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DNA technology help needed!

okay so I'm doing a past paper at the minute (January 11th 2012 CEA) for my AS module 1 exam and I'm so confused on this question. (Q.2 part B)

it says - when a particular bacteriophage DNA is exposed to EcoR1, 6 fragments of DNA are obtained. Another restriction enzyme called HindIII works is a similar way to EcoR1 but acts on a different recognition site. HindIII produces 8 fragments of DNA from the same bacteriophage.
Assuming the recognition sites do not overlap on the bacteriophage DNA, determine the number of fragments produced if both EcoR1 and HindIII were used together.

so I assumed the answer was 14, because 6 + 8 is 14? but the mark scheme says the answer is 13... can anyone explain why this is?
The question got me the first time as well but the key point is that the number of fragments isn't the key piece of information, it is the number of recognition sites. A 6 fragment enzyme will have five recognition sites and the 8 fragment enzyme will have seven recognition sites, which means that in the DNA when they are combined there will be 12 recognition sites, which means that there will be thirteen fragments.

The best way I find to visualise this sort of problem is to draw two lines (the DNA) above each of a piece of paper and mark the recognition sites for one of the enzymes on each with a line (making sure that none of the lines don't match). Then continue the lines from one DNA strip to the other and you will find thirteen fragments. This is how I prove it to myself anyway.

Hope this helps.
Reply 2
makes so much sense now! thanks so much for your help 😄
Just draw a diagram


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