Figure 1.

The protist Paramecium has short introns in which some contain stop codons. Introns in multiples of threes are more likely to contain a stop codon as a fail-safe measure for correct splicing. (a) The failure in removal of an intron that is not a multiple of three will cause a frameshift and this will most likely introduce an out of place stop codon in the resulting mRNA. This transcript can then be degraded by nonsense mediated decay (NMD). (b) When an intron that is a multiple of three long is not removed, it will not cause a frameshift and therefore the mis-spliced transcript will not be degraded. (c) This can be overcome by having stop codons in introns of multiple of three. Therefore when the intron is not removed, NMD can act on the incorrectly placed stop codon and remove the transcript.