What is the Difference Between B DNA and Z DNA

The main difference between B DNA and Z DNA is that the B-DNA is right-handed, whereas the Z-DNA is left-handed. Furthermore, in B-DNA, the bases that occupy the core and sugar-phosphate backbone occurs at the periphery of the helix while, in Z-DNA, the sugar-phosphate backbone forms a zig-zag pattern; therefore, bases occur at the core as well as the periphery. 

B-DNA and Z-DNA are two of the three conformations of DNA that occur in nature. Moreover, the diameter of B-DNA is 20  Å, and it contains 10 residues per turn while the diameter of Z-DNA is 18  Å, and it contains 12 residues per turn. 

Key Areas Covered 

1. What is B DNA
     – Definition, Structure, Importance
2. What is Z DNA
     – Definition, Structure, Importance
3. What are the Similarities Between B DNA and Z DNA
     – Outline of Common Features
4. What is the Difference Between B DNA and Z DNA
     – Comparison of Key Differences

Key Terms 

B DNA, Conformations of DNA, Diameter, Helix Pitch, Helix Sense, Z DNA

What is B DNA 

B-DNA is the most common and the predominant conformation of DNA inside the cell. It means DNA prefers to exist in B-form under natural physiological conditions, including pH and salt concentrations. Moreover, the B-form of DNA was first described by James Watson and Francis Crick However, B-DNA takes other conformations under altering conditions. 

Figure 1: The Structures of A-, B-, and Z-DNA

Furthermore, the main characteristic features of B-DNA include the right-handed coiling, 34 Å pitch with 10 repeating units per turn, mononucleotide repeating units, the helical diameter of 20 Å, etc.  

What is Z DNA  

Z-DNA is another conformation of DNA with a left-handed helical structure and a zig-zag pattern-forming sugar-phosphate backbone. Moreover, it was first described by Andrew Wang and Alexander Rich. Z-DNA forms in response to high salt concentrations. Z-DNA has a role in the regulation of gene expression as it can keep DNA inactive.

Figure 2: The Helix Axis of A-, B-, and Z-DNA

Moreover, the important features of Z-DNA include 45 Å pitch with 6 repeating units per turn, dinucleotide repeating units, the helical diameter of 18 Å, etc. 

Similarities Between B DNA and Z DNA 

• B-DNA and Z-DNA are two of the three conformations of DNA found in nature. 
• Both types of DNA contains a double-stranded helical structure. 
• Moreover, both strands are antiparallel. 
• They contain nucleotide bases attached to the sugar-phosphate backbone. 
• Adenine forms two hydrogen bonds with thymine while cytosine forms three hydrogen bonds with guanine in both forms of DNA. 
• Both types of DNA can occur in vivo under different biological conditions.  

Difference Between B DNA and Z DNA 

Definition 

B-DNA refers to the typical form of double helix DNA in which the chains twist up and to the right around the front of the axis of the helix. But, Z-DNA refers to the left-handed uncommon form of double helix DNA in which the chains twist up and to the left around the front of the axis of the helix. Also, it has 12 base pairs in each helical turn and one groove on the external surface. 

Formation 

Moreover, B-DNA forms under normal physiological conditions, Z-DNA forms under high salt concentrations. 

Helix Sense 

While B-DNA is right-handed, Z-DNA is left-handed. Thus, this is the main difference between B-DNA and Z-DNA.

Major and Minor Grooves 

Furthermore, B-DNA contains a wide and deep major groove and a narrow and deep minor groove while Z-DNA contains a narrow and deep major groove and a wide and shallow minor groove. 

Bases and the Backbone 

In B-DNA, the bases occupy the core and sugar-phosphate backbone occurs at the periphery of the helix while in Z-DNA, the sugar-phosphate backbone forms a zig-zag pattern and therefore, bases occur at the core as well as the periphery.  

The orientation of Sugar Residues 

Also, another difference between B-DNA and Z-DNA is the orientation of sugar residues. The sugar residues in B-DNA has an altering orientation while the sugar residues in Z-DNA is not altering. 

Repeating Unit 

The repeating unit of B-DNA is a mononucleotide while the repeating unit of Z-DNA is a dinucleotide.  

Residues per Turn 

B-DNA contains 10 residues per turn while Z-DNA has 12 residues per turn. 

The Angle of Twist per Repeating Unit  

Besides, the angle of twist per repeating unit in B-DNA is 36° while the angle of twist per repeating unit in Z-DNA is 60°. 

Rise per Residue 

The rise per residue in B-DNA is 3.4 Å while the rise per residue in Z-DNA is 3.75 Å.  

Helix Pitch 

Helix pitch is also a difference between B-DNA and Z-DNA. The helix pitch of B-DNA is 34 Å while the helix pitch of Z-DNA is 45 Å.  

The tilt of Base Pair 

The tilt of the base pair in B-DNA is 6° while the tilt of a base pair in Z-DNA is 7°.  

Rotation of the Repeating Unit 

Furthermore, the rotation of the repeating unit of B-DNA is 36° while the rotation of the repeating unit of Z-DNA is 60°. 

Diameter 

The diameter of B-DNA is 20 Å while the diameter of Z-DNA is 18 Å.  

Glycosidic Torsion Angle  

The glycosidic torsion angle is anti for deoxyguanosine and deoxycytidine in B-DNA while the glycosidic torsion angle is syn for deoxyguanosine and anti for deoxycytidine in Z-DNA.  

Sugar Pucker 

In addition, the sugar pucker is C2‘ endo for deoxyguanosine and C3‘ endo for deoxycytidine in B-DNA while the sugar pucker is C3‘ endo for deoxyguanosine and C2‘ endo for deoxycytidine in Z-DNA. 

The Distance of P from Axis 

The distance of P from the axis is 9.0 Å for both dGpC and dCpG in B-DNA while the distance of P from the axis is 8.0 Å for dGpC and 6.9 Å for dCpG in Z-DNA. 

Conclusion 

B-DNA is the common form of DNA occurring in cells. It is right-handed and consists of bases at the core and sugar-phosphate backbone in the periphery structure. Additionally, it contains 10 nucleotides per turn. The width of the DNA helix is 20 Å. On the other hand, Z-DNA is a left-handed and less common form of DNA, occurring in high salt concentrations. Moreover, its sugar-phosphate backbone shows a zig-zag pattern. It also contains 6 dinucleotides per turn. Additionally, the width of the Z-DNA is 18 Å. Therefore, the main difference between B-DNA and Z-DNA is the helical structure, geometry, and dimensions. 

References:

1. “Z-DNA, a Left Handed DNA Form.” Biocyclopedia, Available Here.

Image Courtesy:

1. “Dnaconformations” By Mauroesguerroto – Own work (CC BY-SA 4.0) via Commons Wikimedia   
2. “B&Z&A DNA formula” By Lankenau at English Wikipedia – Transferred from en.wikipedia to Commons. (Public Domain) via Commons Wikimedia