CH 8: The Cellular Basis of Reproduction and Inheritance

1. CH 8: The Cellular Basis of Reproduction and Inheritance

2. Asexual reproduction Chromosomes are duplicated and cell divides One copy of each chromosome is placed in each cell Each “daughter” cell is genetically identical to the parent and the other daughter Type of eukaryotic cellular division required: mitosis Methods of Reproduction • Advantage = fast and convenient • Disadvantage = very little genetic variation

3. Sexual reproduction Offspring inherit DNA from both of their parents Type of eukaryotic cellular division required: meiosis Offspring can show great variation Advantage = lots of genetic variation Disadvantage = metabolically expensive Methods of Reproduction

4. Prokaryotic cells reproduce asexually by a type of cell division called binary fission The circular DNA molecule replicates to form 2 chromosomes The chromosome copies move apart The cell elongates The plasma membrane grows inward, dividing the parent into two daughter cells  Colorized TEM 32,500

5. Plasma membrane Prokaryotic chromosome Cell wall Duplication of chromosome and separation of copies Continued elongation of the cell and movement of copies Division into two daughter cells

6. Eukaryotic Asexual Reproduction • Mitosis: • Purpose: • Asexual reproduction in single celled organisms • Growth and repair in multicelled organisms • An exact copy of the cell’s DNA is made*, the copies separated, and each copy is put in a new cell. • *Put another way…an exact copy of each chromosome is made

7. Mitosis • Mitosis involves one cellular division. • 1 cell  2 cells (called daughter cells) • Daughter cells are genetically identical • Chromosome number does not change.

8. Eukaryotic Chromosomecondensed form • Sister chromatids have identical DNA • Centromere • Kinetechore on centromere provides binding site for microtubules Sister chromatids

9. Eukaryote Chromosome Structure Histone core is made up of 8 proteins A nucleosome is 2 wraps of DNA around a histone core Histone core shown in greater detail, see page 212

10. Cell Cycle • Cell cycle describes the “life cycle” of a cell- Cell cycle is tightly controlled • G1 • S Interphase • G2 Mitotic Phase • Mitosis • Prophase, metaphase, anaphase, telophase • Cytokinesis

11. Mitosis • Mitosis = division of the cell’s DNA and nucleus in a eukaryotic cell • Cytokinesis = division of the cytoplasm (cell) • Mitosis occurs in somatic cells such as….

12. Cell Cycle

13. Cell Cycle Interphase G 1 - period of cell growth S - DNA synthesis • An exact copy is made of each chromosome • Copies are joined at the ________ G 2 – cell continues to grow and prepares to divide • e.g. centrioles duplicate in animal cells

14. G1 Checkpoint GO SIGNAL Cell Completes Cell Cycle Checks Cell Size, Organelles, Nutrition STOP SIGNAL Waits to Grow Larger Control of the Cell Cycle - Checkpoints G2 Checkpoint M Checkpoint • Chromosomes Aligned? • DNA Replicated? • Cell Division Machinery OK? • Spindle Fibers Attached?

15. Mitosis • Mitosis (division of nucleus/chromosomes) follows interphase – see pages 130/131 • 4/5 phases • Prophase,Prometaphase • Metaphase • Anaphase • Telophase (and cytokinesis)

16. Prophase • Chromosomes condense, become visible under microscope • Centriole pairs* move towards poles (animal only) • Nucleoli disappear • *centriole pair = centrosome

17. Prometaphase • Transition from prophase to metaphase • Nuclear envelope breaks up and forms vesicles • Microtubules arranged as spindle fibers attach the kinetechore on the centromere of each sister chromatid to opposite poles • Attach to centrioles in animal cells

18. Plant Prometaphase • Prometaphase in a plant cell • Chromosomes are visible • Nuclear envelope is breaking down • Spindle fibers cannot be seen in this micrograph.

19. Metaphase • Spindle microtublules push and pull chromo to middle of cell • Centromeres line up across the middle of the cell • Microtubules running pole to pole elongate the cell • Not visible in this micrograph Animal Metaphase

20. Plant Metaphase • Chromosomes tend to be “messier” in plant metaphase

21. Anaphase • Sister chromatids separate at the centromere • MT* pull sister chromatids to opposite poles • MT continue to elongate cell • This also helps to separate chromatids * MT = microtubules • Animal anaphase

22. Plant Anaphase • Separated sister chromatids (daughter chromosomes) clearly visible

23. Telophase • Telophase starts when chromatids reach poles • Goal is to make 2 new nuclei • Chromo. unwind • Nucleoli reappear • Nuclear envelope reforms from vesicles • Animal cell shown

24. Cytokinesis • Cytokinesis – division of cytoplasm • Begins during telophase • Different in plant and animal cells

25. Animal Cytokinesis • Microfilaments wrap around the center of the cell and then contract • Creates cleavage furrow • Cell “squeezed” in 2 Page 132

26. Plant Cytokinesis • Vesicles containing cell wall material line up across middle of cell • Vesicles merge and form cell plate • Cell plate grows until it divides the cell in 2 Cell plate

27. MITOSIS • Interphase • Prophase • Metaphase • Anaphase • Telophase • Cytokenesis 2N 2N 2N Is this a plant or an animal cell?

