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Chromosomes and Mitosis Lecture 6
1 Chromosomal Basis of Heredity <ul><li>A  gene  is a unit of heredity </li></ul><ul><li>Genes are carried on DNA </li></u...
Chromosomes replicate during cell division
The chromosome complement
Chromosome analysis Cri Du Chat results from loss of a small piece of chromosome 5
Gene Map
Chromosome pairs
Non-identical genes
Sex chromosomes <ul><li>These determine the sex of an individual </li></ul><ul><ul><li>Two X chromosomes make a female </l...
Two types of Cell Division <ul><li>Cells divide for two reasons </li></ul><ul><ul><li>To create  genetically identical cop...
The Cell Cycle
S phase Replication Condensation Schematic
DNA replication  Duplex DNA begins Replicating Replication bubbles merge creating two duplexes
Mitosis
The stages of Mitosis
Prophase Detail
Prometaphase
Metaphase
Anaphase
Telophase
The sum total of the process
Karyotypes
Chromosome Length
Chromosome appearance
Meiosis and Gametogenesis
Somatic and Germline cells <ul><li>Development of a fertilized egg into an adult results in two distinct types of cells </...
Meiosis
Meiosis is required for gametogenesis
Meiosis I Somatic cells Germline Cells
Interphase I and Prophase I Leptotene
Prophase I Zygotene
Prophase I Pachytene
Prophase I Diplotene
Recombination
And on the molecular level
Metaphase I and anaphase I
Meiosis I is the reduction division
Non-disjunction
Telophase I
Cytokinesis sperm formation oocyte formation
Meiosis II
A comparison of meiosis and mitosis Germline Somatic Cells involved 4 2 Cells resulting 2 1 Nuclear Divisions Reduces  Mai...
Relationship to Gametogenesis
Sperm and Egg formation
Gametogenesis
Fertilization <ul><li>Entry of a single sperm into an egg prevents entry of other sperm </li></ul><ul><li>The DNA of sperm...
Mitotic Non-disjunction
Cell cycle and apoptosis <ul><li>Cells undergo 3 controlled processes </li></ul><ul><ul><li>The first two are part of the ...
Control of entry into cell cycle and apoptosis <ul><li>Cell cycle is initiated by phosphorylation of transcription factors...
Cyclins and CDK’s <ul><li>CDK levels don’t change while cyclins are destroyed at the end of each phase </li></ul><ul><li>T...
Cyclins bind CDK’s <ul><li>CDK’s are  C yclin  D ependent  K inases </li></ul><ul><li>Association with cyclins activates t...
The exit from Go <ul><li>Go is a quiescent state </li></ul><ul><li>Activation of G1 CDK occurs due to a rising level of G1...
Events during G1 <ul><li>A rising level of G1 cyclins increases the activity of G1 CDK’s </li></ul><ul><li>CDK’s in turn a...
G1/S checkpoint <ul><li>This is controlled by the activity of the  transcription factor E2F </li></ul><ul><ul><li>E2F is a...
And those genes are <ul><li>Three groups </li></ul><ul><ul><li>Cell cycle regulators </li></ul></ul><ul><ul><ul><li>Cyclin...
Other Checkpoints <ul><li>These monitor the completion of DNA synthesis </li></ul><ul><ul><li>The presence of Okazaki frag...
Downregulation of cyclin influenced CDK activity <ul><li>This is accomplished through proteolysis of the cyclins </li></ul...
MPF activates APC which ubiquitinates cyclin B
In the overall <ul><li>Stimulated entry into G1 results in appearance of an initial level of cyclins that promote the prog...
Apoptosis  (apo – toe – sis) <ul><li>This is programmed cell death </li></ul><ul><ul><li>Distinguish it from necrosis </li...
The apoptotic cell <ul><li>Mitochondria break open </li></ul><ul><li>DNA fragments in a regular way </li></ul><ul><li>The ...
