Figure 6–92 An RNA molecule that can catalyze its own synthesis. This hypothetical process would require catalysis both of the production of a second RNA strand of complementary nucleotide sequence (not shown) and the use of this second RNA molecule as a template to form many molecules of RNA with the original sequence. The red rays represent the active site of this hypothetical RNA enzyme.
Figure 6–93 The hypothesis that RNA preceded DNA and proteins in evolution.
In the earliest cells, RNA molecules (or their close analogs) would have had combined genetic, structural, and catalytic functions. In present-day cells, DNA is the repository of genetic information, and proteins perform the vast majority of catalytic functions in cells. RNA primarily functions today as a go-between in protein synthesis, although it remains a catalyst for a small number of crucial reactions.
366 Chapter 6:  How Cells Read the Genome: From DNA to Protein in the chemical differences between them. Ribose, like glucose and other sim-WHAT WE DON’T KNOW ple carbohydrates, can be formed from formaldehyde (HCHO), a simple chemical which is readily produced in laboratory experiments that attempt to simulate • How did the present relationships conditions on the primitive Earth. The sugar deoxyribose is harder to make, and in present-day cells it is produced from ribose in a reaction catalyzed by a pro- evolve? How did the genetic code tein enzyme, suggesting that ribose pre-dates deoxyribose in cells. Presumably, originate?
DNA appeared on the scene later, but then proved more suitable than RNA as a permanent repository of genetic information. In particular, the deoxyribose in • The information carried in genomes its sugar-phosphate backbone makes chains of DNA chemically more stable than specifies the sequences of all proteins chains of RNA, so that much greater lengths of DNA can be maintained without and RNA molecules in the cell, breakage. and it determines when and where these molecules are synthesized.
The other differences between RNA and DNA—the double-helical structure of
DNA and the use of thymine rather than uracil—further enhance DNA stability by making the many unavoidable accidents that occur to the molecule much easier discovered?
to repair, as discussed in detail in Chapter 5 (pp. 271–273).
• Cells go to great length to correct mistakes in the processes of DNA replication, transcription, splicing, and translation. Are there analogous
From our knowledge of present-day organisms and the molecules they contain, it strategies to correct mistakes in the seems likely that the development of the distinctive autocatalytic mechanisms fun- selection of which genes are to be damental to living systems began with the evolution of families of RNA molecules expressed in a given cell type? Could that could catalyze their own replication. DNA is likely to have been a late addition: the great complexity of transcription as the accumulation of protein catalysts allowed more efficient and complex cells to initiation in animals and plants reflect evolve, the DNA double helix replaced RNA as a more stable molecule for storing the such a strategy? increased amounts of genetic information required by such cells.
Which statements are true? explain why or why not.
6–1 The consequences of errors in transcription are less severe than those of errors in DNA replication.
6–2 Since introns are largely genetic “junk,” they do not Figure Q6–1 Supercoils around a moving RNA polymerase (Problem have to be removed precisely from the primary transcript 6–6). during RNA splicing.
6–3 Wobble pairing occurs between the first position 6–7 You have attached an RNA polymerase molecule in the codon and the third position in the anticodon.