Helix Turn in B-form DNA
DNA exists in a double-helix structure, with two strands winding around each other. The B-form DNA is the most common form of DNA found in nature. One turn of the helix in a B-form DNA corresponds to approximately 3.4nm.

Explanation:
- The helix turn in B-form DNA is a complete rotation of the two strands around each other.
- This turn creates the characteristic double-helix structure of DNA.
- The distance covered by one turn of the helix is known as the helical pitch, which in the case of B-form DNA is approximately 3.4nm.
- This distance is the vertical distance between two consecutive nucleotides along the axis of the helix.

Importance:
- Understanding the structure of DNA, including the helix turn, is crucial for various biological processes such as replication, transcription, and translation.
- The precise measurement of the helical pitch helps in studying the interactions between DNA and other molecules, such as proteins and drugs.
In conclusion, a turn of the helix in a B-form DNA corresponds to approximately 3.4nm and plays a significant role in the functioning of DNA in living organisms.