2. Chapter 18 Reading Quiz
Order small to large  eukaryotic cell, bacteria, and virus
Which viral reproductive cycle destroys the host cell?
What are the harmless derivates that stimulate the immune system to create antibodies?
An infectious protein is called a…?
An entire stretch of DNA that is required for enzyme production is known as an …..?

4. 1. Discuss the contributions of Mayer.
Mayer demonstrated that the stunting disease of tobacco plants was contagious  thought it was caused by an unusually small bacteria
- later findings demonstrated that the disease could not be bacteria-caused, but must be a particle much smaller (and unlike) a bacterium
- the infectious particle was finally crystallized and observed and is now known as the tobacco mosaic virus (TMV) 

5. 2. List and describe the structural components of viruses, and explain why viruses are obligate parasites.
It is a genome enclosed in a protective coat
It is organized as single nucleic acid molecules
May have 4 to several hundred genes
Simple composition 
1. Capsid – protein coat that encloses the viral genome
2. Envelope – membrane that cloaks some viral capsids (head, sheath, DNA, tail fibers)
Viruses express their genes and reproduce only within a living cell 

7. 3. Briefly describe what happens when a virus infects a host cell.
A viral infection begins when the genome of a virus makes its way into a host cell 

8. 4. Distinguish between lytic and lysogenic reproductive cycles using phage T4 and phage  as examples.
Lytic Cycle
Viral replication cycle that results in the death (or lysis) of the host cell
T4  phage attaches to cell surface, phage contracts sheath and injects DNA, hydrolytic enzymes destroy the host cell’s DNA, phage genome directs the host cell to make phage components and cell lyses and releases phage particles
Lysogenic Cycle
A viral replication cycle that involves the incorporation of the viral genome into the host cell genome
λ phage binds to the surface of ecoli and injects the DNA and inserts it by genetic recombination 

10. Enveloped Viruses are characterized by:
5. Using viruses with envelopes and RNA as examples, describe variations in replication cycles of animal viruses.
Enveloped Viruses are characterized by:
Attachment
Entry
Uncoating
Viral RNA & protein synthesis
Assembly and release
RNA as viral genetic material:
RNA viruses can be complicated (like retroviruses)
mRNA or the strand that corresponds to mRNA is the + strand and it has the nucleotide sequence that codes for proteins
The – strand is a template for synthesis of the + strand 

12. 6. Describe what vaccines are and how they are manufactured.
Vaccine  a “harmless” variant or derivative of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen 

13. 7. Explain the role of reverse transcriptase in retroviruses.
It is the enzyme that transcribes DNA from an RNA template
Viral genomic RNA
(reverse transcriptase)
Viral DNA 

15. 8. Describe how viruses recognize host cells.
They recognize their host cell by a complementary fit between external viral proteins and specific cell surface receptor sites 

16. 9. Describe several defenses bacteria have against phage infection.
Bacterial mutations can change receptor sites to avoid recognition
- this in turn prevents infection
Restriction nucleases in bacteria recognize and cut up foreign DNA
- self-destruction is avoided because bacterial DNA is chemically altered 

17. They can be toxic or cause infected cells to produce toxins
10. Explain how viruses may cause disease symptoms, and describe some medical weapons used to fight viral infections.
Viruses damage or kill cells (viral infection  lysosome releases hydrolytic enzymes)
They can be toxic or cause infected cells to produce toxins
Cause varying degrees of cell damage
Immune system reacts, causing fever, aches, inflammation
Vaccines – harmless variants or derivatives of pathogenic microbes that mobilize the immune system
Antiviral drugs – fight after the disease 

18. - this insertion is permanent as the provirus never excises
11. List some viruses that have been implicated in human cancers, and explain how tumor viruses transform cells.
Retrovirus – adult leukemia
Herpes virus – Epstein-Barr (mono)  Burkitt’s lymphoma
Papovavirus – human warts; cervical cancer
Hepatitis B virus – chronic hepatitis; liver cancer
Tumor viruses transform cells by inserting viral nucleic acids into host cell DNA
- this insertion is permanent as the provirus never excises
- insertion for DNA tumor viruses is straightforward 

19. 12. Distinguish between horizontal and vertical routes of viral transmission in plants.
Horizontal  route of viral infection in which an organism receives the virus from an external source
Vertical  route in which an organism inherits a viral infection from its parent 

20. - have a genome with the same genetic code as living organisms
13. List some characteristics that viruses share with living organisms, and explain why viruses do not fit our usual definition of life.
CAN: mutate and evolve
- have a genome with the same genetic code as living organisms
CANNOT: reproduce independently
Need a host cell for reproduction 

21. Some viral genes are identical to cellular genes
14. Provide evidence that viruses probably evolved from fragments of cellular nucleic acid.
Genetic material of different viral families is more similar to host genomes than to that of other viral families
Some viral genes are identical to cellular genes
Viruses of eukaryotes are more similar in genomic structure to their cellular hosts than to bacterial viruses
Viral genomes are similar to cellular genetic elements like plasmids and transposons 

22. 15. Describe the structure of a bacterial chromosome.
It is composed of one double stranded, circular molecule of DNA
Structurally simpler and has fewer associated proteins than a eukaryotic chromosome
Found in the nucleoid region of the cell 

23. It is an asexual process, producing clones
16. Describe the process of binary fission in bacteria, and explain why replication of the bacterial chromosome is considered to be semiconservative.
Binary fission is preceded by DNA replication, which begins at a single origin on the chromosome
It is an asexual process, producing clones
2 replication forks move bi-directionally until they meet and replication is complete
Bacteria can divide every 20 minutes 

