Question

Describe the possible outcome of a PCR experiment in which (a) there is a single-stranded break in the target DNA sequence, which is present in only one copy in the starting sample, and (b) there is a doublestranded break in the target DNA sequence, which is present in only one copy in the starting sample.

Short answer

(a) PCR can proceed normally from the second cycle because the strand has the same sequence as broken strand.

(b) A new strand of DNA that is complementary to the template strand is generated in the first cycle of the PCR. Thereafter, the DNA synthesis would terminate as DNA polymerase would not be able to move because of the breaks in both the strands.

Step-by-step solution

Introduction

The polymerase chain reaction (PCR) is a technique for amplifying a particular DNA segment quickly and precisely. In this method, high temperature separates the DNA sample into single strands.

The strands are then processed with DNA polymerase (for example, Taq polymerase), two primers, and deoxynucleotides triphosphate (dNTPs) to synthesize copies of target DNA.

Explanation for (a)

A new complementary strand of DNA is created after the initial PCR cycle. If the template strand is broken, DNA polymerasewill not be able to synthesise DNA. The produced strand, on the other hand, has the same sequence as the broken strand, therefore PCR may resume after the second cycle.

Explanation for (b)

A complementary strand of DNA is created to the template strand during the first cycle of the PCR. When both the strands of the target DNA sequenceare damaged, DNA polymerase is unable to generate DNA. This is because DNA synthesis would halt during the breaks.