Wednesday, September 5

1
18.1 1. Compare the effect on the host cell of a
lytic (virulent) phage and a lysogenic
(temperate) phage.
2

• Lytic
• Can only carry out lysis of host cell

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

3
18.1 2. How do some viruses reproduce without
possessing or ever synthesizing DNA?
4

• 18.1
• 2.
• Genetic material is RNA
• Viral RNA serves as mRNA
• Virus codes for enzymes that replicate RNA

5
18.1 3. Why is HIV called a retrovirus?
6
18.1 3. Because it synthesized DNA from its RNA
genome
7
18.2 1. Describe two ways a preexisting virus
can become an emerging virus.
8

• 18.2
• 1.
• There are three ways
• Mutation
• Jumping to a new host species
• Spreading beyond a previously isolated population

9
18.2 2. Contrast vertical and horizontal
transmission of viruses in plants.
10
18.2 2.
Horizontal plant is infected from external source
Vertical plant inherits virus from a parent
11
18.2 3. Why does the long incubation period of
prions increase their danger as a cause of human
disease?
12
18.2 3. Beef may be distributed from an infected
herd for years before any symptoms appear.
13
18.3 1. Distinguish between the three mechanisms
of transferring DNA from one bacterial cell to
another.
14

• 18.3
• 1.
• 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

15
18.3 2. What are the similarities and
differences between lysogenic phage DNA and a
plasmid?
16

• 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

17
18.3 3. 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.
18

• Hrf x F-
• Bacterial genes are transferred, because the
  F-factor is integrated into the chromosome
• Transferred genes can then recombind with
  recipents genes

• F x F-
• Only plasmid genes are transferred

19
18.4 1. A certain mutation in E. coli changes
the lac operator so that the active repressor
cannot bind. How would this affect the cells
production of ß-galactosidase?
20

• 18.4
• 1.
• The cell would continuously produce
  ß-galactosidase (and the two other enzymes for
  lactose utilization) even without any lactose
  around
• Wastes cell resources

21
18.4 2. 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?
22
18.4 2.

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

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