Page 136 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

Elongation phase
Leu
Peptidyl
transferase
U A C
fMet
fMet
fMet Phe Phe Phe Leu
E P A E P A G A C E P A

5' U A C A A G 3' 5' U A C A A G 3' 5' A A G G A C 3'
AU G UUU CUG AU G UUU CUG CA A AU G UUU CUG CA A
Ribosome moves
toward 3' end of
mRNA at this step

Direction of ribosome movement

acid of all finished polypeptides prior to protein pro- Polyribosome
cessing. Moreover, the ribosome must move along the Growing N RNA subunits
polypeptide
mRNA in the 5′-to-3′ direction so that the polypeptide Ribosome released
can grow in the N-to-C direction.
Once a ribosome has moved far enough away 3'
from the mRNA’s ribosome binding site, that site be- 5'
AUG
comes accessible to other ribosomes. In fact, several mRNA Stop
ribosomes can work on the same mRNA at one time. A
complex of several ribosomes translating from the 5' – 3' direction of N C
same mRNA is called a polyribosome. This complex ribosome movement Released
allows the simultaneous synthesis of many copies of a polypeptide
polypeptide from a single mRNA.

(c) Termination: The ribosome releases the completed polypeptide No normal tRNAs (except for tRNA Sec ) carry anticodons
complementary to any of the three nonsense (stop) codons UAG, UAA, and UGA. Thus, when a nonsense codon moves into
the ribosome’s A site, no tRNAs can bind to that codon. Instead, proteins called release factors recognize the stop codons
and halt polypeptide synthesis. The tRNA specifying the C-terminal amino acid releases the completed polypeptide, the
same tRNA as well as the mRNA separate from the ribosome, and the ribosome dissociates into its large and small subunits.

Termination phase N
N fMet
Polypeptide Phe Polypeptide
fMet
product
Phe
Leu Leu Large subunit
Gln Gln
Val
Pro Ser Gly Val
E P A Pro
tRNA Ser Gly
U C I Release I C
factor C C
Release
factor
5' C C I 3' 5' 3'
A G U G G U U A G A G U G GU U A G
Termination
codon
Small subunit

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