Warm up: 1) How many chromosomes do human somatic (body) cells have? 2) What gametes (sex cells) do men produce? How about women? 3) How many chromosomes do human sex cells have?

It’s…….Meiosis!!!!

Chromosome Number Organisms have different # of chromosomes. Humans: 46 chromosomes, 23 pairs Each of the 23 pairs is called a homologous pair: a similar pair where one came from the mother and one came from the father. What’s this chart called??

Quick review! How many PAIRS of chromosomes do humans have? What is a homologous pair?

Chromosome Number Diploid: A cell that contains both sets of homologous chromosomes. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. Almost all of the cells in your body are diploid (have both sets of chromosomes).

Chromosome Number Haploid: Cells that contain only a single set of chromosomes, and therefore only a single set of genes. (Gametes are Haploid, sperm/egg) These cells are and are represented by the symbol N. Think: HAPloid sounds like HALF-loid

Quick Review A human skin cell has 46 chromosomes (or 23 pairs of homologous chromosomes). We call this diploid / haploid?? What is the haploid number of chromosomes for humans?

How are haploid cells produced? Haploid cells are produced in a process called meiosis. **The purpose of meiosis is to produce four haploid gamete cells that are genetically different from one another and from the original parent cell.

3 Main purposes of Meiosis 1) To cut the # of chromosomes in half in a cell (46 to 23) (haploid). 2) To create gametes (sex cells) egg and sperm. 3) Introduce genetic variation (independent assortment, crossing over).

Phases of Meiosis Meiosis involves two divisions, meiosis I and meiosis II. Overview of what happens: Meiosis I: the chromosomes replicate and the cell divides once Meiosis II: each new cell divides, producing 4 cells total By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

Do you remember the stages of MITOSIS from Unit 4? P M A T C

Telophase I and Cytokinesis Meiosis I Interphase I During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Meiosis I Interphase I: Cells undergo a round of DNA replication, forming duplicate chromosomes.

Meiosis I Prophase I: Each Homologous chromosome pairs up to form a tetrad (with 4 chromatids). When homologous chromosomes form tetrads, they exchange portions of their chromatids in a process called *crossing over*, producing new combinations of alleles (genes). This is why you and your brothers and sisters can look different!

Meiosis I Metaphase I: Spindle fibers attach to the chromosomes *Independent assortment*- Homologous pairs can line up on whichever side they want! This mixes things up and creates genetic variation!

Meiosis I Anaphase I The fibers pull the homologous chromosomes toward opposite ends of the cell. ***HOMOLOGOUS PAIRS separate.

Meiosis I Telophase I and Cytokinesis Nuclear membranes form. The cell separates into two cells. The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

Telophase II and Cytokinesis Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, chromosome replication does NOT occur. Prophase II: chromosomes are condensed. Metaphase II: Chromosomes line up in the center of cell. Anaphase II: ***Sister chromatids separate to opposite ends of the cell. Telophase II: The cell pinches in. Meiosis II results in four haploid (N) daughter cells. Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Telophase I and Cytokinesis I

Quick Write! Name all the phases of Meoisis in order from Prophase I to Telophase/Cytokinesis II.

Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm. Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm.

Gamete Formation In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction. Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction.

Review Questions What do the terms haploid and diploid mean? Describe the main results of meiosis. What do you end up with? What is the result of independent assortment and crossing over? Name two differences between mitosis and meiosis.

3 Main purposes of Meiosis 1) To cut the # of chromosomes in half in a cell (46 to 23) (haploid). 2) To create gametes (sex cells) egg and sperm. 3) Introduce genetic variation (independent assortment, crossing over). *Bonus: What would happen if each of these purposes were not met?

3 main purposes of Meosis? 1) 2) 3)

Why can’t your body use mitosis to make sperm or egg cells? Suppose human sperm and eggs were produced by mitosis. How many chromosomes would each sperm or egg have? 46 chromosomes If a sperm of this type fertilized an egg of this type, how many chromosomes would the resulting zygote have? 92 chromosomes

Obviously, if the body used mitosis to make sperm and eggs, the resulting zygote would have too many chromosomes to produce a normal body. In humans, to produce a normal zygote, how many chromosomes should each sperm and egg have? 23 chromosomes  This is why meiosis is so important and occurs in gametes.