Genetic code and

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Genetic code and protein synthesis
Lodish Chapter 4.4
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Ribosomal RNA is transcribed and processed in
the nucleolus
Pol III transcript
Pol I transcript processed by small
nucleolar ribonucleoproteins (snoRNPs)
S (Svedberg unit) sedimentation coefficient
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All aminoacyl-tRNA synthetases have proofreading
activity
if product doesnt fit correctly then
proofreading activity reverses bond
acceptor stem
enzyme tRNA binding highly dependent on tertiary
structure of tRNA
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3 letter code allows for 3 different reading
frames
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64 possible codons, 61 of which encode for 20
amino acids and 3 for Stops
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Some exceptions to the rule
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The base pair in the 3rd codon position can
wobble
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60 40
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Initiation factors (eIFs) regulate ribosome
assembly
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Polyadenylation mostly in nucleus, but can
also occur in cytoplasm
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Selection of the initiating AUG is determined by
neighboring nucleotides
Shine-Delgarno
In eukaryotes ACCAUGG (Kozak sequence) or
internal ribosomal entry sites (IRES-typical in
viruses with long 5 untranslated regions)

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Crystal structure of prokaryotic ribosome reveals
three distinct tRNA binding sites A, P and E
Switches in rRNA interactions promote conformation
al changes
peptide bond formation requires contact between 23
S rRNA and tRNA acceptor stem (can be carried
out w/o proteins in vitro)
RNA-driven machine
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Elongation factors direct tRNAs to the A site
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Release factors recognize STOP codons
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frameshift mutations
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What you need to know Ribosome assembly rRNAs
are processed by snoRNPs in the nucleolus Subunit
composition of ribosomes in prokaryotic/
eukaryotic cells Universal code 64 codons, 3rd
position is wobble position. Definition of codon
and anticodon. tRNAs are activated by aminoacyl
tRNA synthetases, which have proof- reading
activity (only 1 in 50,000 is wrong) First AUG is
located at a defined distance to the
Shine-Delgarno sequence, which is a ribosomal
docking site. Translation initiation 5cap binds
eIF4, which in turn recruits 30S/40S subunit to
mRNA. Then first tRNA binds, and then the 50S/60S
subunit binds. First tRNA is always tRNA-MET.
First AUG is positioned over P site 3 tRNA
binding sites A (aminoacyl), P (peptide bond)
and E (exit) Elongation requires elongation
factors, termination requires release factors.
Release factors bind to STOP codons. Antibiotics
inhibit aminoacyl-tRNA binding, tRNA
transposition or cause frameshift mutations and
premature chain termination.