Post Transcriptional Modification in Prokaryotes
o rRNA and tRNA go through posttranscriptional processing.
o Almost all mRNA is directly translated to protein.
Post Transcriptional Modification in Eukaryotes
o Each type of RNA undergoes posttranscriptional processing.
o Posttranscriptional processing allows for additional gene regulation.
o The initial mRNA nucleotide sequence arrived at through transcription is called the primary transcript (also called pre-mRNA, or heterogeneous nuclear RNA [hnRNA]).
Type of Post Transcriptional Modification
The primary transcript is processed in three (3) ways:
1) addition of nucleotides
2) deletion of nucleotides
3) modification of nitrogenous bases
o Even before the eukaryotic mRNA is completely transcribed, its 5’ end is capped in a process using GTP.
o The 5’ cap serves as an attachment site in protein synthesis and as a protection against degradation by exonucleases.
o The 3’ end is polyadenylated with a poly A tail, also to protect it from exonucleases.
o The primary transcript is much longer than the mRNA that will be translated into a protein.
o This is due to noncoding regions existing in the primary transcript.
o These regions are called introns.
o Introns = noncoding regions of DNA in a gene; generally much longer than exons; Introns stay IN nucleus
o Exons = coding regions of DNA in a gene; Exons EXIT nucleus
o Enzyme-RNA complexes called small nuclear ribonucleoproteins (snRNPs “snurps”) recognize nucleotide sequences at the ends of the introns.
o Several snRNP’s associate with proteins to form a complex called a spliceosome.
o Inside the spliceosome, the introns are looped bringing the exons together.
o The introns are then excised by the spliceosome and the exons are spliced together.
o The exons of some genes may be spliced together in different order allowing them to encode for different polypeptides.