Unit 9 lesson 2: Meiosis https://superchicbio.files.wordpress.com/2015/06/sexual-reproduction-avatar.gif

Common words in today’s notes gametes (sex cells) sperm egg haploid chromosomes

Meiosis overview Look at the first two letters of the word meiosis. It spells the word me. Meiosis is how ME got here! “Me” got here from sexual reproduction. Two gametes (one egg and one sperm) came together to make an organism (me!) Meiosis is the cell division that makes sex cells or gametes.

Sperm Made in testes Meiosis Cells divide into sex cells (gametes) Gametes are haploid, or n Haploid = 1/2 the correct number of chromosomes for organism Male sex cell Sperm Made in testes Meiosis

Egg Made in ovaries Meiosis Female sex cell Gametes contain ½ the number of original chromosomes.

Human Chromosome Numbers Human body cells have 46 chromosomes. Human sex cells formed from meiosis have 23 chromosomes. Human sex cells or gametes have ½ the number of chromosomes found in the body cells of an organism.

Overview of Meiosis

Meiosis overview (2)

Meiosis produces 4 new cells from one. http://www.wereyouwondering.com/what-is-the-difference-between-mitosis-and-meiosis/ Meiosis produces 4 new cells from one. The new cells each contain ½ number of chromosomes. Notice that no two sex cells are alike!

Meiosis forms 4 male gametes (sperm) Meiosis forms 1 egg and 3 smaller cells in female.

Why Meiosis? Results in sex cells (gametes) for sexual reproduction Cells made by meiosis contain ½ the number of chromosomes for the organism (23 for humans) Sex cells or gametes are haploid or have an “n” number of chromosomes

Why Meiosis? Produces cells with HALF the number of chromosomes so that fertilization can take place The beginning of the word haploid sounds like the beginning of the word HALF! Source of variation in organisms All sperm and egg are different Chromosomes can “exchange” DNA in a process called crossing over =Increases variety! Image: http://www.phschool.com/science/biology_place/labbench/lab3/images/crossovr.gif

Meiosis What is the second letter in the word meiosis? It is the letter e. All of the cells that are formed from meiosis have an e in them! Sex cells Gametes Egg Sperm Meiosis happens in ovaries and testes

Let’s Look at Meiosis 2n n n n n n n http://www.phschool.com/science/biology_place/biocoach/meiosis/geneseg1.html

Why is meiosis important? Meiosis must reduce the number of chromosomes by half for fertilization In fertilization the egg and sperm come together to make one cell called a zygote. Fertilization creates a single-celled zygote with the correct number of chromosomes for the organism (46 for humans). Zygote then grows by mitosis, making body cells.

Fertilization Fertilization egg (n) + sperm (n) http://www.dnalc.org/view/16240-Gallery-8-Sperm-fertilzing-an-egg.html Growth of cells by mitosis Zygote (2n) http://www.americanpregnancy.org/infertility/zift.html http://onepregnancy.files.wordpress.com/2011/02/2cell-embryo.jpg

Father makes sperm by meiosis Mother makes eggs by meiosis Let’s Review Father makes sperm by meiosis Mother makes eggs by meiosis Sperm and eggs have ½ the number of chromosomes for an organism Sperm (n) haploid 23 chromosomes Growth by mitosis Zygote (2n) diploid 46 chromosomes Formed by meiosis Fertilization Egg (n) haploid 23 chromosomes

*Notice meiosis starts with 1 cell with 4 chromosomes http://terra.dadeschools.net/books/Biology/BiologyExploringLife04/0-13-115075-8/text/chapter9/concept9.5.html

*meiosis ends with 4 cells with 2 chromosomes in each http://terra.dadeschools.net/books/Biology/BiologyExploringLife04/0-13-115075-8/text/chapter9/concept9.5.html