The triplet code: frame-shift experiments

Twenty amino acids make all the proteins in living organisms.

§ If a code consisted of one base for one amino acid, only four combinations would be provided (there are four bases).

§ If two bases coded for one amino acid there would be 16 (4²) possible combinations.

§ A three-base (triplet) code provides 64 (4³) possible combinations, more than enough for all 20 amino acids.

Francis Crick and his co-workers confirmed that the genetic code is a triplet code. Using enzymes, they added or deleted nucleotide bases in the DNA of a

virus that infects bacteria. They found that when one or two bases were added or deleted, the viruses were unable to infect the bacteria. But when three bases were added or deleted, the virus was able to infect the bacteria. They concluded that adding or removing one or two bases caused a frame shift which inactivated the gene. However, adding or removing three bases only partially affected the gene. Thus the sequence of bases shown above would contain the following sequences of DNA base triplets and mRNA codons:

DNA base triplet sequence: TAG GCT TGA TCG

mRNA codon sequence: AUC CGA ACU AGC

If one base (for example, guanine) is added to the DNA the frame shifts and the sequence of triplets and codons is changed:

DNA base triplet sequence: G TA GGT TTG АТС G

mRNA codon sequence: C AU CCG AAC UAG С

The results of the frame-shift experiments also showed that the code is non-overlapping:

§ Each triplet in DNA specifies one amino acid.

§ Each base is part of only one triplet, and is therefore involved in specifying only one amino acid.

A non-overlapping code requires a longer sequence of bases than an overlapping code (see box): however, replacing one base for another has a small or no effect.