Meiosis

1. Meiosis

2. Heredity and Variation • Heredity • Is the transmission of traits from one generation to the next • Variation • Shows that offspring differ somewhat in appearance and other traits from parents and siblings • Genetics • Is the scientific study of heredity and hereditary variation

3. Breeding in English Shepherds Mother Father

4. Breeding in English Shepherds - Offspring

5. Genes – A simple definition • Genes • Are the units of heredity • Are segments of DNA • Each gene in an organism’s DNA has a specific locus on a certain chromosome • Sexual organisms inherit one set of chromosomes from the mother and one set from the father

6. The Shallow End of the Gene Pool

7. Asexual Reproduction • For example – budding in Hydra is asexual reproduction that forms clones

8. Asexual Reproduction • Strawberry reproducing by putting out asexual runners – also forming clones

9. Haploid gametes (n 23) Key Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS FERTILIZATION Ovary Testis Diploidzygote(2n 46) Mitosis anddevelopment Multicellular diploidadults (2n 46)

10. Chromosomes • Homologous chromosomes • Are the two chromosomes composing a pair • Have the same characteristics • Autosomes are the non-sex determining chromosomes • Sex chromosomes • Are distinct from each other in their characteristics • Are represented as X and Y in mammals • Determine the sex of the individual, XX being female, XY being male; X and X are homologous in female • A diploid cell • Has two sets of each of its chromosomes • In a human has 46 chromosomes (2n = 46)

12. 5 m Pair of homologousduplicated chromosomes Centromere Sisterchromatids Metaphasechromosome

13. Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes

14. Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes

15. Hairy Ears – a sex linked trait • Square symbol = male • Round symbol = female • Dark square = hairy ear trait

16. Chromosome number in humans if there was no meiosis prior to reproduction • First Generation - Sperm and Egg with 46 chromosomes each • Second generation 92 chromosomes • Third generation 184 chromosomes • Fourth generation 368 chromosomes • Etc.

18. Sexual life cycles Key Haploid (n) Haploid unicellular ormulticellular organism Haploid multi-cellular organism(gametophyte) Diploid (2n) Gametes n n Mitosis Mitosis Mitosis Mitosis n n n n n n n n Spores n MEIOSIS FERTILIZATION n Gametes n Gametes MEIOSIS FERTILIZATION FERTILIZATION MEIOSIS Zygote 2n 2n 2n 2n Diploidmulticellularorganism(sporophyte) Zygote 2n Diploidmulticellularorganism Mitosis Mitosis Zygote (a) Animals (b) Plants and some algae (c) Most fungi and some protists

19. Interphase Pair of homologouschromosomes indiploid parent cell Chromosomesduplicate Duplicated pairof homologouschromosomes Sisterchromatids Diploid cell withduplicatedchromosomes

20. 1 Interphase Pair of homologouschromosomes indiploid parent cell Chromosomesduplicate Duplicated pairof homologouschromosomes Sisterchromatids Diploid cell withduplicatedchromosomes Meiosis I Homologouschromosomes separate Haploid cells withduplicated chromosomes

21. 2 1 Interphase Pair of homologouschromosomes indiploid parent cell Chromosomesduplicate Duplicated pairof homologouschromosomes Sisterchromatids Diploid cell withduplicatedchromosomes Meiosis I Homologouschromosomes separate Haploid cells withduplicated chromosomes Meiosis II Sister chromatidsseparate Haploid cells with unduplicated chromosomes

23. MEIOSIS I: Separates sister chromatids MEIOSIS I: Separates homologous chromosomes Telophase II andCytokinesis Telophase I andCytokinesis Metaphase II Metaphase I Anaphase I Prophase II Anaphase II Prophase I Centrosome(with centriole pair) Sister chromatidsremain attached Chiasmata Centromere(with kinetochore) Sisterchromatids Spindle Metaphaseplate During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes. Cleavagefurrow Homologouschromosomesseparate Sister chromatidsseparate Haploid daughtercells forming Fragmentsof nuclearenvelope Homologouschromosomes Microtubuleattached tokinetochore Each pair of homologous chromosomes separates. Two haploid cellsform; each chromosomestill consists of twosister chromatids. Chromosomes line upby homologous pairs. Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.

