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Chemguide: Support for CIE A level Chemistry Learning outcomes 11.1(i), 11.1(j) and 11.1(k) These statements deal with the mutation of DNA, and the problems that this can cause in the proteins that it codes for. Before you go on, you should find and read the statements in your copy of the syllabus. First, read the Chemguide page about DNA mutation. What you need for exams You have to remember that you are doing a chemistry exam, and so answers should be chemically based. Examiner's Reports keep on stressing this, and mark schemes concentrate on the effects of mutations on the changes to the shapes of proteins. In November 2009 paper 41 question 6, CIE asked a question about sickle cell anaemia. They gave a brief bit of information about it that said that it was caused by by a single mutation in the DNA of a particular gene. That gene was involved in the production of a protein in haemoglobin. They asked you to explain what is meant by mutation. That was worth 1 mark. All they expected you to say was that a base had been changed in some way (miscopied or deleted) in the DNA. The question then went on to ask why such a mutation could alter the bonding in haemoglobin. This was worth 2 marks. All the mark scheme wanted you to say was that the sequence of bases in the DNA is changed, and that this results in a different amino acid sequence. This different sequence can affect the shape (or the tertiary structure) of the protein. Another question (June 2007 paper 4 question 7) asked you to name a disease which results from a genetic defect, and then to explain how the genetic defect can bring about your named disease. There was 1 mark for the name, and 2 for the explanation. There are obviously any number of genetic diseases that you could probably name if you were a biologist. But you aren't doing a biology exam, and the paper probably isn't being marked by people with biology backgrounds. So choose one of the obvious diseases mentioned on the Chemguide page to make it as easy as possible for the examiner. For the explanation, actually all that was needed was for you to say in general terms that there would be a wrong amino acid sequence produced which results in a change on the 3D structure of the protein, or a change in the active site. Or you could explain it in terms of the particular disease you mentioned. So you need to learn the name of at least one disease which is caused by a genetic defect. You should also learn a symptom of the disease. I think I would personally go for haemophilia, because the explanation is so simple. There is a missing protein caused by a gene which has a mutation which produces a stop codon far too soon, and so the whole protein isn't formed. But you should also expect to answer questions based on any disease which the examiners come up with. Your answer will always be on the lines of the one needed for the November 2009 question discussed above. A comment on statement 11.1(k) We have been talking about the fact that changes in a DNA base sequence caused by mutation can affect the function of a protein by changing its tertiary structure. That may make it less effective as an enzyme by changing the active site, or it may alter the shape in a way that stops it performing some other function - for example, in ion transport across cell boundaries as in cystic fibrosis. But it occasionally happens that a mutation produces a protein which improves or adds to the functioning of the cell. That is part of the mechanism for evolution, but is also the stuff of genetic engineering. Up to the time of writing (August 2013), no question had been asked about this. If a question does get asked, I would expect that you could answer it in the same sort of terms as you would answer any other mutation question - because that's all it is. But this time the mutation is beneficial rather than harmful.
© Jim Clark 2011 (modified August 2013) |