Download Ch. 19 Viruses notes

CH. 19 VIRUSES
Objective:
EK 3.C.3: Viral replication results in genetic variation,
and viral infection can introduce genetic variation into the hosts.
L.O. 3.29 The student is able to construct an explanation of how
viruses introduce genetic variation in host organisms.
19.1 A Virus Consists of a Nucleic Acid
Surrounded By a Protein Coat
• 1st discovered viruses – Tobacco Mosaic Virus
• Filtered plant sap still infected plants.
• Something smaller (viruses) was infected them.
RESULTS
1 Extracted sap
from tobacco
plant with
tobacco mosaic
disease
2 Passed sap
through a
porcelain filter
known to trap
bacteria
4 Healthy plants
became infected
3 Rubbed filtered
sap on healthy
tobacco plants
Structure of Viruses
• Nucleic Acid: typically 1 linear strand or circular molecule
• Double stranded DNA
• Single stranded DNA
• Double stranded RNA
• Single stranded RNA
• Protein shell (capsid) encloses nucleic acid. These come in many
shapes/sizes.
• Some have accessory structures:
• Viral envelopes
• Tail fibers
RNA
Capsomere
DNA
Membranous
RNA
envelope
Capsid
DNA
Head
Tail
sheath
Tail
fiber
Capsomere
of capsid
Glycoprotein
18  250 nm
(a) Tobacco
mosaic virus
20 nm
Glycoproteins
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
50 nm
(b) Adenoviruses
50 nm
(c) Influenza viruses
50 nm
(d) Bacteriophage T4
19.2 Viruses Replicate Only in Host Cells
• Viruses do not have organelles to build proteins, replicate
nucleic acids, metabolize, etc. Needs a host cell to do it.
• Viruses can only infect certain hosts (host range)
• Only certain species
• Only specific cells/tissues within species
• Ex: flu only infects cells of the respiratory system
General Features of Viral Replicative Cycles
• Infection begins with the insertion of the viral nucleic acid
into the host cell:
• Use tail fibers to inject DNA into host cell (like a needle)
• Endocytosis of entire virus
• Viral DNA commandeers cells to make more viruses using
the cell’s own machinery.
• Kills cells when done.
The Lytic Cycle
Phage
DNA
• The virus injects DNA
into the host cell
• Host cells starts
copying viral DNA and
making capsid and
accessory parts.
• Once done, the cell
lyses (bursts) releasing
new viruses to infect
other cells.
The phage
injects its DNA.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Lytic cycle
Certain factors
determine whether
The cell lyses, releasing phages.
lytic cycle
is induced
New phage DNA and proteins
are synthesized and assembled
into phages.
or
lysogenic cycle
is entered
Animation: Phage T4 Lytic Cycle
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
The Lysogenic Cycle
• Same beginning and end
as lytic cycle
Daughter cell
with prophage
• Insert DNA …… make
copies and cell lyses
• After the DNA is inserted,
it is integrated into host
cell’s DNA.
• Prophage; 1 gene codes
for a protein that prevents
transcription of the rest of
its genes.
• Cell grows and replicates
(binary fission) normally
with viral DNA still
“hiding” in host DNA.
Cell divisions
produce a
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lysogenic cycle
Certain factors
determine whether
lytic cycle or lysogenic cycle
is entered
is induced
Prophage
The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Animation: Phage Lambda Lysogenic and Lytic Cycles
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Replicative Cycles of
Animal Viruses
• Be aware of (but don’t
memorize) that animal
viruses are classified
according to type of
nucleic acid and presence
of an envelope.
RNA as Viral Genetic Material
• Class IV – viral RNA serves as mRNA to
immediately make proteins.
• Class V – viral RNA used to make many mRNAs.
• Class VI (retroviruses) –
• RNA + reverse transcriptase  DNA
• Viral DNA embeds in host’s DNA
• Viral proteins made with host’s
Figure 19.8
Glycoprotein
Viral envelope
HIV
Capsid
Reverse
transcriptase HIV
RNA (two
identical
strands)
Membrane
of white
blood cell
HOST
CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
0.25 m
DNA
HIV entering a cell
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
mRNA
New virus
New HIV leaving a cell
Animation: HIV Reproductive Cycle
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Evolution of Viruses
• Not living but effects all living things.
• Evolved after life.
• Naked DNA infected injured cells at first, then
evolved to have an envelope and could infect
healthy cells.
• Viral nucleic acids are more similar to host’s
nucleic acids but there are some viral genes that
are similar for all viruses.