Ans: A (Thermostable at temperature at which DNA liquefies) By using a heat stable DNA polymerase9 (for example. Taq polymerase) from a bacterium ('thermus aquations') that normally lives at high temperature, the polymerase is not denatured & therefore, does not have to added at each successive cycle"- Lippintcott. Biochemistry 4th/482Polymerase Chain ReactionPCR provides a sensitive, selective & extremely rapid means of amplifying a desired sequence of DNA. Twenty cycles provide an amplificationQ of 106 & 30 cycles of 109 - Harper 27th/410-11SpecificityQ is based on the use of two oligonucleotide primers that hybridize to complementary sequence on opposite strands of DNA & flank the target sequenceDouble stranded DNA can be disrupted by heat or high pH, giving rise to single stranded DNA. The single stranded DNAQ serves as a template for synthesis of a complementary strand by replicating enzymes, the DNA polymeraseEarly PCR reaction usedanE.coli DNA polymerase that was destroyed by each heat denaturation cycle. Substitution of a heat-stable DNA polymerase (Taq polymerase) from Thermus aquaticus, obviates this problem & has made possible automation of the reaction, since the polymerase reactions can be run at 70degCPCR is a cyclic process & cycle contains three steps that involves (Chatterjea & Shinde)DenaturationQ of DNA duplex (94-98degC)AnnealingQ of primers (37-60degC)ExtensionQ of the primers with a polymerase in the presence of dNTP (about 720C)DNA polymerase amplifies DNA in PCR, Early PCR used an E coli, DNA polymerase that was destroyed by each heat generation cycles i.e. polymerase was denatured. This difficulty is solved by using heats table DNA polymerase from 'thermusaquaticuS'". which can withstand 70deg to 80deg C. Therefore polymerase has not to be added in each cycle.