Meiosis Cell Division

Do Now: Terminology Review Demonstrate understanding of the following terms. Chromosome Chromatid Chromatin

Sexual Reproduction Produces offspring that are genetically different from parents. Accomplished by gametes (sex cells) – one male and one female. Formed during meiosis.

Chromosome Number Somatic cells are diploid cells – they have 2 copies of each chromosome (homologous pairs- 1 from mom, 1 from dad). Gametes (sex cells) are haploid cells- they have only 1 copy of each chromosome from each pair.

Fertilization Because of meiosis, the chromosome number of a species is maintained even after fertilization.

General Information Cells go through 2 divisions Meiosis I Meiosis II Meiosis results in 4 new cells rather than 2 (mitosis) The chromosome number is reduced by half in the 4 new cells.

Prophase I DNA coils tightly into chromosomes Spindle fibers appear The nucleolus and nuclear membrane disappear. Synapsis occurs (Tetrads formed) Crossing-over occurs = genetic variability Source: HOPES: An Introduction to DNA and Chromosomes http://hopes.stanford.edu/basics/dna/b7.html

Crossing Over during Prophase I

Metaphase I The homologous chromosome pairs line up randomly along the middle of the dividing cell. Spindle fibers from one pole attach to one chromosome, while spindle fibers from the opposite pole attach to the other chromosome. Source: HOPES: An Introduction to DNA and Chromosomes http://hopes.stanford.edu/basics/dna/b7.html

Anaphase I Each chromosome moves to opposite poles of the dividing cell. Random separation = independent assortment. Source: HOPES: An Introduction to DNA and Chromosomes http://hopes.stanford.edu/basics/dna/b7.html

Telophase I/ Cytokinesis I The chromosomes reach opposite ends of the cell and cytokinesis begins. The new cell contains half the number of chromosomes as the original cell. Pearson Biology Coach: Meiosis I Animation

Assembling the Stages of Meiosis I Pearson Biology Coach Activity

Meiosis I v. Meiosis II Meiosis I is known as a “reduction division” because the number of chromosomes is reduced by ½. Meiosis II is known as an “equation division” because the number of chromosomes is the same at the end of meiosis II as at the end of meiosis I.

Meiosis II – General Information No copying of DNA takes place before Meiosis II. In some species, meiosis II begins after nuclear membrane forms in the new cells. In other species, meiosis II begins right after cytokinesis.

Prophase II The spindle fibers appear and begin to move the chromosomes toward the middle of the cell. Source: “Cell Division, Mitosis and Meiosis Lecture Notes .” Professor Michael Muller. http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm

Metaphase II Each chromosome is located at the middle of the cell with each chromatid facing opposite poles. Source: “Cell Division, Mitosis and Meiosis Lecture Notes .” Professor Michael Muller. http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm

Anaphase II The chromatid separate and move toward opposite poles of the cell. Source: “Cell Division, Mitosis and Meiosis Lecture Notes .” Professor Michael Muller. http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm

Telophase II/ Cytokinesis II Nuclear membrane re-forms around chromosomes in each of 4 new cells. Cytokinesis results in 4 new cells each which contains ½ the number of chromosomes as parent cell. Source: “Cell Division, Mitosis and Meiosis Lecture Notes .” Professor Michael Muller. http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm Pearson Biology Coach Meiosis II Animation

Pearson BioCoach Activity Assembling the Stages of Meiosis II

Development of Gametes Meiosis occurs in the testes of males and the ovaries of females. In the formation of male gametes, the original cell in testes produces 4 sperm cells by meiosis. In the formation of female gametes, the original cell in the ovary produces 1 egg and 3 polar bodies by meiosis.

General Bio- Do Now What types of cells are made by meiosis? How many cells are made at the end? Are they the same/different as parent? What happens to chromosome number?

CP Bio- Do Now What types of cells are made by meiosis? How many cells are made at the end? Are they the same/different as parent? What happens to chromosome number? Why is meiosis important?