L13 RNA processing in eukaryotes

What are the main steps in producing a functional mRNA

Addition of 5' cap, addition of 3' polyA tail, RNA splicing, RNA editing

What is the purpose of RNA cleavage

To process rRNA and tRNA precursors to the correct length

How are rRNAs and tRNAs processed in E. coli

Transcribed as one long RNA and cleaved by ribonucleases

What is the 45S rRNA

The long precursor RNA in eukaryotes that is processed into 3 rRNA components

How is the 5' cap added to eukaryotic mRNA

Guanine nucleotide added via a 5'

What additional methylation occurs in higher eukaryotes

2'

Functions of the 5' cap

Protects mRNA from degradation, stimulates translation, assists nuclear export

When does capping occur

After the first 20

Steps of capping

RNA triphosphatase removes 5' phosphate, guanylyl transferase adds GMP, methyltransferase methylates guanine

What is polyadenylation

Addition of a polyA tail of ~200 adenosines to 3' end of mRNA

Where is the mRNA cleaved during polyadenylation

After a CA between AAUAAA and U/GU rich region

Which enzyme adds the polyA tail

Poly(A) polymerase

What is RNA splicing

Removal of introns from pre

Where are introns mainly found

In eukaryotic genes

How are introns removed

By transesterification reactions

Which complex removes introns in metazoa

The spliceosome

What are group I introns

Self

What are group II introns

Self

How are intron boundaries recognised

By 5'

What is a spliceosome composed of

Five snRNPs containing snRNA and proteins

What is the exon junction complex

A complex left at splice junctions marking processed mRNA

Function of EJC

Interacts with export and translation proteins and prevents export of unprocessed RNA

What is alternative splicing

Production of different mRNAs from the same pre

What percentage of human genes undergo alternative splicing

Approximately 90%

How is alternative splicing regulated

Developmentally, organ/tissue specifically, or by other signals

How does RNA pol II coordinate mRNA processing

Its phosphorylated CTD recruits capping, splicing, and polyadenylation enzymes

What is RNA editing

Addition, deletion, or chemical alteration of nucleotides in pre

Example of RNA editing in humans

APOB gene codon 2153 CAA to U changes APOB

What determines RNA stability in eukaryotes

5' cap and 3' polyA tail

What are AREs

AU

What is RNA interference

Small RNAs targeting mRNA for degradation or translation repression

Difference between miRNA and siRNA

miRNA binds imperfectly to repress translation

siRNA binds perfectly to cleave mRNA

What is RISC

RNA

Function of miRISC

Repress translation, promote deadenylation, decapping, and exonuclease degradation

Function of siRISC

Cleaves perfectly complementary target mRNA for degradation

Why is RNA degradation important

Regulates gene expression and mRNA half

Which organism first identified RNAi

C. elegans in 1998

What is the role of Argonaute in RNAi

Binds sRNA duplex, retains guide strand, cleaves target mRNA if perfectly matched

How does polyadenylation influence mRNA stability

Longer polyA tail increases mRNA stability

How does the 5' cap enhance translation

Stimulates initiation and protects from exonucleases

Which step of pre

mRNA processing is linked to transcription elongation

What is the function of lariat formation in group II introns

Helps remove intron and join exons efficiently