#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: 3588
#Unit 8. Inheritance Biology
The ability to taste the chemical phenylthiocarbamide is an autosomal dominant phenotype, and the inability to taste it is recessive. If a taster woman with a nontaster father marries a taster man who in a previous marriage had a nontaster daughter. What is the probability that their first child will be tasters’ girl?
#Question id: 1742
#Unit 4. Cell Communication and Cell Signaling
The T-cell receptor link to MHC–peptide is enhanced by interaction between MHC class II on the antigen-presenting cells with the following molecule on the T-cell:
#Question id: 45
#Unit 1. Molecules and their Interaction Relevant to Biology
What is the standard free-energy change, ΔG°, under physiological conditions (E. coli grows in the human gut, at 37 °C) for the following reaction? (Keq = 7.8 × 10^2)
Glucose + ATP → glucose 6-phosphate + ADP
#Question id: 1003
#Unit 1. Molecules and their Interaction Relevant to Biology
The glucose-alanine cycle provides ________.