lesson 9.3: meiosis: the life cycle of sex cells goals: identify male and female gametes compare...
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Lesson 9.3: Meiosis: The Life Cycle of Sex Cells
• Goals: • Identify male and female gametes• Compare chromosome numbers
between somatic cells and gametes• Describe the stages of meiosis• Compare meiosis and mitosis
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Meiosis• Somatic cell = body cell which is
DIPLOID (2n), - it has 2 copies of each chromosome (one from mom and one from dad) these are called HOMOLOGOUS CHROMOSOMES– In humans, somatic cells have 46
chromosomes (skin cells, liver cells etc.)
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Homologous Chromosomes
• Homologous chromosome – one of a matching pair of chromosomes that comes from each parent
MomDad
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• Gametes, or sex cells do not perform mitosis• Human sex cells have 23 chromosomes…
HOW DOES THIS HAPPEN?• Gametes are involved in sexual reproduction
and are HAPLOID (n) (1 set of chromosomes)– Male gamete = sperm– Female gamete = egg– In humans the sperm and egg each have 23
chromosomes
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The Phases of Meiosis
• Consists of 2 separate cell divisions named Meiosis I and Meiosis II
• Starts with one diploid cell and ends with FOUR haploid cells called gametes (sex cells)
• During Meiosis I the number of chromosomes is reduced by ½ therefore meiosis I is known as reduction
• Meiosis II is the same as mitosis (division of the nucleus)
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Meiosis I – PROPHASE I• Preceded by Interphase
(therefore chromosomes have replicated sister chromatids)
• Spindle fibers appear and attach to centromeres
• In meiosis I HOMOLOGOUS chromosomes pair up in a process called SYNAPSIS which results in TETRADS
• While chromosomes are paired, they trade DNA pieces with each other CROSSING OVER
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Meiosis I – Metaphase I
• Tetrads line up randomly along the metaphase plate (tetrads meet in the middle)
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Meiosis I – Anaphase I
• Homologous chromosome separate – move Away/Apart from one another towards opposite ends of the cell
• *Sister chromatids are still together!
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Meiosis I – Telophase I and Cytokinesis
• Each end of the cell now has a haploid nucleus (nucleus only has one set of chromosomes)
• Nuclei and cytoplasm divide in half
• 2 haploid cells form• Each contains 2
copies of the same set of chromosomes
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Meiosis II• Does not begin with DNA
replication
• Prophase II: Spindle fibers attach to sister chromatids
• Metaphase II: spindle fibers move sister chromatids to the middle/center
• Anaphase II: Centromeres divide, sister chromatids separate and move apart to the poles
• Telophase II: Nuclear envelope reforms, cytoplasm divides
• Result: 4 haploid (1/2 the number of chromosomes as the original parent cell), genetically different cells
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Lesson 3 Review
• Please read text pages 256-258 and answer the following questions on page 259
• 1. __________• 2. __________• 3. __________• 4. __________• 5. __________• 6. __________
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Check Mitosis, Meiosis or Both Mitosis Meios
isBoth
No pairing of homologous chromosomes
Two divisions
Four daughter cells produced
Associated with growth and asexual reproduction
Associated with sexual reproduction
One division
Two daughter cells produced
Involves duplication of chromosomes
Chromosome # maintained
Chromosome # is halved
Crossing over between homologous may occur
Daughter cells are identical to parent cell
Daughter cells are not identical to parent cell
Produces gametes
Synapsis occurs in prophase
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State the PhaseState the Phase (for meiosis - indicate meiosis I or II)
• Mitosis– Nucleolus reappears– Sister chromatids line up
at equator– Cleavage furrow forms– Cell elongates– Chromosomes move
AWAY from each other– Spindle fibers stretch
across the cell– Nucleolus and Nuclear
membrane disappear
• Meiosis• Tetrads form• Crossing over occurs• Chromatin condenses• Tetrads line up at equator• Sister chromatids separate
and chromosomes move AWAY…
• Centromere divides• 2 cells are formed• 4 haploid cells are formed• Centrioles move to opposite
ends of the cell
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Review: A comparison of mitosis and meiosiscomparison animation