Supercoiled DNA
Supercoiled DNA is a synonym for closed, circular ( cccDNA ), but also linear DNA molecules, which are extremely packed as spiraled helices in the core equivalent (in prokaryotes ) or in the cell nucleus (in eukaryotes ).
Geometrically, there is another dimension of spiralization on the spiral, which can also be demonstrated in practice. Naturally occurring circular DNA molecules exist in superhelical form. Superhelicality arises from the introduction of helical turns. The twisting of the DNA is compensated by the formation of a supercoil structure. Since the total length of the DNA in a cell is often a thousand times the diameter of the cell, supercoiling is essential for the functionality of some living things.
A right-hand twist is referred to as negative supercoiling (also negative supertwist ), a left-handed over-spiralization as positive supercoiling (also positive supertwist ).
structure
Supercoiled DNA is found in every living cell. It ensures that a minimum number of base pairs is always open in order to enable the polymerases to supplement the original strand to form a new double strand. This applies to replication and transcription . There is an equilibrium between two so-called topoisomerases .
The topoisomerase II cuts two strands and bonded to them. This process requires energy and is therefore dependent on ATP .
The topoisomerase I requires no energy and removes the "supercoiled" by cutting only one strand, revolves around the closed line and closes the gap again.
DNA and RNA synthesis can only take place if there is sufficient ATP. No synthesis takes place under starvation conditions.
The gyrase inhibitors , which inhibit the prokaryotic topoisomerase II ( gyrase ) and partly IV , are among the most powerful known poisons for prokaryotes ; Examples are nalidixic acid or fluoroquinolones .
See also
Individual evidence
- ↑ a b Entry on Supercoil. In: Römpp Online . Georg Thieme Verlag, accessed on February 27, 2014.