Why is Taq polymerase important in PCR?

In IB Biology, Taq polymerase matters because PCR repeatedly heats the reaction to around 95°C during denaturation. Many enzymes would denature at that temperature and stop working after one cycle. Taq polymerase is heat-stable, so it survives cycle after cycle without needing to be replaced. That stability is what makes PCR efficient and automatable in a thermocycler. It also allows the process to run quickly, because the enzyme remains active when the temperature returns to its optimal extension range (around 72°C). In exams, a strong answer explicitly links “heat-stable” to “survives denaturation temperature” and “enables repeated cycles.”

How does PCR amplify only the target DNA sequence?

PCR specificity comes mainly from primer design, which is a key theme in IB Biology biotechnology questions. Primers are engineered to match sequences that flank the target region, so only DNA between those primer binding sites gets copied efficiently. During annealing, the temperature is chosen so correctly matched primers bind well, while mismatched binding is less stable and unlikely to persist. Once primers bind, polymerase extends from them, meaning the reaction is effectively “locked onto” that region. Over many cycles, any correctly primed products dominate because they keep serving as templates. In short: primers set the boundaries, annealing conditions enforce matching, and repetition magnifies the selected region.

Why does the amount of DNA increase exponentially rather than linearly?

Exponential increase is one of the easiest marks to earn in IB Biology if you explain it calmly. After the first cycle, you do not just have the original DNA molecule -- you also have newly synthesized strands. In the next cycle, both the original strands and the newly made strands can act as templates. That means the number of target molecules roughly doubles each cycle, assuming reagents are not limiting. Over 20--40 cycles, doubling compounds quickly into a very large number of copies. A good exam explanation uses the phrase “new strands become templates in subsequent cycles” and explicitly connects that to “doubling each cycle.”