MEIOSIS

1

What is Meiosis??2

Every cell in the body makes more cells through mitosis We call these cells somatic

cells The only cells in your body

that are not produced by mitosis are gametes Gametes are sperm and egg

cells They are made through

meiosis

Meiosis (finally)3

Meiosis, is how cells reduce their chromosome number by half As a bonus, they use it to shuffle DNA

around Start with 1 diploid (2N) cell End with 4 haploid (1N) cells

What is Meiosis?4

WHY WOULD WE WANT TO CUT OUR CHROMOSOME NUMBER IN HALF?!

So that when moms gamete and dad’s gamete join together you get a zygote with 46 chromosomes!

Mitosis Review5

Stages of mitosis Prophase

Chromosomes condense Nuclear membrane

disappears Metaphase

Chromosomes line up Anaphase

Chromosomes pulled apart

Telophase Nuclear membrane re-

forms and cell pinches Chromosomes “dissolve”

Makes 2 genetically identical, diploid (2N=46) cells

Stages of Meiosis6

It happens in two steps: Meiosis 1 and Meiosis 2

Meiosis I – similar to mitosis Prophase I Metaphase I Anaphase I Telophase I & Cytokinesis

Meiosis II – identical to mitosis!!!! Prophase II Metaphase II Anaphase II Telophase II & Cytokinesis

Makes 4 similar, but genetically Makes 4 similar, but genetically different haploid different haploid gametesgametes

Meiosis I (reduction Meiosis I (reduction division)division)

7

Goal: reduce the number of chromosomes in the cells

Prophase I Replicated chromosomes

condense (this means we can see each individual X)

Homologous chromosomes pair up Mom’s #1 and Dad’s #1, all the way

through Moms #46 and Dad’s #46 “Crossing-over” happens

An extra shuffling of the DNA

Prophase IProphase I8

Crossing over Chromatids of homologous

chromosomes exchange material Mom’s #1 and Dad’s #1 switch

some pieces Leads to new genetic

combinations; that way you don’t look like your older or younger siblings!

Prophase IProphase I9

Metaphase I10

Paired homologous chromosomes line up along equator

Moms #1

Dads #1

Anaphase I11

Homologous chromosomes separate & move to opposite poles

Moms #1

Dads #1

**Remember, at this point, mom and dad’s chromosomes are replicated, that is why they look like X’s. Each part of the X is called a sister chromatid

Telophase I12

Nuclear membranes partially reform around two nuclei

Cytokinesis occurs between meiosis I & II

46 Chromosomes

46 Chromosomes

Meiosis II13

We have now achieved haploid cells This means, each of the two cells have 23

X’s each. However, sister chromatids need to be

separated A second round of division pulls the

chromosomes apart Just like in mitosis

Meiosis II is virtually identical to mitosis

Meiosis II14

Prophase II Chromosomes condense and nucleus/nucleolus

disappear Metaphase II

Chromosomes line up in the middle of the cell Anaphase II

Sister chromatids separate to opposite sides of the cell

Telophase II and Cytokinesis 4 new cells, each with 23 chromosomes are

made

Prophase II Metaphase II Anaphase II Telophase IIFour non-

identical haploid

cells

15

16

Spermatogenesis in humans(Meiosis in the male)

17

Comparison of spermatogenesis and oogenesis

18

Why understanding meiosis is important19

A stage of human development where things can go very wrong: Down syndrome

Trisomy 21 Learning disability/MR

Patau syndrome Trisomy 13

Klinefelter’s syndrome Turner syndrome Edwards syndrome

Trisomy 18 5-10% survive first year