This problem is for A-Level and IB readers, and it goes beyond NCERT…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
This problem is for A-Level and IB readers, and it goes beyond NCERT. In PCR, the first molecules with both ends fixed by the two primers appear in the third cycle. Start with one template. The numbers of molecules of exactly the wanted length, from the program of the lesson, are , , , , and after cycles to . The total number of molecules after cycles is .
- How many molecules of the exact length are there after cycles?
- What share of all the molecules is of the exact length after cycles?
- The number of exact molecules after cycles is . Check this for .
Need a reminder? The key ideaShow
PCR needs a template, two primers, nucleotides and Taq polymerase. Each cycle has denaturation, annealing and extension. With a perfect efficiency, . With an efficiency , .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Exact-length molecules after cycles to : . Total after cycles: .
Plan. Read the list, and divide.
Step 1. (a) After cycles the list gives molecules.
Step 2. (b) , which is per cent.
Step 3. (c) molecules. The formula agrees with the list.
Answer: molecules; per cent; molecules.
Check. The share grows with the number of cycles, so after many cycles nearly all the molecules have the exact length.
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