# AQA A2 Biology Unit 4 & 5 Questions HELP!!!

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1. I have loads of past paper questions that I don't understand from Unit 4 and 5. Please can someone help by starting to explain HOW you work out the answer for these two questions about the Polymerase Chain Reaction?

1) How many DNA molecules will have been produced from one molecule of DNA after 6 complete cycles?

And this is a different question from a past paper:

2) One cycle of the polymerase chain reaction takes 5 minutes to complete. Starting with a single gene, calculate the number of copies of the gene produced in 40 minutes.

I have seen two different formulas used to work PCR questions out. One of them is 2^n where n is the number of cycles. Another is 2^n+1 where n is also the number of PCR cycles. I am very confused about which to use.
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3. Use 2^n. You know that through PCR, 1 DNA strand splits into 2, these 2 split into 4, 4 into 8 so on and so forth. By the time you get to 8, the DNA strand has been through 3 PCR cycles. And 2^3 is 8. When 8 splits into 16, it'll be 4 PCR cycles. 2^4 = 16.

Hope I was of help.
4. (Original post by branteo97)
Use 2^n. You know that through PCR, 1 DNA strand splits into 2, these 2 split into 4, 4 into 8 so on and so forth. By the time you get to 8, the DNA strand has been through 3 PCR cycles. And 2^3 is 8. When 8 splits into 16, it'll be 4 PCR cycles. 2^4 = 16.

Hope I was of help.
Thanks so muchhhh! That really makes sense!

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