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Study of RNA processing learned which three important facts
- Eukaryotic genes interrupted by substantial amounts of "extra" DNA (introns) of unknown function
- RNA can act as an enzyme
- RNA can be used as a template to generate DNA "reverse transfer of genetic information from RNA to DNA.
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Final RNA transcripts are ____ than the DNA template, this is because of ____
shorter, splicing
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Splicing removes ____, while preserving ____
introns, exons
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Splicing facts
- -Typical gene has coding (exon) and non-coding (intron) sections.
- -Introns compose 90% of DNA
- -Intronic DNA is removed by splicing
- -Splicing catalyzed by ribonucleoprotein complexes called spliceosomes
- -Some RNA is self-splicing, these are RNA's that can act as enzymes (ribozymes)
- -Splicing takes place in two steps: 5' releases, then attached to 3'.
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Splicing seen far more in
Eukaryotes then prokaryotes
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Splicing can produce different ___ from same ___
- Proteins, gene
- Mix and match order of certain parts of the gene
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4 classes of introns
- 1. Spliceosomal introns
- 2&3. Type I/II: self-splicing
- 4. tRNA introns
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Bacteria lack which class of intron?
spliceosomal
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Steps of splicing:
- 1. Spliceosome assembled
- -Made of snRNP (small ribonucleoproteins)
- 2. Spliceosome activated
- 3. Introns spliced
- 4. exons joined
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Spliceosome composed of RNA and proteins
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Spliceosome RNA base pairs with RNA transcript at:
intron-exon boundaries
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Chemistry of splicing
- 3. OH on intron attacks phosphodiester bond on end of exon 1 (first transesterification)
- 4. OH now at end of exon 1 attacks phosphodiester bond between intron and exon 2
- 5. exons now connected by a phosphodiester bond. Intron is now an excised lariat intron.
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Self-splicing introns
- Type I:
- Rely on exogenous guanosine
- G docks to 5' end of intron (transesterification)
- New OH on 5' end attacks 3' end of exon (transesterification)
- Type II: First attack comes from OH on adenosine in intron
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Differences between Type I/II introns:
- Type I: Relies of exogenous guanine to begin first attack
- Type II: Relies on internal adenosine to begin first attack.
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rRNA/tRNA usually synthesized as:
Extened transcripts that are cut at precise locations to generate functional molecules
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Nuclease that cleaves tRNA out
RnaseR (ribonucleoprotein) (RNA is catalytic subunit).
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Eukaryotic mRNA 5'/3' ends are modified:
Post-transcriptionally
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mRNA 5' end modification
- Eukaryotic mRNAs have 7-methyl guanosine "cap" added to the 5' end.
- Cap aids in forming the translation initiation complex and protects the 5' end from degradation.
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5' end caps added by
- RNA Pol II
- (Capping enzyme (CE), RNA methyltransferase (RNMT), protein involved in translation (elF4E)
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3' ends of mRNA are:
- cleaved and polyadenylated (polyA tails)
- -signals nuclear export
- -stabilizes mRNA
- -promotes translation
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Reverse flow of genetic info:
- Retroviruses
- Carry RNA "genes", transfer genetic material to DNA after infection
- Reverse transcriptase generates double-standard DNA from RNA
- Reverse transcriptase also found in uninfected cells
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Discovered reverse transcriptase for nobel prize
Howard Temin
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Reverse transcriptase is a:
DNA polymerase
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Telomerase:
- keeps chromosome tips from degrading
- Is a reverse transcriptase, RNA catalytic core
- Uses RNA base to add back the bits of telomere that are removed with each replication
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