Chapter 4: Section 2

Sexual Reproduction and Meiosis

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II. Sexual ReproductionA. Introduction to Sexual Reproduction

1. In sexual reproduction, a new organism is produced when sex cells from two parents combine.

a. Each sex cell is produced by a different parent. 1. Sperm are sex cells produced by the male

parent. a. Sperm are usually small with whiplike

tails. 2. Eggs are sex cells produced by the female

parent. a. Eggs are usually large and contain food

material.

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B. Production of Sex Cells1. Human body cells have 23 pairs of

chromosomes, but human sex cells have only 23 total chromosomes.

2. The process of nuclear division that produces sex cells is called Meiosis.

a. Meiosis takes place in cells of reproductive organs in both plants and animals.

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C. Importance of Sex Cells1. In body cells, chromosomes are found in

pairs. a. A cell that has two of every kind of

chromosome is said to be diploid. b. Diploid is written as 2n in Biology.

2. Sex cells contain only one chromosome from each matched pair.

a. A cell that contains only one chromosome is said to be haploid.

b. Haploid is written as 1n in Biology.

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D. Comparing Haploid and Diploid Cells1. A human body cell is considered

diploid. a. Human body cells contain 23 pairs of

chromosomes.

2. A human sex cell is considered to be haploid.

a. A human sex cell contains 23 total chromosomes.

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E. Fertilization1. Sexual reproduction starts with the

formation of sex cells and ends when one sex cell joins with another; a new organism is formed.

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E. Fertilization2. The joining of an egg and a sperm is

called fertilization. a. The cell that forms in fertilization is

called a zygote. b. If an egg (1n) with 23 chromosomes

joins with a sperm (1n) that has 23 chromosomes, a zygote forms that has 46 chromosomes.

1. The zygote is considered to be diploid. (2n)

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F. Chromosomes in Species1. Each species has a fixed number of

chromosomes. a. Humans have 46 chromosomes. b. A donkey has 66 chromosomes. c. A horse has 60 chromosomes.

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III. Meiosis

A. Introduction to Meiosis1. In Meiosis, there are two divisions of

the nucleus, Meiosis I and Meiosis II.a. Mitosis only has one division!!!

2. Meiosis deals only with sex cells. a. Sperm and Egg

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Diagram of Meiosis I

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III. Meiosis B. Meiosis I

1. Interphase 1: Cells undergo a round of DNA replication, forming duplicate chromosomes.

2. Prophase 1: a. Double stranded chromosomes and spindle

fibers appear. b. The nuclear membrane and nucleolus

disappear. c. Like chromosomes come together in matching

pairs to form a TETRAD.

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III. Meiosis B. Meiosis I

3. Metaphase 1a. Pairs of chromosomes line up in the

center of the cell. b. Centromeres become attached to the

spindle fibers.

4. Anaphase 1a. The spindle fibers pull the homologous

(like chromosomes) toward opposite ends of the cell.

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III. Meiosis B. Meiosis I

5. Telophase 1a. The cytoplasm divides and two cells

form. b. Each chromosome is still double

stranded!!

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Diagram of Meiosis II

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III. Meiosis C. Meiosis II (follows Meiosis I)

1. Prophase 2a. The double stranded chromosomes and

spindle fibers reappear in each new cell.

2. Metaphase 2a. The double stranded chromosomes

move to the center of the cell. b. Centromeres attach to the spindle fibers.

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III. Meiosis C. Meiosis II (follows Meiosis I)

3. Anaphase 2a. The centromere divides, and two

strands of each chromosome separate and move to opposite ends of the cell.

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III. Meiosis C. Meiosis II (follows Meiosis I)

4. Telophase 2a. The spindle fibers disappear, and a

nuclear membrane forms around the chromosomes at each end of the cell.

b. Each nucleus contains only half the number of chromosomes that were in the original nucleus.

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Meiosis I and Meiosis II

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III. Meiosis D. Facts about Meiosis

1. A cell with 46 chromosomes at the beginning of Meiosis 1 divides to produce cells that each only have 23 single stranded chromosomes at the end of Meiosis 2.

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III. Meiosis E. Mistakes in Meiosis

1. Mistakes are more common in plants than animals.

2. Mistakes can produce sex cells with too few or too many chromosomes.

a. Sometimes, zygotes produced from these sex cells die.

b. If the zygote lives, the organism may not grow normally because cells will have the wrong number of chromosomes.

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III. Meiosis