1. Chapter 10 How Cells Divide Dr. Joseph Silver

2. as with many other things in biology there is with cell division (mitosis) a progression from primitive to advanced

3. prokaryotes divide by binary fission

4. prokaryotes have only 1 chromosome when it is time to divide the 1 chromosome replicates

7. 1. origin of replication 2. enzyme complex moves along in both directions 3. until 1 chromosome becomes 2 4. enzymes contact terminus site 5. cell elongates 6. pulling chromosomes apart 7. septum forms at mid point 8. cell pinches into 2 9. we now have 2 cells each with 1 chromosome

8. fig 10.3 shows how mitosis has changed during progression form simple to complex in cells having a nucleus

9. from this point on we will study modern eukaryotic cell division usually referred to as mitosis

10. the term mitosis was first used a little more than 100 years ago it means “thread”

11. the number of chromosomes in a cell varies from 1 to more than a 1000

12. in a functioning cell the genetic material is present in an unwound form looking like a bunch of loose random threads known as chromatin

14. the genetic material DNA has a double helix structure

16. one eukaryotic chromosome if laid out in a straight line would be about 2 inches long

17. your text states the following 1 chromosome has about 140,000,000 nucleotides which if they were words would be enough to fill about 280 books of 1000 pages that is a lot of information SO

18. one of our cells has 46 chromosomes 46 x 2 = 92 inches (about 8 feet) so the question is how do you get 8 feet of genetic material into the nucleus of a cell???????

19. the answer is shown in fig. 10.5

20. 1. every 200 nucleotides are wrapped around an 8 protein positively charged complex called a histone 2. this complex is called a nucleosome

21. 3. many nucleosomes wrap around each other to form a thread like structure 4. enzymes called condensins wrap the threads into chromatin loops

22. 5. the loops are then attached to a scaffolding protein just as wires or bristles are attached to a wire brush 6. the rosettes or chromatin loops are tightly wound into what we call a visible chromosome

26. like so many other things is science if you put 3 scientists in the room you will hear 4 different ways that something may happen there are many things that we do not fully understand

27. we have 46 chromosomes 23 from mom 23 from dad the only time that we can actually get to see the 46 chromosomes is during cell division

28. when the chromosomes condense at the beginning of mitosis they are all visible scientists are able to access dividing cells stop them from dividing at the stage when all the chromosomes are visible and use biological stains so that we can see them all

29. look at fig 10.6

34. each pair of chromosomes have genes for the same traits so in a perfect world we have 2 genes one from each parent for every trait this is referred to as diploid a full set of chromosomes

35. each pair of chromosomes are referred to as a homologue and they have genes for the same traits they do not have to be identical genes but they are genes for the same traits they are homologous

36. in order for cell division or mitosis to take place properly all chromosomes must duplicate and remain attached as sister chromatids 2 identical chromosomes attached to each other

38. sister chromatids are held together about the centromere near the center of a chromosome by a complex protein referred to as cohesins

41. The cell cycle consists of 1. interphase 2. prophase 3. metaphase 4. anaphase 5. telophase 6. cytokinesis

43. 1. cell grows, DNA duplicated, cell organelles increase, centrioles duplicate, spindle fibers appear 2. nuclear membrane disappears, spindle fibers increase, chromatin condenses to chromosomes, organelles dispersed 3. centrioles move to poles, chromosomes line up at cell midpoint, spindle fibers attach to centromere 4. spindle fibers shorten, chromosomes pulled to opposite ends of cell 5. nuclear membrane reforms, chromosomes unwind, spindle fibers disappear 6. cell membrane pinches at center to form 2 identical cells

44. 1. the cell cycle goes through a series of steps resulting in 2 smaller and identical cells and as the process proceeds 2. the cell is able to go through a series of checkpoints to make sure that the cell is able to proceed without errors

45. interphase consists of 3 parts 1. G1 2. S 3. G2 which make up about 95% of the cell cycle

47. during interphase the cell grows does the work of the cell usually the longest phase decision is made to divide the cell DNA is duplicated organelles, enzymes, etc. increased spindle begins to appear

48. interphase – the growth phase consists of 3 parts G1 – growth and function S – DNA duplicated G2 – everything that the cell needs in order to become 2 smaller cells is made ready and increased if necessary G0 – growth & division arrested, cell is dormant

49. as with so many things in a living cell there are checks and balances to make certain that all proceeds in a correct manner

50. G1/S checkpoint before making the decision to duplicate the DNA the cell checks that it can proceed 1. nutritional state 2. growth factors 3. size of cell 4. is DNA OK? 5. repair/apoptosis 6. Cdk2 + cyclin e become active

51. S phase checkpoint 1. check for DNA breaks 2. check for raw materials 3. can cell proceed to G2 4. cyclin A + cdk2 become active if all is OK cell proceeds to replicate/copy DNA

52. G2/M checkpoint 1. check for errors in DNA 2. repair/apoptosis 3. condense DNA 4. cyclin B + cdk1 become active 5. M phase promoting factor (MPF) M = mitosis allows cell to continue to M phase

53. APC anaphase promoting complex 1. are chromosomes properly aligned 2. are spindles hooked up to chromosomes 3. is metaphase plate correct 4. Cdc20 and Cdh1 enzymes activate process 5. separases destroys cohesions

56. interphase -growth -DNA copied -centrioles copied -spindle fibers appear -organelles copied -checkpoints OK or not

57. prophase -chromatin condenses -chromosomes visible - sister chromatids appear - nuclear envelope breaks down - organelles dispersed -cytoskeleton disassembled -centrioles move to poles -spindles expand

58. metaphase -chromosomes at mid plate -spindles attached to chromosomes -spindles attached to centrioles

60. telophase -chromosomes at opposite ends -nuclear envelope reforming -Golgi & ER reform -spindles being reabsorbed -chromosomes unwinding - cytoplasm pinched off

65. control of the cell cycle cyclins – cdc and cdk (function???) cyclins – amount go up & down with cell phases MPF – mitosis promoting factor (kinase+cyclin) cyclin dependent kinases – engine that drives cell division kinases = phosphorylation or dephosphorylation APC/C = destroys cohesion complex and cyclins growth factors = 50+, specific & general,

66. cancer loss of control of cell cycle p53 = tumor suppressor gene p53 = halts mitosis if DNA damaged p53 = repair DNA or apoptosis p53 = damaged or absent in many cancers

67. proto-oncogenes – genes which mutate to oncogenes oncogenes – genes that cause cancer tumor suppressor genes – when mutate = cancer see fig 10.24