AP BIOLOGY
CH 18
MICROBIAL MODEL SYSTEMS
• *E. Coli and its viruses because of
their frequent use by researchers
in studies that reveal broad
biological principles.
• *Recall that bacteria are
prokaryotes, with cells much
smaller and more simply
organized than those of
eukaryotes, such as plants and
animals
• *Viruses are even smaller and
simpler
Discovery of viruses
• 1. How and What did Dr. Mayer
discover specifically in 1883?
-Adolf Mayer, a German scientist,
discovered that he could transmit the
tobacco mosaic disease from plant to
plant by rubbing sap extracted from
diseased leaves on to healthy plants.
• Mayer concluded that the disease
was caused by unusually small
bacteria that could not be seen
with the microscope.
Tobacco mosaic disease
stunts the growth of tobacco
plants and gives leaves a
mosaic coloration
2. What did Ivanowsky conclude
that built on Mayer’s work?
Dimitri Ivanowsky, Russian, tested
Mayer’s hypothesis a decade later. He
agreed that bacteria caused the
tobacco mosaic disease. He passed sap
from infected tobacco leaves through a
filter designed to remove bacteria.
After filtering, the sap still produced
mosaic disease.
• Ivanowsky reasoned, the bacteria
were so small that they passed
through the filter or made a filterable
toxin that caused the disease.
• This was ruled out later by Dutch
botanist Martinus Beijerinck.
3. What logic did Beijerinck use
to lead to the idea of a virus?
• -Discovered that the infection agent
in filtered sap could reproduce.
• He rubbed plants with filtered sap,
after these plants developed mosaic
disease, he used their sap to infect
more plants.
• In fact, the pathogen reproduced only
within the host it infected. Unlike
bacteria, the mysterious agent of
mosaic disease could not be
cultivated on nutrient media in test
tubes or petri dishes.
• Beijerinck’s suspicions were
confirmed in 1935 by American
scientist Wendell Stanley.
4. How was the existence of a virus
finally confirmed and by whom?
• Stanley crystallized the in infectious
particle, now known as tobacco
mosaic virus ( TMV)
• TMV and other viruses were seen
by the electron microscope
STRUCTURE OF A VIRUSE
• 5. How small are viruses?
The tiniest viruses are only 20nm in
diameter- smaller than a ribosome
Viruses are infectious particles
consisting of nucleic acid enclosed in a
protein coat and, in some cases, a
membranous envelope.
Viral Genomes
6. What kind of nucleic acids are the
viral genomes made of?
May consist of double stranded DNA, singlestranded DNA, double-stranded RNA, or
single-stranded RNA-depending on the kind
of virus.
• A virus is called a DNA virus or RNA virus,
according to the kind of nucleic acid that
make up its genome.
• *
7. What is the name for a protein
shell enclosing the viral genome?
capsid
• Depending on the type of virus, the
capsid may be rod-shaped,
polyhedral, or more complex.
• -
8. What are the subunits of
capsids?
• Capsids are built from a large number
of protein subunits called
capsomeres.
• The number of different kinds of
proteins is usually small.
9. What are viral envelopes and
what is their function?
• Some viruses have accessory
structures that help them infect their
hosts.
• Viral envelopes are derived from the
membrane of the host cell and
contain host cell phospholipids and
membrane proteins
10. Where are the most complex
capsids found?
• The most complex capsids are found
among viruses that infect bacteria,
called bacteriophages, or simply
phages.
• The first phages studied included 7
that infect E. coli. They are named
type 1 (T1), type 2 (T2)…in the order
they were discovered.
11. Define host range.
Reproductive cycles of phages
12. List the full steps of the simplified
viral reproductive cycle
* Viruses use enzymes, ribosomes, and small molecules of host cells
*Phages
Are the best understood of all viruses
Go through two alternative reproductive
mechanisms: the lytic cycle and the
lysogenic cycle
SIMPLIFIED
DNA
Capsid
VIRUS
1. Virus enters cell and is
Uncoated, releasing viral
DNA and capsid proteins
2. Host enzymes replicate
The viral genome
3. Transcription of viral
Genome into viral mRNA
4. Self-assembly of
new
Figure
18.5
virus particles and their exit from cell
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
13. What is the phage reproductive
cycle that culminates in the death
of
the
host
cell?
• Lytic cycle-refers to the last stage of infection, during which the
bacterium lyses (breaks open) and releases the phages that were
produced within the cell.
• Each of these phages can then infect a healthy cell, and few
successive lytic cycles can destroy an entire bacterial population in
just a few hours.
