Tertiary structure of DNA in prokaryotes. The supercoiled DNA. Circular DNA packaging
Although it was thought that the tertiary structure of DNA was exclusive to the nucleus of the eukaryotic cell and chromosomes, also the DNA of bacteria, mitochondria and chloroplasts has a certain tertiary structure, being associated with other histone- like proteins forming the nucleoid.
The tertiary structure of the circular DNA molecules (not the linear DNA) of bacteria or mitochondria, consists in that the 20 Å fiber coils on itself (by the action of the enzymes DNA-toposiomerases II) forming a species of supercoiled, so it called supercoiled DNA .
Supercoiled DNA has two advantages:
- The structure is more stable because the length of the DNA is shorter.
- It facilitates the DNA duplication process, since the coiling of the supercoiled DNA is to the right, unlike that caused by the enzymes that de-spiralize the DNA to start its duplication, opening the molecule.