RNA processing essay

mRNA processing

For prokaryotes, mRNA is usually transcribed ready to translate. For eukaryotes, mRNA requires some processing. The mRNA is capped at the 5' end of the transcript and is comprised of a 5'-5' pyrophosphate linkage of 7-methylguanosine. This is added early in elongation. There are different types of caps. Cap0 is just 7-methylguanosine. Cap1 is the next base down has a 2' methylation in addition. Cap2 is the next two bases down have a 2' methylation in addition to cap0. The transcript is released by cleavage 10-35 nucleotides dwonstream of the AAUAAA consensus polyadenylation sequence. a poly A tail is added to the end to protect and stabilize the mRNA.

RNA processing is required for the primary transcript to be usable. The primary transcript can either be pre-rRNA, pre-mRNA, or pre-tRNA. SnoRNA and snRNA are also involved in processing.

Introduction to the essay

RNA splicing

Splicing occurs to eliminate unneeded 'junk' sequences called introns that don't encode proteins from the mRNA before translation. The remaining exons are spliced together. Specific base sequences at the exon splice sites specify exact splicing locations to avoid frameshifts. SnRNPs and snRNAs make up the spliceosome that accomplishes this task. The 5' splice site is cleaved first and the 5' end of the intron makes a loop formation called a lariat with the 2' end within the intron. The 3' end site is cleaved and exons joined and the lariat is degraded.

rRNA processing

Ribosomal RNA are comprized of 4 classes in eukaryotes(3 in prokaryotes). They are 28S, 5.8S, 5S, and 18S. 28,5.8,5 are apart of the large subunit designated 60S in eukaryotes(50S in proaryotes) and 18S is apart of the small subunit designated 40S in eukaryotes (30S in prokaryotes). The 28,5.8,and 18S rRNAs are transcribed by RNA polymerase I. These genes are separated by nontranscribed spacers. The transcribed spacers contain half of the pre-rRNA and need to be removed. Methylation groups need to be added to the 2'hydroxyl groups of ribose as well. methylation and removal are done by snoRNAs. rRNA products are released.

tRNA processing

The 5' leader sequence is removed from the pre-tRNA and a CCA sequence is added to their 3' end. 10-15% of the bases are methylated or chemically changed for codon recognition. Sometimes an intron is spliced out. The result is a clover leaf structure.