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mutation - (May/20/2007 )

1. Which is the best evidence that two organisms have a common ancestor? (1) They have the same enzymes and hormones. (2) They live in the same type of environement. I think these two can be possible answers, but i think maybe (1) is the correct one because it shows the similarity in biochemistry between two organisms.
2. Certain strains of bacteria that were susceptible to penicillin in the past have now become resistant. The probable explanation is that (1) the mutation rate increased. (2) a mutation was retained and passed on to succeeding generations because it had high survival value. I think these two are probable answers, but i would choose (1) because in (2), i don't think it can pass to generations. BTW, in my book, it says, Doctor may refuse to give you an antibiotic for a mild illness because he/she may not wish to risk your developing a resistant strain. It sounds more like Lamarck's theory that because your body wants to resist penicillin, then it develops a resistant strain. But Lamarck was wrong. According to Darwin, it should be a mutation just happens naturally, so even if your doctor gives you a lot of antibiotics, if the mutation doesn't happen, then there is no point of not giving you antibiotics. Suppose your doctor just gives you antiobiotic once, then you have mutation, it's a random chance of developing a resistant chain.


I would say that for your first question, (1) is the right answer becasue it is much harder for an organism to develop the same enzymes etc than it is for it to move from one place to another.

For the second question, (2) is the correct answer, of course mutations can be retained, especially if they have high survival value. If your ancestors didn't have the mutations that allowed them to walk upright, and talk then you wouldn't be here today. Bacteria are a special case though, they can pass resistance genes from one bacterium to another (sometimes even between species) on plasmids which are small circular peices of DNA containing a few genes and an origin. If your doctor gives you some antibiotics these will kill off the majority of the population of bacteria, but some will survive, these have a higher tolerance for the drug and will multiply, passing the resistance along to the next generation and so on. If you then stop taking the drug, these bacteria are the only ones left and so you have a resistant strain. Typically your doctor will give you a large dose for a long time (7-10 days is a long time for a bacterium that can divide every 20 minutes = 504-720 generations) in the hope that it will kill off all the bacteria before they become resistant. Next time you have to take some antibiotics, get 2 bacterial agar plates and a source of your drug (NOT the pills you are taking), add the drug to the plates at an appropriate concentration (20-50 ug/ml for ampicillin or penicillin). Before you start taking the drug, press a hand on one of the plates and incubate at 37 deg C overnight then store at 4 degrees, then take your drugs for 3 days and press the hand onto the other plate, incubate and compare the plates, you should see resistant bacteria on one of the plates.