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Meiosis

This text introduces the process of meiosis and explains why it is important for gamete formation and genetic diversity. It covers the stages of meiosis and the reasons behind halving the chromosomes and scrambling the genes in gametes.

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Meiosis

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  1. Meiosis

  2. Bell Work 3-30-2015 • Why halve the chromosomes in gametes? • Why does meiosis scramble the specific forms of each gene that each sex cell (egg or sperm) receives? • Meiosis is also called __. Learning Target I can summarize the events of meiosis.

  3. Bell Work 3-31-2015 • Why might expecting parents be interested in meiosis? • How many cells are produced during meiosis? • Why do we need meiosis? • Logon to internet Learning Target I can summarize the events of meiosis

  4. Bell Work 4-01-2015 • Why do you share some but not all characters of each parent? • Name two things accomplished by meiosis. Learning Target I can review the events of meiosis.

  5. Learning Target I can review the events of meiosis. Agenda • Bell Work / LT • Meiosis Puzzle

  6. Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis.

  7. Research What does homologous mean? What is haploid and diploid? What does meiosis do and why is it important?

  8. Diploid – A cell that contains both sets of homologous chromosomes The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. Haploid – The gametes of sexually reproducing organisms contain only a single set of chromosomes Cells are represented by the symbol N.

  9. Meiosis does two things - 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number. In meiosis, one diploid cells produces four haploid cells.

  10. Why do we need meiosis? • Meiosis is necessary to halve the number of chromosomes going into the sex cells Why halve the chromosomes in gametes? • At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

  11. 2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species.

  12. Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

  13. Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

  14. The Stages of Meiosis: • aka: Reduction Phase / Division

  15. Meiosis I : Separates Homologous Chromosomes • Interphase • Each of the chromosomes replicate • The result is two genetically identical sister chromatids which remain attached at their centromeres

  16. Prophase I • This is a crucial phase for mitosis. • During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. • Extremely IMPORTANT!!! It is during this phase that crossing over can occur. • Crossing Over is the exchange of segments during synapsis.

  17. Metaphase I • The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles. • Still in homologous pairs

  18. Anaphase I • The spindle guides the movement of the chromosomes toward the poles • Sister chromatids remain attached • Move as a unit towards the same pole • The homologous chromosome moves toward the opposite pole • Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

  19. Telophase I • This is the end of the first meiotic cell division. • The cytoplasm divides, forming two new daughter cells. • Each of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell division

  20. Cytokinesis • Occurs simultaneously with telophase I • Forms 2 daughter cells • Plant cells – cell plate • Animal cells – cleavage furrows • NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

  21. Figure 13.7 The stages of meiotic cell division: Meiosis I

  22. Meiosis II : Separates sister chromatids • Proceeds similar to mitosis • THERE IS NO INTERPHASE II !

  23. Prophase II • Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

  24. Metaphase II • The chromosomes are positioned on the metaphase plate in a mitosis-like fashion

  25. Anaphase II • The centromeres of sister chromatids finally separate • The sister chromatids of each pair move toward opposite poles • Now individual chromosomes

  26. Telophase II and Cytokinesis • Nuclei form at opposite poles of the cell and cytokinesis occurs • After completion of cytokinesis there are four daughter cells • All are haploid (n)

  27. Figure 13.7 The stages of meiotic cell division: Meiosis II

  28. Bell Work 4-06-2015 1. If neither of your parents have dimples, could you still have them? Explain. 2. How are gene maps produced? 3. What structure is it that actually assorts independently? Learning Target I can explain how meiosis leads to genetic diversity.

  29. Bell Work 4-07-2015 • Suppose you wanted to explain to your sibling why you have brown hair and they have blonde. Explain how you could do this. You may use a Punnett square. • Compare incomplete dominance and codominance. • Compare haploid to diploid. Learning Target I can earn a 3 on LT 3!

  30. Learning Target I can earn a 3 on LT 3! Agenda • Bell work / LT • Formative • Study guide

  31. One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes. In humans, n = 23 and 223 = 6,000,0000. That’s a lot of diversity by this mechanism alone.

  32. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

  33. Mitosis vs. Meiosis

  34. The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis

  35. Mitosis vs. Meiosis

  36. Y chromosome X chromosome Boy or Girl? The Y Chromosome “Decides”

  37. Boy or Girl? The Y Chromosome “Decides”

  38. Meiosis – division error Chromosome pair

  39. Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

  40. 21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female?

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