Monday, January 30, 2012

Yeast has 'shuffle' option to survive stress conditions

Because ever so often there is a mutation in the DNA, species are able to acquire new capabilities. By changing  its blueprint, life exists in all the variety we find today. Most mutations are simply based on chance: when the genetic code is replicated, the cellular machinery makes a mistake once in a while. Cancer cells are more genetically unstable, which means they have a much higher mutation rate, and therefore are able to adapt quicker. The same is true for antibiotic resistance in bacteria. Those who fail to adapt die, but those that gained the right set of mutations survive, giving rise to a resistant species. Researchers from the Stowers Institute for Medical Research found a similar mechanism in yeast. These micro-organisms appear to have a 'shuffle' button to rearrange the chromosomes, which are the packages in which DNA is tidily kept. It seems to be a marvellous and interesting evolutionary mechanism to keep the species alive during stress conditions by trying to find a new genetic arrangement which is resistant against whatever it is that threatens them.

Cancer cells develop weird karyotypes, which is the number and physical form of the chromosomes. They can have an uneven number of chromosomes, while they normally come in pairs. Or they can have parts of a chromosome swapped with another. It is a form of genetic modification, and gives rise to a bigger variety in the genome of the species. Normally, rearranging chromosomes would be deadly, because chances of developing an improved set is incredibly small. However, yeast has shown to shuffle its chromosomes in stress conditions, presumable as a last resort in order to come up with a variant that can survive the harsh conditions. Because a yeast colony contains many individual cells, even something unlikely as finding a beneficial set of mutations has a chance of turning up in the population.
When the scientists tried to find the mechanism responsible for shuffling the DNA packages, they found that loss of a molecule called H90 resulted in weird genetic behaviour. Apparently, this 'guardian of the genome' stood in the way of achieving chromosomal chaos. H90 is lost when yeast is exposed to stress conditions such as heat, chemicals or radiation. After the scientists induced stress and tried to kill yeast with drugs, the micro-organisms developed resistance, presumably due to the ability to adapt quickly.

As said, cancer cells are also able to adapt rapidly to changing environments because of increased genetic instability. That is why they can develop resistance over time, much in the same way as bacteria do. The shuffle button found in yeast may have implications for cancer. Perhaps we can find the same mechanism in tumours, and if we do, we could use this information to increase our treatment options. Because yeast and humans highly overlap, genetically speaking, it does sound likely. We have already shown that cancer cells have unstable chromosomes, which hints at a similar mechanism.

Achieving chromosomal chaos by hitting the shuffle button is a bold move. It likely results in large numbers of deaths in the population. However, for survival of the species it is probably beneficial. Because yeast colonies consist of millions of cells, a variant with the right adaptations is bound to turn up. It is easy to see how this mechanism was beneficial throughout evolution. 

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