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

• http://www.csun.edu/~cmalone/pdf360/Ch13-2RNAprocess.pdf

• http://www.uky.edu/Classes/BCH/401/daanotes/lec10/daal10.html

• AP Biology Notes• Images: Google