How Cells Reproduce

1. How Cells Reproduce Chapter 8

2. 8.1 Cell Division Mechanisms • Reproduction • Produces a generation of individuals like parents • Cell division • Bridges two generations • Each daughter cell receives • A required number of DNA molecules • Some cytoplasm

3. Dividing HeLa Cells

4. Eukaryotes and Prokaryotes • Eukaryotic cells undergo mitosis and/or meiosis • Separates duplicated chromosomes of parent cell into two daughter nuclei • Another mechanism divides cytoplasm • Prokaryotic cells divide by a different process

5. Mitosis and Meiosis • Mitosis • Basis of growth, cell replacements, and tissue repair in multicelled species • Basis of asexual reproduction in many single-celled and multicelled species • Meiosis • Basis of sexual reproduction • Precedes formation of gametes or sexual spores

6. Cell Division Mechanisms

7. Chromosome Structure • Eukaryotic chromosome • Association of DNA, histones, and other proteins • Proteins structurally organize the chromosome and affect access to its genes • Nucleosome • Smallest unit of organization • Double-stranded DNA looped twice around a spool of histones

8. Structure of a Condensed Chromosome

9. Fig. 8.4a, p.127

10. centromere a One duplicated human chromosome in its most condensed form. Fig. 8.4a, p.127

11. Fig. 8.4b, p.127

12. b When a chromosome is most condensed, the proteins associated with it interact in ways that package loops of DNA, which are already coiled, into higher order levels of coiling. multiple levels of coiling of DNA and proteins Fig. 8.4b, p.127

13. Fig. 8.4c, p.127

14. c At a deeper level of structural organization, the chromosomal proteins and DNA are organized as a cylindrical fiber. fiber d Immerse a chromosome in saltwater and it loosens to a beads-on-a-string organization. What appears to be a “string” is one DNA molecule. Each “bead” is a nucleosome. beads on a string Fig. 8.4c, p.127

15. Fig. 8.4d, p.127

16. DNA double helix core of histone e A nucleosome consists of part of a DNA molecule looped twice around a core of histone proteins. nucleosome Fig. 8.4d, p.127

17. Animation: Chromosome structural organization CLICK HERE TO PLAY

18. Sister Chromatids • A duplicated chromosome consists of two sister chromatids, each with a kinetochore • Sister chromatids remain attached at their centromere until late in mitosis (or meiosis)

19. one chromatid its sister chromatids one chromosome (unduplicated) one chromosome (duplicated) Fig. 8.3, p.126

20. Key Concepts: CHROMOSOMES AND DIVIDING CELLS • Individuals of a species have a characteristic number of chromosomes in each cell • Chromosomes differ in length and shape, carry different portions of cell’s hereditary information • Mechanisms divide information between daughter cells, along with enough cytoplasm for each cell to operate on its own

21. 8.2 Introducing the Cell Cycle • Cell cycle • Starts when a new cell forms • Runs through interphase • Ends when cell reproduces by nuclear and cytoplasmic division

22. Interphase • Most cellular activities occur in interphase • G1: Cell grows in mass, doubles number of cytoplasmic components • S: DNA replication duplicates chromosomes • G2: Cell prepares for division

23. Eukaryotic Cell Cycle

24. G1 S Interval of cell growth before DNA replication (chromosomes unduplicated) Interval of cell growth when the DNA is replicated (all chromosomes duplicated) cytoplasmic division; each daughter cell enters interphase G2 Interval after DNA replication; the cell prepares to divide Interphase ends for parent cell Fig. 8.5, p.128

25. Chromosome Number • Sum of all chromosomes in cells of a given type • In human body cells, chromosome number is 46 • Body cells are diploid (have two of each kind of chromosome)

26. Human Chromosomes: 23 Pairs

27. Mitosis and Chromosome Number • Mitosis maintains parental chromosome number from one generation to the next • Bipolar spindle divides sister chromatids

28. pole microtubule of bipolar spindle chromosomes pole p.129

29. After mitosis and cytoplasmic division, the two daughter cells each have one (unduplicated) chromosome. Both daughter cells start life in interphase. mitosis, cytoplasmic division One of the unduplicated chromosomes in a parent cell at interphase The same two chromosomes, (duplicated) at interphase, prior to mitosis Fig. 8.6b, p.129

30. mitosis, cytoplasmic division One of the unduplicated chromosomes in a parent cell at interphase The same two chromosomes, (duplicated) at interphase, prior to mitosis After mitosis and cytoplasmic division, the two daughter cells each have one (unduplicated) chromosome. Both daughter cells start life in interphase. Stepped Art Fig. 8-6b, p.129

31. Key Concepts:MITOSIS IN THE CELL CYCLE • Cell cycle starts when a daughter cell forms and ends when that cell completes its own division • A typical cycle goes through interphase, mitosis, and cytoplasmic division • In interphase, a cell increases its mass and number of components, and copies its DNA

32. Animation: The cell cycle CLICK HERE TO PLAY

33. 8.3 A Closer Look at Mitosis • Mitosis • A nuclear division mechanism that maintains the chromosome number • Mitosis proceeds in four stages: • Prophase • Metaphase • Anaphase • Telophase

34. Prophase • Duplicated chromosomes become threadlike as they start to condense • Microtubules form a bipolar spindle • Nuclear envelope starts to break apart

35. Transition to Metaphase • Microtubules from one spindle pole harness one chromatid of each chromosome • Microtubules from the opposite spindle pole harness its sister chromatid • Other microtubules extend from both poles and grow until they overlap at the spindle’s midpoint

36. Metaphase • All chromosomes become aligned midway between the two spindle poles • Chromosomes in most condensed forms

37. Anaphase • Sister chromatids detach from each other • Spindles move them toward opposite poles • Microtubules that overlap at spindle’s midpoint slide past each other, push poles farther apart • Motor proteins drive movements

38. Telophase • Two identical clusters (one chromosome of each type) reach opposite spindle poles • Nuclear envelope forms around each cluster • Both new nuclei have the parental chromosome number

39. Mitosis

40. Mitosis

41. Fig. 8.7a, p.130

42. a Cell at Interphase A diploid cell duplicates its DNA and prepares for mitosis. Fig. 8.7a, p.130

43. Fig. 8.7b, p.130

44. nuclear envelope chromosome b Early Prophase Mitosis begins. DNA and its associated proteins have started to condense. Two chromosomes (color-coded purple) were inherited from the female parent. The other two (blue) are their counterparts, inherited from the male parent. Fig. 8.7b, p.130

45. Fig. 8.7c, p.130

46. pair of centrioles c Late Prophase The duplicated chromosomes continue to condense. New microtubules move one of two pairs of centrioles to the opposite side of the nucleus. The nuclear envelope starts to break up. Fig. 8.7c, p.130

47. Fig. 8.7d, p.130

48. microtubule d Transition to Metaphase Microtubules penetrate the nuclear region and collectively form a bipolar spindle. Some tether one sister chromatid of each chromosome to a spindle pole. Others overlap at the spindle equator (not shown). Fig. 8.7d, p.130

49. e Metaphase All of the chromosomes have become lined up midway between the spindle poles. At this stage of mitosis, the chromosomes are in their most tightly condensed form. Fig. 8.7e, p.130

50. Fig. 8.7f, p.130