In the first thread in this series we looked at how the same protein molecule can be assembled by many different sequences of amino acids.
We took the example of cytochrome C and saw that there are many times more possible sequences than atoms in the known universe. However the sequences in humans and chimps are identical, and as we look at species less closely related to us by evolution the more differences we find. This is very compelling evidence for common ancestry.
In this post we are going to look more closely at the DNA code behind those amino acid sequences.
The "language" of DNA is made up of just 4 "letters" - A,C,G and T.
Sequences of letters are read off in groups of 3 called codons.
ACGGCCTCGAATGCCTTC would be read as ACG GCC TCG AAT GCC TTC
If you do the maths you will see that there are 64 different codons or "words" that can be produced by this method.
A codon is comparable to the instructions for making one amino acid. There are a collection of 20 amino acids that living things can choose from to assemble proteins. (Actually a 21st can be assembled from the "stop" codon)
This is where the word redundancy comes in. There are 20 amino acids to be made but many more codons available.
It turns out that codons are often not too fussy about the third letter. If you want to make the amino acid alanine for example the codons GCT, GCC, GCA, or GCG will all do equally well.
So we can now look in more detail at proteins like cytochrome C. We said that the human version and the chimp version have identical sequences of amino acids. We can now ask whether those amino acids were made from identical bits of code.
They differ by 4 letters of code, that's a 1.2% difference.
When geneticists compare different species the very same picture emerges. The more distantly two species are related to each other in evolutionary history the larger the percentage of difference in DNA code that is used to build cytochrome C protein molecules - and yet they all do exactly the same job and can be substituted for each other in the lab.
There is no reasonable explanation for this apart from evolution from a common ancestor.