Download ap ch 18 virus bacteria - Pregitzersninjascienceclasses

Vivacious Viruses
Chapter 19
I. Virus
A. Characteristics
1. Smaller than a ribosome
2. Can form into regular crystals (cells won’t
do this)
3. Made of Nucleic Acid - Genome is made of
one of the following:




DNA - double stranded
DNA - single stranded
RNA - double stranded
RNA - single stranded
A. Characteristics (cont)
4. Range from 4 genes to several hundred
5. Protein coat surrounds nucleic acid
 Capsid - protein coat
 can be rod shaped (helical), polyhedral or more
complex
 capsids are made from large numbers of protein
subunits called capsomeres
A. Characteristics (cont)
6. Accessory Structures (found in some
viruses)
 Viral Envelopes - membranes surrounding
capsid
 come from membrane of their host cell and
contain the same proteins, glycoproteins and
phospholipids used by host
 These envelopes help viruses to infect host
without being detected
 Bacteriophages - have most complex
capsids
B. Reproduction of Viruses
 Happens only in a host
cell because they have no
ribosomes or enzymes to
reproduce on their own
B. Reproduction of Viruses
1. Host Range - limited amount of host cells
that a virus can infect (can be one host
or a few related species)
 Host identified by “lock and key” fit between
proteins on capsid and receptors on outside
of host cell
 Can be Tissue Specific - eukaryotic viruses
may infect only certain tissues of their host
 ex. cold - respiratory tissue, HIV - white blood
cells only
B. Reproduction of Viruses
2. Methods of Reproduction - virus infects host
first and overtakes host cell to make viral
nucleic acids and proteins
 Host provides the following:
 Nucleotides for synthesizing viruses nucleic acid
 Enzymes
 Ribosomes
 tRNA, amino acids, ATP and other parts needed by virus to
go through transciption and translation (to make viral
proteins)
Reproduction of Viruses3. Lytic Cycle - viral reproductive cycle that
ends in the death of the host cell (also
called virulent virus)

ex. T4 bacteriophage
Lytic Cycle (cont)
a. Phage attaches to cell - uses tail fiber to
stick to receptor site on host (e. coli)
b. DNA is injected through cell wall and
membrane into the host cell
c. Empty capsid remains outside the cell and DNA of the
host is hydrolyzed (broken up)
d. Phage overtakes cell using its parts to manufacture
viral nucleic acids and proteins. New phages are
reassembled inside host cell
e. Phage makes host cell produce lysozyme - an
enzyme that digests the host’s cell wall.
Osmosis causes the cell to swell and burst
releasing the new phages (may be 100-200
new ones) Phages go find new host cell to
infect
Reproduction of Viruses
4. Lysogenic Cycle - replicates viral
genome without killing host cell
 If they use both lytic and lysogenic - also
called temperate viruses
B. Lysogenic Cycle (cont)
a. Phage binds to host cell and injects DNA
b. Phage DNA forms a circle inside host - it can now go
c.
d.
through either lytic or lysogenic cycle. (If lytic - see
previous process)
Viral DNA lines up with host DNA and gets
incorporated into the host DNA by crossing over - it is
now called a prophage - one of its genes represses
the other genes so it’s basically not affecting the host
at all at this point
When the host cell replicates and divides, it also
replicates the virus and passes it on to its daughter
cells
B. Lysogenic Cycle (cont)/
e. At some point, phages will leave the host DNA
f.
and return to a lytic cycle. At this point they
will destroy their host cell - trigger can be
chemicals, radiation or other
Some other viral genes may be expressed
when DNA is part of host cell and produce
toxins (like botulism, diptheria, scarlet fever)
C. Animal Viruses
 Vary in type of nucleic acid and
presence/absence of viral envelope
1. Viral Envelopes

Outer membrane (outside capsid) that helps parasite
enter host cell
 Bind to animal cell
 Envelope fuses with plasma membrane and injects
virus + capsid into cell
 Enzymes of host cell remove capsid, virus overtakes
cell (similar to bacteria)
C. Animal Viruses
 Cycle does not always kill the host cell
 Some virus envelopes come from nuclear
membrane and virus is replicated inside the
nucleus of the host (like herpes)
 DNA of virus becomes integrated into host
DNA and becomes a provirus
 Trigger will cause provirus to become active
and destroy host cell
C. Animal Viruses
2. RNA virus
a. Classification - by # of strands and how they
function in host
 Class IV - serve directly as mRNA - can be
translated into viral protein as soon as they
infect
 Class V - RNA of virus serves as template to
make mRNA - must be transcribed into
mRNA before translation
B. RNA viruses in animals
Class VI - Retroviruses - reverse flow of
genetic information
 Contain enzyme - reverse transcriptase - that
transcribes DNA from an RNA template (goes
backward)
 DNA becomes a provirus in nucleus of host
 Viral DNA is now transcribed to RNA (can be
mRNA for translation, or can be packaged and
sent to new host cells)
 HIV is a retrovirus that causes AIDS
E. Viral Diseases in Animals
1. Causes


