They stand for guanine, thymine, adenine, and cytosine. There is also uracil in RNA. Not sure why they have those names – chemical names always sound like aliens.
The main base pairs in DNA are A,T,G,C but there are actually more when you take into account epigenetic modifications (for example methylation/de-methylation around CpG islands). In RNA more than 60 post-transcriptional modifications have been detected but the biological function for most of these remain unknown. Some expected roles include fine tuning gene expression by, for example, promoting changes in structure and in the recruitment of cellular partners.
As Edward said, they are the first letters of the chemical names of the building blocks that make DNA. As for why they are called that – why is any word what it is? Why is a potato called a potato and not a krumpli, for example? (it actually is called that in Hungarian, isn’t language wonderful?)
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