What are the phases of basic PCR reaction?

There are three phases of basic PCR reactions. 

1. The first phase involves heating and separating the two strands of the DNA template, turning it into single strands. This process is called denaturation. 
2. In the second phase, the temperature is lowered as primers attach to each original single strand of DNA at specific locations. This attachment is called annealing and is necessary for the creation of new DNA strands. 
3. In the final step, the temperature increases once more and new DNA strands are built from the primers, extending along the template strands. This process (known as extension) forms complete copies of the original DNA. 

These three phases are repeated between 20 to 40 times. With each cycle, the number of DNA copies doubles. To repeat the synthesis process multiple times, the DNA strands need to be separated again. These reactions can use any DNA polymerase and create specific sections of the original DNA sequence.