Section 11 4 Meiosis cell division that halves

Stages of Meiosis I. Interphase A. cells go through G 1, S, and G

II. Meiosis I A. Prophase I 1. DNA is coiled into chromosomes & nucleus

B. Metaphase I 1. Homologous chromosomes line up randomly in the center of the

C. Anaphase I 1. Spindle fibers pull homologous chromosomes apart. 2. The direction the

D. Telophase I 1. Chromosomes move to opposite ends of the cell 2. Cytokinesis

III. Meiosis II • Begins immediately after Meiosis I. • Both cells from Meiosis

A. Prophase II – spindle fibers reform, centrioles move to opposite ends of cell

IV. Meiosis Forms Gametes A. The 4 haploid cells produced in Meiosis are gametes

V. Genetic Variation in Meiosis A. Crossing Over – switching of DNA between chromatids

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Section 11 -4: Meiosis – cell division that halves the number of chromosomes in new cells, making haploid cells *Recall, haploid cells in human are egg/sperm cells, called gametes, and contain 23 chromosomes.

Stages of Meiosis I. Interphase A. cells go through G 1, S, and G 2 phases; B. DNA is copied and cells grow larger.

II. Meiosis I A. Prophase I 1. DNA is coiled into chromosomes & nucleus breaks down 2. Centrioles move to ends of cell; spindle fibers appear 3. Chromosomes pair up (in homologous pairs) and twist together. – Portions of a chromatid can break off and reattach to the identical chromatid on its homologous chromosome = crossing over

Prophase I and Crossing Over

B. Metaphase I 1. Homologous chromosomes line up randomly in the center of the cell 2. Spindle fibers attach to the centromeres

C. Anaphase I 1. Spindle fibers pull homologous chromosomes apart. 2. The direction the chromosomes are pulled is random = independent assortment

D. Telophase I 1. Chromosomes move to opposite ends of the cell 2. Cytokinesis occurs **Meiosis I results in 2 new haploid cells

III. Meiosis II • Begins immediately after Meiosis I. • Both cells from Meiosis I go through Meiosis II. • DNA is not copied this time!!!!!!!

A. Prophase II – spindle fibers reform, centrioles move to opposite ends of cell B. Metaphase II – single chromosomes line up in the middle of the cell C. Anaphase II – spindle fibers attach at centromeres and pull chromatids apart D. Telophase II – chromatids move to opposite ends, and nucleus forms around them and cytokinesis occurs **Meiosis II results in 4 new Haploid cells.

Stages of Meiosis II

IV. Meiosis Forms Gametes A. The 4 haploid cells produced in Meiosis are gametes (sex cells = egg/sperm) B. Spermatogenesis – production of sperm cells C. Oogenesis – production of mature egg cells or ova (singular = ovum)

V. Genetic Variation in Meiosis A. Crossing Over – switching of DNA between chromatids of homologous pairs; occurs during prophase I B. Independent Assortment – each homologous pair is separated independent of how the other pairs are separated. C. Random Fertilization – of the 4 gametes produced in meiosis, any one can fertilize (join) with another gamete