Understanding the Cell Cycle: Vital Concepts Unveiled

1. Chapter 12The Cell Cycle

2. Cells • The ability of organisms to produce more of their own kind best distinguishes living things from nonliving matter • Cell Theory: Living things are made of cells and cells reproduce from other cells • The continuity of life is based on the reproduction of cells, or cell division

3. Genome • All the DNA in a cell constitutes the cell’s genome • A genome can consist of • Single DNA molecule • common in prokaryotic cells • Multiple DNA molecules • common in eukaryotic cells • DNA molecules in a cell are packaged into chromosomes

4. Chromosomes • Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division • Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus • Somatic cells (nonreproductive cells) have two sets of chromosomes • Diploid cells (2n) • Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells • Haploid cells (n)

5. Cellular Reproduction • Asexual Reproduction: Cells divide creating identical cells • In unicellular organisms, division of one cell reproduces the entire organism • Multicellular organisms depend on cell division for • Development from a fertilized cell • Growth • Repair

6. Binary Fission in Bacteria • Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission • In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart • The plasma membrane pinches inward, dividing the cell into two

7. ChromosomalDNA molecules Mitotic Division Chromosomes Centromere • Most eukaryotes reproduce by mitosis • Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission • Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis Chromosomearm Chromosome duplication(including DNA replication)and condensation Sisterchromatids Separation of sisterchromatids intotwo chromosomes

8. Cellular Reproduction • Sexual Reproduction: Gametes fuse to create zygote • Gametes formed during meiosis • a special type of division that can produce sperm and egg cells • Meiosis creates haploid cells • After fertilization, cell has unique combination of chromosomes

9. Cell Cycle and Mitosis • Eukaryotic cells • Cycle of growth, DNA synthesis, and division • Cellular division= one cell divides into 2 cells • identical to original cell

10. Interphase and Mitosis 1. G1 Phase= Gap 1 Phase -Cell Growth 2. S Phase= DNA Synthesis -Replication 3. G2 Phase= Gap 2 Phase -Cell Growth 4. M Phase= Mitotic Phase -Cellular Division Interphase ~90% of cell life

11. Cell Cycle: G1 • Gap 1 Phase • Growth of cell • Increase in mass • Increase Cytoplasm • Increase Proteins • Produce Organelles • Metabolic activity is high

12. Cell Cycle: S phase • DNA synthesis • During S phase, DNA is replicated • Occurs in nucleus • Eukaryotic cells normally have 2 copies of each chromosome- 1 set from mom, 1 set from dad • Diploid= 2 n • Haploid= n • n= number of different types of chromosomes

13. Human Chromosomes Homologous Chromosomes= pair of chromosomes Haploid (n)=23 Diploid (2n)=46

14. Cell Cycle: S Phase • Prepares DNA for M phase • Duplicates entire genetic code • Creates 2 new strands of DNA from existing strands • DNA synthesis uses a semiconservative model • Each DNA molecule has 1 old strand and 1 new strand

15. Chromosomes • Diploid Cell: 2n = 6 • Haploid= n= 3 • Following DNA synthesis, each of the six chromosomes consists of 2 sister chromatids, connected at the centromere Sister Chromatids Homologous Chromosome Centromere Blue: Paternal Chromosomes Pink: Maternal Chromosomes

16. Chromosomes • Homologous chromosomes are paired chromosomes which possess genes for the same characteristics. One chromosome comes from mom & one from dad Sister Chromatids Homologous Chromosome Centromere Blue: Paternal Chromosomes Pink: Maternal Chromosomes

17. Cell Cycle: G2 • Gap 2 Phase • Growth of cell • Increase in mass • Makes proteins needed for mitosis • Centrosome: the microtubule organizing center • Each centrosome has pair of centrioles • The centrosome replicates during interphase

18. Cell Cycle: M Phase • Mitotic Phase • Mitosis= Nuclear Division • Followed by Cytokinesis • Division of cytoplasm

19. Mitosis: 5 Stages Mitosis is divided into five phases: • Prophase • Prometaphase • Metaphase • Anaphase • Telophase • Cytokinesis overlaps the latter stages of mitosis

20. Mitosis: 5 Stages • Prophase • Chromosomes condense • The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis • The spindle includes the centrosomes, the spindle microtubules, and the asters • Aster • a radial array of short microtubules • extends from each centrosome

21. Mitosis: 5 Stages 2. Prometaphase • Nuclear envelope breaks up • Two centrosomes migrate to opposite ends of the cell during prophase and prometaphase • Some spindle microtubules attach to the kinetochoresof chromosomes and begin to move the chromosomes • Kinetochores are protein complexes associated with centromeres

22. Mitosis: 5 Stages 3. Metaphase • Longest phase of mitosis • 20 minutes • Chromosomes line up at the metaphase plate • an imaginary structure at the midway point between the spindle’s two poles • For each chromosome, sister chromatids are attached at kinetochore to spindle fibers from opposite spindle poles

23. Mitosis: 5 Stages 4. Anaphase • Shortest stage of mitosis • Cohesion proteins between sister chromatids are cleaved • Each chromatid is now an individual chromosome • Individual chromosomes are pulled to opposite ends of cells as microtubules shorten • Cell elongates as microtubules not attached at kinetochores lengthen

24. Mitosis: 5 Stages 5. Telophase • 2 nuclei form • Nucleolus form • Nuclear envelope forms from fragments • Chromosomes become less condensed • Cytokinesis • Division of the cytoplasm • Animal cells: cleavage furrow forms and pinches cell in 2 • Plant cells: Formation of cell plate

25. Cytokinesis

26. Cell Cycle: M Phase • Before Mitosis: • 1 Cell= Diploid (2n) • After Mitosis: • 2 Cells= Diploid (2n) • *End product of mitosis is 2 identical cells

27. Mitosis • Cells vary in rate of mitosis • Age matters: children vs. adult • Varies from continuously (skin cells) to only when necessary to repair damage (internal organ cells) • These differences result from regulation at the molecular level

28. Cell Cycle • The sequential events of the cell cycle are directed by a distinct cell cycle control system • Regulated by both internal and external controls • Example of Internal Control: if kinetochore not attached to spindle microtubules, sends a molecular signal that delays anaphase • Example of External Control: growth factors • Proteins released by certain cells that stimulate other cells to divide • The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

29. Controls on Cell Cycle • Multiple checkpoints in cell cycle • Prevent uncontrolled growth • Repair mistakes • For many cells, the G1 checkpoint seems to be the most important • If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide • If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

30. Controls on Cell Cycle • Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) • Cdks activity fluctuates during the cell cycle because it is controlled by cyclins, so named because their concentrations vary with the cell cycle • MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

31. Controls on Cell Cycle • An example of external signals is density-dependent inhibition, in which crowded cells stop dividing • Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide

32. Controls on Cell Cycle • Cancer is a disease of the cell cycle • Normal cell becomes cancer cell through transformation • Often results from mutations in DNA • Uncontrolled cell growth can lead to tumors • Masses of abnormal cells

33. Controls on Cell Cycle • Benign Tumor= abnormal cells remain at the original site • Malignant tumors= invade surrounding tissues • Metastasize= export cancer cells to other parts of the body, where they may form additional tumors • Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence • Cancer cells have abnormal cell cycle control system • may lack protein checkpoints, produce own growth factors or convey growth signal without growth factor