Download Viruses & Bacteria

Monday, March 8th
Check Chapter #20 Pretest
 Discuss Section 20-1 Notes
 20-1 Review Sheet due next time
 Quiz next time: 20-1 Notes
 Chapter #20 Vocabulary Quiz Friday,
March 12th

Chapter #20
Viruses &
Bacteria
Section 20-1
Viruses
A. History of Viruses
1) The first viruses discovered were ones
that infected tobacco plants, a disease
called TMV (tobacco mosaic virus)
2) One experiment that attempted to
identify the virus involved the use of a
fine porcelain filter:
a) The pores in these filters were so
small that bacteria could not pass
through.
b) However, the infectious agent of
TMV passed through the filters
without difficulty.
c) Scientists concluded that the infectious
agent must be smaller than a bacterial
cell.
d) They discovered the agent could only
reproduce inside living cells.
e) They called the agent a virus, a Latin
word for “poison”.
3) For many years after their discovery,
viruses were erroneously regarded as
primitive forms of life. Scientists thought
viruses were tiny cells that might have been
the ancestors of bacteria.
4) The true nature of viruses was discovered
in 1933 by biologist Wendell Stanley
a) When Stanley tried to purify an extract
of TMV, the extract formed crystals…a
property of chemicals.
b) The crystals retained the ability to infect
healthy tobacco plants & were therefore
the virus itself.
c) Stanley concluded that TMV is chemical
matter rather than a living organism.
5) Within a few years of Stanley’s findings,
scientists were able to disassemble TMV &
confirm Stanley’s conclusion that viruses
were in fact chemicals and not living cells.
In fact, each particle of TMV is made of
only two kinds of molecules:
a) RNA
b) Protein
6) Later, scientists were able to separate the
RNA from the protein. When they
reassembled the two components, the
reconstructed TMV particles were fully
able to infect healthy tobacco plants.
7) Biologists have concluded that viruses
are NOT living organisms.
Instead, a virus is a strand of nucleic acid
encased in a protein coat that an infect
cells and replicate within them.
B. Structure of Viruses
1) Most viruses have a protein sheath
called a capsid surrounding a core of
nucleic acid.
a) Many plant viruses as well as some
animal viruses contain RNA.
b) The nucleic acid found in most
viruses is DNA.
2) Many viruses found in animals, such as the
influenza virus, have a membranous
envelope surrounding the capsid.
a) The envelope helps it gain entry into
cells.
b) It contains proteins, lipids, &
glycoproteins derived from the host cell.
3) Viruses differ greatly in appearance
a) The simplest viruses consist of a single
molecule of a nucleic acid & a capsid
made of a single protein or a few
different protein molecules repeated
several times.
b) More complex viruses may consist of
several different segments of DNA or
RNA contained within a capsid made of
several different kinds of protein.
c) TMV is rodlike in appearance, with
capsid proteins winding around the core
like a helix.
d) The capsid of most polyhedral viruses is
in the shape of an icosahedron, which is a
shape with 20 triangular faces and 12
corners.
The adenovirus, which causes upper
respiratory infections in humans, has the
icosahedral shape.
e) Bacteriophages are viruses that infect
bacteria. They have a very complicated
structure shown in Figure 20-5 on p. 457
C. Viruses Reproduce Inside Living Cells
1) Viruses lack the enzymes for metabolism
& have no ribosomes or other equipment
for protein synthesis. Therefore they
must rely on living cells for reproduction.
2) Before a virus can reproduce, it must
first infect a living cell.
a) Bacteriophages punch a hole in the
bacterial cell wall & inject their DNA
into the cell like a hypodermic needle.
b) Plant viruses, like TMV, enter plant
cells through tiny rips in the cell wall at
points of injury.
c) Animal viruses enter the host cell by
endocytosis.
3) Once they are inside a cell, many viruses
are pathogens….agents that cause disease.
4) Before any virus can be engulfed by a cell,
it must first bind to the cell membrane.
a) The envelope of an animal virus has
spikes of glycoproteins & lipids that are
able to bind to specific receptor
molecules on the cell membrane.
b) An animal virus is able to infect only
cells with surface receptor proteins to
which the virus’ envelope molecules can
attach.
c) This is why viruses affect very specific
cells in their host, the spikes of the virus
will only attach to specific receptor
proteins.
5) Mammals protect themselves from viral
infection by producing antibodies to the
virus envelope’s glycoprotein.
a) An antibody is a protein secreted by
cells in the immune system in response to
a foreign substance in the body.
b) However, mutations in viruses often
change their glycoproteins & make it
difficult for the antibodies to recognize
the virus.
c) Mutations in the virus genes that encode
the structure of its glycoproteins may
also enable the virus to bind to a receptor
protein it failed to recognize earlier.
D. How HIV Infects Human WBC’s
1) HIV gains access to a WBC by first
binding to the cell membrane.
2) The binding triggers endocytosis.
3) The virus enters the cell within a
membrane-bound vesicle, which soon
releases the virus into the cell cytoplasm.
4) Once within the host, HIV sheds its
envelope & capsid, leaving two strands of
the virus’ RNA floating in the cytoplasm.
