Important Differences between B DNA and Z DNA

DNA structure

DNA is a right-handed double helix composed of deoxyribonucleotides. It can take up different forms of structural conformations based on different factors, some are – salt concentration, presence of chemically altered bases, hydration level, sequence of DNA, presence of polyamines in the solution, direction and quantity of supercoiling.

Commonly occurring structural conformations of DNA are – A-DNA, B-DNA and Z-DNA.

The key difference between form B DNA and Z DNA is that the B-DNA is right-handed, while the Z-DNA is left-handed.

B DNA

  • Commonly occurring DNA form in normal physiological conditions, this form of DNA is a right-handed double helix
  • The two strands of this DNA run in two different directions
  • They show an asymmetrical structure, with the alternate presence of major and minor grooves. It is a result of the glycosidic bonds of a base pair not being diametrically opposed to one another
  • Between the adjacent deoxyribonucleotides, there is a distance of 0.34 nm and each turn comprises 10.5 base pairs of length 3.4 nm
  • The helical width of B-DNA is 2 nm and its backbone comprises sugar phosphates associated continuously through phosphodiester bonds. The core comprises nitrogenous bases

Z DNA

  • Structurally differing, this form of DNA is a left-handed double helix
  • The helical width of Z-DNA is 1.8 nm, making it the narrowest compared to the other DNA conformations
  • Its distinguishing factor is its backbone appearing as though a zigzag
  • Each turns comprises 12 base pairs, 4.56 nm long
  • Two adjacent deoxyribonucleotides are 0.37 nm apart with the presence of hydrogen bonds between two strands

Key Difference between B DNA and Z DNA

The table below depicts the differences between B DNA and Z DNA.

B DNA

Z DNA

What it is

One of the three common conformations of DNA helix, the chain twists up and to right around the front of the helical axis

One of the three common conformations of DNA helix, the chain twists up and to the left around the front of the helical axis

Helical type

Right-handed

Left-handed

Occurrence

Common

Less common comparatively

Description of major and minor groove

Major groove – wide and deep

Minor groove – narrow and deep

Major groove – narrow and deep

Minor groove – wide and shallow

Condition for formation

Normal physiological condition

High salt concentration

Repeating unit

Mononucleotides (1 bp)

Dinucleotides (2 bp)

Arrangement of sugar residues

Altering

Not altering

Helical diameter

20 Å

18 Å

Glycosyl angle

anti

C: anti, G: syn

Base pairs per turn

10.5

12

Helical pitch

34 Å

45 Å

The angle of twist per repeating unit

36°

60°

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