Coding for Proteins- How much DNA do we need?

Just to remind ourselves of the facts, there are 20 different amino acids (21 if you include taurine) that go to make up proteins, and 4 different bases in DNA that make up the genetic code. How many bases do we need to code for each amino acid?

If we used just one base, we could code for 4 different amino acids, which isn't enough, with 2 bases we can code for 4 X 4 = 16 amino acids, which still isn't enough, and with 3 bases we can code for 4 X 4 X 4 =64 amino acids, which is enough with lots to spare.

In fact, we do use just three bases (called a codon) to write the code in our DNA for making proteins. But what happens with all the spare bits of code?

Well there are three 'Stop' codons, that tell the machine that makes the protein in your body that it has got to the end of this protein. You might think that there would be a start codon too, but actually the start codon is used for coding for an amino acid, so all proteins start with the same amino acid (Methionine). All the remaining extra codons are used to code for amino acids, so you get a mixture with some amino acids only having one codon, and some having lots of codons. Sadly there are no good resources for learning more about this on the web, but any molecular biology or biochemistry textbook should be able to help you.

Why do you think this happens?

One explanation is called the Silent Mutation theory.

Mutations occur randomly in DNA, but if one occurs in a codon it might affect the protein that is produced. However if there are lots of different ways for coding for some amino acids, then less of these random mutations of DNA will produce an observable effect in the protein. This is called a silent mutation, because you can't tell it has happened until you look at the DNA.

Some mutations have very little effect, because the protein that they make is not critical to your life, for example the protein that makes hair could be changed to give you thicker or thinner hair, straighter or more curly hair, darker or lighter hair. Although that might be important to how you look, it doesn't make a difference to whether you live or die. On the other hand some proteins are absolutely critical, and actually occur in just about all life on Earth. Mutations of the genes that make these can be fatal, but at least with all these extra codons there is a chance that the mutation will not kill you.

In fact, scientists use these critical genes to look at how closely related different species are to each other. Because mutations can occur by chance, and only those that are silent survive you can look at two different sequences for the same gene and count how many different mutations there are in it. Once you know how often mutations occur, you can quite quickly work out how long ago the two genes were the same, and so you can see how closely related two species are. The sorts of diagrams that this produces are called Phylogenetic trees, and the image to the left is one showing how closely related different varieties of a plant called Arabadopsis are.

There is a more complicated theory called (not very elegantly) the Wobble Theory. Scientists noticed that it is usually the last base in the codon that changes to allow the extra codons. They believe that the tRNA is 'lazy' and only looks at the first two bases carefully and will pretend it matches with the third base, (hence the wobble, it makes a wobbly join) and this speeds up the process of making the protein.

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