Essential Question: How do cells divide for sexual reproduction? Unit 2 Lesson 2 Meiosis Essential Question: How do cells divide for sexual reproduction? Copyright © Houghton Mifflin Harcourt Publishing Company

Number Off! How do sex cells differ from body cells? Unit 2 Lesson 2 Meiosis Number Off! How do sex cells differ from body cells? Before sexual reproduction can take place, each parent produces sex cells. Sex cells have half of the genetic information that body cells have. When genetic information from two parents combines, the offspring will have one full set of genetic information. 3

How do sex cells differ from body cells? Unit 2 Lesson 2 Meiosis How do sex cells differ from body cells? In body cells, chromosomes are found in pairs of homologous chromosomes, which have the same structure and size. Homologous chromosomes carry the same genes. However, some may be different versions of the genes. 4

How do sex cells differ from body cells? Unit 2 Lesson 2 Meiosis How do sex cells differ from body cells? One chromosome pair is made up of sex chromosomes, which control the development of sexual characteristics. In humans, the sex chromosomes are called X and Y chromosomes. Cells with a pair of every chromosome are called diploid. 5

Why do organisms need sex cells? Unit 2 Lesson 2 Meiosis Why do organisms need sex cells? Most human body cells contain 46 chromosomes. Gametes (sex cells – sperm and egg cells) have only half the usual number of chromosomes. Gametes are haploid, meaning they have one chromosome from each homologous pair. Gametes are made in the reproductive organs. 6

Why do organisms need sex cells? Unit 2 Lesson 2 Meiosis Why do organisms need sex cells? When sex cells combine during sexual reproduction, the resulting cell has 46 chromosomes: half from the egg and half from the sperm. When an egg is fertilized by a sperm cell, a new diploid cell forms. 7

Unit 2 Lesson 2 Meiosis How are sex cells made? Mitosis produces two new cells that contain exact copies of the chromosomes in the parent cell. A different kind of cell division is needed to produce sex cells. Meiosis is the type of cell division that produces haploid sex cells such as sperm or egg cells. 8

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? In males, the sex cells develop into sperm cells. In females, the sex cells become egg cells. In females of some species, three cells are broken down and only one haploid cell becomes an egg. Copyright © Houghton Mifflin Harcourt Publishing Company 9

One Step at a Time What are the stages of meiosis? Unit 2 Lesson 2 Meiosis One Step at a Time What are the stages of meiosis? Meiosis has two parts: meiosis I and meiosis II. Recall that homologous chromosomes have the same genes but are not exact copies of each other. Copyright © Houghton Mifflin Harcourt Publishing Company 10

What are the stages of meiosis? p104 Unit 2 Lesson 2 Meiosis What are the stages of meiosis? Before meiosis I begins, each chromosome is duplicated. Each half is called a chromatid. Chromatids are connected by centromeres. During meiosis I, pairs of homologous chromosomes and sex chromosomes split apart into two new cells. Copyright © Houghton Mifflin Harcourt Publishing Company 11

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? In prophase I, duplicated homologous chromosomes pair up. Copyright © Houghton Mifflin Harcourt Publishing Company 12

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? In metaphase I, the homologous chromosome pairs line up in the middle of the cell. Copyright © Houghton Mifflin Harcourt Publishing Company 13

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? In anaphase I, the homologous chromosomes separate from their partners and move to opposite ends. Copyright © Houghton Mifflin Harcourt Publishing Company 14

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? In telophase I and cytokinesis, the nuclear membranes re-form and the cell divides into two cells. The chromatids are still joined. Copyright © Houghton Mifflin Harcourt Publishing Company 15

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? Meiosis II involves both of the new cells that formed during meiosis I. The new cells divide during meiosis II. Meiosis II results in four haploid sex cells. Copyright © Houghton Mifflin Harcourt Publishing Company 16

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? In prophase II, the nuclear membrane breaks apart. Copyright © Houghton Mifflin Harcourt Publishing Company 17

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? In metaphase II, the chromosomes line up in the middle of each cell. Copyright © Houghton Mifflin Harcourt Publishing Company 18

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? In anaphase II, the chromatids are pulled apart and move to opposite sides. . Copyright © Houghton Mifflin Harcourt Publishing Company 19

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? In telophase II and cytokinesis, the nuclear membranes re-form and the cells divide. Each cell is haploid. Copyright © Houghton Mifflin Harcourt Publishing Company 20

What are the stages of meiosis? Unit 2 Lesson 2 Meiosis What are the stages of meiosis? 21

Mitosis Meiosis How does meiosis compare to mitosis? Unit 2 Lesson 2 Meiosis How does meiosis compare to mitosis? Mitosis Meiosis 22

How does meiosis compare to mitosis? Unit 2 Lesson 2 Meiosis How does meiosis compare to mitosis? Only cells that will become sex cells undergo meiosis. All other cells divide by mitosis. In meiosis, chromosomes are copied once, and the nucleus divides twice. In mitosis, the chromosomes are copied once, and the nucleus divides once. Copyright © Houghton Mifflin Harcourt Publishing Company 23

How does meiosis compare to mitosis? Unit 2 Lesson 2 Meiosis How does meiosis compare to mitosis? Meiosis produces haploid cells. Mitosis produces diploid cells. Copyright © Houghton Mifflin Harcourt Publishing Company 24

2 1 4 2 Haploid Diploid Sex Cells Body Cells Unit 2 Lesson 2 Meiosis How does meiosis compare to mitosis? 2 1 4 2 Haploid Diploid Sex Cells Body Cells Prophase Metaphase Anaphase Telophase Cytokinesis Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 Cytokinesis Prophase II Metaphase II Anaphase II Telophase II Cytokinesis 25

Mitosis Meiosis How does meiosis compare to mitosis? DNA duplication Unit 2 Lesson 2 Meiosis How does meiosis compare to mitosis? DNA duplication Ends with 2 daughter cells Ends with 4 Haploid daughter cells Purpose for growth and development Creates sex cells for reproduction– sperm and egg Mitosis Meiosis 10 stages Crossing over occurs 5 stages Genetically identical Cell division 26

Unit 2 Lesson 2 Meiosis Down Syndrome Down syndrome is a genetic disease caused by an error during meiosis. The chromatids in chromosome 21 do not separate, so one of the sex cells gets an extra copy of chromosome 21. Down syndrome causes a number of health problems and learning difficulties, but many people with Down syndrome have fulfilling lives. Copyright © Houghton Mifflin Harcourt Publishing Company 27