Overlapping genetic code

The primary goal of peptide mapping is the verification of the amino acid sequence deduced from the genetic code of the recombinant protein. The protein backbone gets cleaved by typically two or three different endoproteinases like Lys-C, trypsin, and Glu-C to achieve maps with sequence-overlapping peptide fragments. These peptide mixtures can then be separated by LC or CE and analyzed on-line by MS to obtain sequence information. Often simple mass analysis matches the predicted primary sequence of the protein. However, sometimes mutations can lead to isobaric masses of peptides that can be overseen, if no further sequence analysis like N-terminal Edman sequencing and MS/MS is carried out. 

FIGURE 27-3 Overlapping versus nonoverlapping genetic codes. In... 

The fact that there are four saline-based chromophores and the experimental evidence that they appear in at least two distinct but overlapping sets of three should be important in inferring the actual relationship between the genes and the chromophores. Comparing the genetic code for the common chromophores between the ultraviolet sensitive members of Arthropoda who are believed to lack a L-channel chromophore and the members of Mollusca and Chordata who do exhibit L-channel sensitivity should prove beneficial. Since most members of Chordata are known to be tetrachromatic, focusing on the presence or absence of the ultraviolet chromophore may not be as fruitful. Recent data has confirmed that the system architecture of the human system is tetrachromatic. Our vision in the ultraviolet is limited primarily by the absorption of our lens group. 

Note the presence of chain initiation and termination codons, respectively, at the beginning and end of the sequence. In translating this sequence, which would occur within a longer mRNA molecule, into a sequence of amino acids, it has been assumed that the codons do not overlap. This has been established experimentally, and for this reason, the triplet genetic code is said to be nonoverlapping. 

FIGURE 12.2 Theoretically possible genetic codes, (a) An overlapping versus a nonoverlapping code, (b) A continuous versus a punctuated code. 

Apart from mitochondria, the genetic code appears to be universal, a particular codon specifies the same amino acid in all species. It is also non-overlapping-, triplets are read sequentially along the mRNA in a 5 —>3 direction, starting at a specific point known as the initiation codon. 

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