25. 17. List and describe the three natural processes of genetic recombination in bacteria.
Transformation  process of gene transfer during which a bacterial cell assimilates foreign DNA from the surroundings 

26. 2. Transduction  gene transfer from one bacterium to another by a bacteriophage
3. Conjugation  the direct transfer of genes between two cells that are temporarily joined 

28. 18. Distinguish between general transduction and specialized transduction.
Generalized  transduction that occurs when random pieces of host cell DNA are packaged within a phage capsid during the lytic cycle of a phage
Specialized  occurs when a prophage excises from the bacterial chromosome and carries with it only certain host genes adjacent to the excision site (AKA restricted transduction) 

30. 19. Explain how the F plasmid controls conjugation in bacteria.
The ability to form sex pili and to transfer DNA is conferred by genes in a plasmid called the F plasmid 

31. Conjugation  transfer of genes only ( no offspring)
20. Explain how bacterial conjugation differs from sexual reproduction in eukaryotic organisms.
Conjugation  transfer of genes only ( no offspring)
Sexual reproduction  donation of genetic material to an offspring 

32. 21. For donor and recipient bacterial cells, predict the consequences of conjugation between the following: 1) F+ and F- cell 2) Hfr and F- cell
1. The F factor (which is in the F+) replicates and one copy is tranferred to the F_
Only some bacterial genes are donated
The recipient F- cell does not become F+ because only part of the F factor transferred
The recipient cell becomes a partial diploid
2. As the integrated F factor of the Hfr cell transfers to the F- cell, it pulls the bacterial chromosome behind its leading end  the conjugation bridge usually breaks before the entire chromosome and tail end of the F factor can be transferred
As a result :
Recombination occurs between the Hfr chromosomal fragment and the F- cell = homologous strand exchange results in a recombinant F- cell 

34. 22. Define transposon, and describe two essential types of nucleotide sequences found in transposon DNA.
Transposons  DNA sequences that can move from one chromosomal site to another
Insertion sequences  the simplest, they contain only the genes necessary for the process of transposition (2 essential types)
- nucleotide sequence coding for transposase (which catalyzes insertion of transposons into new chromosomal sites)
- inverted repeats 

35. 23. Distinguish between an insertion sequence and a composite transposon.
Insertion  the simplest transposons, they contain only the genes necessary for the process of transposition
Composite  transposons which include additional genetic material besides that required for transposition; consist of one or more genes flanked by insertion sequences 

37. 24. Briefly describe two main strategies cells use to control metabolism.
Regulation of enzyme activity  the catalytic activity of many enzymes increases or decreases in response to chemical cues
Regulation of gene expression  enzyme concentrations may rise and fall in response to cellular metabolic changes that switch genes on or off 

38. 25. Explain why grouping genes into an operon can be advantageous.
Operon  a regulated cluster of adjacent structural genes (gene that codes for a polypeptide) with related functions
- common in bacteria and phages 

39. Mechanism for the control of gene expression
26. Using the trp operon as an example, explain the concept of an operon and the function of the operator, repressor, and corepressor.
Mechanism for the control of gene expression
Operator  a DNA segment located within the promoter or between the promoter and structural genes, which controls access of RNA polymerase to structural genes
Repressor  specific protein that binds to an operator and blocks transcription of the operon
Corepressor a molecule, usually a metabolite, that binds to a repressor protein, causing the repressor to change into its active conformation 

41. 27. Distinguish between structural and regulatory genes.
Structural genes  gene that codes for a polypeptide
Regulatory genes  genes that code for repressor or regulators of other genes 

42. 28. Describe how the lac operon functions and explain the role of the inducer allolactose.
Operon that can be switched on or induced
Lactose metabolism in Ecoli is programmed by the lac operon – with 3 structural genes:
1. Lac Z – codes for β galactosidase which hydrolyzes lactose
2. Lac Y – codes for permease which transports lactose into the cell
3. Lac A – codes for transacetylase which has no known role
Has a single promoter and operator
Allolactose  an isomer of lactose, acts as an inducer to turn on the lac operon 

44. 29. Explain how repressible and inducible enzymes differ and how these differences reflect differences in the pathways they control.
Repressible
Genes are switched on until a specific metabolite activates the repressor
Generally function in anabolic pathways
Pathway end product switches off its own production by repressing enzyme synthesis
Inducible
Their genes are switched off until a specific metabolite inactivates the repressor
Function in catabolic pathways
Enzyme synthesis is switched on by the nutrient the path uses 

46. Lac  negative by repressor protein Positive
30. Distinguish between positive and negative control, and give examples of each from the lac operon.
Negative
Binding of active repressor to an operator always turns off structural gene expression
Lac  negative by repressor protein
Positive
Occurs only if an activator molecule interacts directly with the genome to turn on transcription
Lac  positive by cAMP receptor protein (CRP) 

47. 31. Explain how cAMP is affected by glucose concentration.
When no glucose  The cell accumulates cAMP, a nucleotide derived from ATP.
- cAMP activates CRP so that it can bind to the lac promoter
When glucose concentration increases, glucose catabolism decreases the intracellular concentration of cAMP
- thus, cAMP releases CRP 

49. 32. Describe how E. coli uses the negative and positive controls of the lac operon to economize on RNA and protein synthesis.
CRP is an activator of several different operons that program catabolic pathways
Glucose’s presence deactivates CRP, which slows synthesis of those enzymes a cell needs to use catabolites other than glucose
Ecoli prefers using glucose as its primary carbon and energy source and the enzymes for this are coded for by unregulated genes that are continuously transcribed  the end