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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,

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Presentation on theme: "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,"— Presentation transcript:

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2 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.

3 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.

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

5 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.

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

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

8 The Stages of Meiosis: aka: Reduction Division

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

10 Prophase I This is a crucial phase for mitosis. 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. 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. Extremely IMPORTANT!!! It is during this phase that crossing over can occur. Crossing Over is the exchange of segments during synapsis. Crossing Over is the exchange of segments during synapsis.

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

12 Anaphase I The spindle guides the movement of the chromosomes toward the poles 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 The homologous chromosome moves toward the opposite pole – Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

13 Telophase I This is the end of the first meiotic cell division. This is the end of the first meiotic cell division. The cytoplasm divides, forming two new daughter cells. 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 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

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

15 Figure 13.7 The stages of meiotic cell division: Meiosis I

16 Meiosis II : Separates sister chromatids Proceeds similar to mitosis Proceeds similar to mitosis THERE IS NO INTERPHASE II ! THERE IS NO INTERPHASE II !

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

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

19 Anaphase II The centromeres of sister chromatids finally separate The centromeres of sister chromatids finally separate The sister chromatids of each pair move toward opposite poles The sister chromatids of each pair move toward opposite poles – Now individual chromosomes

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

21 Figure 13.7 The stages of meiotic cell division: Meiosis II

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

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24 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.

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26 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

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

29 Boy or Girl? The Y Chromosome “Decides”

30 Meiosis – division error Chromosome pair

31 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 21 st pair – producing a child with Downs Syndrome

32 21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?


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