Chapter_06_Solutions - Chapter 6 How Cells Read the Genome...

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DEFINITIONS 6–1 General transcription factor 6–2 snRNA (small nuclear RNA) 6–3 Promoter 6–4 Exosome 6–5 rRNA (ribosomal RNA) 6–6 mRNA (messenger RNA) 6–7 RNA splicing 6–8 Terminator 6–9 Trans-splicing 6–10 Exon 6–11 Nuclear pore complex TRUE/FALSE 6–12 True. Errors in DNA replication have the potential to affect future genera- tions of cells, while errors in transcription have no genetic consequence. Errors in transcription lead to mistakes in a small fraction of RNAs, whose functions are further monitored by downstream quality-control mecha- nisms. The essential feature is that errors in DNA replication change the gene and, thereby, affect all the copies of RNA (and protein) made in the original cell and all its progeny cells. By contrast, errors in transcription are limited to a small number of defective RNAs (and proteins), and are not passed on to progeny cells. These considerations are reflected in the intrinsic error rates for RNA and DNA polymerases: RNA polymerases typically make 1 mistake in copying 10 4 nucleotides, while DNA polymerases make about 1 error per 10 7 nucleotides. Such significant differences in error rates suggest that natural selection is stronger against errors in replication than against errors in tran- scription. 6–13 False. The s subunit associates with the bacterial RNA polymerase core enzyme to form the RNA polymerase holoenzyme only during the initiation phase of RNA synthesis. The s subunit helps the core enzyme bind to the promoter and stays associated with the core enzyme until RNA synthesis has been properly initiated, and then it dissociates. FROM DNA TO RNA In This Chapter FROM DNA TO RNA A121 FROM RNA TO A132 PROTEIN THE RNA WORLD AND A146 THE ORIGINS OF LIFE Chapter 6 A121 6 How Cells Read the Genome: From DNA to Protein
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6–14 True. At its 3 ¢ end each eucaryotic mRNA has a string of adenine nucleotides, the last of which has a terminal ribose with a free 3 ¢ -OH group. At its 5 ¢ end each mRNA carries a 7-methylguanosine that is linked 5 ¢ to 5 ¢ with the first nucleotide in the mRNA. This linkage leaves a free 3 ¢ -OH group on the ribose of the capping nucleotide. 6–15 False. Although intron sequences are mostly dispensable, they must be removed precisely. An error of even one nucleotide during removal would shift the reading frame in the spliced mRNA molecule and produce an aber- rant protein. 6–16 False. The 3 ¢ ends of most pre-mRNA transcripts produced by RNA poly- merase II are defined, not by the termination point of transcription, but by cleavage of the RNA chain 10–30 nucleotides downstream of the sequence AAUAAA and 30 or so nucleotides upstream of a GU- or U-rich sequence. THOUGHT PROBLEMS 6–17 The answer is best given by Francis Crick himself, who coined the terms ‘the sequence hypothesis,’ which proposes that genetic information is encoded in the sequence of the DNA bases, and ‘the central dogma,’ which states that DNA makes RNA makes protein, in 1957.
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