Transcription And Processing Part 2

Question 1

mRNA capping

Answer 1

• 7-methylguanosine cap added to 5’ end of pre-mRNA after transcription of first 20-30 bases
• GTP added in reverse orientation to 5’ end
• methyl groups added to G and the riboses of one or two of the next bases
• stabilizes it and aligns it with ribosome

Question 2

mRNA tailing

Answer 2

• poly A tail added to 3’ end called polyadenylation
• upstream is AAUAAA signal, downstream is GU rich
• endonuclease cleaves chain and poly-A-polymerase adds poly A tail of 200 bases
• these enzymes attracted to phosphorylated CTD of RNA polymerase

Question 3

Splicing first step

Answer 3

Cleavage at specific 5’ splice site, leaving exon with free 3’ end
-5’ end of intron joined to 2’ hydroxyl of an adenine near 3’ end of intron, forms a lariat

Question 4

Splicing second step

Answer 4

Simultaneous cleavage of 3’ site, releasing lariat shaped intron
-3’ end of exon joined to 5’ end of next exon

Question 5

Spliceosomes

Answer 5

RNA and protein complex where splicing takes place.

-5 types of snRNAs (U1, U2, U4, U5, U6) complexed with snRNPs

Question 6

RNA degradation

Answer 6

• over 90% of pre mRNA is introns and needs degraded
• Lariats targeted by enzymes recognizing the 2’-5’ phosphodiester bonds
• 3’ and 5’ naked ends targeted
• premature stop codons may trigger nonsense mediated mRNA decay
• rate represents control mechanism for protein expression

Question 7

Shorter half lives

Answer 7

Regulatory proteins that rapidly respond

Question 8

Longer half lives

Answer 8

Structural proteins