14 DAY TRIAL // Cells can have their latent memory that makes the previously not observed capacities to suddenly emerge, like single sex cells turning into hermaphrodite in the case of the yeasts.
Now, a team from the Niels Bohr Institute at the University of Copenhagen has used mathematical models and computer simulations to investigate how the cell memory works. Our DNA determines how we look and how we function; this molecule is very stable and transmitted almost completely similar to our descendants. The minute (or not) changes in DNA, named mutations, are the source of the variety of animal and plant life on earth.
Constructive mutations, creating new functions, accumulate through natural selection acting over many generations. But in some cases, big and sudden changes are seen in one individual without any mutation.
This may be common during our development, when similar stem cells form extremely different tissues like skin, brain, or muscles due to cellular differentiation. "The explanation for the sudden changes is that it is not the DNA itself that is altered - it is its immediate surroundings that change and thereby cause a cell to activate some of its dormant capacities" said Kim Sneppen, professor in Biophysics at the Niels Bohr Institute, University of Copenhagen.
The DNA molecule coils itself around protein spheres named nucleosomes. But nucleosomes are not inert; they can turn on or off the DNA wrapped around them. When one DNA multiplies in two single strands, nucleosomes are shared between the two new DNA molecules and the empty loci are completed with new nucleosomes.
This occurs during cell division, changing the nucleosomal composition of a DNA molecule. Also, interconversions between different nucleosome types can change their composition. These changes can trigger a latent capacity that was dormant and which now comes to turn off an active one. The Danish researchers experimented on a mutant yeast cell which was bi-stable, which could develop on either sex or on a hermaphrodite. It resulted that a spontaneous change took place in the yeast cells once in 2000 cell-generations.
The mathematical model based on positive feedback from the microscopic state of the nucleosomes explained how cells with identical DNA display extreme differentiation. It seems that nucleosomes trigger main memory mechanisms in individual cells, offering both stability and openness to new influences affecting the cell.