Forms of DNA : definition, difference

Forms of DNA

Watson and Crick modeled the structure of the DNA molecule, which is now called B-DNA. It is found in all cells. According to modern findings, it has an axial thickness of 0.36 nm instead of 0.34 nm per homologous pair and 10.5 nucleotide pairs instead of 10 in each 5' helix.

Now an A-DNA has also been discovered which has 11 nucleotide pairs in each helix. Probably some specific segments of B-DNA, when there is a lack of water in the nucleoplasm, turn into A-DNA. 

In 1979, Wang and Rich discovered that a segment of B-DNA containing the sequence of bases CGGCCG becomes coiled in the anticlockwise direction under high salinity conditions. It is a zig zag double helix and has 12 pairs of nucleotides in each helix. It has been named Z-DNA.

Different Types of DNA

1. B.DNA - It is the most common form of DNA. The Watson-Crick structure described above is of B.DNA. It has 10 base pairs per turn and diameter is 20Å.

2. A-DNA - It is dehydrated form of DNA, which occurs in high salt concentration. It is also right handedly coiled. It has 11 base pairs per turn. Diameter of A-DNA is 26 Å.

3. Z-DNA - It is stable in synthetic double stranded form Z-DNA is left handedly coiled DNA. Nucleotides are arranged in zig-zag manner. One turn is 45Å long. Diameter is 18 Å.

4. c-DNA - It is also called complementary DNA. It is complementary to mRNA molecule and is syntheseised from the later by the action of reverse transcriptase enzyme. It is of great use in genetic engineering.

Read more - Nucleic Acids

Resemblances between Z-DNA and B-DNA

Resemblances between Z-DNA and B-DNA include the following:

(i) Both are double helical
(ii) Two strands of double helix are antiparallel in both DNAs.
(iii) Both forms exhibit A = T and G = C pairing.


Similarly the differences include the following-

1.  Due to a different arrangement of molecules within Z-DNA polymer, phosphate backbone follows a zig-zag course, while in B-DNA, it is regular.

2.  Z-DNA has left-handed helical sense as against right handed helical sense of B-DNA.

3.  In each strand of Z-DNA, sugar residues have alternating orientation, so that repeating unit is a dinucleotide as against B-DNA where repeating unit is a mononucleotide and the orientation of sugar molecules is not alternating.

4.  In Z-DNA, one complete helix,. i.e., twist through 360°, has twelve base pairs or six repeating dinucleotide units (12 base pairs), while in B-DNA, one complete helix has only ten base pairs or ten repeating units.

5.  Because there are twelve base pairs in one helix n Z-DNA, as against ten in B-DNA, the angle of twist per repeating unit (dinucleotide) in B-DNA.

6.  One complete helix is 45 Å in Z-DNA, while it is 34 Å in B-DNA.

7.  Since bases get more length to spread out in Z-DNA, and since the angle of tilt is 60°, they are closer to the axis and hence the diameter of Z-DNA molecule is 18 Å, whereas it is 20 Å in B-DNA.

Read more - What is Genetic code?

Differences between Z-DNA and B-DNA

1. Z-DNA has left handed helical sense, while B-DNA has right handed helical sense.

2. The phosphate back bone of Z-DNA follows a zig-zag course, while in B-DNA this back bone is regular.

3. In Z-DNA, the adjacent sugar residues have opposite orientation. Due to this, repeating unit is a dinucleotide in Z-DNA as against a mononucleotide unit in B-DNA.

4. In Z-DNA, one complete helix has twelve base pairs or six repeating dinucleotide units, while in B- DNA one complete helix has only ten base pairs or 10 repeating units.

5. The angle of twist per repeating unit in Z-DNA is 60° than the 36° of mononucleotide in B-DNA.

6. In Z-DNA, one complete turn in helix is 45 Å long, while in B-DNA it is 34 Å long.

7. Since base gets more length spread out in Z-DNA and since the angle of tilt is 60°, they are molecule is 10 Å than 20 Å diameter of B-DNA.