gametes Gametes somatic cells diploid (2n) haploid (n)

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2 Overview of Meiosis Meiosis is a form of cell division that leads to the production of gametes. Gametes: egg cells and sperm cells (reproductive) -contain half the number of chromosomes of an adult body cell Adult body cells (somatic cells) are diploid (2n), containing 2 sets of chromosomes. Gametes are haploid (n), containing only 1 set of chromosomes. 2

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4 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.

5 Meiosis does two things - 1) Meiosis takes a cell with two sets of every chromosome (diploid) and makes cells with a single set 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.

6 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

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8 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.

9 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

10 Meiosis

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

12 The Stages of Meiosis: aka: Reduction Division

13 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

14 Prophase I Chromosomes condense Homologous pairs fasten together (synapsis) in a group of four called a tetrad. Crossing Over occurs. Crossing Over is the exchange of like for like segments of homologous chromosomes. Crossing over: genetic recombination between non-sister chromatids

15 What do the arrows represent? What do the x s of the same size and shape represent? What do the different colors represent? What is happening in this picture?

16 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.

17 Metaphase I Homologous pairs of chromosomes line up in the middle of the cell

18 Independent Assortment Occurs in metaphase 1 Homologous chromosomes independently line up on either side of the equator of the cell Results in random shuffling genes.

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20 Anaphase I Homologous chromosomes move to opposite sides of the cell Sister chromatids remain attached

21 Telophase I/ Cytokinesis I The cytoplasm divides, 2 Genetically different haploid cells Concludes Meiosis I - Reduction Division

22 Cytokinesis Occurs simultaneously with telophase I Forms 2 daughter cells Plant cells cell plate Animal cells cleavage furrows

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

24 Meiosis II : Separates sister chromatids Very similar to mitosis (starts with haploid cells) NO INTERPHASE BETWEEN MEIOSIS I AND II!

25 Prophase II Chromosomes condense Nuclear envelope breaks down Spindle fibers form

26 Metaphase II Chromosomes line up in the middle of the cell (just like mitosis)

27 Anaphase II The sister chromatids separate and move toward opposite sides of the cell (just like mitosis)

28 Telophase II and Cytokinesis Nuclei form at opposite poles of the cell and cytokinesis occurs Results in four haploid (n) daughter cells

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

30 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. In humans, n = 23 and 2 23 = 6,000,0000. That s a lot of diversity by this mechanism alone.

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33 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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35 Boy or Girl? The Y Chromosome Decides X chromosome Y chromosome

36 Boy or Girl? The Y Chromosome Decides

37 Meiosis division error Chromosome pair

38 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

39 21 trisomy Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?

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