1. Chapter 10 How Cell Divide Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Bacterial Cell Division Bacteria divide by binary fission –No sexual life cycle –Reproduction is clonal Bacterial genome made up of single, circular chromosome tightly packed in the cell at the nucleoid region. Replication begins at the origin of replication and proceeds in two directions to site of termination New chromosomes are partitioned to opposite ends of the cell Septum forms to divide the cell into 2 cells

3. Prior to cell division, the bacterial DNA molecule replicates. The replication of the double-stranded, Circular DNA mole- cule that constitutes the genome of a bacterium begins at a specific site, called the origin of replica- tion (green area). The replication enzymes move out in both directions from that site and make copies of each strand in the DNA duplex. The enzymes continue until they meet at another specific site, the terminus of replication (red area). As the DNA is replicated, the cell elongates, and the DNA is partitioned in the cell such that the origins are at the ¼ and ¾ positions in the cell and the termini are oriented toward the middle of the cell. 1. 2. 3. Bacterial cell Bacterial chromosome: Double-stranded DNA Origin of replication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4. 5. Septation then begins, in which new membrane and cell wall material begin to grow and form a septum at approximately the midpoint of the cell. A protein molecule called FtsZ (orange dots) facilitates this process. When the septum is complete, the cell pinches in two, and two daughter cells are formed, each containing a bacterial DNA molecule. 4. Septum Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5. Eukaryotic Chromosomes Every species has a different number of chromosomes Humans have 46 chromosomes in 23 nearly identical pairs –Additional/missing chromosomes usually fatal with some exceptions (Chapter 13) 950x Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Biophoto Associates/Photo Researchers, Inc.

6. Chromosomes Composition composed of chromatin – complex of DNA and protein DNA is one long continuous double-stranded polynucleotides Typical human chromosome is 140 million nucleotides long RNA is also associated with chromosomes during RNA synthesis

7. Chromosome Structure Nucleosome –complex of DNA and histone proteins Promote and guide coiling of DNA –DNA duplex coiled around 8 histone proteins every 200 nucleotides –Histones are positively charged and strongly attracted to negatively charged phosphate groups of DNA

8. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mitotic Chromosome Chromatin loop Solenoid Scaffold protein Scaffold protein Chromatin Loop Rosettes of Chromatin Loops Levels of Eukaryotic Chromosomal Organization

9. Chromosome Karyotypes Particular array of chromosomes in an individual organism is called karyotype. –Arranged according to size, staining properties, location of centromere, etc. Humans are diploid (2n) –2 complete sets of chromosomes –46 total chromosomes Haploid (n) – 1 set of chromosomes –23 in humans Only found in egg and sperm Pair of chromosomes are homologous –Each one is a homologue

10. 500x Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © CNRI/Photo Researchers, Inc. A Human Karyotype

11. Chromosome Replication Prior to replication, each chromosome composed of a single DNA molecule After replication, each chromosome composed of 2 identical DNA molecules –Held together by cohesin proteins Visible as 2 strands held together as chromosome becomes more condensed –One chromosome composed of 2 sister chromatids

13. Eukaryotic Cell Cycle 1.G 1 (gap phase 1) –Primary growth phase, longest phase –In G, cell is being cell 2.S (synthesis) –Replication of DNA 3.G 2 (gap phase 2) –Organelles replicate, microtubules organize 4.M (mitosis) –Subdivided into 5 phases 5.C (cytokinesis) –Separation of 2 new cells Interphase

14. Duration of Cell Cycle Time it takes to complete a cell cycle varies greatly Depends on cell type The shortest known animal cell cycles occur in fruit fly embryos = 8 minutes Mature cells take longer than those embryonic tissue to grow –Typical mammalian cell takes 24 hours –Liver cell takes more than a year Growth occurs during G 1, G 2, and S phases –M phase takes only about an hour Most variation in length of G 1 –Resting phase G 0 – cells spend more or less time here