The mechanisms of apotosis <ul><li>Can be classified as externally or internally signaled </li></ul><ul><li>One internal r...
P53 controls entry into S-phase <ul><li>P53 can sense DNA damage by binding mismatches </li></ul><ul><li>In the presence o...
But if the DNA damage is extensive <ul><li>P53 induces apotosis by activating transcription of Bax </li></ul><ul><ul><li>B...
The caspases <ul><li>These are proteolytic enzymes that are held in check by external or internal inhibitors </li></ul><ul...
External apoptotic mechanisms <ul><li>Involve external “death signals” </li></ul><ul><li>Cells may be recognized as a thre...
Fas/Fas ligand signaling <ul><li>Fas ligand (FasL) is a membrane bound cell surface protein  </li></ul><ul><li>It binds to...
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MolBiol #6-7

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MolBiol #6-7

  1. 1. Chromosomes and Mitosis Lecture 6
  2. 2. 1 Chromosomal Basis of Heredity <ul><li>A gene is a unit of heredity </li></ul><ul><li>Genes are carried on DNA </li></ul><ul><li>DNA is contained within chromosomes as chromatin </li></ul>
  3. 3. Chromosomes replicate during cell division
  4. 4. The chromosome complement
  5. 5. Chromosome analysis Cri Du Chat results from loss of a small piece of chromosome 5
  6. 6. Gene Map
  7. 7. Chromosome pairs
  8. 8. Non-identical genes
  9. 9. Sex chromosomes <ul><li>These determine the sex of an individual </li></ul><ul><ul><li>Two X chromosomes make a female </li></ul></ul><ul><ul><li>One X and one Y a male </li></ul></ul>
  10. 10. Two types of Cell Division <ul><li>Cells divide for two reasons </li></ul><ul><ul><li>To create genetically identical copies of themselves </li></ul></ul><ul><ul><ul><li>This is </li></ul></ul></ul><ul><ul><ul><li>mitosis </li></ul></ul></ul><ul><ul><li>To create gametes that contain half of the chromosomes of the original cell </li></ul></ul><ul><ul><ul><li>This is meiosis </li></ul></ul></ul>46 46 46 46 23 23 23 23
  11. 11. The Cell Cycle
  12. 12. S phase Replication Condensation Schematic
  13. 13. DNA replication Duplex DNA begins Replicating Replication bubbles merge creating two duplexes
  14. 14. Mitosis
  15. 15. The stages of Mitosis
  16. 16. Prophase Detail
  17. 17. Prometaphase
  18. 18. Metaphase
  19. 19. Anaphase
  20. 20. Telophase
  21. 21. The sum total of the process
  22. 22. Karyotypes
  23. 23. Chromosome Length
  24. 24. Chromosome appearance
  25. 25. Meiosis and Gametogenesis
  26. 26. Somatic and Germline cells <ul><li>Development of a fertilized egg into an adult results in two distinct types of cells </li></ul><ul><ul><li>Somatic cells </li></ul></ul><ul><ul><ul><li>These create all tissues and organs of the adult except for cells destined to become sperm or egg </li></ul></ul></ul><ul><ul><ul><li>They can only undergo mitosis </li></ul></ul></ul><ul><ul><li>Germline cells </li></ul></ul><ul><ul><ul><li>The final differentiated form of these cells are mature gametes: the sperm and egg </li></ul></ul></ul><ul><ul><ul><li>These cells undergo mitosis until gametogenesis </li></ul></ul></ul><ul><ul><ul><ul><li>They then undergo meiosis </li></ul></ul></ul></ul>
  27. 27. Meiosis
  28. 28. Meiosis is required for gametogenesis
  29. 29. Meiosis I Somatic cells Germline Cells
  30. 30. Interphase I and Prophase I Leptotene
  31. 31. Prophase I Zygotene
  32. 32. Prophase I Pachytene
  33. 33. Prophase I Diplotene
  34. 34. Recombination
  35. 35. And on the molecular level
  36. 36. Metaphase I and anaphase I
  37. 37. Meiosis I is the reduction division
