Translation Occurs in Three Phases

initiator tRNAtRNA carrying the first amino acid in the polypeptide
translation initiation in eukaryotes and bacteriasmall ribosomal subunit binds near the 5 end of mRNA and identifies start codon sequence
final stage of initiationlarge subunit joins small subunit to forma intact ribosome and translation begins

second stage of initiationinitiator tRNA binds to start codon
charged tRNAtRNAs used during translation that carry a specific amino acid
uncharged tRNAtRNA without an amino acid
initiator factor proteinshelp control ribosome formation and binding of the initiator tRNA
E. coli molecular components for initiation1. mRNA
2. small ribosomal subunit
3. large ribosomal subunit
4 initiator tRNA
5. 3 essential initiation factor proteins
6. GTP
initiation factor for translational initiation in bacteriaIF3. affiliated with 30s ribosomal subunit. prevents the 30s subunit from binding to the 50s subunit. small subunit IF3 complex binds near the 5 end of mRNA, searching for AUG sequence that serves as start codon (Shine-Delgarno sequence)
preinitiation complexforms when the authentic start codon is identified by base pairing that occurs between the 16srRNA in the 30s ribosome and a short mRNA sequence

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located a few nucleotides upstream of the start codon in the 5 UTR of mRNA
Shine-Delgarno Sequenceconsensus sequence that is purine-rich located 3-6 nucleotides upstream of the start codon. complementary sequence found near 3 end of 16s rRNA and pairs with this sequence to to position mRNA on 30s subunit
step 2 bacterial translation initiationinitiator tRNA binds to start codon at what will be P site after ribosome assembly.
amino acid on initiator tRNAmodified methionine is called N-formylmethionine (fMet).
end of step 2 bacterial translation initiation30s initiation complex, tRNA fMet located at start codon, 3 initiation factors, and a molecule of GTP formed
final step of bacterial initiation50s subunit joins 30s subunit t oform intact ribosomes. IF1, 2, 3 disassociate and create 70s initation complex.
preinitation complex eukaryotic translationeukaryotic 40s ribosomal subunit complexes with eukaryotic initiation factor proteins and with charged tRNAmet
step 1 eukaryotic translation initiationpreinitiation complex recruited to 5-cap reigion of mRNA
step 2 eukaryotic translation initiationpreinitiation complex joins eIF4 complex. this makes up the initiation complex
eIF4 complexgroup of at least 4 eIF4 proteins that assembles at the 5 cap independently of translational initiation
step 3 eukaryotic translation initiationscanning: uses ATP hydrolysis to move the small ribosomal subunit through the 5 UTR in search of start codon.
Kozak sequencespecific consensus sequence of eukaryotic mRNA that contains the authentic start codon (AUG) sequence
step 4 eukaryotic translation initiation60s complex recruited to complex. 80s ribosome formed. dissociation of the eIF proteins.
elongation factors facilitate three steps of polypeptide synthesis how:1. recruitment of charged tRNAs to A site
2. formation of peptide bond between sequential amino acids
3. translocation of ribosome in 3 direction
step 1 bacterial/eukaryotic translation elongationtRNA recruitment: accompanied by elongation factor protein, charged tRNA enters A site
step 2 bacterial/eukaryotic translation elongationPeptide bond formation: peptidyl transferase catalyzes formation of a peptide bond between the amino acids in the P and A sites. peptide chain moves to the A site
step 3 bacterial/eukaryotic translation elongationTranslocation: elongation factors translocate ribosome; uncharged tRNA released to the E site and a new tRNA is recruited to A site
when does elongation cycle stopone of 3 stop codons enters A site of ribosome. no tRNAs with anticodons complementary to stop codons.
Release factorsused by bacteria and eukaryotics. bind to a stop codon in the A site. polypeptide bound to tRNA at P site released by hydrolysis of GTP, which is complexed with RF.
bacteria release factors2 release factors (RF1 and RF2) recognize stop codons.
eukaryotic release factorssingle release factor called eRF that recognizes all 3 stop codons
three phases of translationinitiation
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