Chapter 4: Section 2

Chapter 4: Section 2Sexual Reproduction and Meiosis

Sexual ReproductionIntroduction to Sexual ReproductionIn sexual reproduction, a new organism is produced when sex cells from two parents combine. Each sex cell is produced by a different parent. Sperm are sex cells produced by the male parent. Sperm are usually small with whiplike tails. Eggs are sex cells produced by the female parent. Eggs are usually large and contain food material. 2

Production of Sex CellsHuman body cells have 23 pairs of chromosomes, but human sex cells have only 23 total chromosomes. The process of nuclear division that produces sex cells is called Meiosis. Meiosis takes place in cells of reproductive organs in both plants and animals. 3

Importance of Sex CellsIn body cells, chromosomes are found in pairs. A cell that has two of every kind of chromosome is said to be diploid. Diploid is written as 2n in Biology. Sex cells contain only one chromosome from each matched pair. A cell that contains only one chromosome is said to be haploid. Haploid is written as 1n in Biology. 4

Haploid CellsDiploid Cells5

Comparing Haploid and Diploid CellsA human body cell is considered diploid. Human body cells contain 23 pairs of chromosomes. A human sex cell is considered to be haploid. A human sex cell contains 23 total chromosomes.

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FertilizationSexual reproduction starts with the formation of sex cells and ends when one sex cell joins with another; a new organism is formed. 7

FertilizationThe joining of an egg and a sperm is called fertilization. The cell that forms in fertilization is called a zygote. If an egg (1n) with 23 chromosomes joins with a sperm (1n) that has 23 chromosomes, a zygote forms that has 46 chromosomes. The zygote is considered to be diploid. (2n)8

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Chromosomes in SpeciesEach species has a fixed number of chromosomes. Humans have 46 chromosomes. A donkey has 66 chromosomes. A horse has 60 chromosomes. 10

III. Meiosis Introduction to MeiosisIn Meiosis, there are two divisions of the nucleus, Meiosis I and Meiosis II.a. Mitosis only has one division!!! Meiosis deals only with sex cells. a. Sperm and Egg11

Diagram of Meiosis I12

III. Meiosis Meiosis IInterphase 1: Cells undergo a round of DNA replication, forming duplicate chromosomes. Prophase 1: Double stranded chromosomes and spindle fibers appear. The nuclear membrane and nucleolus disappear. Like chromosomes come together in matching pairs to form a TETRAD.

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III. Meiosis Meiosis IMetaphase 1Pairs of chromosomes line up in the center of the cell. Centromeres become attached to the spindle fibers. Anaphase 1The spindle fibers pull the homologous (like chromosomes) toward opposite ends of the cell.

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III. Meiosis Meiosis ITelophase 1The cytoplasm divides and two cells form. Each chromosome is still double stranded!!

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Diagram of Meiosis II16

III. Meiosis Meiosis II (follows Meiosis I)Prophase 2The double stranded chromosomes and spindle fibers reappear in each new cell. Metaphase 2The double stranded chromosomes move to the center of the cell. Centromeres attach to the spindle fibers.

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III. Meiosis Meiosis II (follows Meiosis I)Anaphase 2The centromere divides, and two strands of each chromosome separate and move to opposite ends of the cell.18

III. Meiosis Meiosis II (follows Meiosis I)Telophase 2The spindle fibers disappear, and a nuclear membrane forms around the chromosomes at each end of the cell. Each nucleus contains only half the number of chromosomes that were in the original nucleus. 19

Meiosis I and Meiosis II20

III. Meiosis D. Facts about MeiosisA 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 Mistakes in MeiosisMistakes are more common in plants than animals. Mistakes can produce sex cells with too few or too many chromosomes. Sometimes, zygotes produced from these sex cells die. 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 24