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Need to Know: 5’ capping, intron splicing, 3’polyadenylation

Why is RNA processed?

1. First Step 5’ Capping

2. Intron splicing

3. 3’ Polydentation (adding a lot of A’s )

Why?-

1. Reason 1 is the 5’ cap and 3’ tail provides signals to export

2. Both the 5’ cap and 3’ poly a tail protect mRNA from being degraded (stability)

3. Add strictures important for translation of mRNA

4. Why remove the introns? - Removing introns makes sure they are not translated and joins exons together to create a continuous CDS (coding sequence)

5. The reason they happen in this order is because they are linked to the process of transcription

RNA Processing:

5’ Capping

• Addition of 5’ 7- methylguanosine cap that functions on stabi;ity and ribosomes binding during translation

• The cap is literally a methylated guanine nucleotide that is added to the 5’ end

• Capping enzyme is recruited tot he CTD tail of RNA pol II and will cleave some phosphates and is added invertedly

  • The next step is it will be methylated by RNA methyltransferase which will add a methyl group to the added nucleotide and one or more bases after it

  • This functions to add stability, acts as a signal to leave the nucleus, and can facilitate intron splicing

Intron Splicing

• Introns are removed by a process called splicing carried out byt the Spliceosomes

  • recognizes the exon/intron boundaries

  • Catalyze splicing- cutting out introns and joining exons

  • Splicesome- a large flexible complex consisting of proteinf and RNAs that finction in the nucleus of the cells to excises introns from the pre-MRNA

  • Spliceosomes consists of small nuclear RNAs: contain sequence that are antiparallel and complimentary to the sequence patterns ion the pre-mRNA or to the sequences in other RNAs

  • The Spliceosome can recognize consensus sequences or mtifs in the DNA sequence at the boundaries of introns and exons and withing introns

  • Cleave RNA at 5’ site and form lariat

Modification of the 3’ End

• Addition of a ploy (A) tail by cleave and polyadentylation

• Recognizes the 3’ end and cut at cleavage site

• Add Poly-a-tail to the 3’ end

• terminate transcription

• ‘Polyadenylation factors’

  • recognize and binf to the 3’ end

  • Position 3’ end correctly to promote cleavage

  • 3’ end of RNA is cut off at clevage site

• CPSF: cleavage and polyadenylation specific factor is still bound
  to the AAUAAA.

• PAP: Polyadenylate polymerase- Binds near CPSF at the 3’
  end of the transcript and adds adenine nucleotides to the 3’ end.

• Poly-A-Tail: export, translation, and stability (protection form degradation)

Terminate Transcription

• Cleaving the RNA to add the poly-a-tail breaks the RNA and the mRNA is released. The other piece is bound to the RNA polymerase.

  • This triggers breakdown of the bound RNA by Torpedo RNAse
    and termination of transcription in eukaryotes.