Unit 2-2 Meiosis - Phillip's Academy Charter School · Unit 2 Lesson 2 Meiosis. How do sex cells...
Transcript of Unit 2-2 Meiosis - Phillip's Academy Charter School · Unit 2 Lesson 2 Meiosis. How do sex cells...
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Unit 2 Lesson 2 Meiosis
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Number Off!
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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 2 Lesson 2 Meiosis
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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 2 Lesson 2 Meiosis
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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.
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Unit 2 Lesson 2 Meiosis
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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.
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Unit 2 Lesson 2 Meiosis
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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.
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Unit 2 Lesson 2 Meiosis
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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.
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Unit 2 Lesson 2 Meiosis
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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 2 Lesson 2 Meiosis
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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.
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Unit 2 Lesson 2 Meiosis
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What are the stages of meiosis?
• In prophase I, duplicated homologous chromosomes pair up.
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Unit 2 Lesson 2 Meiosis
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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 2 Lesson 2 Meiosis
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What are the stages of meiosis?
• In anaphase I, the homologous chromosomes separate from their partners and move to opposite ends.
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Unit 2 Lesson 2 Meiosis
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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 2 Lesson 2 Meiosis
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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 2 Lesson 2 Meiosis
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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.
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Unit 2 Lesson 2 Meiosis
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What are the stages of meiosis?
• In prophase II, the nuclear membrane breaks apart.
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Unit 2 Lesson 2 Meiosis
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What are the stages of meiosis?
• In metaphase II, the chromosomes line up in the middle of each cell.
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Unit 2 Lesson 2 Meiosis
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What are the stages of meiosis?
• In anaphase II, the chromatids are pulled apart and move to opposite sides.
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Unit 2 Lesson 2 Meiosis
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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 2 Lesson 2 Meiosis
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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.
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Unit 2 Lesson 2 Meiosis
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How does meiosis compare to mitosis?
• Meiosis produces haploid cells. Mitosis produces diploid cells.
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Unit 2 Lesson 2 Meiosis
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Unit 2 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.