Meiosis Notes

1.Mitosis occurs in all somatic (body) cells EXCEPT the sex cells (called gametes) 2. Humans have a total of 46 chromosomes, that is 23 pair of chromosomes: one of each chromosome from each parent (2 parents = 2 of the same chromosome)

3. Since our somatic cells have 2 sets of each chromosome, they are called diploid (ploid means “number of copies of DNA” 4. So, when our somatic cells divide the new cells get the exact same number & kinds of chromosomes (remember DNA replication occurred before the cell split in mitosis)

5.In addition to the somatic cells, we also have sex cells, referred to as gametes from here on. 6. Gamete cells are different than somatic cells because they are NOT used to replace cells in our body (such as skin cells or blood cells) like the somatic cells are. Instead, gamete cells are used for reproduction.

7. The 2 types of sex cells found in humans are egg and sperm a) females produce eggs b) males produce the sperm c) we can’t do both.

What makes gametes special? They have ½ the number of chromosomes as the somatic cell

a) Why is this necessary? So that when the egg & sperm meet the new cell that forms will have the correct number of chromosomes to make the new body cells of the fetus.

B) How many chromosomes do the gamete cells in a human have? 23 chromosomes i. this means that each gamete only has 1 of each type of chromosome (instead of 2) ii. So, we say that they are haploid because they only have 1 copy of each chromosome.

9. What would happen if we used somatic cells to reproduce? We would have too many chromosomes a) Use the human as a specific example =92

10. Since sex cells can only have ½ the number of chromosomes as the rest of the body cells, it must mean that they go through a different type of cell division.

11.Sex cells go through meiosis instead of mitosis because meiosis splits each original cell 2 times 12. the stages of meiosis are generally similar to mitosis, but have some difference.

13.Meiosis is broken down into two phases, called Meiosis I and meiosis II 14. Meiosis I is broken down into 4 stages: Prophase I Metaphase I Anaphase I And telophase I

a) Prophase I i.Nuclear envelope breaks down ii.Chromatin (uncondensed DNA) condenses into visible (duplicated) chromosomes

a) Prophase I iii. Homologous chromosomes pair up Homologous chromosomes are chromosomes that code for the same genes. They contain the same types of information Ex) the chromosome for eye color from your mom & the chromosome for eye color from your dad.

a) Prophase I Cross-overs may occur between chromatids of the other duplicated chromosome-that is where genetic information can be swapped between chromosomes

a) Prophase I v. Homologous pairs move toward the metaphase plate

b) Metaphase I i.Homologous pair of chromosomes line up at the metaphase plate

c) Anaphase I i.Homologous pair of chromosomes split so that one of each duplicated set will be in each new cell. sister chromatids remain attached

c) Anaphase I ii. Duplicated chromosomes move toward the poles

d) Telophase I i.One duplicated chromosome reaches each pole. ii.The resulting 2 cells will each be haploid

Why would these cells be haploid? Because they have only one set of DNA (even though it is duplicated)

* Cytokinesis also occurs here: 1)Animal cells form a cleavage furrow & plant cells form a new cell wall in between the two new cells 2)A total of two new cells are created and ready to move into Meiosis II

Do plants have sex cells?

15. Meiosis II Is also broken down in to 4 stages: Prophase II Metaphase II Anaphase II And telophase II * Both cells from Meiosis I enter into Meiosis II

a) Prophase II i. Duplicated chromosomes move toward the metaphase plate

b) Metaphase II Duplicated chromosomes line up (on the metaphase plate) so that one sister chromatid will end up in each new cell

c) Anaphase II i.Sister chromatids separate forming individual chromosomes ii.Chromosomes move toward poles

d) Telophase II i.The single chromosome reaches the poles ii.The resulting new cells will be haploid * Cytokinesis also occurs here

16. The 2 cells that have entered into meiosis II each make 2 new cells for a grand total of 4 new cells that each have half the original amount of chromosomes.

17. There is a difference between male gamete production and female gamete production.

a. Spermatogenesis: creates 4 equally sized sperm

Oogenesis: creates 1 larger cell that will become the egg (ovum) and 3 smaller cells, called polar bodies, that will dissolve away and NOT get used.

18. Summary of Understanding Remember: A duplicated chromosome is represented by an “X” A single chromosome is represented by a “l”

deo.aspx?xtid=36224&loid=37226&psid=0&sid =0&State=&title=Microscopes and Mutants&IsSearch=Y&parentSeriesID=# Video Segment The Mystery of Sexual Reproduction: From “The Microscopes & Mutants”