In the polymerase chain reaction (PCR) technique, a heating phase and a cooling phase alternate. An original sample of DNA would have to pass through how many total rounds of heating and cooling before a sample is increased eight times in quantity?

3

Explanation:

The polymerase chain reaction, known as PCR, is a technique in molecular biology developed in 1986 by Kary Mullis. Its objective is to obtain a large number of copies of a particular DNA fragment, starting from a minimum; in theory it is sufficient to start from a single copy of that original fragment, or mold.

This technique builds on the natural property of DNA polymerases to replicate DNA strands by using alternating high- and low-temperature cycles to separate newly formed DNA strands from each other after each phase of replication, and then allowing the DNA strands to re-assemble so that they can be duplicated again.

Since the temperatures of the cycle (95°C in the DNA denaturation phases) imply the immediate denaturation of any protein, thermostable DNA polymerases are used, extracted from microorganisms adapted to live at these temperatures, which are restrictive for most living beings. Today, the whole PCR process is automated by an apparatus called a thermal cycler, which allows the reaction tubes to be heated and cooled to control the temperature required for each stage of the reaction.

Once the first cycle is completed, we have 2 copies of the original sample. Because there are 2 strands of DNA and each of one is copied. At the end of the second cycle, there are 4, and at the end of the third cycle 8... If the cycles occur a number "n" of times and assuming that the number of DNA copies doubles in each cycle, we obtain a DNA quantity of 2^n, so the amplification is done in the form of geometric progression.

So, three cycles are needed to increase eight times in quantity, because 2^3= 8.

the correct answer, according to e2020, is the second choice there is an occluded front in region a and a cold front in region b.