More On The Genetic Code

Genes encode proteins. They do this by carrying information in their sequences. Genes are made up of DNA. This is made up of various combinations of 4 different nucleotides (Adenine (A) Guanine (G) Thymidine (T) and Cytosine (C)). It takes three of these nucleotides (called a codon) to code for one amino acid (proteins are covalently linked amino acid chains). There are two complementary strands of a DNA molecule, and Adenine binds Thymidine on its complementary strand and Guanine binds Cytosine on the complementary strand. Only one strand codes for the protein, and when a sequence is written, it is the coding strand that is presented.

DNA is copied (transcribed) into an RNA message (called messenger RNA – or mRNA) Where the DNA contains a G, the mRNA contains a C; where it contains a C, the mRNA contains a G; where there is a T, the mRNA contains an A. It is basically the same pairings as in DNA – this is how the information is transmitted. The only difference is that RNA does not contain T; another nucleotide called Uracil (U) takes its place. So, where the DNA contains a A, the mRNA contains a U. So, this three letter codon ATG, which encodes the amino acid Methionine, has the complementary DNA sequence TAC on the complementary strand. The mRNA that is transcribed would read AUG (basically identical to the coding strand, but with U replacing T). Transcription is illustrated below, with the complementary DNA strands are in Blue and the RNA is in orange. The top DNA strand is the coding one here and AGC is transcribed into AGC etc . The other DNA strand organises the growing mRNA molecule and is called the template strand. To make protein (translation), this mRNA binds to a different type of RNA called transfer RNA (tRNA). There are different types of tRNA that bind to different amino acids. They are recruited in the right order by the mRNA trough the same binding rules as before. So, the mRNA sequence that encodes the amino acid Methionine (AUG) will recognise the sequence UAC on the Methionine bound tRNA. In the figure below, the leucine and Alanine specific tRNAs are shown, containing the sequences GAU and CGC respectively. These bind to CUA and CGC on the mRNA, which are encoded by the codons CTA and CGC in the DNA.

Different amino acids are encoded by different codons, as shown below. You will also notice that most amino acids have more than one possible codon; some have up to six. this means that there will be one a tRNA for each codon.
So, Lets take a closer look at this short sequence from the previous post. It is only 42 amino acids long (average proteins contain over 400 amino acids!)

MCEEEDSTALVCDNGSGLCKAGFAGDDAPRAVFPSIVGRPRHQG

Here is the amino acid breakdown and the number of possible codons that can be used in the above sequence..
So, the number of possible codon strings that could produce the above sequence is (4 x 5 A) x (2 x 3 C) x (2 x 4 D) x…….... (4 x 3 V) = 19 813 556 551 680. Yet human and chimp usage are identical ! Factor in the human and chimp genomes being 98 % identical and the number of possible codon usages genome wide becomes enormous – someone with more time than me can work out that one! Therefore, there is no reason why human and chimp sequences sould be identical if they were designed. They are however identical, suggesting evolutionary relationships.

Another possible place a designer could leave a signature is in the actual genetic code. There is no reason why ATG should have to encode methionine in every species (or indeed why any codon should encode a particular amino acid).

Importantly, it is a different part of the transfer RNA that recognised the mRNA that binds its specific amino acid. It would therefore be possible to mess about and make ATG encode Methionine in one species, but have it encode any other amino acid in another species. Yet, the genetic code is the same across species. Again, this is where evidence of design could be inserted. None is found, so it would appear that if there was a designer, he is unable to write his own name, or just does not want to be found. I'll go with there not being one.