1. The Cell Cycle

3. Types of Cell Division Mitosis Meiosis
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 Meiosis
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
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)
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