DNA: B conformation
The B-conformation of DNA has an important degree of hydration and it is the most usual in vivo conformation.
This structure was discovered by Watson and Crick in their original paper in 1953. Let's see the main features:
A fragment of 12 nucleotide pairs is displayed on the screen:
It consists in two polynucleotides plectonemically wound, forming a right-handed double helix. That means that it is necessary to unwind the double helix
to separate the two polynucleotides:
The two polynucleotides have Opposite Polarity. To illustrate this fact, we'll go back to the original structure:
The two polynucleotides interact by means of hydrogen bonds established between the nitrogen bases. This interaction can only
take place between either Adenine and Thymine, the A-T pair:
Other structural features of B-DNA are the following: the furanose ring of deoxyribose is in endo-2':
DNA: A-conformation
The A-conformation appears in DNA crystals of low hydration and a lesser degree of polymerization than B-DNA. It also
appears in double-helical RNA's and in DNA-RNA hybrids:
It is a structure slightly wider and shorter than B-DNA. It is also a right-handed double helix formed by two polynucleotides
plectonemically wound:
A-DNA presents the same pairing pattern of B-DNA (A-T and G-C), but the planes of the bases are inclined with respect to the
main helix axis.
In A-DNA both grooves have more or less the same width, but the narrow groove is deeper:
DNA: Z-conformation
The Z-conformation appears in GC-rich sequences:
It is also a double polynucleotide:
However, there are important differences between the Z-conformation and the other two. In the first place, the helices are left-handed:
Cuadruplex DNA
Telomers are the terminal regions of chromosomes, and have a basic role in the control of cell division. When a
cell undergoes succesive mitotic divisions, telomers become progressively shorter till a point in which further division is not possible.
In some cells, however, the telomeric structure is repaired by the enzyme Telomerase, thus allowing the cell to divide indefinitely.
Telomeric DNA normally adopts a special structure, called Cuadruplex DBA, in which we find four interacting
bases instead of two (but on the same polynucleotide). Let's see the structure of a cuadruplex DNA:
It consists ina single curved polynucleotide (this structure repeating along the telomer). To better see it, we suppress the
vision of the bases and restrict the display to the pentose-phosphate backbone, labelling the 5' and 3' ends: