Unit 6 Lesson 2 Meiosis Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 Lesson 2 Meiosis Florida Benchmarks • SC.7.L.16.3 Compare and contrast the general processes of sexual reproduction requiring meiosis and asexual reproduction requiring mitosis. • LA.6.4.2.2 The student will record information (e.g., observations, notes, lists, charts, legends) related to a topic, including visual aids to organize and record information and include a list of sources used. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 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. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 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. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 Lesson 2 Meiosis How do sex cells differ from body cells? • One chromosome pair is made up of sex chromosomes. • Cells with homologous chromosomes are called diploid. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 Lesson 2 Meiosis Why do organisms need sex cells? • Normal human body cells contain 46 chromosomes. • Sex cells are also known as gametes. Gametes have only half of 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 6 Lesson 2 Meiosis Why do organisms need sex cells? • Sex cells are needed so that in sexual reproduction offspring receive a total of 46 chromosomes. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 Lesson 2 Meiosis How are sex cells made? • Mitosis produces two new cells that contain exact copies of the chromosomes of 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 6 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 Unit 6 Lesson 2 Meiosis One Step at a Time 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 6 Lesson 2 Meiosis What are the stages of meiosis? • In prophase I, duplicated homologous chromosomes pair up. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 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 Unit 6 Lesson 2 Meiosis What are the stages of meiosis? • In anaphase I, the homologous chromosomes separate from their partners and move to opposite sides. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 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 Unit 6 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 Unit 6 Lesson 2 Meiosis What are the stages of meiosis? • In males, the four sex cells develop into sperm 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 6 Lesson 2 Meiosis What are the stages of meiosis? • In prophase II, the nuclear membrane breaks apart. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 Lesson 2 Meiosis What are the stages of meiosis? • In metaphase II, the chromosomes line up in the middle of the cells. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 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 Unit 6 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 Unit 6 Lesson 2 Meiosis How does meiosis compare to mitosis? • Only 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 6 Lesson 2 Meiosis How does meiosis compare to mitosis? • Meiosis produces haploid cells. Mitosis produces diploid cells. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 6 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