The forms A and B from DNA have a double helix structure to the right, while the Z form of DNA has a double helix structure to the left. The molecule molecule of DNA exists in different forms under different conditions in a cellThese variations in conformations may be due to salt concentrations, hydration level, DNA sequence, presence of metal ions, direction of supercoiling, etc., in the cell.
We are all familiar with the structure of deoxyribonucleic acid (DNA), it is known to carry the information genetics and is a chain or long chain with the combination of 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 process of gene expression.
According to the Chargaff ruleThe two strands that form the two-stranded structure of DNA are made up of base pairs (A,T,C,G) that interact with each other, following the principle of complementarity where the adenine (A) forms a bond of hydrogen bond. and is paired with thymine (T), i.e. A=T and guanine with cytosine, i.e. (G=C), thus creating the double helix. The chain thus formed by these base pairs is very long.
Variation in the conformation or structures of DNA nucleotides results in changes in the structure of DNA. The double helix DNA structure exists in six different forms which are the 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 the Watson and Crick model of DNA.
The role of other forms of DNA is unclear, although it is believed that transitions between different forms of DNA play a vital role in gene expression. In this article, we will look at the difference between the three main forms of DNA, which are A, B and Z, with a brief description.
| Basis of comparison | DNA A | DNA B | Z DNA |
|---|---|---|---|
| Meaning | This type of DNA is formed from DNA B when the relative humidity decreases and also when the heteroduplex is formed with RNA. This form has 11 base pairs per helix and is shorter than another model. | DNA B is an abundant form of DNA, and is the most studied and is present in a cell under common physiological conditions; this model is ten base pairs per helix. | The Z form has 12 base pairs per helix and forms under low humidity conditions in the cell. |
| Helix twist | 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 rotation by base pair | 31°. | 36°. | 51° o 9°. |
| Number of base pairs per helical turn |
11.6. | 10. | 12. |
| Distance between each base pair. | 2,9 Å. | 3,4 Å. | 7,4 Å. |
| Base pair tilt | 20°. | 6°. | 7°. |
| Rotation of the propeller shaft (major grove). | Narrow and deep. | Wide and deep. | Important flat groves. |
| minor grove | Wide and shallow. | Narrow and deep. | Narrow and deep. |
| Conformation of the ribose sugar | Endo C3′. | Endo C2′. | Endo C2′ for pyrimidine and endo C3′ for purine. |
| Glycosidic bond conformation | Anti-. | Anti-. | Anti- for pyrimidine and Syn- for purine. |
Definition of a DNA double helix structure.
When DNA B dehydrates, DNA formation occurs. A. This is the rare type of structural conformation. A-DNA is also the double-stranded helical structure that resembles B-DNA, although in a shorter and more compact form.
Rosalind Franklin discovered the structure of A-DNA.
Structurally it is the right-hand helix and has 11.6 base pairs per turn. The inclination of the base pairs is 20° to the central axis. Compared to B-DNA, A-DNA is much flatter and wider. The diameter of the helix is 26 Å. A-DNA is more stable due to the presence of the OH group group on the ribose and it always exists during the transcription reverse transcription, transcription and RNA primer hybridisation.
Definition of the double helix structure of B DNA.
The form B form of DNA also has the right-handed helix and is the strongest form among all the other types. Watson and Crick described B-DNA and named it Rosalind Franklin; however, this structure is also known as the Watson-Crick DNA model.
The DNA B model is the most common type of common found in the cell, and this structure is the predominant form under physiological conditions such as pH and salt concentrations present in the cell. Each turn in DNA B has ten base pairs extending, with a distance of 3.4 Å. The diameter of the helix is 20 Å. The base pairs have the same width, i.e. (AT and GC) 10.85 Å.
Definition of the double helix structure of DNA Z
The Z-DNA is known for its left-handed double helix conformation, and the polynucleotide strands of the DNA move in a ‘Z’ pattern. zig-zag ‘, hence the name Z-shape. Andres Wang and Alexander Rich discovered the structure of Z-DNA.
Structurally, the Z-form of DNA has 12 base pairs per turn and a turn length of 45 Å. The Z-DNA structure is reported during low humidity and high salt concentration in cells. The exact biological function of Z-DNA is still unclear, but as it is present upstream of the start site, we can predict that it will have some role in gene expression.
Key differences between the double helix structure of A-, B- and Z-DNA
The main and notable differences between the A, B and Z forms of DNA are detailed below:
- The A, B, and Z forms of DNA differ in their conformationswhere the A and B forms are of the right-hand helix type, while Z is of the left-hand helix type. The A-DNA is formed from B-DNA when the relative humidity decreases and also when the heteroduplex with RNA is formed. This form has 11 base pairs per helix and is shorter than another model. DNA B is an abundant and prominent form of DNA, and is the most studied and is present in a cell under common physiological conditions; this model is from ten base pairs per helix. The Z form form of DNA has 12 base pairs per helix and is formed under conditions of low humidity in the cell.
- The helical diameter of A-DNA is 26 Å, B-DNA is 20 Å and Z-DNA is 18 Å.
- The helical pitch (height of the helical turns) is 28.6 Å, 34 Å and 44 Å for DNA forms A, B and Z, respectively.
- The helical turn per base pair is 31° for type A, 36° for type B and 51° or 9° for type Z.
- The number of base pairs per helix turn in the A form is 11.6, while in the B form it is 10 and in the Z form it is 12.
- The distance between each base pair is 2.9 Å in A DNA, 3.4 Å in B DNA and 7.4 Å in Z DNA.
- The inclination of the base pair is 20°, 6° and 7° in the form A, B and Z respectively.
- The rotation of the propeller shaft (major grove) is narrow and deep in DNA A, while it is wide and deep in form B, and the main grove is flat in form Z, with a core solid at the centre.
- The minor groove is wide and shallow in DNA A and narrow and deep in B and Z types.
- Conformation of ribose sugar or sugar puckering in the endo C3′ form of DNA A, in DNA B it is the endo C2′ form and in DNA Z it is endo C2′ for pyrimidine and endo C3′ for purine.
- The conformation of the glycosidic bond in the A and B types is antiform, while in the Z form it is anti- for pyrimidine and Syn- for purine.
Conclusion
In this publication we met the three forms of DNA, although it is believed that, in addition to the double-stranded helical structure, DNA also exists in certain unusual forms, which are essential for molecular recognition of DNA by enzymes and proteins. These unusual forms are triple-stranded DNA, four-stranded DNA, double-stranded DNA, and so on. This structure helps the DNA to perform its function properly.