Biology 1002B Chapter 13.3: Cycle 8 - Processing of mRNA
3 May 2018
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mRNA contain regions that do not specify an AA
• 5' UTR and 3' UTR
13.3a Eukaryotic Protein-Coding Genes are Transcribed into pre-mRNA
• Pre-mRNA must be processed in nucleus to produce translatable mRNA
o 5' end of pre-mRNA is 5' guanine cap
• Consists of guanine nucleotide that is revered so -OH group faces beginning
• Capping enzyme adds 5' cap without need of complementary base pairing
soon after RNA polymerase begins transcription
• Cap protects mRNA from degradation and is the site where ribosomes attach
at start of translation
o 3' end of pre-mRNA has poly-A tail
• Near the 3' end is a sequence of polyadenylation signal
• Proteins bind to this signal in pre-mRNA and cleave it just downstream,
signalling RNA polymerase to stop transcription
• Enzyme poly-A polymerase adds 50-250 A-nucleotides (no complementary
base pairing)
• Poly-A tail enables pre-mRNA to be translated efficiently and protects mRNA
from enzymes
• Sequences interrupting the protein-coding sequence
o Introns interrupt protein-coding sequence and are removed from pre-mRNA
o Introns discovered by comparing nucleotide sequences of mature mRNA and either
pre-mRNA or genes encoding them
o Introns are on average 6x the length of exons
13.3b Introns are Removed During pre-mRNA Processing
• mRNA splicing: process that removes introns from pre-mRNA and joins exons in nucleus
• Spliceosome: complex formed between pre-mRNA and small ribonucleoprotein particles
(complex of RNA and proteins)
• Spliceosome has small nuclear RNA (snRNA) which are the active part of the spliceosome
• Complementary base pairing between snRNA and mRNA ensures that cutting is exact and
precise
13.3c Introns Contribute to Protein Variability
• Introns increase coding capacity of existing genes by alternative splicing and exon shuffling
• Alternative splicing
o Mechanism that joins exons in different combinations to produce different mRNAs
from a single gene
o Regions that are exon in one situation may be removed as intron in another
situation
o As a result, number of diverse proteins far exceed number of genes
o It is the diversity of proteins that determines relative complexity of an organism
o One gene may specify a number of polypeptides, each of which has a related
function
• Exon shuffling
o Existing protein domains are mixed into novel combinations to create new proteins
o This produces new proteins with novel functions MUCH more quickly than changes
in individual nucleotides at random points
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Mrna contain regions that do not specify an aa: 5" utr and 3" utr. Introns interrupt protein-coding sequence and are removed from pre-mrna. Introns discovered by comparing nucleotide sequences of mature mrna and either pre-mrna or genes encoding them. Introns are on average 6x the length of exons. 13. 3b introns are removed during pre-mrna processing: mrna splicing: process that removes introns from pre-mrna and joins exons in nucleus. Spliceosome: complex formed between pre-mrna and small ribonucleoprotein particles (complex of rna and proteins) Spliceosome has small nuclear rna (snrna) which are the active part of the spliceosome: complementary base pairing between snrna and mrna ensures that cutting is exact and precise. Translation: assembly of aa into polypeptides on ribosomes. In eukaryotes, translation takes place in cytoplasm, mitochondria, chloroplast: polypeptide is assembled from n-terminal end to c-terminal end. Same trna anticodon can read codons with u or c in third position: g can pair with u.
