Replication bubble is the space where the strands of double helical DNA structure are separated with the help of DNA helicase. The strands of the double helical DNA are still in the helix form on the both end of the unwinded DNA stand which gives this region an appearance of a bubble.
DNA unwinding is the first step in replication or transcription of the DNA molecule. DNA helicase separates the two types of "DNA" that are connected by hydrogen bonds with the help of ATP hydrolysis.
