The Genetic Code

The genetic code (Figure 27.3) is specified in discrete units of three nucleotides called codons. One codon precisely follows another. The simple, unpunctuated code is found throughout all known prokaryotic cells. Genes in eukaryotic cells are a bit more complex. Some primitive eukaryotic genes have single codon overlaps that are removed by a process called editing (see here). Other eukaryotic genes have sequences in immature RNA that are removed by a process called splicing. Nonetheless, at the time of translation, all editing and splicing is complete, so that the translated code is completely unpunctuated, as shown in Figure 27.1.

The genetic code (also called "the code") is almost, but not quite, universal. That is, it is almost, but not quite, the same in prokaryotes, eukaryotes and viruses. The rare exceptions are known of only in mitochondria and a few protozoans. One alternate coding sequence is the UGA codon, which in the universal genetic code is a STOP codon (i.e., it stops translation, see below), but in many organisms can occasionally specify the rare amino acid, selenocysteine (Table 27.1).

In general, each amino acid is specified by the first two bases of a codon. For example, all four proline codons begin with CC and all four valine codons begin with BU. Thus, redundancy in the genetic code is usually expressed in the third letter. Furthermore, a single tRNA may recognize several different codons for the same amino acid. For this reason, the third base is referred to as the "wobble" position, because it exhibits looser rules for pairing with the anticodon of the tRNA (Table 27.2, Figure 27.4). Part of the wobble can be explained by the presence of the uncommon nucleoside, inosine (Figure 22.4), which is found in a number of anticodons and can pair with A, U, or C.

The mRNA is almost always longer than the message that is to be translated. Thus, specific start and stop signals are needed to start and stop translation.

The codons UAA, UAG, and UGA are called STOP codons because they cause the ribosome to stop translation when it encounters one of them (in almost all cases).

The sequence AUG, which specifies the amino acid methionine, is commonly used to define a translational starting point, but AUG is also found in the middle of coding sequences. Thus the translational starting point must have other sequences that help to define where to begin. In addition, prokaryotic organisms use a modified form of methionine (N-formylmethionine) to start translation. Eukaryotic cells simply use methionine. Once translation has begun, AUG is read simply as methionine in the internal part of a gene in all organisms. Rare start codons include GUG, UUG, and AUU, which also code for valine, leucine, and isoleucine, respectively. When they are used as the first codon, however, N-formylmethionine is still incorporated as the first amino acid.

See also: Structure of Prokaryotic mRNAs, Translation Overview, Structure of tRNAs, Genetic Code (from Chapter 5)

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