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? Meiosis Making 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 Meiosis 1)P rophase I 2)M etaphase I 3)A naphase I 4)T elophase I 5)P rophase II 6)M etaphase II 7)A naphase II 8)T elophase II Prophase 2

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

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 in daughter cells are all different

Meiosis

One final detail…