1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 8: The Cell Cycle : the life cycle of cells

2. Cells divide and cells die Each of a human’s trillions of cells retains the genetic information present in the fertilized egg –Every cell in the body results from countless rounds of cell division Cells also die in predictable ways –Cell death is a normal part of development –Apoptosis – programmed cell death Cell division and cell death must be in balance

3. Chromosome duplication A cell’s entire genome must be duplicated before cell division In prokaryotic cells, genome is single circular DNA molecule In eukaryotic cells, genome divided among multiple chromosomes housed in nucleus –Humans have 46, chickens 78, rice 24 Why is stem cell research so controversial? See p. 158-9 How is DNA used to clone a mammal like “Dolly” the sheep? See p. 160-1

4. Eukaryotic chromosomes Chromatin – DNA plus packaging proteins –Nucleosomes – stretch of DNA wrapped around histones (proteins) Chromatin barely visible when cell is not dividing –DNA loosely packed and accessible for cell

5. DNA condenses into visible chromosomes before cell division –Chromatid – one of 2 identical copies of a replicated chromosome –Sister chromatids – pair with identical DNA sequence –Centromere – point of attachment that holds sister chromatids together

6. Most human cells have 46 chromosomes –23 from each parent; 46 total (23 pairs) –Somatic or body cells –Diploid – 2 full sets of genetic information –Pair members look alike (not genetically identical) –Except sex chromosomes, X and Y –Produced by mitosis Sperm and egg sex cells are haploid –23 total chromosomes –Produced by germ cells in testes and ovaries –Produced by meiosis Chromosomes in humans

7. Human Karyotype: This is a chart called a Karyotype and it is used to display the chromosome pairs that an organism has…for humans a normal karyotype is 22 somatic pairs and 1 sex pair (either XX or XY depending on gender) If the karyotype is abnormal (missing or extra chromosomes), then the person has a “genetic abnormality” Ex: Turner’s Syndrome or Down Syndrome

8. The Cell Cycle Cell cycle – events between one cell division and the next 2 major stages –Interphase – cell not dividing but cell very active –Cell division – mitosis and cytokinesis (cytoplasm division)

9. Interphase –Very active time in cell cycle –G 1 – cell grows and carries out basic functions Signals tell cell to divide, stop for repair of DNA, die or enter G o –S – genetic material replicates (DNA Replication) –G 2 – cell prepares to divide and chromosomes begin to condense The Cell Cycle

10. Mitosis –Overall, separates genetic material evenly between 2 daughter cells –Mitotic spindle pulls half to each new cell –Centrosomes organize mitotic spindle –Kinetochores attach chromosomes to the spindle The Cell Cycle

11. Prophase – chromosomes condense, mitotic spindle begins to form Prometaphase – kinetochores grow on centromeres, nuclear envelop breaks down Metaphase – chromosomes line up on mitotic spindle on equator Anaphase – centromeres split, one chromatid of each pair pulled to opposite pole Telophase – mitotic spindle disassembles, chromosomes begin to unwind, nuclear envelope reforms Phases of Mitosis

14. Cytokinesis (differes in plant & animal cells because of cell wall in plant cells) –Animal cells – cleavage furrow results from contractile ring –Plant cells construct a new cell wall with cell plate Animal Cells Plant Cells

15. Cell cycle regulation Some cells divide more or less constantly Signals to divide come from outside the cell Growth factors – proteins stimulating cell division Checkpoints – ensure cell does not enter next stage until previous stage complete

17. Telomeres –Tips of eukaryotic chromosomes –Lose nucleotides and become shorter at each cell division –Cell division stops after about 50 cell divisions –Most cells lack telomerase, the enzyme needed to add more DNA to extend telomeres Teleomerase allows cells to go beyond 50 divisions –Present in some normal cells and cancer cells

18. Cancer Tumor – abnormal mass of tissue –Forms when body loses control of balance between cell division and cell death Benign tumor – usually slow-growing and harmless Malignant tumor –invades other tissues (metastasizes) – cancer

19. Cancer cells are not normal cells –Look different –Essentially immortal –May produce their own signals to divide –Lack contact inhibition –May not undergo apoptosis when damaged –Send signals to stimulate growth of blood vessels Cancer

20. Causes of Cancer –Oncogene abnormalities – abnormal variants of genes that normally control cell division May accelerate cell cycle and cause cancer –Tumor suppressor gene abnormalities – encode proteins that normally block cancer development Promote normal cell death or prevent cell division Inactivation, deletion, or mutation can cause loss of function –Harmful chemicals, radiation, and viruses can alter DNA –Poor diet and exercise, sun exposure, and smoking can increase risk

21. Cancer treatments –Surgical tumor removal, chemotherapy, and radiation –Chemotherapy and radiation target rapidly dividing cell – both cancerous and healthy Death of healthy cells cause side effects –New more targeted drugs home in on receptors for growth factors –Early detection key to successful treatment

22. Cell death/Apoptosis 2 main functions 1.Eliminates excess cells to carve out functional structures like fingers and toes 2.Weeds out cells that might harm the organism Apoptosis must be overcome for mitosis to occur

23. Caspases – apoptosis-specific enzymes Triggered when “death receptor” protein receives signal Cuts apart cell’s proteins and destroys cell

24. Investigating life: Cutting off a tumor’s supply lines in the war on cancer Rapidly dividing cancer cells frequently develop resistance to drugs Mutations allow some cancer cells to survive treatment and proliferate Indirect attack of angiogenesis Choke off tumor supply lines 1997 – endostatin suppresses tumor growth in mice, no resistance, tumors never grew back –Clinical trials in humans yielded mixed results –Not unusual to find drug works well in animals but not humans