28. Mitosis Review • Comparison Plant and Animal Mitosis • Mitosis • Animal Cell Mitosis • Plant Cell Mitosis

29. Meiosis • Meiosis is needed for sexual reproduction • Purpose of meiosis is to create gametes • Egg and sperm in humans • Needed for sexual reproduction • Gametes have only one copy of each type of chromosome • Occurs in germ cells • Ovaries and testes of humans

30. Related Terms • Diploid = 2 copies of each type of chromosome present (2N) • One copy came from mom’s egg and the other from dad’s sperm • Human diploid number = 46 (also say 2N = 46) • Haploid = 1 copy of each type of chromosome present (N) • Human haploid number = 23 (N = 23) • Gametes are haploid

31. Overview Meiosis • Meiosis separates homologous chromosomes and produces cells with a single set of chromosomes • Homologous Chromosomes: pair of chromosomes with genetic information about the same traits, page 136

32. Human KaryotypeHomologous Chromosomes

33. Meiosis • The process of meiosis requires 2 cellular divisions – page 137 • One division to separate homologous chromosomes • Second division to separate duplicated chromosomes

34. MEIOSIS MEIOSIS I Homologous Pair Begins With: • Duplicated Chromosomes • Diploid (2N) Cells Meiosis 1 Functions: • Separate Homologous Chromosomes • Go From Diploid (2N) to • Haploid (N)

35. MEIOSIS II Begins With: • Duplicated Chromosomes • Haploid (N) cells Function: Meiosis II • Separate Sister Chromatids • Creates gametes

36. 2N = 2 Crossing over occurs in meiosis I Homologous chromosomes separate in meiosis I 2 cells, N = 1 for each Sister chromatids separate in meiosis II 4 cells, N = 1 for each. Chromosomes are different due to crossing over

37. Meiosis I • Prophase I • Cell is diploid • Chromosomes are duplicated • Duplicated chromosomes form tetrads • Tetrad = pair of homologous chromosomes • Crossing over occurs • Exchange of genetic material between homologous chromosomes

38. CROSSING OVER Exchange of genetic material between Homologous Chromosomes M F • During Prophase I occurs at CHIASMA Meiosis 1 Meiosis 2 Produces new genetic combinations --Chromosomes with both Maternal & Paternal components Gametes

39. Meiosis I • Prophase I, continued • Chromosomes condense (super-coil) • Centrioles move towards opposite poles (animal only) • Spindle fibers begin to assemble • Nuclear envelope breaks down (always signals end of a prophase )

40. Meiosis I Metaphase I • Spindle fibers push and pull the tetrads to the middle of the cell. • Spindle fibers attach each chromosome of the pair to one pole

41. Meiosis I • Anaphase I • Homologous chromosomes are separated and pulled to opposite poles by the spindle fibers • Microtubules running pole to pole lengthen and elongate the cell

42. Meiosis I • Telophase I and Cytokinesis • Chromosomes reach the poles – still duplicated • Cell divides in two • Animal cells - cleavage furrow squeezes cell in two • Plant cells – cell plate divides cell in two • Generally, the nucleus does not reform

43. At the end of Meiosis I • Homologous chromosomes have been separated • Chromosomes are still duplicated • Sister chromatids are no longer identical due to crossing over • Chromosome number has been cut in half (to haploid number) • Count centromeres to count chromosomes

44. 2N = 2 Crossing over occurs in meiosis I Homologous chromosomes separate in meiosis I 2 cells, N = 1 for each Sister chromatids separate in meiosis II 4 cells, N = 1 for each. Chromosomes are different due to crossing over

45. Meiosis II • Prophase II – in each cell • Centriole pairs separate and move to opposite poles (animal only) • Spindle fibers attach to kinetechore (centromere) of each chromosome • Remember chromosomes are still duplicated • Notice that each chromo is attached to both poles (as in mitosis)

46. Meiosis II • Metaphase II • Spindle fibers push and pull duplicated chromo. To the center of the cell

47. Metaphase II

48. METAPHASE I – tetrads line up across the center of the cell METAPHASE II – duplicated chromosomes line up

49. Anaphase II • Spindle fibers separate the sister chromatids • One copy of each chromosome moves to each pole • Microtubules running pole to pole lengthen and elongate the cell

50. Telophase II • Telophase II and Cytokinesis • Nucleus reforms in each cell (4 cells in total) • Cytoplasm divides • Meiosis web link