Transcript 1. Discuss the contributions of Mayer.

Chapter 18 Reading Quiz
1.
2.
3.
4.
5.
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 …..?
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) 
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 
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 
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 
5. Using viruses with envelopes and RNA as examples,
describe variations in replication cycles of animal viruses.

1.
2.
3.
4.
5.
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 
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 
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 
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 
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 
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 
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 
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 
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 
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 
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 
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 
17. List and describe the three natural processes of genetic
recombination in bacteria.
1.
Transformation  process of gene
transfer during which a bacterial cell
assimilates foreign DNA from the
surroundings 
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 
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) 
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 
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 
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 :

Only some bacterial genes are donated

Recombination occurs between the Hfr chromosomal fragment and the Fcell = homologous strand exchange results in a recombinant F- cell 
22. Define transposon, and describe two essential types of
nucleotide sequences found in transposon DNA.

1.
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 
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

24. Briefly describe two main strategies cells use to control
metabolism.
1.
2.
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 
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 
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 
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 
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 
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 
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)

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 
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