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Frogger, More Than just a Game December 12, 2010

Posted by zach in Biology, Chemistry, Ecology, Environment/Conservation, Health.

As a child you learned that a frog isn’t born as a litGraphic depicting life stages of a tadpole to a frogtle frog, but rather its life begins as an egg and develops into a frog through metamorphosis. This poses a lot of interesting questions for molecular biology because appendages have to grow at the same time the tadpoles tail is lost. The frog is an ideal model organism for how morphological changes can take place, since frogs are so easily kept in captivity it’s easy to manipulate their environment. When you are able to change their environment it makes it easier to identify what hormones, enzymes, or other environmental cues influence how the tadpole develops into a frog.

One hormone that has been shown to affect the metamorphosis of tadpoles is thyroid hormone (TH). In the presence of dilute concentrations of thyroid hormone the frog’s metamorphosis is accelerated.   The converse is also true, when TH is blocked by compounds such as goitrogens, tadpoles will not develop into frogs but, stay in the tadpole life stage.  We know that TH acts through nuclear receptors to activate transcription or, the process of DNA going to mRNA then translated into proteins.  With many advances taking place in the field of biochemistry my question is how is the structure of the chromatin affected through metamorphosis?  What chemical present in local ponds affect the structure of the chromatin?  There are many different mechanisms in which the structure of the chromatin can be altered.


Two of the most common ways the structure of the chromatin is altered is by methylation, which is adding a methyl group to a cytosine nucleotide, or acetylation, which is the process of adding an acetyl group on to the lysine residue on histones. Both methylation and acetylation can function to change gene expression. Methylation causes a steric hindrance for the transcriptional machinery and acetylation alters how much the gene is exposed to transcriptional machinery.  Both methylation and acetylation takes place through enzymes that cause a change in how the transcriptional machinery operates.  The enzymes can be influenced by a variety of different means, for example acetylation of histone H4 lysine acetylation can be blocked by Nickel compounds.

The question that I want t0 ask is what effects are on the epigenome, when frogs are exposed to pollutants at different life stages? When tadpoles are exposed to high levels of Ni2+ compounds they all die. Is the death of the tadpoles a result of an absence of H4 lysine acetylation or through a different molecular mechanism?  Since epigenetic mechanisms are critical to regulating gene expression it is plausible that the epigenome is tightly correlated with tadpole metamorphosis. Since the habitats of amphibians are becoming more and more polluted, it is urgent to discover the mechanisms in which their metamorphosis acts though in order to reduce a certain group of pollutants.


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