MEIOSIS Vocabulary Important Info Headings Mitosis formation of

MEIOSIS Vocabulary Important Info Headings

Mitosis: • 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. n These cells contain 1/2 the normal # of chromosomes than the regular body cells. n When the sperm and egg unite the zygote will contain the normal # of chromosomes for that organism n

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. 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. 1. Prophase I • chromatin makes a copy of itself • begins to coil up • Homologous chromosomes pair up

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

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. 1. 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? n Testes of the male: – Called Spermatogenesis. – 4 sperm are produced. n 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 P 1 - centrioles move to opposite sides of cell M 1 - 4 chromosomes line up in middle (tetrad) A 1 - chromosomes (intact) pulled to opposite sides by spindle fibers T 1 - cell splits forming 2 new cells

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