Biology, Eighth Edition (Raven)

Chapter 15: Genes and How They Work

Translation Termination

Why does mRNA need a stop codon? When a ribosome reaches the codon UAG, UAA, or UGA on an mRNA strand then protein synthesis is terminated. A protein known as a release factor binds to the ribosome and adds a water molecule to the end of the amino acid chain. This reaction hydrolyzes the polypeptide chain from the tRNA, allowing the protein to exit the ribosome. Without the stop codon, protein synthesis would simply stop without releasing the completed protein.

View the animation below, then complete the quiz to test your knowledge of the concept.


Translation is terminated when a stop codon is presented at the
A)A site.
B)P site.
C)E site.
D)either the A or P site.
E)either the A or E site.

The protein that promotes translation termination is called
C)release factor.
D)doomsday factor.
E)X factor.

Which of the following statements about tRNA is TRUE?
A)No tRNA can recognize a stop codon.
B)Only one tRNA can recognize a stop codon.
C)Some but not all tRNAs can recognize a stop codon.
D)All tRNAs can recognize a stop codon, but they do so rarely.
E)All tRNAs can recognize a stop codon, and they do so all the time.

The completed polypeptide is released from the tRNA that is in the E site.

Translation termination concludes with the dissociation of the two ribosomal subunits and mRNA from each other.
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