Chapter 13: Meiosis and Sexual Life Cycles

1. Chapter 13: Meiosis and Sexual Life Cycles

2. Figure 13.1 • Genetics is the scientific study of heredity and variation • Heredity is the transmission of traits from one generation to the next • Variation is demonstrated by the differences in appearance that offspring show from parents and siblings.

3. Inheritance of Genes • Genes • Are the units of heredity • Are segments of DNA • Recall that DNA is made up of repeating subunits called nucleotides (A C T G). • Locus • Each gene in an organism’s DNA has a specific locus (location) on a certain chromosome

4. Gametes • Are the reproductive cells that are the vehicles that transmit genes from one generation to the next • Male Gamete=sperm, Female Gamete=egg • We inherit • One set of chromosomes from our mother and one set from our father

5. Parent Bud 0.5 mm Comparison of Asexual and Sexual Reproduction • In asexual reproduction (usually results in cloned closed individuals) • One parent produces genetically identical offspring by mitosis • Occurs in single celled and simple multi-cellular organisms such as the hydra • In sexual reproduction • Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

6. Sets of Chromosomes in Human Cells • In humans • Each somatic cell has 46 chromosomes, made up of two sets (one set from each parent) • A chromosome • Is DNA that has been condensed and coiled

7. A somatic cell • Is any cell in a multi-cellular organism except for sperm or egg cells (e.g. liver and muscle cells) • Have 23 pairs of chromosomes, one set from the father, the other from the mother, for a total of 46 chromosomes (2n=46) • Each set of 23 consists of 22 sets of autosomes (a chromosome not directly involved in determining sex) and a single set of sex chromosomes. • Males  XY, Females  XX

8. Gametes (e.g. sperm and egg cells) • Have only 23 chromosomes (n=23) • 22 are autosomes and 1 is a sex chromosome. • Male sperm cell  can have X or Y • Female egg cell  can only have X

9. What type of cell? • Male or female?

10. X Y

11. Pair of homologous chromosomes 5 µm Centromere Sister chromatids • A karyotype • Is an ordered, visual representation of the chromosomes in a cell • Prepared from isolated somatic cells that are treated with drugs that stimulate mitosis • Cells are then arrested in metaphase and stained • Photographs are taken and scanned into the computer, and then chromosomes are electronically arranged according to size, shape and centromere position

12. A karyotype shows: • The two chromosomes in each pair (called homologous chromosomes or homologs) • Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited haracters • The sex chromosomes called X and Y

13. Homologous chromosomes • Are the two chromosomes that have the same staining pattern, centromere position and length • Have the same characteristics, one from mother, one from father • Autosomes are considered to be homologous, but sex chromosomes are not • Only parts of the X and Y are homologous • Most genes on X do not have counterparts on Y and vice versa

14. A haploid cell (haploid number, n) • Has only one homologous pair • Human: n=23 • A diploid cell (diploid number, 2n) • Has two sets of each homologous pair • A human has 46 chromosomes (2n = 46)

15. Figure 13.4 • In a cell in which DNA synthesis (Interphase) has occurred • All the chromosomes are duplicated and thus each consists of two identical sister chromatids Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

16. Behavior of Chromosome Sets in the Human Life Cycle • At sexual maturity in animals • The ovaries (eggs) and testes (sperm) produce haploid gametes by meiosis • Gametes are the only types of human cells produced by meiosis, rather than mitosis • Meiosis results in one set of chromosomes in each gamete

17. During fertilization • These gametes, sperm and ovum, fuse, forming a diploid zygote (2n) • The zygote • Develops into an adult organism by mitosis • At differing points during development, cells will specialize • Those cells that are destined for the testicles or eggs will undergo meiosis to develop gametes

19. Interesting…

20. Key Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm Cell (n) FERTILIZATION MEIOSIS Diploid zygote (2n = 46) Ovary Testis Mitosis and development Multicellular diploid adults (2n = 46) Figure 13.5 The Human Life Cycle • The purpose of meiosis: • To produce 1 set of chromosomes in each gamete instead of two to compensate for the doubling that occurs at fertilization • Produce genetic variability

21. Key Haploid Diploid n n Gametes n MEIOSIS FERTILIZATION Zygote 2n 2n Diploid multicellular organism Mitosis Figure 13.6 A (a) Animals • In animals • Life cycle includes only diploid multi-cellular stages • Gametes are the only haploid cells (unicellular) • Meiosis occurs only during gamete formation • Mitosis occurs only in multi-cellular organisms

