The Genetic Code

Overview

This module will examine how information is encoded in DNA, and how that information is interpreted to bring about changes in cells and tissues.

Objectives

1. Understand the triplet nature of the genetic code, and know the meaning of the term codon.
2. Know that the code is degenerate, and what that means.
3. Know that the code is unambiguous, and what that means.
4. Know the identities of the start and stop codons, and understand how they work.

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:

• The genetic code is composed of nucleotide triplets. In other words, three nucleotides in mRNA (a codon) specify one amino acid in a protein.
• The code is non-overlapping. This means that successive triplets are read in order. Each nucleotide is part of only one triplet codon.
• The genetic code is unambiguous. Each codon specifies a particular amino acid, and only one amino acid. In other words, the codon ACG codes for the amino acid threonine, and only threonine.
• The genetic code is degenerate. In contrast, each amino acid can be specified by more than one codon.
• The code is nearly universal. Almost all organisms in nature (from bacteria to humans) use exactly the same genetic code. The rare exceptions include some changes in the code in mitochondria, and in a few protozoan species.

Some of these properties will be examined in more detail.