In the diagram below, the plane drawn behind the peptide bond indicates the:   

A) absence of rotation around the C—N bond because of its partial double-bond character.

B) plane of rotation around the Ca—N bond.

C) region of steric hindrance determined by the large C=O group.

D) region of the peptide bond that contributes to a Ramachandran plot.

 Which of the following best represents the backbone arrangement of two peptide bonds?

#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: 2004

#Unit 4. Cell Communication and Cell Signaling

A bone marrow transplant may not be appropriate from a given donor (Jane) to a given recipient (Janeʹs cousin Bob), even though Jane has previously given blood for one of Bobʹs needed transfusions. Which of the following might account for this?

#Question id: 12317

#Unit 10. Ecological Principles

#Question id: 27130

#Unit 7. System Physiology – Animal

#Question id: 19016

#Unit 13. Methods in Biology