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Another question now follows: How do we read the mRNA into protein? This is achieved in the next process called translation.

In translation, a new set of molecular apparatus binds to the mRNA to read the code into protein. A central part of this machinery is yet another type of RNA that is folded into a ribosome (sometimes called ribosomal ribonucleic acid or rRNA). The ribosome binds to the mRNA strand. The ribosome provides a scaffold on which other pieces of RNA called transfer ribonucleic acid (tRNA) can bind. tRNAs are the molecules that bring in the amino acids to allow for the assembly of proteins. They can be considered to be adaptor molecules that bind to the mRNA and bring amino acids into alignment to add to a growing polypeptide chain.

Let's go through this process in a simplified way and consider what is happening. You can follow this text and cross-refer to Figure 5.10. We have an mRNA strand, just transcribed from DNA. Around this strand the ribosome forms, made up of subunits which provide the assembly point around which protein synthesis occurs. The ribosome is an impressive structure. In bacteria, it is made up of a large subunit of RNA, consisting of two folded strands of RNA and no fewer than 31 specialized proteins. There is also a small subunit, made up of one folded piece of RNA, called the 16S rRNA, together with another 21 proteins. Remember the 16S rRNA, as it will become important in Chapter 8 when we explore the diversity of life on Earth. This whole apparatus is now ready to make protein.

Figure 5.10 The translation of the genetic code. The protein synthesis apparatus around the mRNA.

The ribosome provides a protected environment in which the tRNA can bind. The tRNA has an amino acid at one end, and each amino acid has its own tRNA molecule associated with it. At the other end is an anticodon. The anticodon is a three-letter sequence of bases that matches three bases on the mRNA molecule (called the codon). Thus, each codon or triplet code on the mRNA corresponds to a specific amino acid (Figure 5.11).

Figure 5.11 The structure of t-RNA. The amino acid is attached at the top of the molecule. The diagram also illustrates the codon-anticodon binding occurring at the bottom of the molecule.

Source: Reproduced with permission of John Wiley & Sons, Ltd.

If we think about this quantitatively, we can immediately see that each codon can code for 64 combinations. There are three positions in each codon, and we already know that at each of these positions there are four possible bases: G, C, A, and U. This gives us 4 × 4 × 4 = 64 possible combinations for each three-letter codon. But we already saw in the previous chapter that there are generally only 20 amino acids used by life. As a result, there is redundancy. Each amino acid can be coded for by more than one codon. We call this the “degeneracy of the genetic code.” Figure 5.12 shows the mRNA codons and their corresponding amino acids. If you look at this table, you can see that each amino acid generally has more than one codon. Apart from the amino acid tryptophan (which just has one codon), all other amino acids have at least two, and many of them have four codons.

Figure 5.12 The table of codons of mRNA corresponding to amino acids. The amino acids are shown with their three-letter designation (see Appendix A6). Note the degeneracy of the code. Apart from small variations in different species, this table is universal across life, one line of evidence that all known life on Earth derives from a common origin.