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Chapter 12: Gene experssion - Coggle Diagram
Chapter 12: Gene experssion
Three stages of transcription
2-Elongation
3-Termination
1-Initiation
1- Initiation
Requires help of ribosomal initiation factors
Also requires input of energy (GTP
hydrolysis)
mRNA, first tRNA and ribosomal subunits
assemble
Initiation in bacteria:
Initiator tRNA recognizes start codon in mRNA.
Start codon is a few nucleotides downstream.
mRNA binds to small ribosomal subunit facilitated
by ribosomal-binding sequence.
Large ribosomal subunit associates.
At the end, the initiator tRNA is in the P site.
Two eukaryotic differences in initiation:
Position of start codon more variable.
Instead of a ribosomal-binding sequence, mRNAs
have guanosine cap at 5’ end.
2- Elongation:
2- Peptide bond is formed between the amino acid at the site A and the growing polypeptide chain
Called the peptidyl transfer reaction.
rRNA catalyzes peptide bond formation (the
ribosome is a ribozyme).
The polypeptide is removed from the tRNA in the P site and transferred to the amino acid at the A site
3- Movement or translocation of the ribosome toward the 3’ end of the mRNA by one codon.
The next codon is now at the A spot.
Uncharged tRNA exits from E spot.
Shifts tRNAs at the P and A sites to the E and P sites.
1- Aminoacyl tRNA brings a new amino acid to the A site.
Elongation factors hydrolyze GTP to provide energy to bind tRNA to A site.
Peptidyl tRNA is in the P site.
Binding occurs due to codon / anticodon recognition.
Aminoacyl tRNA is in the A site.
3- Termination
3 stop codons – UAA, UAG, UGA.
Recognized by release factors.
When a stop codon is found in the A site,
translation ends.
Completed polypeptide is attached to a tRNA
in the P site and stop codon in the A site.
Bond between polypeptide and tRNA hydrolyzed to release polypeptide.
Ribosomal subunits and release factors dissociate.
Release factor binds to stop codon at the A site.