Meiosis NotesMeiosis Notes

Meiosis• Process of reduction division in which the

number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell; happens in gametes (sex cells) – sperm & egg

• Homologous chromosomes – chromosomes that have a corresponding chromosome from the opposite-sex parent (2 sets of chromosomes, one from each parent)

Diploid vs. Haploid• Diploid – a cell that contains both sets of

homologous chromosomes (two sets); represented by the symbol 2N – Found in somatic or body cells (ex. Skin,

digestive tract)– Example : Humans – 2N = 46

• Haploid – a cell that contains only a single set of chromosomes (one set); represented by the symbol N or 1N– Found in gametes or sex cells – sperm & egg– Example: Humans – N = 23

Steps of Meiosis

• Meiosis usually involves two distinct divisions, called Meiosis I and Meiosis II

• By the end of Meiosis II, the one diploid cell that entered meiosis has become 4 haploid cells

Meiosis IMeiosis I• Interphase I

–Cell undergoes a round of DNA replication forming duplicate chromosomes

ChromatinNuclear Envelope

Centrioles

Meiosis IMeiosis I• Prophase I

–Each chromosome pairs with its corresponding homologous chromosome to form a tetrad

TETRAD

CROSSING OVERHomologous chromosomes

Meiosis IMeiosis I• As the homologous

chromosomes pair up and form tetrads in Meiosis I, they exchange portions of their chromatids in a process called crossing-over crossing-over

• This produces chromosomes with different combinations than the parents

Meiosis IMeiosis I• Metaphase I

–Chromosomes attach to the spindle fibers and move to the Middle of the cell

Meiosis IMeiosis I• Anaphase I

–The spindle fibers pull the homologous chromosomes toward opposite ends of the cell

Meiosis IMeiosis I• Telophase I &

Cytokinesis

–Nuclear envelopes form & the cell separates into two cells

Meiosis II• Meiosis I results in two haploid (N)

daughter cells, each with half the number of chromosomes as the original cell

• The two cells produced by meiosis I now enter a second meiotic division, but unlike the first meiotic division, neither cell goes through a round of DNA replication before entering Meiosis II

Meiosis IIMeiosis II• Prophase II

–Spindle fiber forms from the centrioles and the chromosomes attach themselves

Meiosis IIMeiosis II• Metaphase II

–Chromosomes line up in the center of each cell (similar to mitosis)

Meiosis IIMeiosis II• Anaphase II

–The sister chromatids separate and move toward opposite ends of the cell

Meiosis IIMeiosis II• Telophase II &

Cytokinesis–Nuclear

membranes form and the cells separate into 4 haploid (N) daughter cells

Gamete (Sex Cell) Formation• In male animals

(including humans), the haploid gametes produced by meiosis are called sperm

• 4 sperm are produced from one meiotic division

Gamete (Sex Cell) Formation• In female animals

(including humans), the haploid gametes produced by meiosis are called eggs

• The cell divisions at the end of meiosis I & II are uneven, so that 1 large egg is produced and other 3 cells produced, called polar bodies, are not involved in reproduction

Meiosis Animation

Meiosis I Animation

http://wps.prenhall.com/wps/media/objects/487/498728/CDA9_1/CDA9_1b/CDA9_1b.htm

Meiosis II Animation

http://wps.prenhall.com/wps/media/objects/487/498728/CDA9_1/CDA9_1c/CDA9_1c.htm