Genes!

Genes and genetics are an advanced subject but we can introduce some evidence for evolution by taking a brief look at genes. Genes are sections of nucleic acid that code for specific proteins. An organelle in the cell, called the ribosome, translates strings of nucleic acids into proteins by "reading" the code of the nucleic acids sequence and using that information to produce a string of amino acids. The specific sequence of nucleic acids provides the information the ribosome needs to produce the specific amino acid sequence that becomes the protein.

We can isolate the active genes (mRNA) and transfer them to other organisms and other species. (Recombinant genetics technology relies on this fact.) If we transfer the haemoglobin gene from a rabbit to a bacterium the bacterium will manufacture haemoglobin. This shows a remarkable agreement between the gene translation systems of rabbits and bacteria.

The active gene is a sequence of "bases" that the ribosome reads in groups of three at a time. Each group of three bases is called a codon and all twenty amino acids have at least one codon. The three bases designated AUG cause a methionine (a specific amino acid) to be incorporated at that position in the protein. The same is true of other codons and their specific amino acids. There are also three "stop" codons that cause the string of amino acids to come to an end.

Why should the code be universal? Why should AUG always be matched with methionine? There are two possibilities.

One possibility is that the there is a specific chemical binding of methionine to the chemical mechanism responsible for the transfer at the ribosome but this does not appear to be the case. Experiments (requiring knowledge beyond the level of genetics appropriate to this course) show that it is possible to change the genetic code. This "artificial code" will cause different codons to be used for amino acids other than those coded by the "universal code". But other than this artificial code produced in laboratory experiments, the genetic code is the same between all species! Why?
What is the other explanation for this remarkable conservation of the genetic code?
"I know!"