What are the major conformations of DNA?

The three major conformations of DNA are B-DNA, A-DNA, and Z-DNA. 

B-DNA 

B-DNA is the most commonly recognized and studied conformation of DNA. It is also the most abundant form of DNA in cells under normal physical conditions. 

B-DNA or B-form DNA is differentiated by these characteristics: 

• It consists of two strands of DNA that wind around the same axis in a right-handed helical structure. 
• The plectonemic coiling of the two DNA strands creates a structure that is compact and efficient.
• The two strands of the double helix are antiparallel - they run in opposite directions. The nucleotides on one strand align in a 5′ to 3′ orientation, while their complementary nucleotides on the opposite strand align in a 3′ to 5′ orientation.
• The base pairs are nearly centered over the helical axis.
• The sugar ring is in the C2′ endoconformation.
• The helix diameter is approximately 9 nanometers. 
• The helix pitch is 34 degrees, base-pair tilt is -6 degrees, and twist angle is 36 degrees. 
• The hydrogen bonding between the bases, specifically in the anti-conformation helps to maintain the stability of the B-DNA structure.  
• The B-form of DNA is observed at high water concentration (high humidity).

A-DNA

The A-form of DNA has several similarities with the B-form of DNA but it can be differentiated by a few distinctive characteristics:  

• It consists of two strands of DNA that wind around the same axis in a right-handed helical structure. 
• The helix has a ribbon-like shape with a more open cylindrical core. This is because of the arrangement of base pairs within the double helix. 
• The base pairs are displaced away from the central axis and closer to the major groove.
• The sugar ring is in the C3′ endoconformation
• The helix pitch is 28 degrees, base-pair tilt is 20 degrees, and twist angle is 33 degrees. 
• The helix diameter is approximately 2.3 nanometers, which is slightly smaller than the B-form DNA helix.  
• The A-form of DNA is observed at low water concentration (low humidity).

Z-DNA 

Z-DNA or Z-form DNA is differentiated by these characteristics: 

• It has a distinct structure that’s significantly different from both B-DNA and A-DNA. 
•  
• Z-DNA consists of two strands of DNA that wind around the same axis in left-handed helices, resulting in a prominent zig-zag pattern in the phosphodiester backbone.
•  
• It has antiparallel strands similar to B-DNA and A-DNA but its helix is narrower and more elongated compared to the compact structures of the other two major conformations of DNA. 
• The helix pitch is 45 degrees, base-pair tilt is 7 degrees, and twist angle is -30 degrees. 
• The helix diameter is approximately 1.8 nanometers. 
• The Z-form of DNA is observed in high salt concentrations.