Which of the following samples of DNA in the table given below will give the desired result during polymerase chain reaction ?

The correct option is Sample - I: Temperature used for Denaturation - High temp. / 90° C, Enzyme used for extension - Heat stable.

To understand why this is the correct choice, let us break down the steps of the Polymerase Chain Reaction (PCR) and analyze the requirements for each step:

1. Denaturation:
During the denaturation step, the double-stranded DNA template must be separated into two single strands. This separation occurs by breaking the hydrogen bonds between complementary bases. A high temperature, typically between $90\circ C$ and $98\circ C$, is required to disrupt these strong bonds. Therefore, a denaturation temperature of 50° C (as mentioned in Samples II and III) is too low to separate the DNA strands, rendering them ineffective.

2. Extension and Enzyme Properties:
During the extension step, a DNA polymerase enzyme synthesizes new DNA strands complementary to the templates. Since the PCR process cycle involves repeatedly heating the mixture to high temperatures (around 90° C or above for denaturation), the polymerase enzyme must remain functional and not denature or degrade under these conditions. This necessitates the use of a thermostable (heat-stable) DNA polymerase, such as Taq polymerase. A heat-unstable enzyme (as in Sample IV) would denature during the very first denaturation cycle, halting the reaction.

Comparing the options based on these requirements:
- Sample I correctly uses a high temperature (90° C) for denaturation and a heat-stable enzyme for extension.
- Sample II and Sample III use an insufficient temperature (50° C) for denaturation.
- Sample IV uses an enzyme that is heat-unstable, which will get deactivated during heating.

Thus, only Sample I provides the correct conditions necessary to yield the desired result during PCR.