Considering the information in this chapter on B- and Z-DNA and right- and left-handed helices, carefully analyze structures (a) and (b) below and draw conclusions about their helical nature. Which is right handed and which is left handed?

Key Concepts

B-DNA and Z-DNA Structures

B-DNA is the most common form of DNA in cells, characterized by a right-handed helix with about 10.5 base pairs per turn. In contrast, Z-DNA is a left-handed helix that forms under certain conditions, such as high salt concentrations or specific sequences. Understanding these structural differences is crucial for analyzing the helical nature of DNA.

Right-Handed vs. Left-Handed Helices

The terms 'right-handed' and 'left-handed' refer to the direction in which the helix twists. Right-handed helices, like B-DNA, twist clockwise when viewed from the top, while left-handed helices, like Z-DNA, twist counterclockwise. This distinction affects the physical properties and biological functions of the DNA.

Helical Parameters

Helical parameters, such as pitch, rise per base pair, and base pair orientation, are essential for characterizing DNA structures. These parameters help determine the overall shape and stability of the helix. Analyzing these parameters allows for the identification of whether a given DNA structure is right-handed or left-handed.