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Toefl ibt internet based test 2006 - 2007 part 87

Tham khảo tài liệu 'toefl ibt internet based test 2006 - 2007 part 87', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 726 ANSWERS AND AUDIO SCRIPTS FOR ACTIVITIES QUIZZES AND MODEL TESTS This is the spirilla They look a little like corkscrews and they re responsible for a number of diseases in humans But I don t want you to get the wrong idea. It s true that some species of bacteria do cause diseases but for the most part bacteria are benign. There s a lot of bacteria in this room in fact. We all have it on us. They live harmlessly on the skin in the mouth and in the intestines. In fact oactena are very helpful to researchers because bacterial cells resemble the cells of other life forms in many ways and may be studied to give us insights For example we have a major research project in genetics here at the university. Since bacteria reproduce very rapidly we re using them to determine how certain characteristics are inherited. Okay now let me review these three types with you . cocci are spheres bacilli are rods and spir Illi are spirals. One of my students came up with a way to remember them. Just try to visualize the first letter in the name of each of the different types Cocci starts with C like the shape of half a sphere. Bacilli starts with a straight line on the B. and a rod is straight. Spinlla starts with S. and that s a spiral shape if it helps you use it. In any case although I want you to know the three major classifications within these basic groups there are virtually hundreds of variations that make them somewhat more difficult to identify and classify than the rather straightforward specimens that I showed you a minute ago. Because you see. bacteria can join in chains clusters pairs. And sometimes more than one type of bacteria may be found together in a specimen. I think you get the picture. Okay then in addition to identifying bacteria by their shape which we now know isn t really a very good method for distinguishing them easily if we really want to identify what type of bacteria we re dealing with it s better to study the biochemistry or genetic structure of the