Post Transcriptional Modification: Definition, Types, and Mechanism

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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.



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