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Ch. 13 - The Genetic Code and Transcription
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 13 - The Genetic Code and TranscriptionProblem 25
Chapter 13, Problem 25

Substitution RNA editing is known to involve either C-to-U or A-to-I conversions. What common chemical event accounts for each?

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1
Understand that substitution RNA editing involves the conversion of one nucleotide to another, specifically C-to-U (cytidine to uridine) and A-to-I (adenosine to inosine).
Recognize that these conversions are catalyzed by specific enzymes: cytidine deaminase for C-to-U and adenosine deaminase acting on RNA (ADAR) for A-to-I.
Identify the common chemical event in both conversions: deamination, which is the removal of an amino group from the nucleotide base.
In C-to-U conversion, the cytidine deaminase enzyme removes an amino group from cytidine, converting it into uridine.
In A-to-I conversion, the ADAR enzyme removes an amino group from adenosine, converting it into inosine, which is read as guanosine by the cellular machinery.

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Key Concepts

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

RNA Editing

RNA editing is a molecular process through which the nucleotide sequence of an RNA molecule is altered after transcription. This modification can lead to changes in the protein that is produced, affecting its function. Substitution editing, specifically, involves the conversion of one nucleotide to another, such as C-to-U or A-to-I, which can significantly impact gene expression and protein diversity.
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C-to-U Conversion

C-to-U conversion is a specific type of RNA editing where cytidine (C) is deaminated to uridine (U). This reaction is catalyzed by enzymes known as cytidine deaminases. The resulting change can affect the coding potential of the RNA, potentially altering the amino acid sequence of the resulting protein, which can have functional implications in various biological processes.
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A-to-I Conversion

A-to-I conversion refers to the editing process where adenosine (A) is converted to inosine (I) through the action of adenosine deaminases. Inosine is interpreted as guanosine (G) during translation, which can lead to changes in the protein sequence. This type of editing is particularly important in regulating gene expression and can influence the stability and function of RNA molecules.
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Related Practice
Textbook Question

Present an overview of various forms of posttranscriptional RNA processing in eukaryotes. For each, provide an example.

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Textbook Question

One form of posttranscriptional modification of most eukaryotic pre-mRNAs is the addition of a poly-A sequence at the 3' end. The absence of a poly-A sequence leads to rapid degradation of the transcript. Poly-A sequences of various lengths are also added to many bacterial RNA transcripts where, instead of promoting stability, they enhance degradation. In both cases, RNA secondary structures, stabilizing proteins, or degrading enzymes interact with poly-A sequences. Considering the activities of RNAs, what might be general functions of 3'-polyadenylation?

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Textbook Question

Describe the role of two forms of RNA editing that lead to changes in the size and sequence of pre-mRNAs. Briefly describe several examples of each form of editing, including their impact on respective protein products.

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Textbook Question

It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis.

Can you find any support for the doublet code notion in the existing coding dictionary?

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Textbook Question

It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis.

The amino acids Ala, Val, Gly, Asp, and Glu are all early members of biosynthetic pathways and are more evolutionarily conserved than other amino acids. They therefore probably represent 'early' amino acids. Of what significance is this information in terms of the evolution of the genetic code? Also, which base, of the first two within a coding triplet, would likely have been the more significant in originally specifying these amino acids?

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Textbook Question

It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis.

As determined by comparisons of ancient and recently evolved proteins, cysteine, tyrosine, and phenylalanine appear to be late-arriving amino acids. In addition, they are considered to have been absent in the abiotic Earth. All three of these amino acids have only two codons each, while many others, earlier in origin, have more. Is this mere coincidence, or might there be some underlying explanation?

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