#Question id: 4950
#Unit 3. Fundamental Processes
Temperature-sensitive (ts) mutations, which are one form of conditional lethal mutation, allow cells to grow at one temperature (for example, 30°C) but not at a higher temperature (for example, 42°C). A large number of temperature-sensitive replication mutants have been isolated in E. coli. These mutant bacteria are defective in DNA replication at 42°C but not at 30°C. If the temperature of the medium is raised from 30°C to 42°C, these mutants stop making DNA in one of two characteristic ways. The “quick-stop” mutants halt DNA synthesis immediately, whereas the “slow-stop” mutants stop DNA synthesis only after many minutes. Extracts from quick-stop mutants halt DNA synthesis immediately at 42°C, whereas extracts from slow-stop mutants do not stop DNA synthesis for several minutes after a shift to 42°C. Suppose extracts from a temperature-sensitive DNA helicase mutant and a temperature-sensitive DNA ligase mutant were mixed together at 42°C. Which of the following statements are correct?
A. The mixed extracts should be fully competent for DNA replication at 30°C; that is, the mixture should exhibit a nonmutant phenotype.
B. The defective DNA helicase extract would provide normal DNA ligase, and the defective DNA ligase extract would provide normal DNA helicase.
C. In practice, the behaviors of extracts from slow-stop mutants and from nonmutant cells are easy to distinguish.
D. Because of the extreme complexity of DNA replication and the large number of proteins involved, cell-free extracts are not capable of maintaining DNA replication indefinitely.
#Question id: 4951
#Unit 11. Evolution and Behavior
The synthesis of new DNA requires the prior existence of oligonucleotides to serve as primers. On Earth, these primers are small RNA molecules. This latter observation is evidence in support of the hypothesized existence of
#Question id: 4952
#Unit 11. Evolution and Behavior
Several scientific laboratories across the globe are involved in research concerning the origin of life on Earth. Which of these questions is currently the most problematic and would have the greatest impact on our understanding if we were able to answer it?
#Question id: 4953
#Unit 11. Evolution and Behavior
Several scientific laboratories across the globe are involved in research concerning the origin of life on Earth. Which graph below, if the results were produced abiotically, would have the greatest promise for revealing important information about the origin or Earthʹs first genetic system?
#Question id: 4954
#Unit 11. Evolution and Behavior
If natural selection in a particular environment favored genetic systems that permitted the production of daughter ʺcellsʺ that were genetically dissimilar from the mother ʺcells,ʺ then one should expect selection for
I. polynucleotide polymerase with low mismatch error rates.
II. polynucleotide polymerases without proofreading capability.
III. batteries of efficient polynucleotide repair enzymes.
IV. polynucleotide polymerases with proofreading capability.
V. polynucleotide polymerases with high mismatch error rates.
#Question id: 4955
#Unit 11. Evolution and Behavior
If relatively small carbonaceous chondrites from space were a significant source of Earthʹs original amino acids, then which two of these would have been most important in permitting their organic materials to survive impact with Earth?
I. Carbonaceous chondrites must contain no D-amino acids.
II. Earthʹs early atmosphere must have had little free oxygen.
III. The chondrites must have arrived on Earth before 4.2 billion years ago.
IV. Earthʹs early atmosphere must have been dense enough to dramatically slow the chondrites before they impacted.
V. The chondrites must have impacted land, rather than a large body of water.