There are many ways that a cell can 2) RNA editing 3) Control of mRNA stability
control gene expression: Post-transcriptional editing of mRNA mRNAs can be translated may
1) Alternative splicing – single pre- further increases the number of times to its stability determines
mRNAs produce multiple products distinct proteins that can be how much protein is produced.
depending on where the introns are encoded in a genome. This is UTR sequences can be important
spliced out (not that the order of common in parasites like Leishmania in controlling mRNA lifetimes &
exon remains the same although but can occur in other animals. how well they are translated. E.g.
some may be missed) In RNA editing, mature mRNA iron regulation in mammals.
This can be specific to the tissue sequences don’t correspond to the At low iron levels, Iron Response
depending on the protein the tissue gene sequence. Can involve: Protein (IRP) binds to the Iron
requires and its function. addition or deletion or uridine to Receptor Element (IRE) in the 3’
E.g. Drosphila melanogaster DSCAM mRNA, deamination of adenine to UTR for the transferrin receptor
pre-mRNA has 95 alternative exons produce inosine (A-to-I editing), protein (involved in iron uptake).
and only one exon from each block deamination of cytosine to produce This causes translation of this
is processed into the final product, uracil (C-to-U editing). protein and iron uptake. In high
giving 38,016 isoforms. DSCAM in Occurs in around 1000 human iron levels, the iron binds to IRPs,
the NS is needed for wiring of genes. E.g. Apo-lipoprotein B protein preventing them binding to IREs on
neurones. In the immune system it product has 4563 codons in the liver mRNA. mRNA is degraded from
mediated phagocytosis. but 2152 in the intestine, as the RNA the 3’ end, no translation, no iron
for the liver has C-to-U editing which uptake.
causes a stop codon to be produced, Similar for ferritin, which prevents
giving a shorter protein. iron from getting to toxic levels. In
low levels IRP binds to IRE on
ferritin mRNA in 5’ UTR, so no
Active Regulation: Post-transcription ferritin produced.
Repressors binds over splice sites
preventing them being spliced. modification microRNA gene silencing
Activators bind on introns close to
splice sites -> attract splicing RNAi gene silencing miRNA regulate expression of specific
machinery. 1) DsRNA is processed & binds mRNA & degradation of specific mRNAs.
to an argonaut or piwi protein & Transcribed by RNA pol.II & form hairpin
RNAi becomes single stranded structures of around 80 nucleotides.
Double-stranded RNAs can control 2) ssRNA binds to target mRNA 1) 5’ & 3’ ends cut off in nucleus
mRNA stability & translation. & forms a complex which causes 2) miRNA transported to cytosol where
miRNAs, siRNAs and piRNAs all either: they are diced (loop removed)
regulate gene expression. Cleavage of target RNA (no 3) Interact with argonaut protein to get
RNAi was discovered in C.elegans translation) ; Translational ssRNA. Complex is called RISC (RNA
in 1998 by Andrew Fire and Craig repression and eventual induced silencing complex)
Mello when injecting dsRNA into it degradation of target RNA ; 4) RISC binds to a complementary
reduced expression of specific formation of heterochromatin mRNA to give one of 2 effects:
genes through reducing levels of on DNA from which the target - Extensive match: silencing &
mRNA. RNA is being transcribed. degradation
RNAi is sequence-specific. It (depends on match between - less extensive match: rapid
regulated resistance to target RNA and ssRNA) translational repression & eventual
endogenous parasitic nucleic acids degradation of mRNA
(transposable elements) and Importance
exogenous pathogenic nucleic miRNAs – shown to be key regulators of
acids (dsRNA viruses), and gene expression in many biological
regulates expression of protein- processes
coding genes. RNAi is an experimental tool of specific gene
‘knockdown’ in organisms through e.g.
intake or injection. It’s applicable to many
eukaryotes with an RNAi pathway and has
potential applied and clinical uses in
biotechnology and medicine. E.g by knocking
down a pathway that causes disease.
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