Chemguide: Support for CIE A level Chemistry

These statements deal with the structure of DNA, the information it holds, and how this information is used in the synthesis of proteins.

Before you go on, you should find and read the statements in your copy of the syllabus.

There is a sequence of six pages on Chemguide about DNA - they follow on from each other with links at the bottom of each page to get you to the next one.

For these statements you need to read the first five of these pages. For now, you can ignore the last page in the sequence titled "DNA mutations" - that is needed for a later statement.

You do NOT need to remember the structures of the deoxyribose (or ribose in RNA), or the phosphate groups, or the four bases. You can use the sort of block diagrams that you will find from the second half of the first page onwards.

CIE seem to ask very straightforward questions about this, and the best thing that you can do is to find as many past papers, mark schemes and Examiner's Reports as you can.

A couple of minor points:

You may well be asked to work out the sequence of amino acids produced from a particular RNA molecule given a chart of the various codes. In the Chemguide pages, you will find that I have drawn these as a table (because that is the easiest way to draw them). CIE have used a similar table, but have also done it with a circular arrangement.

If you are given a circular arrangement, start from the middle and work out to the edge to find which amino acid is coded for by a particular codon. It is fairly obvious when you look at it.

I have found a inconsistency in the marking of questions involving this.

In June 2007, CIE gave you the sequence -AUGUCUAGAGACGGGUAA- to decode. The first codon, AUG, codes for either methionine (met) or is used as a start codon. The mark scheme deducted a mark if you included the word "start" because it isn't an amino acid. Similarly, you would also have lost that mark if you had included the word "stop" (for UAA), because that obviously isn't an amino acid either.

In November 2010, they gave you the sequence -AUGAGCCGACUUGACGUG-. This time, the Examiner's Report said "The only common error was not to include Met or Start at the beginning of the sequence."

Well, of course nobody used the word "start". They had read (or been told about) the previous mark scheme which penalised you for using it!

What to do about this? In either version, you would have been safe if you had interpreted AUG as "met". Unless it is obvious from the question that they want you to identify the first codon as the start codon, just treat it as the amino acid methionine (met).

Update August 2013: In November 2012, CIE asked a very similar question. In this one, they allowed either Met or Start at the beginning of the sequence, and Stop at the end. A bit of common sense seems to have crept in!

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