  38. 38. Non-disjunction
  39. 39. Telophase I
  40. 40. Cytokinesis sperm formation oocyte formation
  41. 41. Meiosis II
  42. 42. A comparison of meiosis and mitosis Germline Somatic Cells involved 4 2 Cells resulting 2 1 Nuclear Divisions Reduces Maintains Chromosome number Meiosis Mitosis
  43. 43. Relationship to Gametogenesis
  44. 44. Sperm and Egg formation
  45. 45. Gametogenesis
  46. 46. Fertilization <ul><li>Entry of a single sperm into an egg prevents entry of other sperm </li></ul><ul><li>The DNA of sperm and egg are initially kept separate in “pronuclei” of the zygote </li></ul><ul><li>Timing of a pregnancy extends from the “last menstrual period” (LMP) rather than the time of fertilization </li></ul>
  47. 47. Mitotic Non-disjunction
  48. 48. Cell cycle and apoptosis <ul><li>Cells undergo 3 controlled processes </li></ul><ul><ul><li>The first two are part of the cell cycle, the last an exit from the cell cycle </li></ul></ul><ul><ul><li>Division (the cell cycle) </li></ul></ul><ul><ul><li>Quiescence </li></ul></ul><ul><ul><ul><li>This is where most of the work of being a cell lies </li></ul></ul></ul><ul><ul><ul><ul><li>During division the energy of the cell is devoted to making a new cell </li></ul></ul></ul></ul><ul><ul><li>Death </li></ul></ul><ul><ul><ul><li>This can be a normal process creating a final functional form or an induced suicide </li></ul></ul></ul><ul><ul><ul><ul><li>Epithelium and reticuloendothelial cells undergo active transitions towards terminally differentiated states in which the final forms are unable to divide </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>The stratum corneum consists of cells that have become bags of crosslinked keratin protein with no internal metabolism </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><li>Suicide can be induced because the organism senses a threat to the entire organism </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Infection, cancer, avoidance of autoimmunity </li></ul></ul></ul></ul></ul>
  49. 49. Control of entry into cell cycle and apoptosis <ul><li>Cell cycle is initiated by phosphorylation of transcription factors </li></ul><ul><li>These activate transcription of a set of proteins known as cyclins </li></ul><ul><li>The appearance of cyclins is progressive and determines the types of proteins that will be phosphorylated at a particular point during the cell cycle </li></ul>
  50. 50. Cyclins and CDK’s <ul><li>CDK levels don’t change while cyclins are destroyed at the end of each phase </li></ul><ul><li>There are 3 general groups of each </li></ul><ul><ul><li>G1 cyclins </li></ul></ul><ul><ul><ul><li>Cyclin D </li></ul></ul></ul><ul><ul><li>S-phase cyclins </li></ul></ul><ul><ul><ul><li>Cyclin A </li></ul></ul></ul><ul><ul><li>G2 cyclins </li></ul></ul><ul><ul><ul><li>Cyclin B (maturation promoting factor MPF) </li></ul></ul></ul><ul><ul><li>Cyclin E is shared between G1 and M phase </li></ul></ul><ul><ul><li>Cyclin A is shared between M phase and G2 </li></ul></ul>
  51. 51. Cyclins bind CDK’s <ul><li>CDK’s are C yclin D ependent K inases </li></ul><ul><li>Association with cyclins activates their kinase function </li></ul><ul><ul><li>A cyclin tethers a target protein to the CDK </li></ul></ul><ul><li>The targets of CDK’s are transcription factors among other proteins </li></ul><ul><ul><li>CDK’s are serine/threonine kinases </li></ul></ul>
  52. 52. The exit from Go <ul><li>Go is a quiescent state </li></ul><ul><li>Activation of G1 CDK occurs due to a rising level of G1 cyclins </li></ul><ul><li>G1 cyclins are transcriptionally activated by growth factors </li></ul>
