Intro to Meiosis

Meiosis What is it? Who does it? Where does it happen? Why does it happen? Why is a different process needed? Making gametes (sperm & eggs) All sexually reproducing organisms In males- in the testes In females- in the ovaries To allow organisms to reproduce Chromosome # needs to be cut in half Creates genetic diversity (can help evolution of species)

Meiosis allows sex cells to join to form a complete set of chromosomes/instructions to make a new organism. Diploid adult MITOSIS FERTILIZATION MEIOSIS Haploid gametes Diploid zygote

Remember, humans have 23 pairs of chromosomes Pairs 1-22 are said to be Homologous chromosomes (they go together because they have same traits but are not identical) Gene for eye color Gene for hair color

Now let’s look at how sex cells are made…

Eight Stages of Meiosis Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

Prophase I: 1.) Homologous chromosomes pair up 2.) Crossing over occurs between replicated homologous chromosomes Crossing over (exchanging parts)

Crossing over

Note: chromosomes in daughter cells are all different

Metaphase I: Homologous chromosomes get next to each other in the cell, and line up in pairs Along the middle of the cell. Question: How is metaphase I of meiosis different from metaphase of mitosis? Note: “X’s” are next to each other, not above each other

Anaphase I: homologous chromosomes separate

Telophase I: sister chromatids still need to be separated still looks like an “X”

Now Meiosis II occurs (looks like mitosis)

Meiosis

Meiosis animation

Practice 1.) If a haploid cell has 7 chromosomes, how many chromosomes does a diploid cell have?

2.) If a gamete has 23 chromosomes, how many chromosomes are in a body cell of that organism?

3.) How are the cells at the end of meiosis I different from the cells at the end of meiosis II?

4.) What is the same and what is different about mitosis and meiosis?

5.)Why is crossing over important to the survival of a species?

And now let’s see a more technical version… http://www.pbs.org/wgbh/nova/baby/divi_flash.html Meiosis technical

http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

One final detail…

The End!