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RNA, transcription and processing
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A summary of the lectures which describe the properties or RNA, how DNA is transcribed and the processing of mRNA to mature mRNA.
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The central dogma tells us that How does DNA and RNA differ? The respective structures ofthere is a flow of the genetic DNA RNA DNA and RNA suit their
information held by DNA to RNA Double stranded Single strand (but functions. DNA, with its more
and then to proteins. can form double stable deoxyribose, needs to be
stranded areas
accurate for long-term storage
Double helix Forms a variety of
RNA – there are various types of RNA as any mutations will affect all
shapes
e.g mRNA and tRNA all with varying More stable Less stable due to 2’ progeny cells. DNA also can’t
functions. mRNA is important as it hydroxy group contain uracil as it wouldn’t all
acts as the messenger, directing the Function is to store Many varying for identification of uracil bases
production of proteins. the genetic code functions
which arose through cytosine
Long-term storage Transient
The transcriptome is the set of all deamination (a mutation)
Bases: A,C,G,T Bases: A,C,G,U
RNA molecules. As RNA is so variable, Deoxyribose ribose RNA on the other hand needs to
this can change from cell to cell but Likely predates DNA be transient to allow for
also in the same cell depending on its distinction between cells over a
developmental stage. temporal gradient. If it didn’t
degrade protein production of a
Translation synthesises mRNA. cell wouldn’t end. Its accuracy
For this to take place we need the isn’t as important as it will only
enzyme RNA polymerase, DNA affect the protein it encodes for
locally single stranded (available
template strand), NTPs and the loss
RNA, transcription and there are many e.g mRNAs
per protein molecule.
of a pyrophosphate. Note that RNA
is synthesised 5’ to 3’. DNA is locally and processing In eukaryotic cells, mRNA needs
unwound and fed through RNA to be processed and marked
polymerase (part of the before it can leave the nucleus
Alternative splicing can also take
transcription initiation complex). for the ribosomes in the
place in eukaryotic cells (not
The DNA template strand in the cytoplasm.
prokaryotes which don’t have
groove has complementary The 5’ end gets ‘capped’ with an
introns). This is when more than
nucleosides (which entered from atypical nucleotide and bond e.g
one protein isoform is expressed
another groove) base pair with it, an atypical guanine. This protects
from a single gene.
forming phosphodiester bonds this end from exonuclease
between the bases releasing a degradation. As pre-mRNA
pyrophosphate. This forms mRNA. contains introns, these are then
The released pyrophosphates This allows roughly 90% of human
spliced out by the spliceosome at
provide the energy to drive this genes to produce multiple mRNa
the 5’ and 3’ splice site – the
process and hence protein variant. So
intron forms a bond at its branch
around 500,000 proteins can be
point using the 2’ hydroxyl group
formed from around 20-30
and forms a lariat. When the 3’
thousand protein-coding genes.
end of the exon binds to the 3’
Splicing is necessary as introns are
splice site this lariat is released.
non-coding. If they were included
Once the introns are removed,
in the final mRNA they would
the 3’ end of then gets a tail of
produce the wrong/ a disfunctional
poly-A-nucleotides
protein.
(polyadenylation). It is now ready
Splicing of the 5’ site usually occurs
for exportation.
at a G,U and for a 3’ splice site, an
This mature mRNA is then
A,G. (with an adenine at the branch
selectively exported from the
point).
nucleus. This is done through
mRNAs are eventually degraded by their markers, 5’ cap, poly-A-
the cell. They will have different binding proteins and nuclear
similarities based on their transport receptors, which
sequences. It is these stabilities identify them for export through
which determine how many times a nuclear pores via an active
mRNA is used to translate a protein process. The nuclear transport
and hance the amount of protein receptor is then returned to the
produced. This can be show through nucleus.
their half-life e.g 10 hours or 30 mins
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