Double helix
The double helix ( plural double helices ; ancient Greek ἕλιξ hélix 'winding') is a geometric structure in which - in contrast to the simple helix - the motif of the winding appears twice.
There are two types of double helices:
- Type 1 consists of two simple helices running around each other, it is also known as a double-start screw (Fig. 1)
- Type 2, in contrast, consists of only one strand. This is wound into a helix, which in turn is wound into a helix with a larger radius. This type is also called double helix (Fig. 2).
biochemistry
In biochemistry , the term double helix to describe a secondary structure is mostly used for type 1. It describes two parallel strands of macromolecules that revolve around each other like a screw. The most prominent example is the structure of occurring in all living things deoxyribonucleic acid - the molecule . The course of the DNA strands is also in opposite directions ( anti-parallel ). The actual helices are formed from the backbone of DNA, which consists of phosphates and sugars. The so-called "rungs" are the bases that connect the two individual strands via hydrogen bonds.
The DNA double helix is often wrongly represented as a double strand, in which the individual strands wrap around each other (see Figure 1, red / blue). In fact, however, both strands run antiparallel to each other around a common axis (see figure below). In this arrangement, the large and small grooves of the DNA can also be seen, which cannot be seen in the simplified representation.
Within the chromosomes , these DNA double helices are again arranged in a compressed spiral around nucleosomes , which in turn are presumably wound spirally in a "super-super-helix".
A special form of the double helix can be found in the structure of keratins in actin filaments . The special thing about this structural protein is that the single strands in this double helix of type 1 are themselves wound into a helix, i.e. they represent a double helix of type 2.
In such cases, one speaks of a coiled coil structure, in which up to seven individual strands can be involved.
Double helices of different dimensions and permeability are z. B. A-DNA , B-DNA and Z-DNA .
technology
In incandescent filaments in light bulbs to type is 2. This double helixes consist of a coiled wire, in turn, is shaped again into a helix. This makes it possible to accommodate a longer wire over a short distance so that the individual windings illuminate each other, which increases the emissivity of the overall structure and thus the light yield and luminance according to the principle of cavity radiation .
One application in architecture is the arrangement of type 1 spiral staircases , similar to the thread of a screw with two turns. Two flights of stairs offset by 180 ° can form the two corridors of a spiral staircase that can be used independently of each other for ascent or descent. The best-known examples of such double-flight spiral staircases include the one in the center of the Loire Castle Chambord from the first half of the 16th century and the one in Rome in 1932 in the Vatican Museum .
astronomy
Near the center of the Milky Way there is an approximately 80 light-year long plasma cloud in the form of a double helix in which two strands of nebula wind around each other. The longitudinal axis of the gigantic double helix structure is aligned with the supermassive black hole Sagittarius A * in the center of the Milky Way, 300 light years away , whose extremely strong magnetic field interacts with the matter of the dust cloud.
See also
- Single helix : α-helix
- Triple helix: Collagen triple helix (a super helix)
