Question

Compare and contrast pre-mRNA splicing with Group 1 and Group 2 intron self-splicing.

Answer 1

Transcription is the process of mRNA synthesis by using DNA as a template. In eukaryotes, the mRNA synthesized undergoes further processing to generate the mature mRNA which is used for translation. These post-transcriptional modifications include capping, spicing and tailing.

The precursor mRNA contains sequences which code for proteins (exons) and also sequences which do not encode for proteins (introns). Splicing involves the removal of introns and joining together the exons.

There are 3 classes of RNA splicing -

Nuclear pre-mRNA splicing - involves spliceosome machinery.

Class II introns - these are rare. Present in protein coding genes and some rRNA and tRNA genes in mitochondria and chloroplasts of plants and fungi. Mechanism involves 2 transesterification reaction. Branch site is residue is A. these fold into a secondary structure to excise themselves. Involves an RNA enzyme (Ribozyme).

Class I introns - are similar to class II introns. These are also rare and involve ribozyme. Mechanism involves 2 transesterification reactions. Present in rRNA genes of protozoans.

However, the maiin difference between class I and II introns is that, class I introns have a branch site residue G.