Vocabulary Important Info Headings MEIOSIS Vocabulary Important Info Headings

Mitosis: Meiosis: formation of somatic cells (body cells) At the end of mitosis each of the 2 daughter cells has 46 chromosomes These are your body cells; cells that make up tissues, organs, etc. Meiosis: Formation of sex cells called gametes. At the end of meiosis each of the 4 daughter unique cells has 23 chromosomes. They contain 1/2 the # of chromosomes

Meiosis Gametes are special cells used in sexual reproduction. These cells contain 1/2 the normal # of chromosomes than the regular body cells. When the sperm and egg unite the zygote will contain the normal # of chromosomes for that organism

Egg(23 chrom) + sperm(23 chrom) = zygote(46 chrom) CELL DIVISION CELL TYPE # OF DAUGHTER CELLS # OF CHROMO-SOMES Mitosis Somatic (body) 2 46 Diploid Meiosis Gamete (sex) 4 23 Haploid Meiosis reduces the number of chromosomes to ½ the # in body cells. This is called the haploid number (46  23) Reductive division is necessary so that when reproduction occurs = Zygote contains the necessary 46 chromosomes. Egg(23 chrom) + sperm(23 chrom) = zygote(46 chrom)

2 Divisions of Meiosis There are 2 divisions of meiosis: Meiosis I and Meiosis II. Must be in order to end up with: Each sex cell contains: = 23 chromosomes

Meiosis I During the first meiotic division, chromosomes are replicated. Prophase I chromatin makes a copy of itself begins to coil up Homologous chromosomes pair up

Meiosis I Prophase I Crossing over occurs which is the exchange of chromosome segments between homologous chromosomes

This forms a tetrad (meaning 4). 2. Metaphase I homologous pairs of chromosomes line up together at the middle of the cell. This forms a tetrad (meaning 4). *Homologous chromosomes are paired chromosomes. *Each contains genes for the same trait.

3. Anaphase I- Homologous chromosomes that form each tetrad separate and move to opposite ends. The centromeres (hold chromatids together) do not split.

4. Telophase I- Spindle fibers disappear Cell divides by: Cytokinesis (a cleavage forms)

Meiosis II During the second meiotic division NOTHING is replicated. Prophase II- centrioles double spindle forms

2. Metaphase II- chromosomes line up at the middle

3. Anaphase II- Individual chromatids move to opposite ends

4. Telophase II- 2 daughter cells divide into 4 new, unique daughter cells Each new daughter cell has 23 chromosomes. This is ½ the # of the original parent cell.

Where Does Meiosis Occur? Testes of the male: Called Spermatogenesis. 4 sperm are produced. Ovaries of the female: Called Oogenesis. 4 eggs are produced Only 1 becomes an egg used for reproduction!!! Other 3 dissolve

Meiosis I in Motion I- chromatin  chromosome & nuclear membrane disappears P1- centrioles move to opposite sides of cell M1- 4 chromosomes line up in middle (tetrad) A1- chromosomes (intact) pulled to opposite sides by spindle fibers T1- cell splits forming 2 new cells

Meiosis II in Motion P2- centrioles move to opposite sides of each cell M2- 2 chromosomes line up in middle A2- chromatids pulled apart by spindle fibers T2- cell splits forming 4 daughter cells that contain haploid set of chromosomes