Ch. 14 - Translation and Proteins

Klug - Concepts of Genetics 12th Edition

Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook

Klug 12th Edition

Ch. 14 - Translation and Proteins

Problem 11

Chapter 14, Problem 11

When a codon in an mRNA with the sequence 5'-UAA-3' enters the A site of a ribosome, it is not recognized by a tRNA with a complementary anticodon. Why not? What recognizes it instead?

Verified step by step guidance

Understand the role of codons and anticodons: Codons are sequences of three nucleotides in mRNA that specify amino acids during translation. Anticodons are complementary sequences on tRNA molecules that pair with codons to deliver the correct amino acid.

Identify the codon in question: The codon 5'-UAA-3' is one of the three stop codons (UAA, UAG, UGA) in the genetic code. Stop codons do not code for any amino acid.

Explain why tRNA does not recognize stop codons: Stop codons are not recognized by tRNA because there are no tRNAs with anticodons complementary to stop codons. This ensures that translation stops at the appropriate point.

Describe what recognizes stop codons: Instead of tRNA, stop codons are recognized by proteins called release factors. These release factors bind to the stop codon in the A site of the ribosome.

Explain the role of release factors: Release factors trigger the hydrolysis of the bond between the polypeptide chain and the tRNA in the P site, releasing the completed polypeptide and terminating translation.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Codons and Anticodons

Codons are sequences of three nucleotides in mRNA that specify an amino acid or signal termination during protein synthesis. Each codon is matched by a complementary anticodon found on tRNA molecules, which carry specific amino acids. In the case of the UAA codon, it is a stop codon, meaning it does not have a corresponding tRNA with an anticodon, as it signals the termination of translation.

Stop Codons

Stop codons are specific codons in mRNA that signal the end of protein synthesis. The three stop codons—UAA, UAG, and UGA—do not code for any amino acids and are recognized by release factors instead of tRNA. When a ribosome encounters a stop codon in the A site, it triggers the release of the newly synthesized polypeptide chain from the ribosome.

Release Factors

Release factors are proteins that recognize stop codons during translation. When a ribosome reaches a stop codon, such as UAA, a release factor binds to the A site, prompting the ribosome to release the completed polypeptide chain. This process is crucial for terminating protein synthesis and ensuring that proteins are properly formed and released.

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Related Practice

Textbook Question

What are isoaccepting tRNAs? Assuming that there are only 20 different aminoacyl tRNA synthetases but 31 different tRNAs, speculate on parameters that might be used to ensure that each charged tRNA has received the correct amino acid.

Textbook Question

Individuals with phenylketonuria cannot convert phenylalanine to tyrosine. Why don't these individuals exhibit a deficiency of tyrosine?

Textbook Question

Summarize the steps involved in charging tRNAs with their appropriate amino acids.

Textbook Question

Early detection and adherence to a strict dietary regimen have prevented much of the intellectual disability that used to occur in those with phenylketonuria (PKU). Affected individuals now often lead normal lives and have families. For various reasons, such individuals tend to adhere less rigorously to their diet as they get older. Predict the effect that mothers with PKU who neglect their diets might have on newborns.

Textbook Question

To carry out its role, each transfer RNA requires at least four specific recognition sites that must be inherent in its tertiary structure. What are they?

Textbook Question

Discuss the potential difficulties of designing a diet to alleviate the symptoms of phenylketonuria.

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