  53. 53. Events during G1 <ul><li>A rising level of G1 cyclins increases the activity of G1 CDK’s </li></ul><ul><li>CDK’s in turn activate proteins and in turn genes that prepare the cell to begin DNA replication </li></ul><ul><li>At the G1 S boundary, the cell encounters a checkpoint </li></ul>
  54. 54. G1/S checkpoint <ul><li>This is controlled by the activity of the transcription factor E2F </li></ul><ul><ul><li>E2F is a family of related proteins (E2F 1 to E2F5) </li></ul></ul><ul><li>E2F is found complexed throughout the cell cycle to another family of proteins: Rb </li></ul><ul><ul><li>At the G1/S checkpoint, Rb is phosphorylated by CDK2/cyclinA </li></ul></ul><ul><ul><li>E2F is freed from sequestration and activates transcription at genes containing an E2F consensus sequence </li></ul></ul>
  55. 55. And those genes are <ul><li>Three groups </li></ul><ul><ul><li>Cell cycle regulators </li></ul></ul><ul><ul><ul><li>Cyclin A </li></ul></ul></ul><ul><ul><ul><li>E2F, Rb, myc, myb </li></ul></ul></ul><ul><ul><ul><ul><li>Note that these are not all positive regulators of cell cycle </li></ul></ul></ul></ul><ul><ul><li>Enzymatic machinery for DNA synthesis </li></ul></ul><ul><ul><ul><li>DNA polymerase </li></ul></ul></ul><ul><ul><ul><li>PCNA </li></ul></ul></ul><ul><ul><ul><li>Enzymes involved in nucleotide metabolism </li></ul></ul></ul><ul><ul><li>DNA synthesis regulators </li></ul></ul><ul><ul><ul><li>Enzymes that recognize the origins of replication for example </li></ul></ul></ul>
  56. 56. Other Checkpoints <ul><li>These monitor the completion of DNA synthesis </li></ul><ul><ul><li>The presence of Okazaki fragments prevents entry into G2 </li></ul></ul><ul><li>DNA damage </li></ul><ul><ul><li>This occurs before, during and after completion of S phase </li></ul></ul><ul><li>Spindle attachment </li></ul><ul><ul><li>Failure to attach spindle to centromere results in blockage of mitosis at metaphase </li></ul></ul><ul><ul><li>Failure to align the spindle during cytokinesis results in blockage at anaphase </li></ul></ul>
  57. 57. Downregulation of cyclin influenced CDK activity <ul><li>This is accomplished through proteolysis of the cyclins </li></ul><ul><ul><li>G1 phase cyclins disappear during S and G2 phase </li></ul></ul><ul><ul><li>M-phase promoting factor (CDK2 + cyclin B) concentrations rise just prior to onset of mitosis </li></ul></ul><ul><ul><ul><li>Cyclins associated with MPF are degraded by anaphase promoting complex </li></ul></ul></ul><ul><ul><ul><ul><li>Cyclin B levels peak at G1/M </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Degradation during anaphase </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><li>APC promotes polyubiquitination of cyclin B </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Ubiquitinated cyclin B is degraded by a proteosome </li></ul></ul></ul></ul><ul><li>Cyclin transcription is also turned off and the mRNA is unstable </li></ul><ul><ul><li>So no new cyclin is made until transcription is restored </li></ul></ul>
  58. 58. MPF activates APC which ubiquitinates cyclin B
  59. 59. In the overall <ul><li>Stimulated entry into G1 results in appearance of an initial level of cyclins that promote the progressive activation of genes enabling the cell to synthesize DNA </li></ul><ul><li>A series of progressive steps result in </li></ul><ul><ul><li>Activation of genes further into the cycle </li></ul></ul><ul><ul><li>Degradation of cyclins that promoted earlier steps </li></ul></ul><ul><ul><li>Passage through checkpoints that insure mechanistic fidelity of each step </li></ul></ul>
