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Lecture 20

Lecture 20: "Chromosome Mutation"

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Biology 1002B
Tom Haffie

Biology Lecture No. 20: Chromosome Mutation st Wednesday March 21 , 2012 RECALL: -Telomerase would be active on both ends of a cat chromosome. -Primase, DNA polymerase and poly-A polymerase can all extend a 3’ OH on RNA. Telomerase however cannot extend a 3’ OH on RNA. In fact DNA polymerase extends RNA all the time; every primer consists of RNA. Biological Mutagens: -Mutagens are not restricted to just being harmful chemicals, mutagens can be biological as well. For example, mobile elements can be very active in plants. The gene that makes a yellow flower red has a mobile element in it, which can jump in and out of genes to produce various observable streaks in colour. This is a prime example of mutation. Physical & Chemical Mutagens: -Mutagens are not only biological, but can also be physical and chemical as well. 5-bromouracil is an example of a chemical mutagen. The IAP mobile element for the agouti gene in mice is an example of a biological mutagen. UV radiation is an example of a physical mutagen. The Effects Of UV Radiation: -UV radiation is an example of physical mutagenesis and different organisms have different kinds of repair for this occurrence. If a cell is suffering UV damage, from the kind that known as white (or blue) light, photolyase activates and assists in breaking apart the thymine dimers. -This method of repair is specific to plants and E. coli and is not found in humans. This is because plants have dramatic DNA repair mechanisms due to their prolonged exposure in the sun. The repair mechanism observed in humans is called excision repair and involves cutting out the thymine dimer and replacing the missing nucleotides with regular thymines. -If little or no excision repair is present in a human being, it is said that they suffer from xeroderma pigmentosum and may have a high predisposition for skin cancer. They would have to dramatically lower their time spent in the sun. Chernobyl Accident & Radioactive Fallout: -Failure of safety mechanisms led to explosions and fires and the meltdown of the nuclear reactor in Chernobyl (now in Belarus), which led to the release of radioactive material (Caesium-137) in the atmosphere (mostly Eastern Europe). Few of the emergency responses survived due to lethal dosages. Radioactivity: -Radioactivity arises from the decay of unstable nuclei. For example, Carbon-12 is stable, with its 6 protons and 6 neutrons, but Carbon-14, which has 6 protons and 8 neutrons, is not stable and decays over time. The decay of Carbon-14 does not release very much into the atmosphere, in terms of high- energy particles, but the decay of other atoms does. Ionizing Radiation & Reactive Oxygen Species: -When particular atoms such as Caesium and Iodine decay, they create ionizing radiation, which is called ionizing because it creates reactive oxygen species (ROS). The formation of ROS by ionizing radiation entering the cell damages everything, in particular the proteins and DNA present in the cell. -In the event of a nuclear accident it would be essential to flood the bodily systems with cold iodine (used by the thyroid) to
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