5) HIV utilizes the enzyme reverse
transcriptase, which manufactures DNA
from its RNA. This ability makes HIV a
retrovirus.
6) After the viral RNA is transcribed into
DNA, the genes are translated into HIV
proteins.
7) The host cell’s machinery is then used to
produce & assemble many copies of the
HIV virus.
8) Some of the newly assembled virus
particles leave the cell by exocytosis, and
eventually the host cell ruptures, releasing
thousands of additional virus particles.
9) These newly released virus particles are
then free to infect other WBC’s and
continue the cycle of infection.
E. Origins of Viruses
1) Viruses are considered to be escaped
fragments of host genomes.
2) This is why viruses are almost always
highly specific to the host they infect.
Tuesday, March 16th
 Check
Directed & Active Reading
 Discuss
Sections 20-2 & 20-3 Notes
 Review
Sheet due Thursday
 Quiz
Thursday: 20-2 & 20-3 Notes
Section 20-2
Bacteria
A. Bacteria are small, single cells and are the
only ones characterized by prokaryotic
organization.
B. A bacterial cell is usually one of three
basic shapes:
1) Bacillus: Rod-shaped cells
2) Coccus: Spherical cells
3) Spirillum: Spiral cells
C. Bacteria differ in cell wall structure.
1) A bacterium’s plasma membrane is
encased within a cell wall.
2) Some eubacteria have a cell wall
covered with an outer membrane layer
made of large molecules called
lipopolysaccharides.
3) Outside of the cell wall & membrane,
many bacteria have a gelatinous layer
called a capsule.
D. Bacteria are commonly classified by
differences in their cell walls.
1) Gram Positive bacteria have a cell wall
containing a large amount of
peptidoglycan.
2) Gram Negative bacteria have a cell wall
containing a thin layer of peptidoglycan
covered by an outer membrane.
E. Danish microbiologist Hans Gram
developed a staining procedure in 1884 that
allows scientists to distinguish between
Gram+ and Gram- bacteria.
F. The staining method is based on the fact
that the thick cell wall of Gram+ bacteria
will hold on to a stain, while the stain will
be washed out of the thin cell wall of Grambacteria.
G. Steps in the Gram Stain
1) Cover the smear with crystal violet stain
and let it react for 60 seconds.
2) Pour off the excess stain and gently rinse
with water.
3) Cover the smear with Gram’s iodine
and let it react for 60 seconds.
4) Gently rinse the smear with ethanol
until no stain rinses off.
5) Rinse off the alcohol with water.
6) Cover the smear with Safranin & let it
react for 15 seconds.
7) Rinse the slide with water.
8) Gently blot the slide dry.
H. Medical Importance of Gram Staining
1) Gram+ bacteria tend to be killed by
penicillin.
2) Gram- bacteria tend to be resistant to
penicillin, but are much more susceptible
to tetracycline.
3) This is why doctors try to identify the
type of bacteria causing a disease before
prescribing a certain antibiotic.
I. Some bacteria form thick-walled
endospores around their chromosomes and
a small bit of cytoplasm when they are
exposed to harsh conditions.
1) These endospores are highly resistant to
environmental stress & may germinate
after years to form new, active bacteria.
2) Endospores of Clostridium botulinum are
responsible for botulism, a very serious
form of food poisoning.
J. Bacteria vs. Eukaryotes
1) Bacteria are prokaryotic. They lack a
nucleus & membrane-bound organelles.
2) Eukaryotic cells are about 10x larger
than bacterial cells.
3) All bacteria are single-celled.
Eukaryotes can be unicellular or
multicellular.
4) Bacterial chromosomes consist of a
single circular strand of DNA.
Eukaryotes have more complex DNA.
5) Bacteria divide by binary fission. Most
eukaryotes divide by mitosis & meiosis.
6) Eukaryotic flagella are more complex
than bacterial flagella. Some bacteria
have shorter, thicker outgrowths called
pili.
7) Bacteria have many different types of
metabolism. All eukaryotes have the
same general metabolic requirements.
K. Bacterial Metabolism
1) Photosynthetic bacteria utilize sunlight
for their energy source. Examples
include cyanobacteria, green sulfur
bacteria, purple sulfur bacteria, and
purple nonsulfur bacteria
2) Chemoautotrophic bacteria utilize
electrons in inorganic molecules.
Examples include Nitrosomonas and
Nitrobacter.
3) Heterotrophic bacteria utilize organic
material from other organisms. Most
bacteria are heterotrophic.
Section 20-3:
Bacteria & Viruses
as Pathogens
Diseases caused by Bacteria: Table 20-1
Diseases caused by Viruses: Table 20-2
B. Treating Bacterial Diseases
1) In 1928, the British bacteriologist
Alexander Fleming observed a Pennicillium
mold growing on a bacterial culture
2) He noticed the bacteria did not grow near
the mold.
3) He isolated the substance that killed the
bacteria and named it penicillin.
4) Penicillin is an antibiotic, which is a drug
used to fight pathogenic microorganisms.
a) Antibiotics work by interfering with the
microorganism’s cellular processes.
b) Antibiotics can’t be used to fight viral
diseases.
5) Scientists are concerned because many
microorganisms are becoming resistant to
antibiotics.