18.11.Compare the effect on the host cell of a lytic (virulent) phage and a lysogenic (temperate) phage.

Lytic•Can only carry out lysis of host cell

Lysogenic•Can integrate into host chromosome•Later, can exit chromosome and initiate lytic cycle

18.12.How do some viruses reproduce without possessing or ever synthesizing DNA?

18.12.•Genetic material is RNA•Viral RNA serves as mRNA•Virus codes for enzymes that replicate RNA

18.13.Why is HIV called a retrovirus?

18.13.Because it synthesized DNA from its RNA genome

18.21.Describe two ways a preexisting virus can become an emerging virus.

18.21.There are three ways:1.Mutation2.Jumping to a new host species3.Spreading beyond a previously isolated population

18.22.Contrast vertical and horizontal transmission of viruses in plants.

18.22.Horizontalplant is infected from external source

Vertical plant inherits virus from a parent

18.23.Why does the long incubation period of prions increase their danger as a cause of human disease?

18.23.Beef may be distributed from an infected herd for years before any symptoms appear.

18.31.Distinguish between the three mechanisms of transferring DNA from one bacterial cell to another.

18.31.•Transformation: bacteria uptake naked, foreign DNA•Transduction: phage viruses carry bacterial genes from one to another•Conjugation: bacteria “mate” and exchange DNA across a pilus

18.32.What are the similarities and differences between lysogenic phage DNA and a plasmid?

similarities•Both are episomes (can exists as part of chromosome or independently)

differences•Virus can leave the cell in a protein coat•Viruses are harmful while plasmids are beneficial

18.33.Explain why the process of conjugation can lead to genetic recombination of chromosomal DNA in an Hrf x F- mating, but not in an F+ x F- mating.

Hrf x F-

•Bacterial genes are transferred, because the F-factor is integrated into the chromosome•Transferred genes can then recombind with recipent’s genes

F+ x F-

•Only plasmid genes are transferred

18.41.A certain mutation in E. coli changes the lac operator so that the active repressor cannot bind. How would this affect the cell’s production of β-galactosidase?

18.41.•The cell would continuously produce β-galactosidase (and the two other enzymes for lactose utilization) even without any lactose around•Wastes cell resources

18.42.How does the binding of the trp corepressor and the lac inducer to their respective repressor proteins alter repressor function and transcription in each case?

trp corepressor•Binds to repressor•Activates repressor•Shuts off genes

lac inducer•Binds to repressor•Inactivates repressor•Turns on genes

18.42.