Cell Reproduction: Mitosis & Meiosis

1. Cell Reproduction: Mitosis & Meiosis Chapter 8 (and the beginning of Chapter 10)

2. Overview • DNA replication • Overview of cell division • Mitosis • Meiosis

3. DNA Replication Occurs during interphase of cell cycle 1 DNA molecule untwisted Each parent strand serves as template for new strand = 2 new DNA molecules, each ½ old & ½ new = semi-conservative replication

4. Enzymes break H bonds between 2 strands = unwinds & exposes nucleotide bases Free nucleotides pair with exposed bases Each parent strand has new one made on it = twist together to form double helix

5. DNA replication in a little more detail … Sugar-phosphate backbones of 2 DNA strands run in opposite directions 5’ end = Phosphate group on sugar’s C 3’ end = –OH group on sugar’s C

6. DNA polymerase adds nucleotides to 3’ ends only Daughter strand grows in 5’ to 3’ direction = 1 daughter strand synthesized continuously = Other daughter strand synthesized disjointedly

7. Replication Enzymes: Helicases Catalyze breaking of H bonds so double helix can unwind Work with small proteins to prevent rewinding of parent strands

8. Replication Enzymes: DNA Polymerases Catalyze addition of free nucleotides to exposed bases on each strand Also have proofreading abilities

9. Replication Enzymes: DNA Ligases Work on discontinuously-assembled strand Seal together short stretches of new nucleotides

10. Transcription Only part of DNA strand unwound RNA polymerase adds nucleotides to growing strand Results in 1 free mRNA strand DNA replication Whole DNA molecule unwound DNA polymerase adds nucleotides to growing strand Results in 2 double-helix DNA molecules Transcription vs. DNA replication

11. Mistakes occur that can be lethal if not caught e.g. wrong base-pairing DNA proofreading mechanisms fix most replication errors & breaks in strands (proofread & correct mismatches) Repair enzymes repair some changes by snipping out damaged sites or mismatches If mismatch can’t be fixed, replication is stopped

12. Cell Division: An Overview Parents reproduce to produce new generation of cells or multicellular organism Offspring inherits all information & metabolic machinery from parent

13. Prokaryotic Cell Division Prokaryotic cells reproduce asexually = binary fission

14. Eukaryotic Cell Division DNA in eukaryotic cells is in nucleus Eukaryotic cells can’t divide by fission Must copy & package DNA into > 1 nucleus before cytoplasm can split

15. Two Types of Cell Division Mitosis: • Produces 2 genetically identical cells • Happens throughout body Meiosis: • Produces 4 genetically different cells • Cells only have ½ of genetic info • Happens only in gonads

16. Mitosis One part of the cell cycle Growth, cell replacement, tissue repair Also used for asexual reproduction = organisms clone selves Unique to eukaryotes

17. The Cell Cycle The period from one cell division to next

18. Interphase: The Longest Phase 90% of cell cycle length

19. Interphase

20. G1: Gap / Growth Phase Cell growth # of cytoplasmic components doubled

21. S: Synthesis Phase DNA duplicated

22. Chromosome & copy = sister chromatids Joined at centromere

23. G2: Gap or Growth Phase II Makes proteins necessary for cell division Cell prepares to divide

24. Cells stay in G1 if making macromolecules Enter S when DNA & accessory proteins are copied Rate of DNA replication is same for all cells of a species

25. Same cycle length for same type of cells Different cycle lengths for different types of cells e.g. cells in red bone marrow divide every second e.g. nerve cells stay in G1 indefinitely Rate of cell division is under control (checkpoints, molecular brakes, etc.)

26. After G2, cell enters mitosis Mitosis maintains cell’s chromosome #

27. Chromosome Number Humans have 46 chromosomes = diploid (2n) 2 of each type of chromosome = one set from mother, one from father

28. During mitosis: Each 2n parent cell produces two 2n daughter cells Each daughter cell has each pair of chromosomes = 23 pairs

29. During mitosis, 2 sister chromatids (duplicated chromosomes) separate Each becomes independent chromosome that ends up in 1 of daughter cells

30. The Mitotic Spindle Present in every cell Made of microtubules = change length by addition or removal of tubulin subunits Originates from pair of centrioles

31. Early in cell division, duplicated chromosome is condensed = coils up DNA winds twice around histones = nucleosome Keeps chromosomes organized during nuclear division

32. Late Interphase / Pre-Prophase Outside of nucleus, 2 centrioles duplicate selves

33. Early Prophase Inside nucleus: Chromosomes begin to condense Outside nucleus: Spindle begins to form Nuclear envelope begins to fall apart

34. Late Prophase Nuclear envelope completely falls apart Spindle fibres from each pole attach to sister chromatids of each chromosome

35. Metaphase Chromosomes line up halfway between spindle poles

36. Anaphase Sister chromatids of each chromosome separate & move to opposite poles (motor proteins attached to kinetochores drag chromatids along microtubules) Spindle poles pushed apart by growing microtubules

37. Telophase 1 of each type of chromosome reaches each spindle pole = 2 identical groups of chromosomes at each cell pole Chromosomes decondense Nuclear envelope forms around each cluster of chromosomes = two nuclei, each with 2n # of chromosomes

38. Cytokinesis Cytoplasm of cell divides Results in 2 daughter cells, each with same number of chromosomes as parent cell

39. Cytokinesis in Animal Cells Contractile ring mechanism Halfway between cell’s poles, plasma membrane constricts = cleavage furrow (ATP energy causes contraction of actin filaments) Cleavage furrow deepens until cytoplasm split into 2

40. Cytokinesis in Plant Cells Cell plate formation Golgi vesicles move to cell equator & fuse Vesicle membranes become cell membranes Contents become cellulose cell wall

41. Summary of Mitosis Nuclear & cellular division that maintains chromosome # Used for growth, repair, asexual reproduction

42. Cell division & DNA replication regulated so that: DNA only replicated once before cell division Cells that never divide do not replicate DNA Cells don’t try to replicate DNA if lack the energy & raw materials to complete process

43. Cellular Controls over Mitosis Anchorage dependence Animal cells must be in contact with a solid surface to divide Density-dependent inhibition Crowded cells stop dividing Growth factors Required to start & continue dividing Secreted by other cells

44. Cell Cycle Checkpoints Cell cycle has checkpoints: • Structure of chromosomal DNA monitored • Completion of phases monitored • Determines if good time for cell division Rely on internal & external cues

46. G1 checkpoint is most important: If no go-ahead signal, cell will switch to non-dividing G0 phase e.g. nerve & muscle cells remain in G0 indefinitely

47. Cancer & Cell Division • If immune system doesn’t recognize & destroy a cancerous cell, it may divide multiple times & form a tumor • Benign • Cells remain localized • Malignant • Spreads to other parts of body & disrupts function

49. Why don’t cancer cells follow the rules? Don’t exhibit density-dependence Have defective control systems Ignore / over-ride checkpoints Some synthesize own growth factors so continue dividing Divide indefinitely

50. Types of Cancers Carcinomas Internal & external coverings of body e.g. skin Sarcomas Supportive tissues e.g. bone & muscle Leukemias & Lymphomas Blood-forming tissues e.g. bone marrow, spleen, lymph nodes