Chapter 7: Amplification by Polymerase Chain Reaction

The mechanism operates through repeated thermal cycles that systematically denature double-stranded DNA, allow short oligonucleotide primers to anneal to complementary target sequences, and facilitate extension of these primers by heat-stable DNA polymerase enzymes. Each complete cycle theoretically doubles the target DNA concentration, yielding exponential amplification after 25-35 rounds of replication. The chapter details essential PCR components including the genomic or extracted DNA template serving as the amplification source, precisely designed primers that define the target region boundaries, deoxynucleoside triphosphates providing nucleotide building blocks, an appropriate buffer maintaining optimal pH and ionic conditions, and thermostable polymerase such as Taq enzyme derived from thermophilic bacteria. Successful amplification requires careful management of numerous variables including template DNA quality and quantity, removal or neutralization of inhibitory substances that may originate from biological samples or extraction procedures, and rigorous contamination prevention protocols essential in forensic contexts where trace evidence integrity is paramount. The chapter extends beyond conventional PCR to address reverse transcriptase PCR, which adapts the amplification strategy for ribonucleic acid analysis by first synthesizing complementary DNA from messenger RNA templates through reverse transcription before proceeding with standard PCR amplification cycles. This modification enables analysis of gene expression patterns and detection of RNA viruses in forensic and clinical applications where traditional DNA-based approaches prove inadequate.