On page 158, Moalem talks about mice and their agouti gene. Moalem says that the agouti gene was flicked into the “off
position”. The DNA still contained the gene however the gene itself wasn’t
expressed. The process that suppresses the genes is called DNA methylation. DNA
methylation “occurs when a compound called methyl group binds to a gene and
changes the way that gene expresses itself, without actually changing the DNA”
(158). DNA methylation was not only shown with the mice and the agouti gene but also with the identical
twins Eleanor and Elizabeth (Eleanor was diagnosed with breast cancer,
Elizabeth was not).
The idea of DNA methylation relates to Big Idea 3, Living
systems store, retrieve, transmit, and respond to information essential to life
processes. DNA methylation affects gene regulation and gene expression as shown
with the mice and the identical twins.
Find other examples of how DNA methylation has affected gene
expression and explain the significance. Also, identify other examples of regulation
of gene expression (not limited to just eukaryotes) and explain how the
regulation works. What are the advantages/disadvantages of gene regulation?
(Hervy Ong, hong3@students.d125.org)
One example of DNA methylation of great significance was an experiment by McGhee & Ginder in 1979 which looks at the beta-globin locus in various cells. The scientists discovered that the expresses beta-globin gene was unmethylated when the beta-globin gene was expressed and methylated when the beta-globin gene was not expressed. This was among the first clues to the fact that methylation is significant in gene expression.
ReplyDeleteA different but very important aspect of DNA methylation is the methylation that occurs during embryonic development, in which various genes “turn on” or “off” based on the gender of the individual, as to ensure proper development of the fetus.
Another example of gene regulation are operons. These are found in prokaryotic organisms. The operon is made up of the promoter region (the region to which RNA polymerase binds), the operator (the region which inhibits or permits RNA polymerase binding), and the gene cluster (the region that contains the genetic information). Two operons worthy of note are the lac and trp operon. The lac operon is normally off, but in the presence of cAMP and lactose (or more specifically, allolactose, created ), the allolactose binds to the inhibitor attached to the operator, turning the inhibitor off, thus allowing for gene transcription to occur. The trp operon, on the other hand, is normally on, allowing for the synthesis of tryptophan. When tryptophan reaches a certain level, it acts like a corepressor and turns the inhibitor of the operon to the active shape. The inhibitor then binds to the operator, turning off gene expression.
Another way that gene expression occurs is with histone acetylation, occurring in eukaryotes. The binding of acetyl to histone tails causes the histones to spread out, thus facilitating in the binding of RNA polymerase to induce transcription.
DNA methylation is important for survival. A study by Suzuki and Bird in 2008 showed that lack of DNMT (DNA methyltransferase enzyme responsible for methylation) in mice lead to premature death. A study by Robertson in 2005 claims that diseases such as cancer, lupus, and muscular dystrophy, as well as other birth defects come from defective imprinting mechanisms, which requires methylation to occur successfully. In short, the advantage is that one can survive and reproduce, to be able to function properly. The disadvantage is that errors in methylation can spell death.
(Gabriel Antoniak gantoni4@students.d125.org)