Organisms receive half of their DNA from each parent

Sperm and eggs cells (haploid) combine to create a zygote (diploid)

Used by most animals, fungi, plants, and algae

Haploid gametes (n = 23)

Egg cell

Sperm cell

Fertilization

n

n

Meiosis

Ovary Testis

Diploidzygote(2n = 46)

2n

MitosisKey

Haploid stage (n)Diploid stage (2n)

Multicellular diploidadults (2n = 46)

A life cycle

Humans body cells are diploid (2N) 2N = 46

Gametes - sex cells Males – sperm Females – eggs

Gametes are HAPLOID N = 23

Fertilization restores the diploid state

(N + N = 2N) Zygote - a fertilized

egg (2N)

What happens during Interphase???DNA REPLICATION!!!

2N

2N

Occurs in Prophase I Chromosomes exchange portions of their

chromatids Increases genetic variation

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Homologous chromosome pair

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Centromere

• Prophase I – homologous chromosomes form tetrads & crossing over occurs• Metaphase I – tetrads line up along the middle

• Anaphase I – homologous chromosomes separate

• Telophase I – two nuclei form

• Cytokinesis I – cells separate

2N

N N

• Prophase II – spindle forms, prepares for division

• DNA replication DOES NOT occur between Meiosis I & II

• Metaphase II – chromosomes align along the center

• Anaphase II – sister chromatids separate

• Telophase II – nuclei reform; cytokinesis

N N

N

N NN

Looks A LOT like MITOSIS!

Prophase II Metaphase II Anaphase II

Haploid daughtercells forming

Telophase IIand Cytokinesis

3

SUMMERY OF THE MEIOSIS PROCESS

Process to make haploid gametes

Chromosome # is cut in half

2N N Occurs in ovaries & testes TWO cell divisions Increases genetic variation

Down syndrome

Down syndrome

Men: 4 sperm are made

Women: only 1 egg is made

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Deletion

Duplication

Inversion

Reciprocal translocation

Homologouschromosomes Nonhomologous

chromosomes

Meiosisvs.Mitosis

In meiosis, “crossing over” occurs during metaphase I.

Chromosomes form tetrads

Homologous chromosomes “swap” parts of their DNA leads to genetic variation

Mitosis makes cells for growth, development, repair, and asexual reproduction

Meiosis makes cells for sexual reproduction

The following table lists the most common human sex chromosome abnormalities.

© 2012 Pearson Education, Inc.

Campbell &Reece,(2010) Biology Williamson, J. LS3AFET . Unit 3. University of

Johannesburg: Science Department, 2014. Shilma, S. Meiosis. (2011). http://www.worldstarhiphop.com/videos/vi

deo.php?v=wshhk0A758v5lEkmCKrk