  60. 60. Apoptosis (apo – toe – sis) <ul><li>This is programmed cell death </li></ul><ul><ul><li>Distinguish it from necrosis </li></ul></ul><ul><ul><li>Necrosis results from traumatic forces outside the cell </li></ul></ul><ul><ul><li>Necrotic tissue provokes inflammation as the immune system moves in to clear out damaged and dead cells </li></ul></ul><ul><li>Apoptosis is an ordered stepwise self-destruction that permits surrounding cells to utilize the breakdown products of the dead cell </li></ul><ul><ul><li>There is no inflammation involved </li></ul></ul>
  61. 61. The apoptotic cell <ul><li>Mitochondria break open </li></ul><ul><li>DNA fragments in a regular way </li></ul><ul><li>The cell loses a regular shape </li></ul><ul><ul><li>Undergoes blebbing </li></ul></ul><ul><ul><li>This is an irregular bubbling appearance of the plasma membrane </li></ul></ul>
  62. 62. The mechanisms of apotosis <ul><li>Can be classified as externally or internally signaled </li></ul><ul><li>One internal route involves p53 </li></ul><ul><li>p53 is a transcription factor that is involved in cell cycle control and sensing the presence of DNA damage </li></ul><ul><li>The central role p53 plays is at the G1/S checkpoint </li></ul>
  63. 63. P53 controls entry into S-phase <ul><li>P53 can sense DNA damage by binding mismatches </li></ul><ul><li>In the presence of damage, p53 activates transcription of p21 </li></ul><ul><ul><li>P21 binds and inactivates CDK2-cyclin E complexes </li></ul></ul><ul><ul><li>The complex is unable to phosphorylate Rb and free E2F </li></ul></ul><ul><ul><li>Thus entry into S phase is inhibited </li></ul></ul><ul><ul><li>If the damage is repaired, p53 levels and p21 levels drop and S phase ensues </li></ul></ul>
  64. 64. But if the DNA damage is extensive <ul><li>P53 induces apotosis by activating transcription of Bax </li></ul><ul><ul><li>BAX protein competes with BCL-2 to form pores in mitochondrial membranes </li></ul></ul><ul><ul><ul><li>BCL-2 prevents the release of cytochrome c from mitochondria into the cytoplasm </li></ul></ul></ul><ul><ul><ul><li>BAX permits release of cytochrome c </li></ul></ul></ul><ul><ul><li>When released, cytochrome c stimulates caspase activation </li></ul></ul>
  65. 65. The caspases <ul><li>These are proteolytic enzymes that are held in check by external or internal inhibitors </li></ul><ul><li>Activation results in an explosive proteolytic cascade </li></ul><ul><ul><li>Caspase 9 cleaves and activates other caspases </li></ul></ul><ul><ul><li>The caspases also activate quiescent nucleases </li></ul></ul>
  66. 66. External apoptotic mechanisms <ul><li>Involve external “death signals” </li></ul><ul><li>Cells may be recognized as a threat to the whole organism </li></ul><ul><ul><li>The immune system moves in to kill them </li></ul></ul><ul><ul><li>One mechanism of killing involves a command to the cell to initiate apoptosis </li></ul></ul>
  67. 67. Fas/Fas ligand signaling <ul><li>Fas ligand (FasL) is a membrane bound cell surface protein </li></ul><ul><li>It binds to Fas receptor </li></ul><ul><li>Binding results in trimerization and activation of APAF </li></ul><ul><li>APAF in turn activates caspase 8 by proteolysis of a caspase 8 zymogen </li></ul><ul><ul><li>Caspase 8 cleaves a BCL-2 family member BID </li></ul></ul><ul><ul><li>BID translocates to the mitochondria and binds BAX </li></ul></ul><ul><ul><li>Bax permits leakage of cytochrome c and activation of the caspase 9 cascade via APAF-1 again </li></ul></ul>

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