Conditional lethal mutants have proven indispensible in genetic and biochemical

Question

Conditional lethal mutants have proven indispensible in genetic and biochemical analyses of complex processes such as DNA replication. Temperature-sensitive (ts) mutations, which are one form of conditional lethal mutation, allow growth at one temperature (for example, 30oC) but not at a higher temperature (for example, 42oC). A large number of ts mutants have been isolated in E. coli. These mutant bacteria are defective in DNA replication at 42oC but not at 30oC. If the temperature of the medium is raised from 30oC to 42oC, 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 a few minutes. Which of the following mutant proteins would you predict would display a 'quick stop' phenotype?

a. DNA topoisomerase I, a replication initiator protein, DNA ligase, and DNA helicase

b. DNA topoisomerase I, single-stranded DNA-binding protein, DNA ligase, and DNA helicase

c. DNA topoisomerase I, single-stranded DNA-binding protein, DNA primase, and DNA helicase

d. A replication initiator protein, single-stranded DNA-binding protein, DNA ligase, and DNA helicase

e. DNA topoisomerase I, single-stranded DNA-binding protein, DNA ligase, and DNA helicase

Explanation / Answer

The answer should be:- a. DNA topoisomerase I, a replication initiator protein, DNA ligase, and DNA helicase.

This is because the single-stranded DNA-binding proteins are usually formed at the later stages of the initiation of DNA replication when the replication is about to start so they won't fall under the criteria for 'quick-stop' as the replication would have already reached to a certain extent when the single-stranded DNA-Proteins come into the picture. This eliminates b.,c.,d. and e. On the other hand, the replication initiator protein is required at the very starting of the replication process hence inhibiting its action immediately stops the replication process hence it is a 'quick stop' protein. DNA topoisomerase I, DNA ligase, and DNA helicase also are required in the early stages of DNA replication (before the single-stranded DNA-binding protein) hence same logic applies in their case as well. Therefore A is the correct option.

Hope you can understand the solution and find it helpful!

Related Questions

Navigate

• Browse (All)
• Browse C
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.