1. Unit 2 Lesson 2 Meiosis Copyright © Houghton Mifflin Harcourt Publishing Company

2. Number Off! Copyright © Houghton Mifflin Harcourt Publishing Company 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. Unit 2 Lesson 2 Meiosis

3. 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. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Meiosis

4. 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. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Meiosis

5. Why do organisms need sex cells? Most human body cells contain 46 chromosomes. Sex cells are also known as gametes. Gametes 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. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Meiosis

6. 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. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Meiosis

7. 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. When an egg is fertilized by a sperm cell, a new diploid cell forms. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 2 Lesson 2 Meiosis

8. One Step at a Time Copyright © Houghton Mifflin Harcourt Publishing Company 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. Unit 2 Lesson 2 Meiosis

9. 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 Unit 2 Lesson 2 Meiosis

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

11. 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 Unit 2 Lesson 2 Meiosis

12. 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 Unit 2 Lesson 2 Meiosis

13. 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 Unit 2 Lesson 2 Meiosis

14. 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 Unit 2 Lesson 2 Meiosis

15. What are the stages of meiosis? In males, the four 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 Unit 2 Lesson 2 Meiosis

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

17. 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 Unit 2 Lesson 2 Meiosis

18. 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 Unit 2 Lesson 2 Meiosis.

19. 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 Unit 2 Lesson 2 Meiosis

20. 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 Unit 2 Lesson 2 Meiosis

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

22. Down Syndrome Copyright © Houghton Mifflin Harcourt Publishing Company 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.