1–1 Each member of the human hemoglobin gene illustrated in Figure Q1–1. Only one in a million computfamily, which consists of seven genes arranged in two clus-er-generated “random” codes is more error-resistant than ters on different chromosomes, is an ortholog to all of the the natural genetic code. Does the extraordinary mutation other members. resistance of the genetic code argue in favor of its origin as a frozen accident or as a result of natural selection? Explain 1–2 Horizontal gene transfer is more prevalent in sin- your reasoning. gle-celled organisms than in multicellular organisms.
1–3 Most of the DNA sequences in a bacterial genome code for proteins, whereas most of the DNA sequences in the human genome do not.
Discuss the following problems.
1–4 Since it was deciphered four decades ago, some have claimed that the genetic code must be a frozen acci number of codes (thousands) dent, while others have argued that it was shaped by nat susceptibility to mutation ural selection. A striking feature of the genetic code is its inherent resistance to the effects of mutation. For example, Figure Q1–1 susceptibility to mutation of the natural code shown a change in the third position of a codon often specifies the relative to that of millions of computer-generated alternative genetic codes (Problem 1–4). susceptibility measures the average change in same amino acid or one with similar chemical properties.
amino acid properties caused by random mutations in a genetic code.
The natural code resists mutation more effectively (is less
A small value indicates that mutations tend to cause minor changes.susceptible to error) than most other possible versions, as (data courtesy of steve Freeland.) 1–5 You have begun to characterize a sample obtained from the depths of the oceans on Europa, one of Jupiter’s moons. Much to your surprise, the sample contains a life-form that grows well in a rich broth. Your preliminary analysis shows that it is cellular and contains DNA, RNA, and protein. When you show your results to a colleague, she suggests that your sample was contaminated with an organism from Earth. What approaches might you try to distinguish between contamination and a novel cellular life-form based on DNA, RNA, and protein?
1–6 It is not so difficult to imagine what it means to feed on the organic molecules that living things produce. That is, after all, what we do. But what does it mean to “feed” on sunlight, as phototrophs do? Or, even stranger, to “feed” on rocks, as lithotrophs do? Where is the “food,” for example, in the mixture of chemicals (H2S, H2, CO, Mn+, Fe2+, Ni2+, CH4, and NH4+) that spews from a hydrothermal vent?
1–7 How many possible different trees (branching patterns) can in theory be drawn to display the evolution of bacteria, archaea, and eukaryotes, assuming that they all arose from a common ancestor?
1–8 The genes for ribosomal RNA are highly conserved (relatively few sequence changes) in all organisms on Earth; thus, they have evolved very slowly over time. Were ribosomal RNA genes “born” perfect?
1–9 Genes participating in informational processes such as replication, transcription, and translation are transferred between species much less often than are genes involved in metabolism. The basis for this inequality is unclear at present, but one suggestion is that it relates to the underlying complexity of the two types of processes. Informational processes tend to involve large aggregates of different gene products, whereas metabolic reactions are usually catalyzed by enzymes composed of a single protein. Why would the complexity of the underlying process—informational or metabolic—have any effect on the rate of horizontal gene transfer?