22. Haploid multicellular organism (gametophyte) n Mitosis Mitosis n n n n Spores Gametes MEIOSIS FERTILIZATION Diploid multicellular organism (sporophyte) 2n 2n Zygote Mitosis (b) Plants and some algae The Variety of Sexual Life Cycles • Plants and some algae exhibit an alternation of generations • The life cycle includes both diploid and haploid multicellular stages • The two generations are: gametophyte (n-haploid) and sporophyte (2n-diploid)

23. Polytrichum Moss Sporophyte Gametophyte

24. Mitosis Spore making a gametophyte Mitosis Gametophyte making gametes Mitosis Sporophyte development

25. Alternation of generations • Plants and some algae • Plants have a life cycle that involves spores, which form as a result of meiosis. These spores are haploid. Notice that both haploid and diploid cells can divide by mitosis. • Meiosis always begins with cells that are diploid (2n), and a result, daughter cells formed are always haploid (n).

26. Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Figure 13.7 Haploid cells with unreplicated chromosomes The Stages of Meiosis • An overview of meiosis • Meiosis I • Reduces the number of chromosomes from diploid to haploid • Meiosis II • Produces four haploid daughter cells

27. MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (with centriole pairs) Chiasmata Metaphase plate Sister chromatids Spindle Nuclear envelope Homologous chromosomes separate Microtubule attached to kinetochore Tetrad Chromatin Pairs of homologous chromosomes split up Chromosomes duplicate Tertads line up Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example • Interphase and meiosis I Figure 13.8

28. MEIOSIS II: Separates sister chromatids TELOPHASE II AND CYTOKINESIS TELOPHASE I AND CYTOKINESIS METAPHASE II ANAPHASE II PROPHASE II Cleavage furrow Haploid daughter cells forming Sister chromatids separate Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Figure 13.8 • Telophase I, cytokinesis, and meiosis II

29. Prophase I • Synapsis: Duplicated homologous chromosomes line up and are held together by a protein called the synaptonemal complex. This eventually dissassembles in late prophase. • Crossing Over: Non-sister chromatids exchange DNA segments • Each pair of chromosomes forms a tetrad, a group of four chromatids. • Each tetrad usually has one more more chiasmata or X shaped criss-cross regions where crossing over has occurred.

30. Metaphase – Mitosis vs. Meiosis • In metaphase of mitosis, the sister chromatids are aligned at the metaphase plate, whereas in metaphase I of meiosis, the homologous chromosomes (tetrads) are lined up.

31. Separation of homologues • At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell • Sister chromatids remain attached • Resultant daughter cells (after anaphase I and telophase I) are haploid (n) • In anaphase II of meiosis, the sister chromatids separate • Chromosomes never duplicate in meiosis! (Interphase is not technically part of meiosis…or mitosis!)

32. MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Chromosome replication Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) 2n = 6 Tetrads positioned at the metaphase plate Chromosomes positioned at the metaphase plate Metaphase I Metaphase Sister chromatids separate during anaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II Figure 13.9 • A comparison of mitosis and meiosis

34. Spermatogenesis vs. Oogenesis

35. http://www.youtube.com/watch?v=zGVBAHAsjJM

36. The original source of genetic variation in individuals are mutations • Most of these are repaired • The vast majority of genetic variability is as a result of the “reshuffling of genetic material” that occurs in meiosis • Independent Assortment • Crossing Over • Random Fertilization

37. Independent Assortment of Chromosomes • Independent Assortment • Homologous pairs of chromosomes orient randomly at metaphase I of meiosis • Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

38. Key Maternal set of chromosomes Possibility 1 Possibility 2 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 • Number of chromosomes possible when chromosomes assort independently into gametes is 2n, where n is the haploid number n=2 22=4 combos of gametes

39. Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes Figure 13.11 Crossing Over • Crossing over • Produces recombinant chromosomes that carry genes derived from two different parents

40. Random Fertilization • Random Fertilization • Fertilization: the fusion of gametes • Any sperm can fuse with any ovum (egg), each containing different combination of genes

41. In humans… • Number of possible different combinations of chromosomes in sperm: 223 = approx 8 million combos • Number of possible different combinations of chromosomes in egg: 223 = approx 8 million combos • Number of possible diploid combinations as a result of fertilization: 8 million x 8 million = 70 trillion WE ARE SO UNIQUE AND SPECIAL!

42. Mutations!