Genetic code andprotein synthesis

Lodish Chapter 4.4

Pol I transcriptprocessed bysmall nucleolarribonucleoproteins(snoRNPs)

Pol III transcript

S (Svedberg unit): sedimentation coefficient

Ribosomal RNAis transcribed and processed in thenucleolus

All aminoacyl-tRNA synthetases have proofreading activity

enzyme tRNA binding highly dependenton tertiary structure of tRNA

if product doesn’t fit correctlythen proofreading activity

reverses bondacceptor stem

3 letter code allows for 3 different reading frames

64 possible codons, 61 of which encode for 20 amino acids and 3 for Stops

Some exceptions to the rule

The base pair in the 3rd codon position can “wobble”

60% 40%

Initiation factors (eIFs) regulate ribosome assembly

Polyadenylation mostlyin nucleus, but can alsooccur in cytoplasm

Selection of the initiating AUG is determined by neighboring nucleotides

In eukaryotes: ACCAUGG (Kozak sequence)or internal ribosomal entry sites (IRES-typical in viruses with long5’ untranslated regions)

Shine-Delgarno

Crystal structure of prokaryotic ribosome reveals threedistinct tRNA binding sites: A, P and E

Switches in rRNAinteractionspromoteconformationalchanges

RNA-drivenmachine

peptide bondformationrequirescontactbetween23S rRNAand tRNAacceptor stem(can be carried outw/o proteins in vitro)

Elongation factors directtRNAs to the A site

Release factors recognizeSTOP codons

frameshift mutations

What you need to know:Ribosome assembly: rRNAs are processed by snoRNPs in the nucleolusSubunit composition of ribosomes in prokaryotic/ eukaryotic cellsUniversal 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:5’cap 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 site3 tRNA binding sites: A (aminoacyl), P (peptide bond) and E (exit)Elongation requires elongation factors, termination requires releasefactors. Release factors bind to STOP codons.Antibiotics inhibit aminoacyl-tRNA binding, tRNA transposition or cause frameshift mutations and premature chain termination.