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MeiosisFormation of Gametes

(Eggs & Sperm)

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Facts About Meiosis ü 4 haploid daughter cells are

produced (each contain half the number of chromosomes as the original cell)

ü Produces gametes (eggs & sperm) ü Occurs in the testes in males

(Spermatogenesis) ü Occurs in the ovaries in females

(Oogenesis)

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Meiosis: Two Part Cell Division

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Homologous Chromosomes separate

Sister chromatids separate

Diploid

Meiosis I

Meiosis II

Diploid

Haploid

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Why Do we Need Meiosis? ü sexual reproduction ü two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote

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Fertilization

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1n =3

2n = 6

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Meiosis I: Reduction Division

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Nucleus Spindle fibers

Nuclear envelope

Early Prophase I (Chromosome number doubled because has gone through S phase of interphase)

Late Prophase I Metaphase I

Anaphase I Telophase I (haploid)

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Prophase I

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Early prophase ü Homologs pair through synapsis forming tetrads. ü Crossing over occurs.

Late prophase ü Chromosomes condense. ü Spindle forms. ü Nuclear envelope breaks down.

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Crossing-Over

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

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Metaphase I

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Homologous pairs of chromosomes align across from each other along the equator of the cell. Random alignment is called Independent Assortment.

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Anaphase I

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Homologs separate and move to opposite poles. Sister chromatids remain

attached at their centromeres.

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Telophase I

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Nuclear envelopes reassemble.

Spindle disappears. Cleavage furrow forms and cytokinesis divides cell into two.

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Prophase II

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Nuclear envelope breaks down. Spindle forms.

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Metaphase II

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Chromosomes align On the equator of cells.

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Anaphase II

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Sister chromatids separate and move to opposite poles.

Equator

Pole

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Telophase II

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Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.

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Results of Meiosis

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Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome

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Gametogenesis Oogenesis or Spermatogenesis

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Spermatogenesis ü Occurs in the testes ü Meiosis produces 4

spermatids ü Spermatids mature into

sperm ü Men produce about

250,000,000 sperm per day

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Spermatogenesis in the Testes

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Spermatid

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Oogenesis ü Occurs in the ovaries before birth

ü Two divisions produce 3 polar bodies that die and 1 egg

ü Polar bodies die because of unequal division of cytoplasm

ü Immature egg called oocyte

ü Starting at puberty, one oocyte matures into an ovum (egg) every 28 days

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Oogenesis

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Oogonium (diploid)

Mitosis

Primary oocyte (diploid)

Meiosis I

Secondary oocyte (haploid)

Meiosis II (if fertilization occurs)

First polar body may divide (haploid) Polar

bodies die

Ovum (egg)

Second polar body (haploid)

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A

X

X

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A X

a

X

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X

Mature egg

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X

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X

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Comparison of Divisions

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Mitosis Meiosis Number of

divisions 1 2

Number of daughter cells 2 4

Genetically identical cells

made? Yes No

Chromosome # Same as parent Half of parent

Where Somatic cells Germ cell to make gametes

When Throughout life At sexual maturity

Role Growth and repair Sexual reproduction