Entropy driven force behind segregation of DNA in bacteria
Researchers of the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam have discovered that entropy plays an important part in the segregation of DNA during fission of bacteria. In fission the DNA chain is duplicated at first and then segregated. Up to now researchers have been expecting that in segregation a molecular mechanism plays a part, just as this is the case in more complex organisms like the human body. However, for bacteria it proves that entropy takes part, as a standard for ‘chaos’ in a system. This result also puts a different light in evolution and origin of early live in which DNA plays a part. The researchers have their results published in the scientific magazine Proceedings of the National Academy of Sciences (PNAS) on 15 August 2006
Living creatures distinguish themselves from dead material by being able to multiply. Basic to this multiplication is fission, in the course of which two genetic identical daughter cells originate from one cell. In order to manage this the DNA - the genetic code - of the mother cell has to be duplicated accurately at first. This is called the replication of DNA. The next step is at least as important: the two multiplied DNA cells have to be separated dimensionally. This is called segregation. Cells of ‘higher graded’ organisms, such as plants and animals, build up a wonderful molecular mechanism for this latter task, the so-called mitotic spindle. This construction consists of a complex of long and strong polymers and crude proteins, which literally draws out the duplicated chromosomes.
Segregation in bacteria
Surprisingly, researchers do not know how segregation is being established in the more common bacteria. One of the causes of the lack of understanding is the fact that bacteria are generally very small. They are about cylindrical shaped, one micrometre wide and a few micrometres long. This makes it hard to see the progress of processes in these organisms that are important to us, for example, to our health and nutrition.
Researchers are investigating a great deal in the exact mechanism of segregation in bacteria. As yet, most of the cell biologists do believe that there must be an active molecular mechanism, just as in higher graded cells. Researchers are now able to actually test their predictions on mechanisms, because recent technical breakthroughs have made it possible to follow the movement of the indicated pieces of DNA in time.
Role for entropy?
An alternative mechanism devised by microbiologist Koenraad Woldring at the University of Amsterdam, has inspired Bela Mulder and Suckjoon Jun of the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam by asking themselves whether entropy can play a part in the dimensional segregation of duplicated DNA chains. Entropy, one of the basic forces in nature, is a standard for a number of states that a system can adopt under prevailing circumstances, also described as the ‘chaos’ of a system. Every system strives after a state of maximum entropy.
The DNA of a microbe is a 1.5 millimetre long circular polymere chain that is locked up in the very small volume of the microbe. In a computermodel the Amsterdam researchers have simulated the segregation of two circular DNA chains in a volume that is comparable to the shape and dimension of that of a microbe. To their surprise they noticed that the two chains segregated spontaneously and that each of them took up one half of the volume. In an open space two polymer chains also reject one another. However, this force can be neglected and thus segregation does not take place. The repulsive force proves to be large enough in order to cause segregation, when locked up in an extremely small closed volume.
Entropy and DNA duplication
Inspired by the result, Mulder and Jun have also considered the state in which DNA chains have not yet been completely duplicated. The DNA chain exists of three circular segments: one not-duplicated mother strand and two equally long daughter strands, which are connected in two so-called replication forks (see figure 1). It appears from the simulations that this strangely shaped molecule organizes itself dimensionally in a cell in a very specific way. This structure is dependent on the limited measurement of the three parts. It appears that entropy again is the driven force of the entire molecule.
Light on evolution
Thus, the researchers are able to follow the entire process of duplication and segregation of DNA. They have also compared the results to recent experimental data in the bacteria Escherichia coli and Caulobacter crescentus. In both cases the discovered mechanism seems to duplicate the observations almost entirely. The Amsterdam results put another light on a question that microbiologists have been occupying for decades. Moreover, there are also eventual but theoretical implications on understanding the origin of early life that has been based on DNA. After all it seems that a chain shaped molecule that duplicates itself, provided it is in an adequately small cell, will get the necessary segregation by nature itself for free. It need not wait for the evolutionary development of quite a lot of assistant-proteins. In the near future researchers expect to confirm the results also by experiment by removing DNA from bacteria in order to observe and manipulate them in micromachined channels.
To the editor:
For more information, please contact Professor dr. Bela Mulder, FOM-Institute for Atomic and Molecular Physics in Amsterdam, phone: (020) 608 12 34 or Suckjoon Jun, FOM-Institute for Atomic and Molecular Physics in Amsterdam, phone: (020) 608 12 34.
For the electronical versions of the pictures, please contact Annemarie Zegers, public relations department, FOM, phone (030) 600 12 18.