14. What kind of phage only
reproduces by lytic cycle
• *virulent phage
15. How do bacteria defend
themselves against phages?
• 1. natural selection favors bacterial mutants
with receptor sites that are no longer recognized
by a particular type of phage.
• 2. When phage DNA successfully enters a
bacterium, the DNA often is recognized as
foreign and cut up by cellular enzymes called
restriction enzymes. The cell’s own DNA is
chemically modified that prevents attack.
• 3. Many phages coexist with bacteria –lysogenic
cycle
16. Steps of lytic cycle of T4 phage
• Virulent phage
1.
• 1. attachment
• 2. entry of phage DNA and
degradation of host DNA
5. Release
2.
3. Synthesis
4. Assembly
17. What is the phage reproductive
cycle that replicates the phage
genome without destroying the host?
• Lysogenic cycle
18. Phages capable of using both modes of reproducing
within a bacterium are called temperate phagescalled lambda and written with Greek letter λ
19. When λ DNA is integrated into the bacterial
chromosome, the viral DNA is known as prophage.
20. What is an example of the
interaction between a prophage and a
bacterium?
• The bacteria that causes the diseases
diphtheria, botulism, and scarlet fever
would be harmless to humans if it were
not for certain prophage genes that
cause the host bacteria to make toxins.
21. What is the use of a viral
envelope in animal viruses?
• An animal virus equipped with an
outer membrane, or viral envelope,
uses it to enter the host cell.
Protruding from the outer surface of
the envelope are viral glycoproteins
that bind to specific receptor
molecules on the surface of the host
cell.
22. Does this reproductive cycle kill
the host cell?
• This reproductive cycle does not
necessarily kill the host cell, in
contrast to the lytic cycles of phages.
23. What are retroviruses and how
do they use reverse transciptase?
• The RNA animal viruses with the most
complicated reproductive cycles are the
retroviruses (class VI). These viruses are
equipped with an enzyme called reverse
transcriptase, which transcribes an RNA
template into DNA which flow is opposite
of the usual direction.
• Gave rise to the name retroviruses”backwards”
26. Is it believed that viruses evolved before
or after the first cells appeared and what
evidence is used to support the idea?
• Viruses do not really fit our definition
of living organisms
• Since viruses can reproduce only
within cells
• They probably evolved after the first
cells appeared, perhaps packaged as
fragments of cellular nucleic acid
• Concept 18.3: Rapid
reproduction, mutation, and
genetic recombination
contribute to the genetic
diversity of bacteria
• By studying the mechanism by which
viruses are replicated, researchers
also learn the mechanisms that
regulate DNA replication and the gene
expression in cells.
32. What is the main
component of most bacterial
genomes?
• The main component of the genome
in most bacteria is one doublestranded, circular DNA molecule that
is associated with a small amount of
protein.
33. How is the DNA arranged
in the nucleoid region of the
bacterial genome?
• Proteins cause the chromosome to
tightly coil and “supercoil,” densely
packing it so that it fills only part of
the cell.
34. What is a plasmid?
• Smaller circles of DNA. Each plasmid
has only a small number of genes,
from just a few to several dozen.
35. Describe the process of
binary fission
• Bacterial cells divide by binary fission,
which is preceded by replication of
the bacterial chromosome.
• Binary fission is an asexual processthe production of offspring from a
single parent.
36. Why do mutations make such a
large contribution to bacterial
genetic variation as compared to
humans
• Since bacteria can reproduce rapidly
– New mutations can quickly increase
a population’s genetic diversity
when reproductive rates are very
high because of short generation
spans.
Further genetic diversity
Can arise by recombination of
the DNA from two different
bacterial cells
• 37. Explain the experiment and the
results that demonstrated evidence of
genetic recombination in bacteria.
Researchers had two mutant strains, one that could make arginine but not
tryptophan (arg+ trp–) and one that could make tryptophan but not arginine (arg
trp+). Each mutant strain and a mixture of both strains were grown in a liquid
medium containing all the required amino acids. Samples from each liquid culture
were spread on plates containing a solution of glucose and inorganic salts
(minimal medium), solidified with agar.
Mixture
Mutant
strain
arg trp+
Results
• Only the samples from the mixed culture,
contained cells that gave rise to colonies
on minimal medium, which lacks amino
acids.
CONCLUSION
•
Because only cells that can make both arginine
and tryptophan (arg+ trp+ cells) can grow into
colonies on minimal medium, the lack of
colonies on the two control plates showed that
no further mutations had occurred restoring this
ability to cells of the mutant strains. Thus, each
cell from the mixture that formed a colony on
the minimal medium must have acquired one or
more genes from a cell of the other strain by
genetic recombination.
