102009-BCH311

102009-BCH311 - Major ways transcription is regulated (only...

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Fig. 10-1, p.241 Crick’s Central Dogma, Part II(b): Post-transcriptional processing/RNA processing In prokaryotes transcription and translation are coupled: mRNA is translated as soon as it is transcribed In eukaryotes, prior to translation, extensive processing of the mRNA takes place In eukaryotes, the mRNA is produced in the nucleus and translated in the cytoplasm
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Fig. 9-20a, p.232 Transcription and Translation are coupled in prokaryotes In prokaryotes transcription and translation are coupled: mRNA is translated as soon as it is transcribed This is due in part to the absence of compartmentalization in prokaryotes - no defined nucleus, no nuclear membrane
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Fig. 9-20b, p.232 Extensive RNA processing occurs in eukaryotes
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Eukaryotic RNA processing When RNA is first produced in eukaryotes it is referred to as hnRNA ( heterogeneous nuclear RNA ) or pre-mRNA Modifications of the hnRNA include capping of the 5’ end, polyadenylation of the 3’ end, and splicing of the coding sequences (exons) Capping The cap attached to the 5’ end of eukaryotic hnRNA is a guanylate residue that is methylated at the N-7 position This modified guanylate residue is attached to its neighboring residue by a 5’-5’ triphosphate linkage The 2’ hydroxyl of the β - D -ribose sugar is also frequently methylated
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Eukaryotic RNA processing Polyadenylation A polyadenylate tail (poly-A) approximately 100 - 200 nucleotides long is added to the 3’ end of the hnRNA It is thought that the presence of the poly-A tail protects the mRNA from nucleolytic cleavage (exo) and degradation The poly-A tail also signals the mRNA for transport out of the nucleus for translation An enzyme called polyadenylate polymerase (PAP) synthesizes the poly-A tail
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Eukaryotic RNA processing Splicing Prokaryotic genes are continuous - every base pair in a continuous prokaryotic gene is reflected in the base sequence of the mRNA Eukaryotic genes, however, are often discontinuous, and contain intervening sequences called introns that are non-coding The DNA sequences that are expressed (are retained in the final mRNA and translated into protein) are called exons (coding) During the processing of eukaryotic hnRNA these introns must be removed
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Messenger RNA Least abundant, ~5-10% total cellular RNA Sequences of the bases specify the order of amino acids in proteins The sequence of mRNA bases (that encodes for the amino acids in a protein) reflects the sequence of DNA bases in the gene mRNAs are heterogeneous in length, e.g. the BRCA1 hnRNA precursor molecule is 81,154 bp ( or 81 kb ) while the FANCF hnRNA precursor molecule is only 3,308 bp ( or 3.3 kb ) This heterogeneity in length is dependent on the size of the protein and the number of exons BRCA1 is a 23 exon gene while is a single exon gene
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Eukaryotic RNA processing: Splicing
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102009-BCH311 - Major ways transcription is regulated (only...

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