MEIOSISchapter 11 section 4

SC SCIENCE STANDARD B-4.5Students will demonstrate

understanding of characteristics of phases of Meiosis I and II.

Meiosis Vocabulary

Homologous: term used to refer to chromosomes that each have corresponding chromosome from opposite-sex parent› Example: Your # 12 chromosome from

your mom is homologous to your # 12 chromosome you received from your dad

Diploid: refers to a cell that contains both sets of homologous chromosomes› Referred to as 2n› 2n in humans = 46

Haploid: refers to cells that have one set of chromosomes› Referred to as n› In humans n = 23

Meiosis Phases

Meiosis is a process of reduction division in which the # of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.› Only occurs in sex cells› Involves 2 distinct divisions:

Meiosis I Meiosis II

Meiosis I

Prior to:› each chromosome has replicated

Meiosis I: Prophase I

Homologous chromosomes are paired forming a tetrad

Prophase I

Chromatids (#4) in tetrad exchange portions with their homologous partner = crossing-over

Results in new combinations of alleles

Meiosis I continued

Metaphase I› Homologous

pairs line up on metaphase plate

Anaphase I› Homologous

chromosomes separate and move to opposite poles

Meiosis I continued

Telophase I› Same as

telophase in mitosis

Cytokinesis I› 2 daughter cells› 1set of

duplicated chromosomes (n)

› Chromosomes are different from parent cell

Meiosis II

2 cells made in Meiosis I immediately enter Meiosis II

Except there is no replication of chromosomes prior to Prophase II

Meiosis II

Prophase II› Each

chromosome has 2 sister chromatids

› No crossing over› Spindles start to

form› Nuclear envelope

& nucleolus fade away

› (a lot like prophase in mitosis)

Meiosis II continued

Metaphase II› Chromosomes

line up on metaphase plate

› Spindles attach to centromeres

› (just like metaphase in mitosis)

Anaphase II

Centromeres split separating sister chromatids

Individual chromosomes move toward poles of cell

Telophase II & Cytokinesis II

Nuclear envelopes and nucleoli reform

Spindle disappears

Cleavage furrow forms

4 daughter cells formed each with haploid # (n) of chromosomes

Meiosis

Comparing :Mitosis Meiosis Parent cell

produces 2 genetically identical daughter cells

Parent cell produces 4 genetically different daughter cells

Gamete Formation

Males:› Even cell

divisions at end of Meiosis I & II

Females:› uneven cell

divisions in Meiosis I & II (most of cytoplasm goes to 1 cell)

GametesMales: Females:

Females:

Meiosis Animations

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html

http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html