24. Telophase I andCytokinesis Anaphase I Metaphase I Prophase I Centrosome(with centriole pair) Sister chromatidsremain attached Chiasmata Sisterchromatids Centromere(with kinetochore) Spindle Metaphaseplate Cleavagefurrow Homologouschromosomesseparate Fragmentsof nuclearenvelope Homologouschromosomes Microtubuleattached tokinetochore Each pair of homologous chromosomes separates. Two haploid cells form; each chromosomestill consists of two sister chromatids. Chromosomes line upby homologous pairs. Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.

25. Telophase II andCytokinesis Metaphase II Prophase II Anaphase II During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes. Haploid daughtercells forming Sister chromatidsseparate

26. MEIOSIS MITOSIS Parent cell MEIOSIS I Chiasma Prophase Prophase I Chromosomeduplication Chromosomeduplication Duplicatedchromosome Homologouschromosome pair 2n 6 Metaphase I Metaphase Anaphase I Telophase I AnaphaseTelophase Daughter cells ofmeiosis I Haploidn 3 MEIOSIS II 2n 2n Daughter cellsof mitosis n n n n Daughter cells of meiosis II

27. SUMMARY Property Mitosis Meiosis Occurs during interphase beforemitosis begins Occurs during interphase before meiosis I begins DNAreplication One, including prophase, metaphase,anaphase, and telophase Two, each including prophase, metaphase, anaphase,and telophase Number ofdivisions Occurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion Does not occur Synapsis ofhomologouschromosomes Four, each haploid (n), containing half as manychromosomes as the parent cell; genetically differentfrom the parent cell and from each other Two, each diploid (2n) and geneticallyidentical to the parent cell Number of daughter cellsand geneticcomposition Role in the animal body Enables multicellular adult to arise fromzygote; produces cells for growth, repair,and, in some species, asexual reproduction Produces gametes; reduces number of chromosomesby half and introduces genetic variability among the gametes

28. Possibility 2 Possibility 1 Two equally probablearrangements ofchromosomes atmetaphase I Independent Assortment

29. Possibility 2 Possibility 1 Two equally probablearrangements ofchromosomes atmetaphase I Independent Assortment Metaphase II

30. Possibility 2 Possibility 1 Two equally probablearrangements ofchromosomes atmetaphase I Independent Assortment Metaphase II Daughtercells Combination 1 Combination 2 Combination 3 Combination 4

31. Mitosis and meiosis have several key differences. • The chromosome number is reduced by half in meiosis, but not in mitosis. • Mitosis produces daughter cells that are genetically identical to the parent and to each other. • Meiosis produces cells that differ from the parent and each other.

32. Three events, unique to meiosis, occur during the first division cycle.1. During prophase I, homologous chromosomes pair up in a process called synapsis. • A protein zipper, the synaptonemal complex, holds homologous chromosomes together tightly. • Later in prophase I, the joined homologous chromosomes are visible as a tetrad. • At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged. • Chiasmata is the physical manifestation of crossing over, a form of genetic rearrangement. • Prophase I is longest and most important phase

33. 2. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate. • In humans, you would see 23 tetrads. 3. At anaphase I, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell. • Sister chromatids remain attached at the centromere until anaphase II. • The processes during the second meiotic division are virtually identical to those of mitosis.

34. The synaptonemal complex binding together four homologous chromosomes

35. Chiasmata and crossing over

36. Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs

37. Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM

38. Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I

39. Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II

40. Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughtercells Recombinant chromosomes

42. Human Female vs Male Meiotic Timelines

43. Sexual reproduction leads to genetic variation via: • Independent assortment during meiosis • Crossing over during meiosis • Random mixing of gametes (sperm and egg)