CHAPTER 8 The Cellular Basis of Reproduction and Inheritance

1. CHAPTER 8The Cellular Basis of Reproduction and Inheritance Modules 8.12 – 8.18

2. MEIOSIS AND CROSSING OVER 8.12 Chromosomes are matched in homologous pairs • Somatic cells of each species contain a specific number of chromosomes • Human cells have 46, making up 23 pairs of homologous chromosomes Chromosomes Centromere Sister chromatids Figure 8.12

3. 8.13 Gametes have a single set of chromosomes • Cells with two sets of chromosomes are said to be diploid • Gametes are haploid, with only one set of chromosomes

4. Repeated mitotic divisions lead to the development of a mature adult • The adult makes haploid gametes by meiosis • All of these processes make up the sexual life cycle of organisms • At fertilization, a sperm fuses with an egg, forming a diploid zygote

5. Haploid gametes (n = 23) Egg cell • The human life cycle Sperm cell MEIOSIS FERTILIZATION Diploidzygote (2n = 46) Multicellulardiploid adults (2n = 46) Mitosis anddevelopment Figure 8.13

6. 8.14 Meiosis reduces the chromosome number from diploid to haploid • Meiosis, like mitosis, is preceded by chromosome duplication • However, in meiosis the cell divides twice to form four daughter cells

7. While they are paired, they cross over and exchange genetic information • The homologous pairs are then separated, and two daughter cells are produced • In the first division, meiosis I, homologous chromosomes are paired

8. Draw pix on pgs. 162 and 163 in your text. MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes(withcentriolepairs) Microtubules attached tokinetochore Metaphaseplate Sister chromatidsremain attached Sites of crossing over Spindle Nuclearenvelope Sisterchromatids Tetrad Centromere(with kinetochore) Homologouschromosomes separate Chromatin Figure 8.14, part 1

9. The sister chromatids of each chromosome separate • The result is four haploid daughter cells • Meiosis II is essentially the same as mitosis

10. MEIOSIS II: Sister chromatids separate TELOPHASE IAND CYTOKINESIS TELOPHASE IIAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavagefurrow Sister chromatidsseparate Haploiddaughter cellsforming Figure 8.14, part 2

11. 8.15 Review: A comparison of mitosis and meiosis • For both processes, chromosomes replicate only once, during interphase

12. MITOSIS MEIOSIS PARENT CELL(before chromosome replication) Site ofcrossing over MEIOSIS I PROPHASE I Tetrad formedby synapsis of homologous chromosomes PROPHASE Chromosomereplication Chromosomereplication Duplicatedchromosome(two sister chromatids) 2n = 4 Chromosomes align at the metaphase plate Tetradsalign at themetaphase plate METAPHASE I METAPHASE ANAPHASE I TELOPHASE I ANAPHASETELOPHASE Sister chromatidsseparate duringanaphase Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together Haploidn = 2 Daughtercells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cellsof mitosis n n n n Daughter cells of meiosis II Figure 8.15

13. 8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring • Each chromosome of a homologous pair comes from a different parent • Each chromosome thus differs at many points from the other member of the pair

14. The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes • Random fertilization also increases variation in offspring

15. POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16

16. 8.17 Homologous chromosomes carry different versions of genes • The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci

17. Coat-color genes Eye-color genes C E Brown Black C E C E c e c e c e White Pink Tetrad in parent cell(homologous pair ofduplicated chromosomes) Chromosomes ofthe four gametes Figure 8.17A, B

18. 8.18 Crossing over further increases genetic variability • Crossing over is the exchange of corresponding segments between two homologous chromosomes • Genetic recombination results from crossing over during prophase I of meiosis • This increases variation further

19. Tetrad Chaisma Centromere Figure 8.18A

20. Coat-colorgenes Eye-colorgenes Tetrad(homologous pair ofchromosomes in synapsis) 1 Breakage of homologous chromatids • How crossing over leads to genetic recombination 2 Joining of homologous chromatids Chiasma Separation of homologouschromosomes at anaphase I 3 Separation of chromatids atanaphase II and completion of meiosis 4 Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Figure 8.18B Gametes of four genetic types

21. Spermatogenesis • Spermatogenesis, or sperm production, begins around puberty and continues for the remainder of a man's life • A young healthy man produces several hundred million sperm per day.

22. Spermatogenesis

26. 24 days

27. Oogenesis • During embryonic development, diploid cells in the ovaries called oogonia divide by mitosis to produce primary oocytes.  The primary oocytes are diploid. • Primary oocytes start meiosis.  They complete interphase and prophase I.  They are “frozen” at the end of prophase I and remain this way until the female reaches puberty. • A female is born with about 2 million primary oocytes.  By the time she reaches puberty, about 400,000 are left.

28. Oogenesis cont…. • After puberty, each month one of the primary oocytes is selected for ovulation (release from the ovary).  Just prior to ovulation, the primary oocyte is “unfrozen” and completes meiosis I, forms the first polar body, and is then frozen in meiosis II at metaphase II.  • After ovulation, if a sperm penetrates the secondary oocyte (egg) after it is released from the ovary, the secondary oocyte will be stimulated to complete meiosis II forming one more polar body and a mature egg.  The first polar body formed from meiosis one may also complete meiosis II to form another polar body.

30. Prophase I

32. Fertilization takes place in the fallopian tubes