15. Duration of Cell Cycle Most variation in the length of the cell cycle between organisms or cell types occurs in G 1 –Cells often pause in G 1 before DNA replication and enter a resting state called G 0 –Resting phase G 0 – cells spend more or less time here before resuming cell division. –Most of cells in animal’s body at any given time are in G 0 phase, they may be like that for days, months or years (depending on cell type) nerve cells remain there permanently –That is why damage to nerve cells is permanent, they will never enter into replication –Liver cells can resume G 1 phase in response to factors released during injury

16. Cell Cycle (2 stages) 1.Interphase (G1, S, G2) 2.Mitosis (prophase, metaphase, anaphase, telophase)

17. Interphase: Preparation for Mitosis G 1, S, and G 2 phases –G 1 – cells undergo major portion of growth –S – synthesis of new DNA strands replicate DNA produce two sister chromatids attached at the centromere –G 2 – chromosomes coil more tightly using motor proteins; centrioles replicate; organelles replicate

18. Centromere – point of constriction Kinetochore – attachment site for microtubules to pull chromosomes apart during mitosis

19. Mitosis is divided into phases: Prophase Prometaphase Metaphase Anaphase Telophase

20. Prophase Individual condensed chromosomes first become visible with the light microscope Spindle apparatus assembles –2 centrioles move to opposite poles forming spindle apparatus (no centrioles in plants) Nuclear envelope breaks down

21. Chromosomes condense and become visible Chromosomes appear as two sister chromatids held together at the centromere Nuclear envelope breaks down Mitotic spindle beginning to form Condensed chromosomes Prophase

22. Prometaphase Transition occurs after disassembly of nuclear envelope Microtubule attachment –Each sister chromatid connected to opposite poles Chromosomes begin to move to center of cell – congression –Assembly and disassembly of microtubules –Motor proteins at kinetochores

23. Chromosomes attach to microtubules at the kinetochores Each chromosome is oriented such that the kinetochores of sister chromatids are attached to microtubules from opposite poles. Centromere and kinetochore Mitotic spindle Prometaphase

25. All chromosomes are aligned at equator of the cell, called the metaphase plate Polar microtubule Chromosomes aligned on metaphase plate Kinetochore microtubule Metaphase

26. Anaphase Begins when centromeres split Key event is removal of cohesin proteins from all chromosomes Sister chromatids pulled to opposite poles

27. Chromosomes Proteins holding centromeres of sister chromatids are degraded, freeing individual chromosomes Chromosomes are pulled to opposite poles (anaphase A) Kinetochore microtubule Polar microtubule Anaphase

28. Telophase Spindle apparatus disassembles Nuclear envelope forms around each set of chromosomes Chromosomes begin to uncoil Nucleolus reappears in each new nucleus

29. Polar microtubule Nucleus reforming Chromosomes are clustered at opposite poles and decondense Nuclear envelopes re- form around chromosomes Golgi complex and ER re- form Kinetochore microtubule Telophase

30. Cytokinesis Cleavage of the cell into equal halves Animal cells – constriction of actin filaments produces a cleavage furrow Plant cells – cell plate forms between the nuclei Fungi and some protists – nuclear membrane does not dissolve; mitosis occurs within the nucleus; division of the nucleus occurs with cytokinesis

33. At the end of mitosis, you get 2 daughter cells that have the exact same chromosomes as the original parent cell 33

34. Control of the Cell Cycle Cell cycle as to be regulated Do not want uncontrolled cell division, this leads to CANCER! Cell must receive the proper signals to move into mitosis and there are checkpoints (to check the proper signal, to check the integrity of the DNA, to check that one of each replicated chromosome moved correctly, etc)

