Intro to Meiosis

Remember Mitosis

• Produces two daughter cells with chromosomes identical to the parent cell. Each having 46 chromosomes.

• There is said to be a diploid number of chromosomes in somatic cells (all body cells except sperm and egg)

• Important for growth, repair and replacing worn out cells.

What is it?

Who does it?

Where does it happen?

Why does it happen?

Why is a different process needed?

MeiosisMaking sex cells (sperm & eggs)

All sexually reproducing organisms

To allow organisms to reproduce

1) Creates genetic diversity (can help evolution of species)2) Chromosome # needs to be cut in half

In males- in the testes (all the time)In females- in the ovaries (every month)

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

Meiosis is the process of making sex cells

called gametes (Sperm and Egg)•Each gamete has half the number of chromosomes (23). We call this haploid. So when the egg and sperm come together in fertilization the new ZYGOTE will have the correct # 46. The zygote undergoes mitosis then and grows into a baby.

Eight Stages of Meiosis1)Prophase I2)Metaphase I3)Anaphase I4)Telophase I

5)Prophase II6)Metaphase II7)Anaphase II8)Telophase II

Prophase 2

Remember, humans have 23 pairs of chromosomes

Pairs 1-22 are said to beHomologous chromosomes(they go together becausethey have same traits but arenot identical)

Gene for eye color

Gene for hair color

Prophase I: Crossing over occurs between replicated homologous chromosomes

Crossing over(exchanging parts)

Note: “X’s” are next to each other, not above each other

Metaphase I: Replicated chromosomes get next to each other in the cell

Anaphase I: homologous chromosomes separate

Telophase I: replicated chromosomes still need to be separated

still looks like an “X”

Meiosis II

•Looks like mitosis but no Interphase this time so no further DNA replication.

Note: chromosomes indaughter cells are all different

Meiosis

One final detail…