Table of Contents
Chapter: Cell Reproduction
Section 1: Cell Division and Mitosis
Section 2: Sexual Reproduction and Meiosis
Section 3: DNA

Sexual Reproduction and Meiosis
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A. Sexual Reproduction
1. During sexual reproduction , two sex cells, sometimes called an egg and a sperm, come together.
2. Sex cells are formed from cells in reproductive organs. Sperm are formed in the male reproductive organs and eggs are formed in the female reproductive organs.

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A. Sexual Reproduction
3. The joining of an egg and a sperm is called fertilization , and the cell that forms is called a zygote (ZI goht).
4. Following fertilization, cell division begins. A new organism with a unique identity develops.

Sexual Reproduction and Meiosis
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B. Diploid Cells
1. A typical human body cell has 46 chromosomes or 23 pairs of chromosomes.
2. Each chromosome has a mate that is similar to it in size and shape and has similar DNA.
3. When cells have pairs of similar chromosomes, they are said to be diploid
(DIH ployd).

Sexual Reproduction and Meiosis
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C. Haploid Cells
1. Because sex cells do not have pairs of chromosomes, they are said to be haploid
(HA ployd).
2. They have only half the number of chromosomes as body cells. Haploid means “single form.”
3. Human sex cells have only 23 chromosomes—one from each of the 23 pairs of similar chromosomes.

Sexual Reproduction and Meiosis
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D. Meiosis and Sex Cells
1. A process called meiosis (mi OH sus) produces haploid sex cells.

Sexual Reproduction and Meiosis
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D. Meiosis and Sex Cells
2. Meiosis ensures that the offspring will have the same diploid number as its parent.

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D. Meiosis and Sex Cells
3. After two haploid sex cells combine, a diploid zygote is produced that develops into a new diploid organism.

Sexual Reproduction and Meiosis
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D. Meiosis and Sex Cells
4. During meiosis, two divisions of the nucleus occur. These divisions are called meiosis I and meiosis II.

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D. Meiosis and Sex Cells
5. The steps of each division have names like those in mitosis and are numbered for the division in which they occur.

Sexual Reproduction and Meiosis
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E. Meiosis I
1. Before meiosis begins, each chromosome is duplicated, just as in mitosis.

Sexual Reproduction and Meiosis
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E. Meiosis I
2. When the cell is ready for meiosis, each duplicated chromosome is visible under the microscope as two chromatids.

Sexual Reproduction and Meiosis
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E. Meiosis I
3. The events of prophase I are similar to those of prophase in mitosis.
4. In meiosis, each duplicated chromosome comes near its similar duplicated mate.
5. In mitosis they do not come near each other.

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E. Meiosis I
6. In metaphase I, the pairs of duplicated chromosomes line up in the center of the cell.
7. The centromere of each chromatid pair becomes attached to one spindle fiber, so the chromatids do not separate in anaphase I.

Sexual Reproduction and Meiosis
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E. Meiosis I
8. In anaphase I, the two pairs of chromatids of each similar pair move away from each other to opposite ends of the cell.
9. Each duplicated chromosome still has two chromatids.

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Sexual Reproduction and Meiosis
E. Meiosis I
10. In telophase I, the cytoplasm divides, and two new cells form.
11. Each new cell has one duplicated chromosome from each similar pair.

Sexual Reproduction and Meiosis
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F. Meiosis II
1. The two cells formed during meiosis I now begin meiosis II.

Sexual Reproduction and Meiosis
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F. Meiosis II
2. The chromatids of each duplicated chromosome will be separated during this division.

Sexual Reproduction and Meiosis
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F. Meiosis II
3. In prophase II, the duplicated chromosomes and spindle fibers reappear in each new cell.

Sexual Reproduction and Meiosis
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F. Meiosis II
4. In metaphase II, the duplicated chromosomes move to the center of the cell.
5. Unlike what occurs in metaphase I, each centromere now attaches to two spindle fibers instead of one.

Sexual Reproduction and Meiosis
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F. Meiosis II
6. The centromere divides during anaphase II, and the chromatids separate and move to opposite ends of the cell.
7. Each chromatid now is an individual chromosome.

Sexual Reproduction and Meiosis
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F. Meiosis II
8. As telophase II begins, the spindle fibers disappear, and a nuclear membrane forms around the chromosomes at each end of the cell.
9. When meiosis II is finished, the cytoplasm divides.

Sexual Reproduction and Meiosis
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Summary of Meiosis
•
This animation will help you further understand meiosis.
•
Remember that meiosis produces haploid sex cells.
Click box to view movie.

Sexual Reproduction and Meiosis
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G. Mistakes of Meiosis
1. Meiosis occurs many times in reproductive organs.
2. Mistakes can produce sex cells with too many or too few chromosomes.

Sexual Reproduction and Meiosis
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G. Mistakes of Meiosis
3. Sometimes, zygotes produced from these sex cells die.
4. If the zygote lives, every cell in the organism that grows from that zygote usually will have the wrong number of chromosomes.
5. Organisms with the wrong number of chromosomes may not grow normally.

Section Check
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Question 1
_______ is the joining of an egg and a sperm.

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Section Check
Question 2
_______ and _______ are the two types of cells your body forms.
A. body and sex
B. egg and sperm
C. meiosis and mitosis
D. zygote and embryo

Section Check
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Question 3
Which best describes the sperm and egg?

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Section Check
A. typical body cells
B. haploid cells
C. zygotes
D. diploid cells