35. 3 Checkpoints 1.G 1 /S checkpoint –Cell “decides” whether or not to divide –Primary point for external signal influence; did it receive signal for replication? 2.G 2 /M checkpoint –Cell makes a commitment to mitosis –Assesses success of DNA replication – are there mistakes? –Can stall the cycle if DNA has not been accurately replicated. 3.Late metaphase (spindle) checkpoint –Cell ensures that all chromosomes are attached to the spindle

37. Cyclin-dependent kinases (Cdks) Progress through cell cycle depends on cyclin dependent kinases (Cdks) Cdks are not active by themselves –Have to bind to cyclin to be activated Cyclins can be produced in response to stimulus, or can be inhibited so it can’t bind to kinase If cyclin binds to Cdk it is now cyclin-Cdk –This cyclin-Cdk can now transfer phosphate group from ATP to target protein –That target protein (like RB) will then be involved in moving cell past the checkpoint »Phosphorylation of the RB inhibits the RB and allows cell to move past the checkpoint

38. Cdk – cyclin complex –Also called mitosis-promoting factor (MPF) Activity of Cdk is also controlled by the pattern of phosphorylation –Phosphorylation at one site (red) inactivates Cdk –Phosphorylation at another site (green) activates Cdk Cyclin-dependent kinase (Cdk) Cyclin P P

39. The Action of MPF(mitosis promoting factor or cyclin-Cdk) Damage to DNA acts through a complex pathway to tip the balance toward the inhibitory phosphorylation of MPF Example – p21 –p21 is a protein that is made in response to DNA damage due to radiation –If p21 is present, it binds the G1-S Cdk, preventing activation by the cyclin –So the cell cycle stops until damage is repaired or until the cell is killed

40. 40

41. Growth factors Act by triggering intracellular signaling systems Platelet-derived growth factor (PDGF) one of the first growth factors to be identified PDGF receptor is a receptor tyrosine kinase (RTK) that initiates a MAP kinase cascade to stimulate cell division If you cut yourself and bleed, platelets gather to make clot, release protein (PDGF, growth factor) that stimulates adjacent cells to divide and heal wound Growth factors can override cellular controls that otherwise inhibit cell division

42. A cascade of events that lead to certain genes being transcribed and translated (in other words, genes will be used to cause the cell to respond to the stimulus)

43. Cancer Unrestrained, uncontrolled growth of cells Failure of cell cycle control Two kinds of genes can disturb the cell cycle when they are mutated Tumor-suppressor genes Proto-oncogenes

44. Tumor-suppressor genes p53 plays a key role in G 1 checkpoint p53 protein monitors integrity of DNA –If DNA damaged, cell division halted and repair enzymes stimulated –If DNA damage is irreparable, p53 directs cell to kill itself Prevent the development of mutated cells containing mutations p53 gene is absent or damaged in many cancerous cells

45. Tumor-suppressor genes First tumor-suppressor identified was the retinoblastoma susceptibility gene (Rb) –Predisposes individuals for a rare form of cancer that affects the retina of the eye Inheriting a single mutant copy of Rb means the individual has only one “good” copy left –During the hundreds of thousands of divisions that occur to produce the retina, any error that damages the remaining good copy leads to a cancerous cell –Single cancerous cell in the retina then leads to the formation of a retinoblastoma tumor

46. Proto-oncogenes Proto-oncogenes are normal cellular genes that become oncogenes when mutated –Oncogenes can cause cancer Some encode receptors for growth factors –If receptor is mutated in “on” position, cell no longer depends on growth factors, it will just continue to divide on its own –The cell will then replicate uncontrollably

47. Proto-oncogenes Growth factor receptor: more per cell in many breast cancers. Ras protein: activated by mutations in 20–30% of all cancers. Src kinase: activated by mutations in 2–5% of all cancers. Tumor-suppressor Genes Rb protein: mutated in 40% of all cancers. p53 protein: mutated in 50% of all cancers. Cell cycle checkpoints Ras protein Rb protein p53 protein Src kinase Mammalian cell Nucleus Cytoplasm Key Proteins Associated with Human Cancers