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Mutations Mutations in DNA can alter cells Mutations: What is a mutation? Any change in the nucleotide sequence of DNA Here a single base is changed: 2 general ways to alter DNA Change a single DNA base Or entire sections of DNA can move from one place to another Misspelling Changes in a single DNA base A small change can make a BIG difference Caused by errors in DNA replication, UV radiation, or chemical carcinogens Normal Hb Mutant Hb A change in a single nucleotide in the gene for hemoglobin A change in a single amino acid A change in the shape of the protein and RBC and causes DISEASE Changes in a single DNA base Insertions and deletions Can have disastrous effects. Change the reading frame of the gene. But mistakes happen – DNA repair Mutations are rare The cell uses DNA proofreading mechanisms to correct these mistakes The daughter strand is compared to the parent DNA to check for mistakes Proofreading is not perfect – mutations are still possible, although rare Mutations–key features Mutations are rare changes in DNA can affect genes and the proteins they encode Mutations are inherited only if they occur in germ-line cells Mutations are important for genetic variation and evolutionary change How Genes Are Controlled How do different kinds of cells arise? 4 of the over 200 types of human cells They all contain the same DNA, so what makes them different? Gene activity is regulated When cells differentiate Certain genes are turned on and off. Cells become specialized Fertilized egg Blastula Different types of cells express different combinations of genes Muscle cells make contractile proteins (actin and myosin) Blood cells make hemoglobin How do cells switch genes on and off? Main switch point occurs at transcription X Genes Glycolysis enzymes X X X Hemoglobin – – X Myosin X – – Insulin – – – Example of transcriptional control: Lactose intolerance Turning on a gene Proteins called transcription factors interact with DNA → switch on transcription Lactose is a disaccharide found in milk. Many adults have trouble digesting lactose they lack the digestive enzyme that breaks down lactose. Genetic basis of lactose intolerance C→T mutation in the promoter of the lactase gene allows some groups to digest milk Transcription of the gene is not turned off in adults Other ways cells control gene expression RNA processing Lactase gene C→T The introns are removed The remaining RNA is spliced together Different cells start with the same RNA transcript but cut out different introns different proteins Turning on a gene Cells secrete chemical signals, or hormones, that turn on transcription Important for coordinating cell activities. The Genetic Basis of Cancer Cancers are caused by mutations in genes Cancer = most frequent cause of death in U.S. Cancer Cells Have Abnormal shape Not specialized Immortal (able to divide continually) Normal cervical cells Cervical cancer cells What Is Cancer? Cells growing out of control They lose control over cell division 3D Medical Animation - What is Cancer? 1:06 http://www.youtube.com/watch?v=LEpTTolebqo&NR=1&feature=fvwp What Is Cancer? A disease of the cell cycle Often results from mutations in cyclins, proteins that help regulate the cell cycle What causes Cancer? Mutations that damage DNA Environmental carcinogens Many “oncogenes” code for cyclin proteins Cancer cells do not respond normally to the cell cycle checkpoints Some viruses = Cell cycle checkpoints Tumor suppressor proteins Tumor suppressor proteins keep division in check by killing damaged cells Growth factors Cyclins Tumor-suppressor genes DNA repair proteins HPV virus Hepatitis B and C viruses Epstein-Barr virus Mutations in DNA repair genes cause cancer Mutations in different genes cause cancer Tobacco smoke X-rays UV radiation in sunlight and tanning lamps DNA repair or proofreading genes make enzymes that help the cell fix mutations in its DNA. When a DNA repair gene is mutated, the cell can’t repair mistakes in its DNA. These mistakes build up until an oncogene is hit. Then the cell becomes cancerous. It takes several mutations to create a cancer cell Is Cancer Inherited? Usually not Most mutations occur in the organ where the cancer starts e.g. the colon or lung Usually germ-line cells aren’t affected But cancer-causing mutations sometimes do the mutations are not passed from parent to child occur in gametes Case study: 52-year old woman with colon cancer Workup Anemic Stool blood test was positive BRCA1, an inherited cancer gene In the vast majority of cases, breast cancer is not caused by inherited mutations The normal BRCA1 codes for a tumor suppressor protein Some mutations in the BRCA1 gene inactivate the tumor suppressor protein → puts a woman at high risk for breast and ovarian cancers. Mammograms of healthy (left) and cancerous (right) breasts Case study: 52-year old woman with colon cancer Colonoscopy Treatment Mass was found in the descending colon Biopsy tested positive for cancer Abdominal CT scan Lesion in the same area of her colon Liver and chest CT scans were negative Questions Surgery to remove the tumor Pathology report: stage II lesion Chemotherapy What characteristics of cancer cells distinguish them from normal cells? Why do cancer cells form tumors? Why did they do a CT scan of the liver and chest? Why would the doctor recommend both surgery and chemotherapy? Why cancer cells form tumors Normal cells stop dividing when they come in contact with a neighbor Cancer cells have lost all restraint Pile on top of one another Grow in multiple layers