Journal Questions: At one level, the answers lie in meiosis. 1.How are genes passed on to offspring? 2.Where are the cells made that carry these genes? 3.What is special about these cells? 4.How are these cells made?

Meiosis: Cell division in which the daughter cells have half the DNA as the parent cell. Purpose: To create cells to be used in sexual reproduction= sex cells/gametes

I. Regular Cells vs. Sex Cells A.Regular cells 1.Also called Somatic Cells 2. Have 2 sets of chromosomes= Diploid Cell 3. Each set is inherited from one parent.

4. Examples -- muscle, nerve, skin cell, etc. are somatic, diploid, have 46 chromosomes (2 sets of 23)

B.Sex Cells 1. also called gametes 2. have 1 complete set of chromosomes = haploid

3. Examples: Sperm and Egg cells = gametes; haploid; have 23 chromosomes (1 set of 23)

II. Sexual Reproduction sperm(haploid) + egg(haploid) zygote: fertilized egg; 1 st diploid cell.

III. More on Chromosomes A. Chromosome # 1. Each species has specific number of chromosomes. 2. # is not necessarily related to complexity.

B.Homologous chromosomes. 1. same type of chromosome a. carry same type of genes b. same length c. same centromere position

2. If homologs are in a cell, it is diploid; If not, the cell is haploid.

3. Chromosomes that contain genes that determine sex/gender = sex chromosomes; in animals there are 2 sex chromosomes in all somatic cells. 4. All other chromosomes = autosomes; in humans there are 44 autosomes in all somatic cells. X chromosome Y chromosome humans = 46 chromosomes 2 are sex chromosomes (XX = female; Xy male 44 are autosomes

Journal Questions 5. Is this cell diploid or haploid? Explain your answer. 6. A mouse cell has 40 chromosomes. How many are autosomes and how many are sex chromosomes? 7. Before meiosis, cells in the ovaries/testes have 46 chromosomes. During the S phase, these chromosomes replicate. What do you think will happen in meiosis to create daughter cells with 23 chromosomes? Explain & draw.

IV. Making Sex Cells A.Cell cycle 1. Normal until M phase; now M = meiosis 2. Cells that are programmed to do meiosis are called germ cells (located in ovaries and testes).

3. During Meiosis, 2 divisions must occur because there is double DNA after the S phase and goal is to get to half DNA. Overall View of Meiosis

Meiosis – mouse testes Parent cell 4 gametes 1 st division 2 nd division

B. Stages of Meiosis Prophase 1: Prophase 1:

* When homologs are in a tetrad, they can cross chromatids and swap genes= crossing over; results in new combinations of genes on a chromosome that were not together in either parent.

Metaphase 1: pairs line up in middle *Either homolog can end up on either side of the line-up which means a unique combination of chromosomes can end up in the egg or sperm.

Example of how metaphase Line-up affects chromosomes In gametes.

Anaphase 1: Telophase 1: 2 nuclei form, cell begins to split. Cytokinesis 1: cell splits into 2 cells that are haploid, but still have double standed (replicated) chromosomes.

Figure 13.7 The stages of meiotic cell division: Meiosis I

Prophase 2: Metaphase 2: Anaphase 2:

Telophase 2: 2 nuclei form, cells begin to split Cytokinesis 2: both cells split giving 4 haploid cells with single stranded (normal) chromosomes.

Figure 13.7 The stages of meiotic cell division: Meiosis II

Journal Questions 8.Make a venn diagram comparing mitosis and meiosis. 9.How does a son end up with an X & y chromosome? 10.How could a daughter end up with 3 X chromosomes? (47XXX)

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis