Download Chapter 14 – RNA molecules and RNA processing

Chapter 14 – RNA molecules
and RNA processing
Gene organization
• Francis Crick – 1958
– Nucleotide sequence of a
gene directly codes for
amino acid sequence of
polypeptide
• Gene contains
interruptions of
nucleotides that do not
code for amino acids
– Eukaryotic genes
Gene organization
• DNA and RNA transcripts
within the nucleus are larger
than transcripts found in the
cytoplasm
– Exons are coding regions
– Introns are intervening
sequences in eukaryotic genes
• Occasionally seen in prokaryotes
as well
• Are spliced out of premRNA/primary transcript before
leaving the nucleus
Messenger RNA (mRNA)
• Structure
– 5′ untranslated region
• Bacteria contain ShineDalgarno sequence
– Serves as ribosome
attachment site
– Protein coding region
• Codes for amino acids
• Codon – 3 nucleotide
sequence that codes for
one amino acid
– 3′ untranslated region
• Aids in stability of molecule
Prokaryotic mRNA
• Since introns are rare, mRNA can begin to
be translated before transcription is
complete
– Ribosome associates with Shine-Dalgarno
sequence, and moves down mRNA molecule
in 5′→3′ direction
Eukaryotic mRNA
• mRNA requires posttranscriptional
modifications before
exiting nucleus to
cytoplasm (site of
translation)
• 5′ cap
– Guanine nucleotide added 5′
to 5′
– Several nucleotides at the 5′
end are methylated
– Stabilizes molecule; aids in
initiation of translation
Post-transcriptional modifications
cont
• polyA tail
– At 3′ end there is at
least one possible
cleavage site where
nucleotides are
removed
• After removal, 50-250
adenine nucleotides
are added
– Polyadenylation
– With associated
proteins, stabilizes
molecule
Post-transcriptional modifications
cont
• RNA splicing
– Splicing out of introns
requires 5′ splice site, 3′
splice site, and branch
point
– Spliceosome
• 5 snRNPs and several
other proteins
– Process of splicing
• 5′ end of intron is cut
and folds back on itself
to attach to branch point
sequence
– Forms a lariat
• 3′ end of intron is cut
and intron is released
• 3′ of exon #1 is ligated to
5′ end of exon #2
• Intron reverts to linear
form and is degraded
Alternative processing
• Approx. 30,000 genes
in genome; 120,000
polypeptides
– A single pre-mRNA
molecule can give rise to
different mature mRNA –
each results in a different
polypeptide
• Alternative splicing
– In addition to intron
removal, exons may be
removed as well
Alternative processing cont
• Multiple 3′
cleavage sites
– Cleavage may
occur at different
sites before polyA
tail is added
– Any exons not
included will yield
a different
polypeptide
Transfer RNA (tRNA)
• 74-95 nucleotides
long
• Can have chemically
modified bases in
addition to the normal
4 normally present in
RNA
tRNA cont
• Complementary base pairs
form a cloverleaf shape
(folds into an “L” 3D)
• 3′ end is the acceptor arm –
where a specific amino acid
attaches
• Anticodon arm contains 3
nucleotides (anticodon) that
recognize codon of mRNA
• Initial transcript contains
introns that are removed
Ribosomal RNA (rRNA)
• Original transcript for ribosomal RNA is cleaved by
snoRNA
– snoRNA + proteins form snoRNPs
• Sizes of rRNA measured in Svedberg untis (S) –
how fast substances sediment out in a centrifugal
field)
– Based on molecular weight and structure – not cumulative
• Ribosomes
– 1 or more molecules of rRNA and approx 50 proteins
– Complete ribosome consists of 2 subunits – large and
small
RNA interference (RNAi)
• miRNA (microRNA) and
siRNA (small interfering
RNA)
• Both arise from doublestranded RNA, which is
cute by enzyme Dicer –
fragments are miRNA and
siRNA
• Small fragments bind to
mRNA
– miRNA inhibits translation
– siRNA – degrades mRNA