This module will examine how information is encoded in DNA, and how that information is interpreted to bring about changes in cells and tissues.
The Genetic Code
It has been mentioned in a variety of modules that DNA stores genetic information. That much was clear from the experiments of Avery, Macleod, and McCarty and Hershey and Chase. However, these experiments did not explain how DNA stores genetic information. Elucidation of the structure of DNA by Watson and Crick did not offer an obvious explanation of how the information might be stored. DNA was constructed from nucleotides containing only four possible bases (A, G, C, and T). The big question was: how do you code for all of the traits of an organism using only a four letter alphabet?
Recall the central dogma of molecular biology. The information stored in DNA is ultimately transferred to protein, which is what gives cells and tissues their particular properties. Proteins are linear chains of amino acids, and there are 20 amino acids found in proteins. So the real question becomes: how does a four letter alphabet code for all possible combinations of 20 amino acids?
By constructing multi-letter "words" out of the four letters in the alphabet, it is possible to code for all of the amino acids. Specifically, it is possible to make 64 different three letter words from just the four letters of the genetic alphabet, which covers the 20 amino acids easily. This kind of reasoning led to the proposal of a triplet genetic code.
Experiments involving in vitro translation of short synthetic RNAs eventually confirmed that the genetic code is indeed a triplet code. The three-letter "words" of the genetic code are known as codons. This experimental approach was also used to work out the relationship between individual codons and the various amino acids. After this "cracking" of the genetic code, several properties of the genetic code became apparent:
Some of these properties will be examined in more detail.
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