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The Cell Cycle: Cell Growth and Cell Division AP Biology 2007-2008 Why do cells divide? Metabolic needs sets upper limit to the size of a single cell. As a cell increases in size, its volume increases faster (X3) than its surface area (X2). Cell membrane cannot keep up supplying nutrients needed by an enlarging cytoplasm…. The solution to the problem is to split into two cells. In Biology, we multiply by dividing! AP Biology Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Why do cells divide? More surface area means more food comes in and more garbage moves out! Fig. 7.5 AP Biology Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings What are the benefits of Cell Division? For reproduction asexual reproduction one-celled organisms For growth from fertilized egg to multi-celled organism For repair & renewal replace cells that die or from injury AP Biology amoeba Function Nucleus protects DNA and houses the DNA nuclear pores Kind of like an all-reference book Library! AP Biology nucleolus What is passed on to daughter cells? Copy of genetic material = DNA Mitosis (division of the nucleus) Organelles, cytoplasm, cell membrane, etc. Cytokinesis (division of the cell) chromosomes (stained orange) in kangaroo rat epithelial cell notice cytoskeleton fibers AP Biology Interphase 90% of cell life cycle cell doing its “everyday job” produce RNA, synthesize proteins/enzymes prepares for duplication if triggered I’m working here! Time to divide & multiply! AP Biology Interphase Divided into 3 phases: Gap1 = Growth cell doing its “everyday job” cell grows S = DNA Synthesis copies chromosomes Gap2 = Growth prepares for division cell grows (more) produces organelles, proteins, membranes AP Biology G0 Cell has a “life cycle” cell is formed from a mitotic division M Mitosis G2 Gap 2 S Synthesis cell grows & matures to divide again cell grows & matures to never divide again G1, S, G2, M epithelial cells, blood cells, stem cells AP Biology brain / nerve cells muscle cells G1 Gap 1 G0 Resting green = key features Interphase Nucleus well-defined DNA loosely packed in long chromatin fibers Prepares for mitosis replicates chromosome produces proteins & organelles AP Biology S phase: Copying / Replicating DNA Synthesis phase of Interphase replicates DNA must separate DNA copies correctly to 2 daughter cells AP Biology ACTGGTCAGGCAATGTC Organizing DNA DNA DNA is organized in chromosomes double helix DNA molecule wrapped around histone proteins histones like thread on spools DNA-protein complex = chromatin organized into long thin fiber condensed further during mitosis http://www.dnalc.org/ resources/3d/DNAWr apAdvanced06.html AP Biology double stranded chromosome duplicated mitotic chromosome chromatin Mitotic Chromosome Duplicated chromosome 2 sister chromatids narrow at centromeres contain identical copies of original DNA AP Biology homologous = “same information” homologous chromosomes homologous chromosomes sister chromatids single-stranded double-stranded homologous = “same information” AP Biology double-stranded mitotic human chromosomes AP Biology Mitosis Dividing cell’s DNA between 2 daughter nuclei 4 phases prophase metaphase anaphase telophase AP Biology Mitosis in whitefish blastula AP Biology green = key features Prophase Chromatin condenses Becomes visible chromotids chromatids Centrioles move to opposite poles of cell (animal cell) Protein fibers cross cell to form mitotic spindle coordinates movement of chromosomes Nucleolus disappears Nuclear membrane breaks down AP Biology green = key features Metaphase Chromotids align along middle of cell meta = middle AP Biology spindle fibers coordinate movement of chromotids green = key features Anaphase Sister chromatids separate move to opposite poles pulled at centromeres Poles move farther apart AP Biology green = key features Telophase Chromotids arrive at opposite poles daughter nuclei form nucleoli form chromosomes disperse no longer visible under light microscope Spindle fibers disperse Cytokinesis begins AP Biology cell division Cytokinesis Animals Constriction of microfilaments