Virus could damage or kill cells
Virus may produce toxins that cause
symptoms
C. Viral Diseases in Animals
2. Effects depend on ability of affected
tissue to make new cells
a. If virus affects rapidly dividing cells – can
have complete recovery (throat)
b. If virus affects areas where cells have
stopped dividing, damage is permanent (like
nerve cells - polio virus)
3. Vaccines
 Harmless variants of pathogenic microbes
that stimulate the immune system to
mount defense against virus
 sensitizes immune system to the virus so
that it reacts vigorously if ever truly
exposed to the virus
4. New Viruses

Result from a variety of causes
a. Mutation of existing virus - seen often in RNA
viruses because they have no proofreading
mechanism
 May lead to new varieties that individuals were already
immune to (flu)
b. Spread of existing viruses from one species to
another
c. Dissemination from small population to large
population - due to increased travel, blood transfusions,
IV drug use
5. Cancer causing viruses
 Oncogenes - genes that trigger
cancerous characteristics in cells
a. Proto-oncogenes - versions of oncogenes
found in normal cells - code for proteins that
control growth factors and cell division
b. Virus may trigger proto-oncogenes to turn
on causing uncontrolled cell division Usually only cause cancer in combo with a
mutagen
F. Plant Viruses
 stunt growth in plants and diminish crop
yields
1. Horizontal transmission of virus - insects,
high winds, injury, freezes - cause plants
outer layer of epidermis to become
damaged
 increases chance of virus to penetrate epidermis
and infect plant
 farmers transmit from plant to plant using same
pruning tools
D. Plant Viruses
2. Vertical transmission - plant inherits viral
infection from a parent
 Occurs during asexual reproduction or in
seeds of sexual reproduction
3. Virus spreads through plant via
plasmodesmata
G. Viroids
 Tiny molecules of naked RNA that infect
plants only
 Make no proteins but replicate in plant
cells and stunt their growth
H. Prions
 infectious proteins - usually affect brain
tissue (mad cow)
Breathtaking Bacteria
Chapter 27
Chapter 18
I. Major Characteristics
A. DNA




One, double stranded, circular molecule
Tightly packed into “nucleoid” region
No membrane surrounding it
Plasmids - smaller circular pieces of DNA
outside nucleoid region
B. Reproduction
Divide by binary fission from single origin of
replication
Asexual- no mating involved - Most offspring are
genetically identical to parent
Fast process - many can divide every 20
minutes
Relatively high rate of mutation due to speed of
reproduction
Mutation rate helps bacterial colonies to survive
better
C. Genetic Recombination
 While bacteria do not reproduce sexually,
they are able to have some recombination
of genes with other bacteria through one
of the following methods:
1. Transformation
 Uptake of “naked” foreign DNA from the surrounding
environment (like the S-strain/R-strain killing mice)
 Live, nonpathogenic cell takes up a piece of DNA that
includes the allele to make it pathogenic
 Foreign allele is incorporated into the bacterial
chromosome and replaces the original allele by crossing
over
 Many bacteria have receptors on their surface proteins
that aid in uptake of naked DNA only from closely related
species
 Calcium - can be added to bacteria without these
receptors (like e.coli) and will artificially stimulate the
bacteria to take up naked DNA
2. Transduction
Phages carry bacterial genes from one
host to another
Transduction (cont.)
When a virus is reassembling in it’s host a
small piece of bacterial DNA from host is
accidentally packaged into capsid of virus
Phage can attach to another host and
inject the bacterial DNA into it
Crossing over incorporates this DNA into
the host cell
RANDOM
3. Conjugation
 Direct transfer of genetic material between two
bacterial cells that are temporarily joined.
bacterial version of “sex”
One way transfer
donor = male - uses sex pili to attach to
recipient = female
bridge forms between two bacteria and DNA can
be transferred
D. Plasmids
 Small, circular, self-replicating DNA molecule





