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1. The problem that has been annoying cancer researchers for years is the difficulty in telling __________ how cancer cells are formed why cancer cells never stop dividing why normal cells can turn into cancer cells how different normal cells are from cancer cells 2. Whiteheads scientists have succeeded in__________ developing malignant cells in human bodies making normal human cells cancerous controlling the change of human cells changing the genes of cancer cells 3. In the 1983 experiment, human cells didnt work the way mouse cells did because the former__________ were easier to become aged checked telomerase in dividing had short ends of chromosomes lacked telomeres for cell division 4. To make human cells more mouse like scientists tried__________ to subject them to two more genes to keep the division from slowing down to promote the production of telomerase to prevent excessive cells copying 5. One key factor in creating tumor with human cells is __________ lengthening the ends of chromosomes altering the structure of telomeres | 
