What is a likely cause for an unexpected band in a PCR reaction?

An unexpected band in a PCR reaction can often be attributed to the annealing temperature being too low. When the annealing temperature is significantly lower than what is optimal, it allows for non-specific binding of primers to the template DNA. This non-specific binding can result in the amplification of unintended sequences, leading to the appearance of extra bands on a gel.

In PCR, the annealing step is critical for ensuring that the primers bind specifically to their complementary sequences on the target DNA. If the temperature is too low, the binding is less selective, potentially amplifying non-target regions that may share similar sequences with the intended template. Consequently, this reinforces the importance of optimizing the annealing temperature to enhance specificity and yield of the desired product.