The Cell Cycle

1. The Cell Cycle

3. Types of Cell Division • Mitosis • provides cells for growth, cell replacement, and asexual reproduction • maintains chromosome number • Meiosis • produces cells for sexual reproduction • reduces chromosome number by half

4. Chromosome Replication • All the DNA is duplicated. • Duplicated chromosomes separate as cell divides.

5. Asexual Reproduction in Bacteria • Bacteria reproduce asexually by binary fission. • A bacterium possesses a single, circular chromosome which is replicated; and then the replicates (chromosomes) are distributed to two new cells formed by division of the original cell.

9. The Eukaryotic Cell Cycle • Interphase • G1 growth phase; synthesis of organelles • S synthesis of DNA (replication) • G2 growth; synthesis of proteins essential to cell division • Mitosis • prophase • metaphase • anaphase • telophase

11. Important Terms • chromatin - the form taken by chromosomes when a eukaryotic cell is not dividing • monad chromosome - unreplicated chromosome (single chromosome) • dyad chromosome - replicated chromosome (double chromosome) • chromatid - half of a dyad chromosome • centromere - structure by which chromatids of a dyad chromosome are held together

12. Mitosis* • 1. Prophase: dyad (replicated) chromosomes resemble long threads (chromatin); replicated centrioles move apart toward opposite poles; (animal cell); nucleolus disappears; dyad chromosomes condense (coil tightly); nuclear membrane disintegrates; spindle fibers appear between centrioles; dyad chromosomes migrate toward the equator of the cell

14. Mitosis* • 2. Metaphase: dyad chromosomes line up along the equator of the cell

16. Mitosis* • 3. Anaphase: dyad chromosomes separate forming monad chromosomes; monad chromosomes move toward opposite poles

18. Mitosis* • 4. Telophase: monad chromosomes cluster at poles; cytokinesis occurs gradually; nuclear membrane forms; nucleolus reappears; chromosomes uncoil; cytokinesis is completed; two daughters cells are formed

20. Daughter Cells of Mitosis • have the same number of chromosomes as each other and as the original cell from which they were formed • e.g. Mitosis in a cell which possesses 4 chromosomes will result in the formation of two daughter cells each having 4 chromosomes.

23. Can you identify the stages (phases) of mitosis in both plant cells and animal cells?

24. Can you cite differences between plant cell mitosis and animal cell mitosis?

25. Types of Cell Division • Mitosis • provides cells for growth, cell replacement, and asexual reproduction • maintains chromosome number • Meiosis • produces cells for sexual reproduction • reduces chromosome number by half

26. Meiosis • preceded by interphase which includes chromosome replication • two meiotic divisions • four daughter cells produced • daughter cells contain half the number of chromosomes as the original cell

27. Important terms in the study of meiosis • monad chromosome - unreplicated chromosome • dyad chromosome - chromosome composed of two chromatids (replicates) • homologous - similar in size and shape • synapsis - pairing of homologous chromosomes • crossover - exchange of segments between chromatids of homologous chromosomes

28. Chromosomes are replicated during S-phase of interphase. As a result, monad chromosomes become dyad chromosomes.

29. Meiosis I(Figure 6.6, p. 106) • Prophase I: homologous chromosomes of diploid cell synapse; homologous pairs migrate toward equator of cell; crossover may occur during synapsis • Metaphase I: dyad pairs of homologous chromosomes align at the equator of the cell • Anaphase I: homologous chromosomes move apart toward opposite poles • Telophase 1: chromosomes cluster at poles of cell; cytokinesis occurs forming two haploid daughter cells each containing one member of each homologous pair thus reducing chromosome by half

30. Meiosis II (resembles mitosis)(Figure 6.6, p. 106) • Prophase II: dyad chromosomes move toward the equator of each cell (2) • Metaphase II: dyad chromosomes align at equator of cell • Anaphase II: dyad chromosomes separate forming monad chromosomes (chromatids separate); monads migrate toward opposite poles of each cell • Telophase II: cytokinesis occurs at right angles to the first plane of division in each cell (2); four daughter cells are formed each having half the number of chromosomes as the original cell

31. Gamete formation - meiosis • meiosis in males - spermatogenesis • meiosis in females - oogenesis

32. Spermatogenesis • 4 mature sperm cells produced from meiosis in a single spermatocyte

33. Oogenesis • one mature ovum (egg cell) and 3 polar bodies formed from meiosis in a single oocyte • human female embryo - oocytes formed by mitosis and then complete prophase I of meiosis by time of birth • puberty - one oocyte in alternating ovaries resumes meiosis under the influence of hormones; at ovulation the oocyte is at metaphase II • oogenesis will be completed in the event of fertilization

34. Comparison of Mitosis and Meiosis(Table 6.2; Figure 6.8) • Mitosis • synapsis does not occur • one division occurs • two daughter cells are formed • chromosome number is maintained • Meiosis • synapsis occurs during Prophase I • two divisions occur • four daughter cells are formed • chromosome number is reduced by half

35. Nondisjunction(Essay 6.1, p. 109) • failure of chromosomes to separate during anaphase • results: daughter cells contain abnormal number of chromosomes

36. Chromosome anomalies • Down syndrome • Turner syndrome • Klinefelter syndrome

37. Down syndrome • Cause: trisomy 21 • Effects: mental retardation, weakened cardiovascular system, shortened limbs, widely-spaced eyes, receding forehead and chin, protruding tongue

38. Turner syndrome • Cause: one X chromosome in female (XO) • Effects: underdeveloped ovaries; failure of secondary sex characteristics to appear; sterility; webbing of neck; weakened pancreas, liver, and heart in 40%; short stature

39. Klinefelter syndrome • Cause: extra X chromosome in males (XXY) • Effects: underdeveloped testes, sterility, failure of secondary sex characteristics to appear, taller than average

40. Detection of chromosome anomalies in a fetus: • amniocentesis • chorionic villi sampling

41. Questions for next class meeting: • During which weeks of a pregnancy, may the following procedures be conducted: • amniocentesis • chorionic villi sampling