Unit 7 Lesson 2 Meiosis
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Unit 7 Lesson 2 Meiosis
Indiana Standards
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• 8.3.3 Explain that genetic information is transmitted from parents to offspring mostly by chromosomes.
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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 7 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.
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Unit 7 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.
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Unit 7 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.
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Unit 7 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.
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Unit 7 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.
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Unit 7 Lesson 2 Meiosis
One Step at a Time
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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 7 Lesson 2 Meiosis
One Step at a Time
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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.
Unit 7 Lesson 2 Meiosis
What are the stages of meiosis?• In prophase I, duplicated homologous
chromosomes pair up.
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Unit 7 Lesson 2 Meiosis
What are the stages of meiosis?• In metaphase I, the homologous chromosome
pairs line up in the middle of the cell.
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Unit 7 Lesson 2 Meiosis
What are the stages of meiosis?• In anaphase I, the homologous chromosomes
separate from their partners and move to opposite sides.
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Unit 7 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.
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Unit 7 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.
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Unit 7 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.
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Unit 7 Lesson 2 Meiosis
What are the stages of meiosis?• In prophase II, the nuclear membrane breaks
apart.
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Unit 7 Lesson 2 Meiosis
What are the stages of meiosis?• In metaphase II, the chromosomes line up in the
middle of the cells.
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Unit 7 Lesson 2 Meiosis
What are the stages of meiosis?• In anaphase II, the chromatids are pulled apart
and move to opposite sides.
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Unit 7 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.
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Unit 7 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.
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Unit 7 Lesson 2 Meiosis
How does meiosis compare to mitosis?• Meiosis produces haploid cells. Mitosis produces
diploid cells.
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Unit 7 Lesson 2 Meiosis
Unit 7 Lesson 2 Meiosis
Down Syndrome
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• 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.
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