Lecture 14 Outline of Last Lecture I. PhotosynthesisII. PlantsIII. How It WorksIV. The Circle of LifeV. Cell CycleOutline of Current Lecture I. Cell DivisionII. CytokinesisIII. Control of Cell DivisionIV. Important Cell Cycle Control PointsV. Cell Control in Cancer CellsCurrent LectureI. Cell Divisiona. 2 phases:i. Interphase – a period of growth ii. Mitosis + Cytokinesis – division of the nucleus and cytoplasmb. Interphasei. G1 – cell grows and gets biggerii. S – DNA replicated, cell continues to growiii. G2 – cell gets ready for mitosis and cytokinesis  makes special proteins needed for thatc. Mitosis – division of the nucleusd. Prophase i. Replicated chromosomesii. Nucleoli break downiii. Spindle apparatus is formed  serves to move chromosomes arounde. Prometaphasei. Nuclear envelope breaks downii. Spindle apparatus attaches to replicated chromosome at the kinetochore  protein complexes located at the centromere where sister chromatids are connectedf. Metaphase - replicated chromosomes line up in single file in the center of the cell BIOL 1st Editiong. Anaphase – sister chromatids separate and are pulled to opposite poles of the cell – once the sister chromatids separate, then they are bone fide chromosomesh. Telophasei. Chromosomes decondense  chromatidii. Spindle apparatus disappearsiii. Nuclear envelope reformsII. Cytokinesisa. Division of the cytoplasmb. Animal cells -i. Cells are pinched apartii. First a “cleavage furrow” forms between 2 newly formed nucleiiii. Microfilaments form a ring at the cleavage furrow and tighten to pinch 2 cells apartc. Plant cells - i. Vesicles from golgi meet and fuse in center of the cellii. Forms 2 new membranes within single cell walliii. New cell wall forms between 2 new membranesIII. Control of Cell Divisiona. Important for a cell to know when to divide and when to stop dividingb. Varies with cell typei. Skin cells – divide frequently throughout lifeii. Liver cells – only divide for repairiii. Nerve and muscle – do not divide, only get biggerc. Mechanisms that control cell cycle are of intense interesti. How normal cells regulate growthii. Understand how cancer cells escape the usual controlsd. Studied using tissue cultures – grow cells in flask using artificial growth mediai. Normal cells in tissue culture grow and divide until they touch one another, and then they stop growing  “contact inhibition”ii. Limited number of cell divisions  normal cells divide 20-50 times, then stop, they age and diee. Cancer Cells in tissue cultures behave differently from normal cellsi. Don’t show contact inhibitionii. Immortal  they divide indefinitelyIV. Important cell cycle control points:a. Called “checkpoints”  points in cycle where STOP and START signals are givenb. Molecular sensors  check to see if all the right things have happened so farc. Molecular brakes  slow things down if things are going wrongd. Apoptosis  cell suicide – if cell controls don’t result in correction of a problem, the cell may kill itselfe. 3 Checkpoints:i. G1/S checkpoint1. Cell determines if its going to divide or not2. Based on signals from environment and also on internal signals (is cell big enough to divide?)3. If cell passes the G1/S checkpoint, then it is committed to divide – it will go stepwise ( G1 S  G2  M) always completing each step before starting the next4. If it decides not to divide, it will exit the cell cycle and enter a non-dividing stage = G05. Most of the cells of the body are in G0ii. Checkpoints G2 and M are understood at some point at a molecular levelV. Cancer Cell Checkpointsa. Don’t respond to cell cycle controlsb. If they do stop dividing, it’s often at the wrong pintc. Results (sometimes) in cancer cell having wrong amount of DNA (if they stop in the middle of S