1. Do Now ● Do Now ● Do Now
Write down the following questions in your journal. Answer each question in complete sentences. 1.) List the phases of the cell cycle. Interphase, Prophase, Metaphase, Anaphase, Telophase 2.) What are the phases of mitosis? Prophase, Metaphase, Anaphase, Telophase 3.) Describe the events of each stage of mitosis. Prophase=Nucleus and nucleolus disappear, chromatids are visible Metaphase=Spindle fibers connect to chromatids, chromatids meet in the middle Anaphase= Chromatids are pulled apart to opposite ends of the cell Telophase = Nuclei begin to form, cell membrane pinches/cell plate forms, cytokinesis

2. Section 2: Sexual Reproduction and Meiosis
Ch. 4: Cell Reproduction
Section 2: Sexual Reproduction and Meiosis

3. Sexual Reproduction
During sexual reproduction, two sex cells, sometimes called an egg and a sperm, come together.
These sex cells are formed from cells in reproductive organs.
Sperm are formed in the male reproductive organs.
Eggs are formed in the female reproductive organs.
The joining of an egg and a sperm is called fertilization, and a cell that forms is called a zygote.

4. Sexual Reproduction
Generally, the egg and sperm come from two different organisms of the same species.
Following fertilization, mitosis and cell division begin. A new organism with a unique identity develops.
Your body forms two types of cells – body and sex cells. Body cells far outnumber sex cells.
Your skin, bones, and other tissues and organs are formed from body cells.

5. Sexual Reproduction
Remember, a typical human body cell has 46 chromosomes. Each chromosome has a mate that is similar to it in size and shape and has similar DNA.
Human body cells have 23 pairs of chromosomes.
When cells have pairs of similar chromosomes, they are diploid.
Most body cells are diploid (46 chromosomes) while sex cells are haploid (23 chromosomes)

6. Meiosis and Sex Cells
A process called meiosis produces haploid sex cells. Mitosis produces diploid body cells.
Meiosis ensures that the offspring will have the same diploid number as the parent.
After two haploid sex cells combine, a diploid zygote is produced that develops into a new diploid organism.

7. Meiosis and Sex Cells
During meiosis, two divisions of the nucleus occur. These divisions are called meiosis I and meiosis II. Meiosis I 1.) Before meiosis begins, like mitosis, each chromosome is duplicated. 2.) When a cell is ready for meiosis, each duplicated chromosome is visible under the microscope as two chromatids. 3.) Prophase I: The nucleus and nucleolus disappear. Each duplicated chromosome comes near its similar duplicated mate. This does NOT happen in mitosis. Crossing over occurs between P1 and M1 of meiosis. Segments of the chromosomes are switched. This leads to genetic diversity.

8. Meiosis and Sex Cells
4.) Metaphase I: the pairs of duplicated chromosomes line up in the center of the cell.
The centromere of each chromatid pair becomes attached to one spindle fiber, so the chromatids won’t separate in Anaphase I.
5.) Anaphase I: The two pairs of chromatids of each similar pair move away from each other to opposite ends of the cell.
Each duplicated chromosome still has two chromatids

9. Meiosis and Sex Cells
6.) Telophase I: The cytoplasm divides, and two new cells form. Each new cell has one duplicated chromosome from each similar pair. Two new haploid cells are formed at the end of meiosis I.

11. Meiosis II:
The two cells formed during meiosis I now begin meiosis II. The chromatids of each duplicated chromosome will be separated during this division.
1.) Prophase II: the duplicated chromosomes and spindle fibers reappear in each new cell.
2.) Metaphase II: the duplicated chromosomes move to the center of the cell. Each centromere now attaches to two spindle fibers instead of one.

12. Meiosis II:
3.) Anaphase II: the centromere divides and the chromatids separate and move to opposite ends of the cell. Each chromatid is now an individual chromosome. 4.) Telophase II: the spindle fibers disappear, and a nuclear membrane forms around each set of chromosomes. When meiosis II is finished, the cytoplasm divides. Four new haploid cells are formed at the end of meiosis II.