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04. RNA splicing
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75% dos genes humanos sofrem splicing
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Modelo geral de splicing
DNA
hnRNA
mRNA
Nucleo
Citoplasma
53
5
3 5
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Tipos de processamento (splicing)
-Nuclear(eucariotas)Sequencias consenso intro/exo (GT.....AG)
snRNPs (small nuclear Ribonuclear Particles) U1.....U6
-Self splicingMitocondria em algumas bacterias
Grupo I -> Guanosina como factor (thermophila)
Grupo II -> sequencia especfica (GT....AG) Intro/Exo
(I e II possuem estruturas secundrias muito complexas)
(RNA funciona como uma emzima Ribozyma)
-Enzimaticaligase de splicing (S.cerevisiae, Xenopus)
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O splisossoma utiliza os RNAs dos snRNPs para orientar
a reao
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Fungos filamentosos
Mitocondria
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Exon skipping or cassette exon: in this case, anexon may be spliced out of the primary transcript
or retained. This is the most common mode inmammalian pre-mRNAs.
Mutually exclusive exons: One of two exons isretained in mRNAs after splicing, but not both.
Alternative donor site: An alternative 5' splice
junction (donor site) is used, changing the 3'
boundary of the upstream exon.Alternative acceptor site: An alternative 3' splice
junction (acceptor site) is used, changing the 5'
boundary of the downstream exon.
Intron retention: A sequence may be spliced out asan intron or simply retained. This is distinguished
from exon skipping because the retained sequence
is not flanked by introns. If the retained intron is in
the coding region, the intron must encode aminoacids in frame with the neighboring exons, or a
stop codon or a shift in the reading frame will
cause the protein to be non-functional. This is the
rarest mode in mammals.
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Splicing Alternativo
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Splicing occurs as transcription takes place
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Alternative splicing: how to get more proteins out ofyour genome
30,000 human genes probably give more than 100,000 different proteins
21 3 4 calcitonin 5 CGRP 6
5 3
21 3 4 calcitonin
21 3 4 calcitonin 5 CGRP 6
21 3 4 calcitonin
21 3 5 CGRP 6
AAAA
AAAA
DNA
NuclearTranscripts
mRNAs
ProteinsThyroid
Neurons
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Down Syndrome Cell Adhesion Molecule
Axon guidance in the CNS
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Alternative splicing of the -tropomyosin gene from rat. -tropomyosin is a coiled-coil proteinthat regulates contraction in muscle cells. The primary transcript can be spliced in different
ways, as indicated in the figure, to produce distinct mRNAs, which then give rise to variant
proteins. Some of the splicing patterns are specific for certain types of cells. For example, the
-tropomyosin made in striated muscle is different from that made from the same gene in
smooth muscle. The arrowheads in the top part of the figure demark the sites where cleavageand poly-A addition can occur.
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Escolha do local de processamento
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Splicing Alternativo (fast and slow Pol II)
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RNA editing
EucariotasVirus
tRNAs
rRNAs
mRNAs
miRNAs
No RNA editing in prokaryotes
mRNA - Can alter AA sequence, splice site
RNA degradation change from Adenosine to Ionosine leads to RNA degration
RNA structure different protein binding site
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RNA editing
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Editing in trypanosomes
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Mechanism of A-to-I RNA editing in mammals. The position of an edit issignaled by RNA sequences carried on the same RNA molecule. Typically, a
sequence complementary to the position of the edit is present in an intron,and the resulting double-stranded RNA attracts the A-to-I editing enzyme
ADAR. Mice and humans have three ADAR enzymes: ADR1 is required in theliver for proper red blood cell development, ADR2 is required for proper
brain development and the role of ADR3 is not yet known.
Adenine:Inosine
adenosine deaminases acting on RNA (Purine Biosynthesis)
Inosine:citosine
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Apolipoprotein B