around equator of cell cleavage furrow forms splits cell in two like tightening a draw string AP Biology Cytokinesis in Animals The Stages of Mitosis http://www.youtube.com/watch?v=VGV3fv-uZYI AP Biology Cytokinesis in Plants Plants cell plate forms new cell wall fuses with existing cell wall AP Biology Mitosis in plant cell AP Biology onion root tip AP Biology AP Biology “Go-ahead” signals - Kinases Protein signals that promote cell growth & division internal signals “promoting factors” external signals “growth factors” Primary mechanism of control phosphorylation kinase enzymes activates or inactivates cell signals AP Biology inactivated Cdk Cell cycle signals Cell cycle controls cyclins regulatory proteins levels cycle in the cell Cdks cyclin-dependent kinases phosphorylates cellular proteins activates or inactivates proteins activated Cdk Cdk-cyclin complex triggers passage through different stages of cell cycle AP Biology 1970s-80s | 2001 Cyclins & Cdks Interaction of Cdk’s & different cyclins triggers the stages of the cell cycle AP Biology Leland H. Hartwell checkpoints Tim Hunt Cdks Sir Paul Nurse cyclins Apoptosis Apoptosis is programmed or controlled cell suicide Components of the cell are chopped up and packaged into vesicles that are digested by scavenger cells Apoptosis prevents enzymes from leaking out of a dying cell and damaging neighboring cells AP Biology © 2011 Pearson Education, Inc. Apoptotic Pathways and the Signals That Trigger Them Apoptosis can be triggered by An extracellular death-signaling molecules DNA damage in the nucleus (p53) AP Biology © 2011 Pearson Education, Inc. Example of a Growth Factor Platelet Derived Growth Factor (PDGF) made by platelets in blood clots binding of PDGF to cell receptors stimulates cell division in connective tissue heal wounds AP Biology Apoptosis evolved early in animal evolution and is essential for the development and maintenance of all animals Interdigital tissue Cells undergoing apoptosis 1 mm Space between digits Apoptosis may be involved in some diseases (for example, Parkinson’s and Alzheimer’s); interference with apoptosis may contribute to some cancers AP Biology © 2011 Pearson Education, Inc. Growth Factors and Cancer Growth factors can create cancers proto-oncogenes normally activates cell division growth factor genes become oncogenes (cancer-causing) when mutated if switched “ON” can cause cancer tumor-suppressor genes normally inhibits cell division if switched “OFF” can cause cancer example: p53 AP Biology Cancer & Cell Growth Cancer is essentially a failure of cell division control unrestrained, uncontrolled cell growth What control is lost? lose checkpoint stops gene p53 plays a key role in G1/S restriction point p53 protein halts cell division if it detects damaged DNA p53 is the options: Cell Cycle Enforcer stimulates repair enzymes to fix DNA forces cell into G0 resting stage keeps cell in G1 arrest causes apoptosis of damaged cell ALL cancers have to shut down p53 activity AP Biology p53 discovered at Stony Brook by Dr. Arnold Levine Development of Cancer Cancer develops only after a cell experiences ~6 key mutations (“hits”) unlimited growth turn on growth promoter genes ignore checkpoints turn off tumor suppressor genes (p53) escape apoptosis turn off suicide genes immortality = unlimited divisions turn on chromosome maintenance genes It’s like an out-of-control car with many systems failing! promotes blood vessel growth turn on blood vessel growth genes AP Biology overcome anchor & density dependence turn off touch-sensor gene What causes these “hits”? Mutations in cells can be triggered by UV radiation chemical exposure radiation exposure heat AP Biology obesity cigarette smoke pollution age genetics Tumors Mass of abnormal cells Benign tumor abnormal cells remain at original site as a lump p53 has halted cell divisions most do not cause serious problems & can be removed by surgery Malignant tumor cells leave original site lose attachment to nearby cells carried by blood & lymph system to other tissues start more tumors = metastasis impair functions of organs throughout body AP Biology
