Triple-stranded DNA

 

DNA STRUCTURE

If you imagined your DNA right now, you probably sure thought that double helix you learned in school at some points. Probably. But it may look a lot different !!

"That ladder twisted on itself bonds between four bases Adenine,Guanine,Cytosine,Thymine.That is the DNA double helix,the thing you have probably picturing in your head".

But it is now thought ,DNA, in times may have different configurations.

The helix can twist into other way ,have a couple extra strands and can have twisted within its interdisciplinary shape.

There are three biologically active types of DNA double helix, Type A, Type B and Type Z.

1. B-DNA is recognized as the first one with right handed helix and common in all living cells.
2. A-DNA has a wider spiral.
3. Z-DNA twists in other way, it is a left handed helix with zig zag structure.

The biological purpose of Z-DNA isn't quite clear. It twists the other way it starts to release the stress on the structure and some researches show it may play role in transcription where the genetic info is being copied.

Beyond, the double helix, new researches says the DNA molecules may also coil into range of shapes called SuperCoils.

The science helps us to know how the DNA cramps into a tiny structure by nucleotides. Keep in mind that most of researches on DNA are invitro in an laboratory.

But, by using the enzyme Human Topoisomerase II alpha,it has been described that supercoiled DNA is possible. so the DNA may be backwards or zig-zag or even twisted. It might also have doubled strands. 

It is now recognized that besides double stranded structure, DNA also exists in certain unusual structures.It is believed that such structures are important for molecular recognition of DNA by proteins and enzymes. This is infact needed for the DNA to discharge its functions in an appropriate manner, some selected unusual structures of DNA are briefly described.

Bent DNA 

 

 In general, adenine base containing DNA tracts are rigid and straight. Bent conformation of DNA occurs when A-tracts are replaced by other bases or a collapse of the helix into the minor groove of A-tract. Bending in DNA structure has also been reported due to photochemical damage of mispairing of bases.

Certain antitumour drugs (e.g. cisplatin) produce bent structure in DNA. Such changed structure can take up proteins that damage the DNA. 

Triple-stranded DNA 

 

Triple-stranded DNA formation may occur due to additional hydrogen bonds between the bases. Thus, a thymine can selectively form two Hoogsteen hydrogen bonds to the adenine of A-T pair to form T-A-T. Likewise, a pronated cytosine can also form two hydrogen bonds with guanine of G-C pairs that results in C+-G-C.

Triple-helical structure is less stable than double helix. This is due to the fact that the three negatively charged backbone strands in triple helix results in an increased electrostatic repulsion.  

Four-stranded DNA  

 

Most DNA have two-strands but many people don't know it can also have four and its called G-Quadraplex DNA (G4 DNA).

Cambridge university researchers claimed that they found the G4 DNA in human cells using florescence biomarker. They found G4 DNA seems to turnout during the s-phase just before the cells about to divide which precise us to the point why should we care aboUt four-stranded DNA.

 Polynucleotides with very high contents of guanine can form a novel tetrameric structure called G-quartets. These structures are planar and are connected by Hoogsteen hydrogen bonds. Antiparallel four-stranded DNA structures, referred to as G-tetraplexs have also been reported. 

The ends of eukaryotic chromosomes namely telomeres are rich in guanine, and therefore form G-tetraplexes. In recent years, telomeres have become the targets for anticancer chemotherapies.

G-tetraplexes have been implicated in the recombination of immunoglobulin genes, and in dimerization of double-stranded genomic RNA of the human immunodeficiency virus (HIV).

References:

http://www.pnas.org/content/pnas/98/15/8490.full.pdf

https://www.dnastar.com/genequest_help/index.html#!Documents/predictingdnabending.htm

https://en.wikipedia.org/wiki/Triple-stranded_DNA

https://www.sciencedirect.com/topics/neuroscience/triple-stranded-dna

https://en.wikipedia.org/wiki/G-quadruplex

https://www.nature.com/news/four-strand-dna-structure-found-in-cells-1.12253

https://www.sciencedaily.com/releases/2016/05/160516083458.htm