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6.2 Process of Meiosis

KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

6.2 Process of Meiosis

!  Gametogenesis

•  What is gametogenesis? Meiosis that creates gametes

2 Types 1. Spermatogenesis - Creates 4 sperm

cells 2. Oogenesis -  Creates 1 viable

ovum & 3 unavailable polar bodies

-  Conserve cytoplasm

Sex cell To make

Viable – survive Polar bodies – broken down & discarded from body

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6.2 Process of Meiosis

!  Haploid cells develop into mature gametes.

6.2 Process of Meiosis

!  Meiosis – Sex Cells Mitosis – everything else

Sperm •  Haploid = 23 •  Has no cytoplasm, no

organelles “ONLY a NUCLEUS” DNA

Ova •  Haploid = 23 •  Has all the stuff

mitochondria, golgi bodies, ER, cyotplasm

N= 23 2n = 46 (23 + 23) = Diploid

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6.2 Process of Meiosis

!  Meiosis •  Nuclear division •  Begin with diploid cell •  egg (n) haploid + sperm (n) haploid = 2n

diploid •  DNA replication •  Copy chromosomes •  End with 4 haploid cells •  Sexual reproduction •  2 cell divisions Meiosis I and Meiosis II

Zygote – union of sex cells Gametes – egg/sperm

6.2 Process of Meiosis

!  Cells go through two rounds of division in meiosis. •  Meiosis reduces chromosome number and creates genetic

diversity. •  Meiosis I and meiosis II each have four phases, similar to

those in mitosis.

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6.2 Process of Meiosis

!  Homologous Chromosomes

•  2 separate chromosomes •  (1) mom & (1) dad = 23 each •  Similar, same length &

genes •  They are NOT duplicates of

each other

homologous chromosomes

sister chromatids

Sister chromatids

Meiosis I – chromosomes divide Meiosis II – Sister chromatids divide

6.2 Process of Meiosis

!  1) Meiosis •  Called reduction division •  Daughter cells will have ½ number of chromosomes as

the parent (mother cell)

46 23

23

2. How is this done? - Replicate DNA once -  Divide twice

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6.2 Process of Meiosis

46

92

46 46

23 23 23 23

Parent

Replicate MI

Divide Twice MII

3) 1st Division is Meiosis I - Crossing over - Homologous chromosomes pair up by a process called synapsis &

make a terad (4)

4) 2nd Division Meiosis II - Reduction occurs

6.2 Process of Meiosis

!  Meiosis I

•  Homologous chromosomes pair-up

•  Crossing over is when homologous chromosomes exchange portions of their chromatids

•  Creates new combinations or increase in genetic variety

chiasma

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6.2 Process of Meiosis

!  Meiosis II •  NO DNA repliction •  Once Meiosis II is done, 4 haploid (n) daughter cells will

be produced •  Humans 2n = 46 diploid, N = 23 halpoid

6.2 Process of Meiosis

•  Meiosis I occurs after DNA has been replicated.

•  Meiosis I divides homologous chromosomes in four phases.

DNA is mixed Chromosomes pair

Independent assortment Different chromosome combinations

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6.2 Process of Meiosis •  Meiosis II divides sister chromatids in four phases.

•  DNA is not replicated between meiosis I and meiosis II.

If gametes are sperm produce = 4 sperm cells If gametes are ova produce = 1 egg (other 3 cells will die or perform another function)

6.2 Process of Meiosis

•  Meiosis differs from mitosis in significant ways.

–  Meiosis has two cell divisions while mitosis has one. –  In mitosis, homologous chromosomes never pair up. –  Meiosis results in haploid cells; mitosis results in diploid

cells.