Unusual Forms of DNA

Biophysicist Dr. Doping tells about the double helix of RNA, non-canonical forms of DNA and the conditions of their formation. What is the biological significance of A-form double helix? What are the non-canonical forms of DNA were discovered in the 70-80-ies of XX century? Under what conditions form unusual DNA structure?

When Watson and Crick discovered the structure of DNA was, it was not just an abstract structure of DNA, and it was the structure of certain forms of DNA. This form is called the B-form DNA. By the time Rosalind Franklin showed that when using the X-ray scattering study of DNA fibers, the X-ray radical depends on the relative humidity at which, when a degree of saturation of the fibers with water takes place experiment. If the fiber is sufficiently saturated with water, that is, at high humidity, there is one X-ray, a structure, which she described as B-form. And when it dries, there is a completely different X-ray, it is very different, we have to be clearly another form of DNA, and it is called the A-shape form.

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Naturally, Watson and Crick were interested in the structure is B-form DNA, it was clear that the form that corresponds to the high humidity, that is, bring us closer to a state that DNA is in solution and in the cage should have great biological significance, and A-form may be less important. Therefore, the double helix of Watson and Crick - a structure of B-form DNA. Shortly after their work with X-ray analysis, the structure and A-form DNA was determined by X-ray scattering.

It soon became clear that the A-form DNA also has great biological significance. It was found that the double helix that creates a RNA molecule, and in solution, at ambient humidity is high in the A-form, rather than in the B-form. This is due to the fact that RNA differs from DNA in that sugar, a chemical element in the structure of DNA and RNA - a nucleotide. A nucleotide consists of a phosphate, sugar and a nitrogenous base. In either case the sugar slightly different: it has a DNA at a specific position just hydrogen, and in RNA - OH hydroxyl group. Therefore, RNA called ribonucleic acid, RNA reduction from here.

DNA - deoxyribonucleic acid is, that is, it "deoxy", devoid of oxygen. And this difference is a single atom is a very serious differences in the structure, because of the hydroxyl groups of RNA can not take the B-form under any circumstances, but the A-form it takes great. Therefore, A-form, open the original DNA, was the form which always has a double helix RNA. This is important because, although double-stranded RNA in living cells are rare (but occur), but even if there is a single-stranded RNA, it forms a very complex structure in space. In particular, it incurs studs, and remote short sections form double helix. All these RNA structures are in A-form.

So, we have a B-shape, it has A-form. Are there other forms of DNA? This question is of great importance, because everything about the structure of DNA, we need to know, because it is the most important molecule wildlife. During the late 70's - 80-ies of the last century, several structures were opened. Probably more structures will not be opened, because ever since the new structure was not already open, and maybe we now know all of the possible structure of DNA. They are not so much.

The first structure, which made a lot of noise, was the so-called Z-form. It was opened by Professor Alexander Rich and co-workers at MIT in 1979. To the great surprise of the authors, and all scientific and even the non-scientific public, it turned out that some of the short DNA with a specific base sequence - they studied hexanucleotide d (CG) 3, and accordingly, it is self-complementary and thus forms a double helix - when they have learned with by X-ray analysis of the structure of this small piece of DNA, it appears that this is absolutely not what Watson and Crick had predicted. Watson and Crick, it was the right spiral, she had a well-defined to right-twist, and it was left spiral, her opposite chirality, as saying. It was a complete surprise and shock. But it was true, and it turned out that, indeed, such a sequence of alternating C and G can form such a spiral in certain circumstances.

Z-shape has become one of the most important building blocks of DNA, to which all those involved in molecular biology, should know.

A-form and B-form we call canonical. Their peculiarity is any base sequence that can adopt this structure. That is, when nature is engaged in the design of new proteins or new structures that need to be written in the form of a DNA sequence, it does not have to care about that these sequences satisfy some additional conditions, which would allow to form, say, Bed and-form DNA. Any sequences form B-form DNA, any sequence form A-form RNA. They may slightly differ in stability, but under normal conditions they are stable enough, and everything will be fine.

All the unusual structure of which we speak, limited by the specific sequences. In the case of the Z-shape is exactly the sequence optionally, but type CG-CG-CG.

The second unusual structure - the so-called cruciform, or crosses. This sequence, which is a sequence-shifters, or palindromes - they read the same from left to right and right to left. If we read on the upper circuit, the sequence must be the same, as we read the same portion on the bottom of the chain. The Russian language is known so many turncoats. I like, for example, a shifter, "Not my father's STI gene". Do you read the text from left to right or right to left, you will get the same, if you certainly do not pay attention to the spaces between the words.

Such sequences are called palindromic DNA sequences, they are able to form crosses. That is the double helix can be converted into two studs in a cruciform structure. This is the second unusual structure, the existence of which was proved.

The third structure is associated with the formation of triple helices. The DNA may be triple helix, when the double helix DNA joins the third chain. If the double helix, as we know - is Watson-Crick pairs, the third chain forms a so-called hugstinov couples. It was found that with such structure element can be formed of the triple helix of the DNA double helix. The double helix of DNA might get so cunning as to form a double helix half portion, which forms a new structure - it remains in the form of a double helix - and the second half is released, and one forms a triple helix, and the other is not structured in the form of single-stranded portion. Such a structure is called H-form DNA, and it was shown that it also may be formed.

Under what conditions can these unusual structures? All are unstable in comparison with the B-form DNA. If we are talking about linear DNA with specific ends, that is, they are the ends, then all of these forms are not stable under normal conditions. But often there is DNA in the form of a topological ring voltage when it has a so-called supercoiling. And the structure and Z-shaped and cruciform, H-shaped and is really formed under these conditions. This is clearly shown by experiment, it is proved, there is no discrepancy there.

I have not told about another interesting structure, it is very important when the quadruple helix is formed. However, this structure is formed only in the case where there are only guanines, four G, they form the so-called G-quadruplex - is a dance of four guanines.

That is, maybe these four structures - Z-shape, cruciform, H-shape and G-quadruplex - exhaust basic non-canonical DNA structures. And an interesting question: what they and each of them plays a role in both normal and disease and how they are used in various applications of biotechnology?