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Chapter 12: Problem 17
Suppose that in the use of polynucleotide phosphorylase, nucleotides A and C are added in a ratio of 1A:5C. What is the probability that an AAA sequence will occur?
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In this chapter, we focused on the genetic code and the transcription of genetic information stored in DNA into complementary RNA molecules. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions: (a) How did we determine the compositions of codons encoding specific amino acids? (b) How were the specific sequences of triplet codes determined experimentally? (c) How were the experimentally derived triplet codon assignments verified in studies using bacteriophage MS2? (d) How do we know that mRNA exists and serves as an intermediate between information encoded in DNA and its concomitant gene product? (e) How do we know that the initial transcript of a eukaryotic gene contains noncoding sequences that must be removed before accurate translation into proteins can occur?
Alternative splicing is a common mechanism for eukaryotes to expand their repertoire of gene functions. Studies by Xu and colleagues (2002. Nuc. Acids Res. 30: 3754-3766) indicate that approximately 50 percent of human genes use alternative splicing, and approximately 15 percent of disease-causing mutations involve aberrant alternative splicing. Different tissues show remarkably different frequencies of alternative splicing, with the brain accounting for approximately 18 percent of such events. (a) Define alternative splicing and speculate on the evolutionary strategy alternative splicing offers to organisms. (b) Why might some tissues engage in more alternative splicing than others?
An alanine residue exists at position 180 of a certain plant protein. If the codon specifying alanine is GCU, how many singlebase substitutions will result in an amino acid substitution at position \(180,\) and what are they?
Illustrating the importance of triphosphate and monophosphate molecules, explain the process of RNA biosynthesis by RNA polymerase.
One form of postranscriptional modification of most eukaryotic RNA transcripts 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 prokaryotic 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 the general functions of \(3^{\prime}\) -polyadenylation?
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