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11-4 Meiosis. Chromosome Number. Chromosomes are homologous Have one set from mother and one set from father Drosophila , a fruit fly, contains 8 chromosomes - 4 came from mother, 4 came from father. Chromosome Number.

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Presentation Transcript
chromosome number
Chromosome Number
  • Chromosomes are homologous
    • Have one set from mother and one set from father
    • Drosophila, a fruit fly, contains 8 chromosomes - 4 came from mother, 4 came from father
chromosome number3
Chromosome Number
  • A cell that contains both sets of homologous chromosomes is diploid
    • Represented by 2N
    • For Drosophila, 2N = 8
    • Body cells (i.e. skin cells, liver cells, etc.)
  • A cell that contains only one set of homologous chromosomes is haploid
    • Represented by N
    • For Drosophila, N = 4
    • Sex cells - gametes (egg or sperm)
prophase i
Chromosomes pair with homologous chromosomes to form a tetrad

Contains 4 chromatids

Chromatids cross-over

Exchange portions of their chromatids

Leads to genetic diversity

Prophase I

Tetrad

Crossing Over

prophase ii
Chromosomes DO NOT replicate

Start with two haploid cells each with half the number of chromosomes as the original cell

Prophase II
gamete formation
Meiosis is how gametes are formed

In males, gametes = sperm

In female, gametes = eggs

Gamete Formation
comparison of mitosis and meiosis
Mitosis

Starts with one diploid cell

Cell divides once

Occurs in 4 phases

Results in 2diploid cells (all body cells)

Cells are genetically identical

Meiosis

Starts with one diploid cell

Cell divides twice

Occurs in 8 phases

Results in 4 haploid cells (gametes)

Cells are genetically different

Comparison of Mitosis and Meiosis