separate from bacterial chromosome
Can remain separate from chromosome
Episome: Can become part of bacterial
chromosome and replicate with it
No protein coats and can’t exist outside the cell
Generally beneficial to bacteria
Small number of genes that may help bacteria
survive in stressful environments
1. F Plasmid
 Contains 25 genes required for production of
sex pili
F+ = cell that contains F plasmid - donaters
F- = cell without F plasmid - recipients
Heritable F+ bacteria give rise to F+ offspring
F+ x F-  F+ (male) replicates it’s DNA,
transfers copy to F- (female) converting it to F+
(male). Now the newly converted F+ bacteria
can make sex pili and transfer its plasmid to new
bacterial cells
2. Hfr cell
 F factor becomes part of bacterial chromosome.
During conjugation - F factor replicates and gets
transferred to F-, but some of the bacterial
chromosome can be taken with it.
Recipient cell is temporarily “diploid” for some
genes and crossing over can occur
When bacteria divides, it has the new genes
Any pieces of Hfr DNA left will be degraded
3. R-plasmids
 “Resistance” plasmids - contain genes that code for
resistance to antibiotics
 When a bacterial population is exposed to antibiotics,
any “sensitive” bacteria are killed by the antibiotics
 Bacteria that contain the R plasmid with resistance to the
antibiotic survive
 These bacteria then reproduce, increasing the number of
antibiotic resistant bacteria (natural selection)
 R-plasmids can also be transferred during conjugation
and may carry as many as 10 genes for resistance to
different antibiotics
II. Control of Gene Expression in
Bacteria
A. Purpose
 helps individual bacteria cope with
changes in their surroundings

ex. E. coli in human intestine - rely on what
the host eats for it’s nutrition. Bacteria must
be able to turn genes on and off depending
on its nutritional needs
B. Metabolic control
 Regulates which metabolic pathways are
turned on and off. Done by one of the
following 2 methods.
1. Adjust activity of enzymes already
present in cell. Depends on sensitivity of
enzymes to chemical cues

ex. Feedback inhibition - when the endproduct starts to accumulate, it will inhibit
one or more of the enzymes in the pathway
turning it off
B. Metabolic control
2. Regulation of gene expression  Control of enzyme/protein production by
controlling transcription and translation
(turning genes on and off).
 In bacteria - this regulation occurs with
Operons
C. Operon

contains all the necessary components for
controlling a metabolic pathway including:
1. Operator - on/off switch located in the
promoter region of the DNA (before the genes)
2. Promoter - binding site for RNA polymerase
3. Transcription unit - all of the genes necessary
for a certain metabolic pathway
C. Operon
4. Regulatory gene - occurs somewhere
else in the DNA and codes for a
repressor (off switch)
5. Repressor – a protein that switches an
operon off. Specific to one operon
6. Corepressor - binds to repressor and
activates it, changes it’s shape so it can
bind to the operator and turn the pathway
off
Negative Control.. The Repressible
Operon

This pathway is called repressible because the
system is normally on but can be turned off
when there is enough resources available for
the bacteria
1. Normally the operon is in the on position, one
long mRNA is made for the 5 enzymes
required in the pathway. The mRNA will attach
to a ribosome, produces the enzymes and the
enzymes will make tryptophan
2. Host eats Thanksgiving dinner
 Turkey contains lots of tryptophan
 Why would the bacteria use its
resources/energy to make tryptophan
when it can get it from its host? - it doesn’t
Trp operon (cont)
3. Tryptophan is a corepressor in this pathway. It
will bind to the repressor molecules (floating in
cytoplasm but inactive)
 When trp binds to repressor, shape of
repressor changes and it becomes active
 Active repressor then binds to operator region
of DNA and turns the operon OFF
 No mRNA is made and the enzymes to make
trp are no longer produces
4. After that great turkey dinner is digested
by the host and the tryptophan levels
decrease again, the repressor becomes
inactive again, releases from the
operator, and the operon is turned back
on
E. Negative Control.. The
Inducible Operon
 In this pathway, the repressor is made in
its active form and automatically binds to
the operator. The system is normally OFF
but can be turned on (induced) when it is
needed. (Why make the enzymes to
break down lactose if there’s no lactose
around)
 EX. The lac operon produces 3 enzymes to
break down lactose (milk sugar) for energy.
Lac operon (cont)
1. Host drinks a big glass of milk (contains
lots of lactose plus some allolactose)
2. Allolactose binds to the repressor and
releases it from the operator region
3. RNA polymerase can then go on and
transcribe the mRNA needed to make
the enzymes for breaking down lactose
Positive Control of Gene
Expression
 There is another factor controlling the lac
operon
 Glucose is the best energy source for the
bacteria. Lactose doesn’t provide as
much ATP as glucose does.
 Enzymes to break down lactose will only
be made if lactose is present AND glucose
levels are LOW!
Positive Control of Gene
Expression
1. cAMP accumulates (when glucose is low)
2. cAMP binds to a regulatory protein called
cAMP receptor protein (CRP)
3. This complex (cAMP+CRP) is an activator of
transcription and binds near the promoter of the
operon and makes transcription faster by
making it easier for RNA polymerase to bind to
promoter
4. If glucose levels build up, cAMP/CRP will
release and transcription will slow down