

Meiosis II

Only one homolog of each chromosome is present in the cell.

Gene X

Slide 86

Meiosis II: Reducing Chromosome Number

Prophase II

Metaphase II

Anaphase II

Telophase II

4 Genetically Different haploid cells

Slide 87

Prophase II

Nuclear envelope fragments.

Spindle forms.

Slide 88

Metaphase II

Chromosomes align

along equator of cell.

Slide 89

Anaphase II

Sister chromatids separate and move to opposite poles.

Equator

Pole

Slide 90

Telophase II

Nuclear envelope assembles.

Chromosomes decondense.

Spindle disappears.

Cytokinesis divides cell into two.

Slide 91

Results of Meiosis

Gametes (egg & sperm) form

Four haploid cells with one copy of each chromosome

One allele of each gene

Different combinations of alleles for different genes along the chromosome

Slide 92