Transcript The Genetics of Viruses

The Genetics of Viruses
I.
II.
III.
Background
Structure
Life Cycles
Viral infections, past & present
Viral infection have been one
of the major infectious
challenges of the human
species, for as far back as we
can tell.
Polio
Ebola Virus
Herpes Virus
Severe acute respiratory
syndrome (SARS)
(b) The SARS-causing agent is a coronavirus
(a) Young ballet students in Hong Kong
like this one (colorized TEM), so named for the
wear face masks to protect themselves
“corona” of glycoprotein spikes protruding from
from the virus causing SARS.
the envelope.
Figure 18.11 A, B
Let’s size them up…
Compare the size of:
eukaryotic cell, bacterial
cell and a virus
I. Background
Everyone is at risk
for infection!




Bacteriophages
infect bacteria only
All eukaryotes
(animals, plants,
fungi, protist) all
vulnerable
Size 20nm-250nm
Ghost phage
0.5 m
Viral Diseases in Plants
More than 2,000 types of viral diseases of plants are known.
Spots on leaves and fruits, stunted growth, and damaged flowers or roots
Tobacco Mosaic Virus
I. Background


Virus Discovery:
Tobacco mosaic
disease (1930’s)
Stunts growth
produces the
speckled
coloration
I. Background




Viral Evolution proposal: Fragments of
cellular nucleic acid
Reproduce within host cells only
(non-living)
Obligate intracellular parasites
Host range
II. Structure of viruses
II. Structure of Viruses
1. Nucleic acid

Enclosed in a protein coat

Genomes may be


ds/ss DNA
ds/ss RNA
II. Structure of Viruses
2. Capsids



Protein
Various shapes &
structures
Capsids are
produced by host
Capsomere
of capsid
RNA
Capsomere
DNA
Glycoprotein
70–90 nm (diameter)
18  250 mm
20 nm
50 nm
(a) Tobacco mosaic virus (b) Adenoviruses
Viral structure
II. Structure of Viruses
3. Envelopes




Found in many
animal viruses
Glycoprotein and
lipids
Derived by host
“Spikes” fuse with
membrane or
receptor mediated
entry
Membranous
envelope
Capsid
RNA
Glycoprotein
80–200 nm (diameter)
50 nm
(c) Influenza viruses
Receptor mediated model
Fusion Model
Basic Infection:
Just Genome and Capsid
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
III. Life Cycles
Bacteria and Eukaryotic Models
III. Life Cycles:
1. Bacteriophages complete two
reproductive mechanisms:
Head


Tail
sheath
lytic cycle
lysogenic cycle
DNA
Tail
fiber
80  225 nm
Figure 18.4d
50 nm
(d) Bacteriophage T4
III. Life Cycles
A. Lytic Cycle:




Digests the host’s cell wall, releasing
the progeny viruses
Kills host
Virulent phage
1 Attachment. The T4 phage uses
its tail fibers to bind to specific
receptor sites on the outer
surface of an E. coli cell.
5 Release. The phage directs production
of an enzyme that damages the bacterial
cell wall, allowing fluid to enter. The cell
swells and finally bursts, releasing 100
to 200 phage particles.
2 Entry of phage DNA
and degradation of host DNA.
The sheath of the tail contracts,
injecting the phage DNA into
the cell and leaving an empty
capsid outside. The cell’s
DNA is hydrolyzed.
Phage assembly
4 Assembly. Three separate sets of proteins
self-assemble to form phage heads, tails,
and tail fibers. The phage genome is
packaged inside the capsid as the head forms.
Head Tails
Tail fibers
3 Synthesis of viral genomes
and proteins. The phage DNA
directs production of phage
proteins and copies of the phage
genome by host enzymes, using
components within the cell.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Life Cycles
B. Lysogenic cycle




Incorporate viral DNA into bacteria
genome (propahge)
Phage genome is replicated (for free!)
w/o destroying the host
Temperate phages capable of using
both cycles
The lytic and lysogenic cycles of
phage , a temperate phage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
The cell lyses, releasing phages.
Lysogenic cycle
Certain factors
determine whether
Lytic cycle
is induced
Figure 18.7
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
New phage DNA and
proteins are synthesized
and assembled into phages.
or
Lysogenic cycle
is entered
Prophage
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
III: Life Cycles
2. RNA viruses typically infect animals

Retroviruses (HIV), use reverse
transcriptase to make cDNA

Integrated into genome, provirus
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
• The reproductive cycle of HIV, a retrovirus
HIV
Membrane of
white blood cell
1 The virus fuses with the
cell’s plasma membrane.
The capsid proteins are
removed, releasing the
viral proteins and RNA.
2 Reverse transcriptase
catalyzes the synthesis of a
DNA strand complementary
to the viral RNA.
HOST CELL
3 Reverse transcriptase
catalyzes the synthesis of
a second DNA strand
complementary to the first.
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
4 The double-stranded
DNA is incorporated
as a provirus into the
cell’s DNA.
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Chromosomal
DNA
RNA genome
for the next
viral generation
Provirus
mRNA
5 Proviral genes are
transcribed into RNA
molecules, which serve as
genomes for the next viral
generation and as mRNAs
for translation into viral
proteins.
6 The viral proteins include
capsid proteins and reverse
transcriptase (made in the cytosol)
and envelope glycoproteins (made
in the ER).
Figure 18.10
New HIV leaving a cell
9 New viruses bud
off from the host cell.
8 Capsids are
assembled around
viral genomes and
reverse transcriptase
molecules.
7 Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane.
III: Life Cycles
Life after infection





Viruses may damage or kill cells
Tissue damage
Toxins that lead to disease symptoms
may be produced
Asymptomatic
Sources







http://www.aapsj.org/
http://www.stanford.edu/group/virus/1999/jchow/rep
.html
http://pathmicro.med.sc.edu/mhunt/RNA-HO.htm
http://www.brooklyn.cuny.edu/bc/ahp/LAD/C5/C5_V
iruses.html
http://www.biology.com
Campbell Reece Mitchell. Biology, Prentice Hall
1999, 2001
Sherris. Medical Microbiology: An introduction to
infectious disease, Appleton and Lange,1990
http://teachers.eastern.k12.nj.
us/nabi/biology/index.html
Good links