W6 mRNA Processing in Eukaryotes Flashcards
Eukaryotic RNA polymerases:
RNA Polymerase 1:
-Synthesises ribosomal RNA
RNA polymerase 2:
-Transcribes the template to form a pre-mRNA and also synthesises microRNAs
RNA polymerase 3:
-Synthesises tRNAs and small nuclear RNAs
Promoter region: TATA box
-Consensus sequence: 5’-TATAA-3’
-Almost always found at -30 position
-Mutations do not affect level of transcription but change initiation point
-Important to specify start point of transcription
Promoter region: CAAT box
-Consensus sequence: 5’-GGCCAATCT-3’
-Found at -75 position in many genes (can operate in many locations)
-Mutations of this sequence cause a big reduction in transcription rate
-May act in initiation of transcription
Promoter region: GC box
-Consensus sequence: 5’-GGGCCG-3’
-Often found around -90 position
-Often more than one copy
-May operate in either orientation
-May help to bind RNA polymerase
-Often found in promoters of constitutively expressed genes
Enhancer regions:
-Enhancers are required for maximal transcription rate
-Operate in either orientation
-No consensus sequence
-May act as binding sites for regulatory proteins
-May interact with proteins at promoter region
Adding a 5’ cap:
-Guanine plus ~2 methyl groups added to mRNA (occurs soon after start of transcription)
5’-5’ link (This G nucleotide is in the reverse orientation to all other nucleotides)
-Capping enzyme involved (Guanylyl transferase)
-Required for functionality (All eukryotic mRNAs are capped - protects from degradation
Addition of poly(A) tail
-200 adenines (A) added at 3’ end of eukaryotic mRNAs (not coded by the DNA)
-Carried out by poly(A) polymerase
-Poly(A) recognition signal sequence (5’-AAAUAAA-3’)
-Strand cleavage and rejoining
-Functional signifance for mRNA processing (stabilises mRNA against degradation)
Introns and Exons:
-Introns are intervening sequences, initially transcribed, but not found within the functional mRNA in the cytoplasm
-Exons are sections fo the mRNA that code for amino acids
