Download Viral Replication - Hartland High School

Viruses
Viruses
• Nonliving particles
• Very small (1/2 to 1/100 of a bacterial cell)
• Do not perform respiration, grow, or
develop
• Are able to replicate (only with the help of
living cells)
• Host cell—a cell where a virus replicates
• Bacteriophage (phage)—virus that infects a
bacterium
T4 bacteriophage infecting an E. coli cell
0.5 m
Comparing the size of a virus, a bacterium,
and an animal cell
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Viral Structure
• 2 main parts:
• inner core of nucleic acid (DNA or RNA)
– instructions for making copies of the virus
• outer coat of protein (capsid)
– determines shape of virus (which cells &
how cells are infected)
» polyhedral
» helical
» envelope with projections
» classic phage shape
Viral structure
Capsomere
of capsid
RNA
Capsomere
Membranous
envelope
DNA
Head
Capsid Tail
sheath
RNA
DNA
Tail
fiber
Glycoprotein
18  250 mm
20 nm
(a) Tobacco mosaic virus
Glycoprotein
70–90 nm (diameter)
80–200 nm (diameter)
50 nm
50 nm
(b) Adenoviruses
(c) Influenza viruses
80  225 nm
50 nm
(d) Bacteriophage T4
Infection by tobacco mosaic virus
(TMV)
Attachment to Host Cell
• Order of events:
• virus recognizes host cell
• virus attaches to receptor site on
membrane of host cell
–Receptor site on host matches
with viral proteins (like a puzzle)
• virus enters host cell
• virus replicates inside host cell
Attachment is Specific
• viruses have specifically shaped attachment
proteins
• each virus infects only certain types of cells
– most are species specific
• Smallpox, polio, measles—affects only humans
– although some are not
• West Nile virus—mosquitoes, birds, humans,
horses
– some are cell-type specific
• polio—affects intestine & nerve cells
Simplified viral reproductive cycle
Entry into cell and
uncoating of DNA
DNA
VIRUS
Capsid
Transcription
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Self-assembly of new
virus particles and their
exit from cell
Lytic cycle of phage T4, a 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.
Lytic vs Lysogenic
• Lytic cycle (virulent phage)
– Release of virus bursts and kills host cell
(lysis)
• Lysogenic cycle (temperate phage)
– Viral DNA integrates into host genome
(provirus)
– Can be transmitted to daughter cells
– Can initiate lytic cycle in response to
environmental signal (stress)
The lytic and lysogenic cycles of phage , a
temperate phage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
Lysogenic 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
Prophage/Provirus
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage (provirus).
The structure of HIV, the retrovirus that
causes AIDS
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
RNA
(two identical
strands)
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
Provirus
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.
RNA genome
for the next
viral generation
mRNA
6 The viral proteins include capsid
proteins and reverse transcriptase
(made in the cytosol) and envelope
glycoproteins (made in the ER).
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.
Complete the Following Venn Diagram. Describe in detail
similarities and differences, give examples.
Lytic Cycle
Lysogenic Cycle
Retrovirus Cycle