# Calculating PCR Cycles

I have a problem understanding a PCR exercise, it says to calculate the number of cycles to get 1µg of the DNA fragment, starting from 10ng and then from 500ng, I know about 2^n, and the exercise even gave the answers:

For 10ng, we need 29 cycles and for 500ng, we need 24 cycles

But I don't know how they did it, I tried with 2^n and ended up with something totally different. I would be grateful if someone explained it to me. Thank you.

With every round of PCR, your sample doubles (ideally) and you are right when using $$2^n$$ for calculating the amount of DNA produced.

The formula would basically be like this:

$$X_{n} = X_0 2^n \\[1em] n=\log_2(X_n/X_0)$$

Just substitute the values of $$X_0$$ and $$X_n$$ — 10ng and 1000ng. The answer would be ~6.64 cycles.

The 500ng case is easier to imagine. The amount is doubled between 1µg and 500ng. So it is just one cycle of PCR.

If this was your answer then you are right. If book says otherwise then it is wrong or you could have read the question wrong.

• Thank you, i got exactly that, 6,64 and 1 cycle, but i was so confused.... there probably was a mistake in the book, thank you again, i appreciate it ^^ – Skylar Knight Dec 4 '18 at 15:29