#Question id: 4945
#Unit 11. Evolution and Behavior
Which is a defining characteristic that all protobionts had in common?
#Question id: 4946
#Unit 11. Evolution and Behavior
Although absolute distinctions between the ʺmost evolvedʺ protobiont and the first living cell are unclear, biologists generally agree that one major difference is that the typical protobiont could not
#Question id: 4947
#Unit 11. Evolution and Behavior
The first genes on Earth were probably
#Question id: 4948
#Unit 11. Evolution and Behavior
RNA molecules can both carry genetic information and be catalytic. This supports the proposal that
#Question id: 4949
#Unit 11. Evolution and Behavior
What probably accounts for the switch to DNA-based genetic systems during the evolution of life on Earth?
#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.