RNA Processing By: Kelvin Liu, Jeff Wu, Alex Eishingdrelo

Purpose The purpose of RNA processing is to prepare RNA for translation

Background mRNAs have three main parts: - 5’ untranslated region - Exons (coding sequence) - 3’ untranslated region

Prokaryotes

Does not occur in prokaryotic cells because they do not have a nuclei Protein synthesis goes happens before transcription is even complete, no processing necessary!

Eukaryotes

Steps in Process -Addition of 7-methyl guanosine cap at the 5’ end -Addition of a poly-A tail at the 3’ end -Removal of introns (RNA splicing) -Transport to the cytoplasm

5’ Cap -The first step of RNA processing -Occurs during transcription -A Guanine group is added to the 5’ end of triphosphate (ATP) linkage -This end is methylated to create 7-Methylguanosine Triphosphate using the enzyme Methyltransferase -Its purpose is to protect the mRNA against degradation from nucleases and act as a point of attachment in rRNA

Poly-A Tail - The second step of RNA processing -After transcription, the 3’ end of a mRNA sequence is cleaved by Ribonuclease downstream from a specific AAUAAA site - The enzyme poly-A polymerase adds a few hundred adenine nucleotides at the 3’ end - It’s purpose is to protect the mRNA sequence from enzymatic degradation and help the mRNA sequence move through the nuclear envelope

RNA Splicing - A strand of DNA contains exons and introns – Exons are the coding sequences – Introns are noncoding sequences – Only the coding sequence is translated - The introns are removed from the mRNA sequences by spliceosomes with subunits called small nuclear ribonucleoproteins, or snRNPs.

RNA Splicing Mechanism Splice donor (GU) – Start, 5’ Splice acceptor (AG) – End, 3’ Branch site (A) – Close to the end

Alternative Splicing Splicing allows the cutting and pasting of different combinations of exons together Each exon can code for part of more than one protein Introns can sometimes change to exons and vice versa

Prokaryotic Regulation Remember that prokaryotes do not use RNA processing They can regulate specific protein production by synthesizing a strand of RNA with multiple coding regions for multiple proteins – Clustered genes called an operon

Relation to other processes - The processing of adding the 5’ Cap, Poly-A Tail, and splicing of RNA occurs in the nucleus. After this process is complete, the RNA strand is ready for translation - RNA Processing is the step in between transcription and translation that allows protein synthesis to occur.

Sources Ch13-2RNAprocess.pdf Ch13-2RNAprocess.pdf aanotes/lec10/daal10.html aanotes/lec10/daal10.html AP Biology Notes Images: Google