38. What is the process of alteration
of a bacterial cell’s genotype by the
uptake of naked, foreign DNA from
the surrounding environment?
• Transformation
39. What famous experiment
in the previous unit described
this process?
• Frederick Griffith’s experiment on the
pathogenicity being transferred
between bacteria
• “transformation”
40. Define transduction
• Phages carry bacterial genes from one
host cell to another as a result of
aberrations in the phage reproductive
cycle.
41. List the generalized steps of
transduction
42. What is the process of direct transfer
of genetic material between two
bacterial cells that are temporarily
joined?
• Conjugation
• AKA bacterial “sex” cojugation
43. What structure joins them?
• Sex pili
44. What generally must be present for
the sex pili to donate DNA during
conjugation?
• A special piece of DNA called an F factor (F
for fertility)
45. What is special about the F
plasmid?
• They can undergo reversible
integration into the cell’s
chromosome.
46. What is an episome?
A genetic element that can replicate
either as part of the bacterial
chromosome or independently of it.
47. What are R plasmids and why
are these a problem to humans?
• Resistant genes that code for enzymes that
specifically destroy antibiotics
48. How does this relate to natural
selection?
• The theory of natural selection predicts
that under these circumstances, the
fraction of the bacterial population
carrying genes for antibiotic resistance
will increase. Medically, resistant strains
of pathogens are becoming more
common making the treatment of certain
bacterial infections more difficult to treat.
49. Define transposable
elements
• Can move around within a cell’s
genome
• 51. Are often called “jumping genes”
• Contribute to genetic shuffling in
bacteria
50. Do transposable elements
exists independently?
• No, transposable elements never exist
independently but are always part of
chromosomal or plasmid DNA
• 52. What is the name for the
simplest transposable elements?
• Insertion sequences-they only exist in
bacteria.
53. What is the name for transposable
elements that are longer and more
complex than insertion sequences?
• Transposons
54. What is an example of the benefit
to bacteria
They can help them adapt to a new
environment and can add a gene for
antibiotic resistance
55. What are the two ways that
metabolic control can occur within
bacteria?
• 1. Cells can adjust the activity of
enzymes by adapting to short-term
fluctuations in the supply of a
substance it needs.
• 2. Cells can regulate the expression of
genes encoding enzymes.
56. What is the key advantage of
grouping genes of related function
into one transcription unit?
• A single on-off “switch” can control the
whole cluster of functionally related
genes.
• 57. What is the switch called?
operator
operon model-discovered by Francois Jacob
and Jacques Monod in 1961
58. Where is the operator
positioned?
• Within the promoter and the enzymecoding genes
59. What does the operator control?
The access of RNA polymerase to the
genes
60. What is the name for the
operator, promoter, and the genes
they control? operon
61. What can happen if the trp
operan is turned “on”?
• RNA polymerase can bind to the promoter
and transcribe the genes of the operon.
• 62. switch turned “off”?
• transcription of the genes is prevented
• 63. How does repressor work?
• It binds to the operator and blocks
attachment of RNA polymerase to the
promotor, preventing transcription of
genes
64. What gene controls the making
of the trp repressor protein?
• Regulatory gene (trpR), which is located
some distance away from the operon it
controls and has its own promoter.
• 65. What are the two states that the
operator vacillates?
• one without repressor bound and
one without repressor bound
66. How is the trp repressor
protein an allosteric protein?
• It has two alternative shapes, active and
inactive.
• 67. Define corepessor
• A small molecule that cooperates with a
repressor protein to switch an operon off
68. What are the two methods of
negative gene regulation?
• Repressible and inducible operons
•
69. Why is the trp operan
considered repressible?
• Because its transcription is usually on
but can be inhibited when a specific
small molecule binds allosterically to
a regulatory protein
• Regulation of both the trp and lac
operons
– Involves the negative control of
genes, because the operons
are switched off by the active
form of the repressor protein
70. What is the definition of an
inducible operan?
Binding of an inducer to an innately inactive
repressor inactivates the repressor and
turns on transcription
71. what does the inducer do?
inactivates the repressor
Why are repressible enzymes
generally associated with
anabolic pathways and how this
an advantage to the organism?
• They can synthesize essential end products
from raw materials then the cell can
allocate its organic precursors and energy
for other uses.
73. How does positive gene
regulation work?
• Use stimulatory activator protein
which promotes transcription when
bound to a site with the promoter.