Stages of Translation

three binding sites for tRNA

A site, P Site, E Site

A site

holds tRNA that carries the next amino acid to be added

P Site (peptidyl site)

holds tRNA that carries growing polypeptide chain

E Site (exit site)

where discharged tRNAs leave the ribosome

to begin the assembly of the initiation complex:

ribosomal sequence promotes binding of mRNA to small ribosomal subunit

16S rRNA sequence

complementary to the ribosomal binding sequence; hydrogen bonds with mRNA to attach small ribosomal subunit

Inititator tRNA

AUG; start codon is usually few nucleotides downstream

when do the small and large ribosomal subunits associate with each other?

ONLY when they are on the mRNA partaking in translation

similarities in initiation between prokaryotes and eukaryotes

• binding of small ribosomal subunit, then methionyl tRNA, followed by large subunit

• translation begins at AUG start codon

differences in initiation between prokaryotes and eukaryotes

• protein synthesis initiated with unmodified methionine residue

• small ribosomal subunit bind 7-methylguanosine 5’ cap

• requires many non-ribosomal proteins (translation factors)

codon recognition (elongation)

charged tRNA binds to A site

peptide bond formation (elongation)

formed between amino acid in A site and growing polypeptide

peptidyl transfer reaction

polypeptide removed from tRNA in P site and transferred to amino acid in A site by peptidyl transferasetransla

translocation

once peptidyl transfer reaction is complete, the ribosome moved toward 3’ end of the mRNA by ONE codon

where does the P site tRNA move to in translaocation?

E site; uncharged tRNA exits

where does the tRNA in the A site move dto in translocation?

P site; A site now unoccupied, allows next tRNA to enter

elongation steps

1. codon recognition/tRNA entry

2. peptide bond formation

3. translocation

small ribosomal subunit function

“decodes” triplets present on mRNA

Large ribosomal sununit function

peptide bond synthesis

termination

stop codons recognized by release factor proteins

where does the release factor bind to?

A site, where the release factor binds to stop codon

what happens to the bond between the tRNA and polypeptide

it’s hydrolyzed, releasing the tRNA and polypeptide from the ribosome

final step in termination

mRNA, small and large ribosomal sunbunits, and release factors dissociate

polysome

single mRNA strand with multiple ribosomes translating it