Cellular Division

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Two meiotic divisions --- Meiosis I and Meiosis II

Called Reduction- division

Original cell is diploid (2n)

Four daughter cells produced that are monoploid (1n)

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Facts About Meiosis

Daughter cells contain half the number of chromosomes as the original cell

Produces gametes (eggs & sperm)

Occurs in the testes in males (Spermatogenesis)

Occurs in the ovaries in females (Oogenesis)

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Start with 46 double stranded chromosomes (2n)

After 1 division - 23 double stranded chromosomes (n)

After 2nd division - 23 single stranded chromosomes (n)

Occurs in our germ cells that produce gametes

More Meiosis Facts

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Why Do we Need Meiosis?

It is the fundamental basis of sexual reproduction

Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote

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Fertilization – “Putting it all together”

1n =3

2n = 6

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Replication of Chromosomes

Replication is the process of duplicating a chromosome

Occurs prior to division

Replicated copies are called sister chromatids

Held together at centromere

Occurs in Interphase

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A Replicated Chromosome

Gene X

Homologs separate in meiosis I and therefore different alleles separate.

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Meiosis Forms Haploid Gametes

Meiosis must reduce the chromosome number by half

Fertilization then restores the 2n number

The right number!

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Meiosis: Two Part Cell Division

Homologs

separate

Sister

chromatids

separate

Diploid

Diploid

Haploid

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Meiosis I: Reduction Division

Early Prophase I

(Chromosome number doubled)

Late Prophase I

Metaphase I

Anaphase I

Telophase I (diploid)

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Prophase I

Early prophase

Homologs pair.

Crossing over occurs.

Late prophase

Chromosomes condense.

Spindle forms.

Nuclear envelope fragments.

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