Section 11.4

1. Section 11.4 Meiosis

2. Chromosome Number Mendel’s principles require at least 2 events: An organism with 2 parents must inherit a single copy of every gene from each parent When that organism produces gametes, those two sets of genes must be separated so that each gamete contains just one set of genes Chromosomes (strands of DNA and protein inside a cell nucleus) are carriers of genes The genes are located in specific positions on chromosomes

3. Diploid Cells Body cell (somatic) of adult fruit fly = 8 chromosomes 4 from male parent + 4 from female parent Homologous – each of the chromosomes from male parent has a corresponding chromosome from female parent Diploid (2 sets) – cell that contains both sets of homologous chromosomes Diploid cells of most adult organisms contain 2 complete sets of inherited chromosomes and 2 complete sets of genes Diploid number represented by 2N N represents single set of chromosomes found in an egg or sperm cell

4. Haploid Cells Haploid (one set) – cells contain only a single set of chromosomes and therefore a single set of genes Gametes (sex cells) of sexually reproducing organisms are haploid N represents number of chromosomes because there is only one set

5. Phases of Meiosis Meiosis produces haploid cells from diploid cells Meiosis – process in which the number of chromosomes per cell is cut in half through separation of homologous chromosomes in a diploid cell Usually involves 2 distinct divisions: meiosis I & meiosis II By the end of meiosis II  diploid cell becomes 4 haploid cells

6. Meiosis I Just prior to Meiosis I  cell undergoes a round of chromosome replication during interphase Each replicated chromosome consists of 2 identical chromatids joined at the center

7. Prophase I After Interphase I  cell begins to divide  chromosomes pair up Prophase I of meiosis: each replicated chromosome pairs with its corresponding homologous chromosome Pairing forms a structure called a tetrad – contains 4 chromatids As homologous chromosomes form tetrads they undergo a process called crossing over Chromatids of homologous chromosomes cross over one another Crossed sections of the chromatids (containing alleles) are exchanged - Crossing over produces new combinations of alleles in the cell Spindle forms and attaches to each tetrad

8. Metaphase I Metaphase I: paired homologous chromosomes line up across the center of the cell

9. Anaphase I Anaphase I: spindle fibers pull each homologous chromosome pair toward opposite ends of the cell

10. Telophase I & Cytokinesis Separated chromosomes cluster at opposite ends of the cell Telophase I: a nuclear membrane forms around each cluster of chromosomes Cytokinesis: separates cytoplasm & forms 2 new cells

11. Results of Meiosis I 2 cells called daughter cells Neither daughter cell has the 2 complete sets of chromosomes that it would have in a diploid cell because each pair of homologous chromosomes was separated 2 sets have been shuffled and sorted (almost like a deck of cards) 2 cells produced by meiosis I have sets of chromosomes and alleles that are different from each other and from the diploid cells that entered meiosis I

12. Meiosis II 2 cells now enter second meiotic division Unlike 1st division  neither cell goes through a round of chromosome replication before entering Meiosis II NO CHROMOSOME REPLICATION!!

13. Prophase II Chromosomes (consist of 2 chromatids) – become visible Chromosomes do not pair to form tetrads Because homologous pairs were already separated during meiosis I

14. Metaphase II Chromosomes line up in the center of each cell

15. Anaphase II Paired chromatids separate

16. Telophase II & Cytokinesis The final 4 phases of meiosis II are similar to those in Meiosis I Result in 4 haploid daughter cells In this example: each cell receives 2 chromosomes

17. Results of Meiosis II 4 daughter cells Each have 2 chromosomes Contain haploid number (N)

18. Gametes to Zygotes Haploid cells produced by meiosis II = gametes (sex cells)  important to heredity Male animal gametes = sperm Female animal gametes Only 1 of cells produced by meiosis is involved in reproduction After female cell (egg) is fertilized = called a zygote Zygote undergoes cell division by mitosis and eventually forms a new organism

19. Compare Mitosis and Meiosis

23. Comparing Mitosis and Meiosis Mitosis can be a form of asexual reproduction Meiosis is an early step in sexual reproduction 3 differences:

24. Replication and Separation of Genetic Material Both preceded by complete copying of genetic material (chromosomes) Mitosis: 2 sets of genetic material separate Each daughter cell receives 1 complete set of chromosomes Meiosis: homologous chromosomes line up and then move to separate daughter cells 2 alleles for each gene are segregated  end up in different cells Sorting and recombination of genes result in a greater variety of possible gene combinations than could result from mitosis

25. Changes in Chromosome Number Mitosis does not normally change the chromosome number of the original cell Diploid cell enters with 8 chromosomes will divide to produce 2 diploid daughter cells Each cell will have 8 chromosomes Meiosis reduces the chromosome number by half diploid cell enters with 8 chromosomes will pass through 2 meiotic divisions Produces 4 haploid gamete cells Each cell will have 4 chromosomes

26. Number of Cell Divisions Mitosis: single cell division 2 daughter cells Genetically identical Diploid Meiosis: 2 rounds of cell division 4 daughter cells Genetically different Haploid

27. Gene Linkage and Gene Maps Genes that are located on different chromosomes assort independently What about genes located on the same chromosome? Wouldn’t they generally be inherited together?

28. Gene Linkage Thomas Hunt Morgan says yes Violates principle of independent assortment Fruit flies: reddish-orange eyes and miniature wings in test crosses  almost always inherited together Principle of gene linkage – genes for 2 traits almost always inherited together Grouped flies genes into 4 linkage groups  groups assorted independently, but all genes in one group were inherited together

29. Gene Linkage Conclusions Each chromosome is a group of linked genes Principle of independent assortment still holds true - Chromosomes assort independently  not individual genes Alleles of different genes tend to be inherited together from one generation to the next when those genes are located on the same chromosome Genes Mendel studied are on different chromosomes Other genes Mendel studied are so far apart that they also assort independently

30. Gene Mapping 1911 – Columbia University – Morgan’s lab – student: Alfred Sturtevant – tested frequency of crossing over b/w genes during meiosis – might give clues to genes’ locations Reasoning: farther apart 2 genes were on a chromosome  more likely that crossing over would occur between them If 2 genes are close together, then crossovers b/w them should be rare If 2 genes are far apart, then crossovers b/w them should be more common Used frequency of crossing-over b/w genes to determine distances from each other This method has been used to construct gene maps ever since this discovery

31. Mystery Clue – p. 329 White is the least common colr found in parakeets. What does this fact suggest about the genotypes of both green parents?

32. Solve the Mystery – p. 333 Draw a Punnett square that accounts for the inheritance of blue pigment. Construct a Punnett square that explains the inheritance of a white pigment. Solve the mystery by determining the genotypes and phenotypes of the parents and offspring. What ratio of colored offspring would you expect if Susan breeds her original pair of parakeets in the years ahead? Would any offspring be green?