Question

Describe the biochemical mechanism by which RNA splicing occurs precisely at the exon-intron borders considering that a single nucleotide offset by a mistake in splicing will reset the open reading frame register for the encoded protein.

Answer 1

Splicing needs to be precise and consistent. It involves careful cutting and pasting specific parts of the pre-mRNA (introns ans exons).

Introns are recognized are recognized and removed by the spliceosome, an enzyme complex made of protein and small RNAs.

Introns have a GU (guanine, uracil) nucleotide sequence at the 5´end splice site, and an AG (adenine, guanine) at the 3´end splice site. The 3´ site can also have a polypyrimidine tract (RPT) which recruits factors to the 3´splice site and the first adenosine required for the first step of splicing.

These are the borders recognized by the spliceosome which executes a two step transesterification reactions between RNA nucleotides.

After the first cutting step, the 3´ end of the released 5´exon is joined to the next one releasing the intron.

During translation, the mRNA sequence is read in groups of three nucleotides, thats why the RNA splicing has to occur precisely at the exon-intron borders as described in order to avoid resetting the open reading frame.