In the extension phase of PCR, what is synthesized?

During the extension phase of PCR (Polymerase Chain Reaction), new DNA strands are synthesized. This step occurs after the denaturation and annealing phases, wherein the double-stranded DNA is separated into single strands and short primers bind to the target sequences.

In the presence of deoxynucleotides (dNTPs) and a DNA polymerase enzyme, the polymerase adds nucleotides one by one to the primer that has annealed to the single-stranded template. This process continues, effectively creating new strands of DNA that are complementary to the original template strands. The result is an exponential amplification of the target DNA sequence, with each cycle doubling the number of DNA molecules present.

The other options do not accurately represent what occurs during this phase. Single-stranded RNA hybrids, specific probes, and signal amplification products are not the primary products of the extension phase in the context of traditional PCR. Therefore, the correct choice reflects the fundamental mechanism of PCR where new DNA strands are synthesized, leading to the amplification of the desired DNA segment.