The A and B forms of DNA are right-handed double helical structure, while Z form of DNA is left-handed double helical structure. The DNA molecule exists in different forms under the different condition in a cell, these variations in the conformations can be due to the salt concentrations, hydration level, DNA sequence, the presence of metal ions, direction of supercoiling, etc., in the cell.
We all know the deoxyribonucleic acid (DNA) structure, is known to carry the genetic information and it is the long string or chain with the combination of the monomeric compounds or nucleotides, these nucleotides are linked together to form a long chain-like structure, which carries the information to encode the structure of proteins and thus contribute to the gene expression process.
According to the Chargaff’s rule, the two chains forming the DNA double-stranded structure, consist of the base pairs (A,T, C,G) which interact with each other, following the principle of complementarity where adenine (A) forms hydrogen bond and pairs with thymine (T), i.e. A=T and guanine with cytosine, i.e. (G=C) and thus creating the double helix. The chain thus formed of these base pairs is very long.
Variation in the conformation or structures of nucleotides of DNA results in the changes in the structure of DNA. The structure of Double-helical DNA, exist in six different forms which are A, B, C, D, E, and Z forms. Among all these forms B, A and Z are the essential forms, but B-DNA is the most prominent type, also known as Watson and Crick model of DNA.
The role of other forms of DNA is unclear, though it is believed that the transitions between different forms of DNA play a vital role in gene expressions. In this article, we will be studying the difference between the main three significant forms of DNA, which are A, B, and Z, with a brief description.
Content: A Vs B Vs Z DNA Double Helix Structure
| Basis for Comparison | A-DNA form | B-DNA form | Z-DNA form |
|---|---|---|---|
| Meaning | This type of DNA is formed from the B-DNA when the relative humidity gets lowered and also when the heteroduplex is formed with the RNA. This form has 11 base pair per helix and is shorter than another model. | B-DNA is an abundant form of DNA, and it is most studied one and present in a cell under common physiological conditions, this model as ten base pair per helix. | The Z-form has 12 base pair per helix, and it is formed under low humidity conditions in the cell. |
| Helix turn | Right-handed. | Right-handed. | Left-handed. |
| Helical diameter | 26 Å. | 20 Å. | 18 Å. |
| Helical pitch (height of helical turns) | 28.6 Å. | 34 Å. | 44 Å. |
| Helical twist per base pair | 31°. | 36°. | 51° or 9°. |
Number of base pairs per helical turn | 11.6. | 10. | 12. |
| The distance between each base pair | 2.9 Å. | 3.4 Å. | 7.4 Å. |
| Base pair tilt | 20°. | 6°. | 7°. |
| Helix axis rotation (Major grove) | Narrow and deep. | Wide and deep. | Flat major groves. |
| Minor grove | Wide and shallow. | Narrow and deep. | Narrow and deep. |
| Ribose sugar conformation | C3' endo. | C2' endo. | C2' endo for pyrimidine and C3' endo for a purine. |
| Glycosidic bond conformation | Anti-. | Anti-. | Anti- for pyrimidine and Syn- for a purine. |
Definition of A DNA Double Helix Structure
When B-DNA is dehydrated, it results in the formation of the A-DNA. It is the rare type of structural conformation. A-DNA is also the double-stranded helical structure resembling B-DNA, though in a shorter and compact form.
The Rosalind Franklin discovered the structure of the A-DNA.
Structurally it is the right-handed helix and has 11.6 base pairs per turn. The tilting of the base pairs is 20° away from the central axis. As compared to the B-DNA, the A-DNA is much flatter and wider. The helix diameter is 26 Å. A-DNA is more stable because of the presence of the additional OH group in the ribose and always exist during the reverse transcription, transcription and RNA-primer annealing.
Definition of B DNA Double Helix Structure
The B form of DNA also has the right-handed helix and is the most potent form among all other types. Watson and Crick described the B-DNA and named by Rosalind Franklin; though this structure is also known as Watson-Crick Model of DNA.
B-DNA model is the most common type found in the cell, and this structure is the predominant form under the physiological conditions like the pH and salt concentrations present in the cell. Each turn in the B-DNA has ten base pairs spanning, with the distance of the 3.4 Å. The helix diameter is of 20 Å. The base pairs have the same width, i.e. (A-T and G-C) 10.85 Å.
Definition of Z DNA Double Helix Structure
The Z-DNA is known for the left-handed double helix conformation, and the polynucleotide strands of the DNA moves in a ‘zig-zag‘ pattern, and so the name is Z form. The Andres Wang and Alexander Rich discovered the structure of Z-DNA.
Structurally the Z form of DNA has 12 base pairs per turns and 45 Å turn length. The structure of Z-DNA is reported during low humidity and high salt concentration in the cells. The exact biological function if the Z-DNA is still unclear, but as it is present in the upstream of the start site and so we can predict that it some role in gene expression.
Key Differences Between A, B, and Z DNA Double Helix Structure
Following are the major and noteworthy difference between the A, B, and Z forms of DNA:
- A, B, Z forms of DNA differ in their conformations, where A and B form are right-handed helix type, while Z is the left-handed helix type. The A-DNA is formed from the B-DNA when the relative humidity gets lowered and also when the heteroduplex is formed with the RNA. This form has 11 base pair per helix and is shorter than another model. B-DNA is an abundant and prominent form of DNA, and it is most studied one and present in a cell under common physiological conditions, this model as ten base pair per helix. The Z-form of DNA has 12 base pair per helix, and it is formed under low humidity conditions in the cell.
- The helical diameter of A-DNA is 26 Å, B-DNA has 20Å and Z-DNA has 18 Å.
- Helical pitch (height of helical turns) is 28.6 Å, 34 Å and 44Å of A, B, and Z form of DNA respectively.
- Helical twist per base pair is of 31° of A type, 36°of B type and 51° or 9° of Z type.
- The number of base pairs per helical turn in A form is 11.6, while in B form it 10 and in Z form it is 12.
- The distance between each base pair 2.9 Å in A DNA, 3.4 Å in B DNA and 7.4 Å in Z DNA.
- Base pair tilt is of 20°, 6° and 7°of A, B and Z form respectively.
- Helix axis rotation (Major grove) is narrow and deep in A DNA, whereas it is wide and deep in B form and flat major grove in Z form, with a solid core at the center.
- Minor groove is wide and shallow in A DNA and narrow and deep in B and Z type.
- Ribose sugar conformation or sugar pucker in A DNA C3′ endo form, in B DNA it is C2′ endo form, and in Z DNA it is C2′ endo for pyrimidine and C3′ endo for a purine.
- Glycosidic bond conformation in A and B type is Anti- form whereas in Z form it is Anti- for pyrimidine and Syn- for a purine.
Conclusion
In this post, we came to know about the three forms of DNA, while it is believed that apart from the double-stranded helical structure, DNA also exists in certain unusual shapes, which are essential for the molecular recognition of DNA by enzymes and proteins. These unusual forms are triple-stranded DNA, four-stranded DNA Bent DNA, etc. This structure helps DNA to discharge its function in an appropriate way.
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