VIRUSES Chapter 19

1
VIRUSES Chapter 19
2
What is a virus?

• A virus is a submicroscopic infectious particle
  composed of a protein coat (capsid) and a nucleic
  acid core (either DNA or RNA).
• Viruses are similar in size to a large protein
  macromolecule, generally smaller than 200 nm in
  diameter.

3
Discovery of Viruses

• Search for cause of tobacco mosaic disease led to
  viruses
• Beijerinck proved that the disease was caused by
  a virus.
• The elusive virus was crystallized in 1935 by
  Wendell Stanley.

4
(No Transcript)
5
Beijerincks experiment
Fig. 19-2
RESULTS
Extracted sap from tobacco plant
with tobacco mosaic disease
Passed sap through a porcelain filter known to
trap bacteria
Rubbed filtered sap on healthy tobacco plants
1
2
3
Healthy plants became infected
4
6
Viral Capsids

• Capsids are built from protein subunits called
  capsomeres
• May be rod-shaped (helical viruses), polyhedral
  (icosahedral viruses) or more complex
• Some viruses have membranous envelopes that help
  them infect hosts (flu virus)
• Bacteriophages, also called phages, infect
  bacteria

7
Fig. 19-3
RNA
Membranous envelope
Head
DNA
RNA
DNA
Capsomere
Capsid
Tail sheath
Tail fiber
Capsomere of capsid
Glycoproteins
Glycoprotein
18 ? 250 nm
7090 nm (diameter)
80200 nm (diameter)
80 ? 225 nm
50 nm
50 nm
50 nm
20 nm
(a) Tobacco mosaic virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4
8

• Viruses are obligate intracellular parasites,
  which means they can reproduce only within a host
  cell
• Each virus has a host range, a limited number of
  host cells that it can infect

9
Viral Reproduction

• Once a viral genome has entered a cell, the cell
  begins to manufacture viral proteins using the
  host cells materials (enzymes, ribosomes, tRNAs,
  amino acids, ATP, etc.)
• RNA viruses may have codes for their own
  enzymes however.

10
Fig. 19-4
VIRUS
Entry and uncoating
1
DNA
Capsid
Transcription and manufacture of capsid
proteins
3
Replication
2
HOST CELL
Viral DNA
mRNA
Capsid proteins
Viral DNA
Self-assembly of new virus particles and
their exit from the cell
4
11

• Phages are the best understood of all viruses
• Phages have two reproductive mechanisms the
  lytic cycle and the lysogenic cycle

http//www.nsf.gov/news/news_videos.jsp?orgNSFcn
tn_id100420media_id51295
12
The Lytic Cycle

• The lytic cycle culminates in the death of the
  host cell by producing new phages and digests the
  hosts cell wall, releasing the progeny viruses
• A phage that reproduces only by the lytic cycle
  is called a virulent phage
• Bacteria have defenses against phages, including
  restriction enzymes that recognize and cut up
  certain phage DNA

13
Fig. 19-5-5
Attachment
1
2
Entry of phage DNA and degradation of host DNA
5
Release
Phage assembly
4
Assembly
3
Synthesis of viral genomes and proteins
Head
Tail
Tail fibers
14
The Lysogenic Cycle

• The lysogenic cycle replicates the phage genome
  without destroying the host
• The viral DNA molecule is incorporated into the
  host cells chromosome and is called a prophage.
• Every time the host divides, it copies the phage
  DNA and passes the copies to daughter cells
• Viruses that can be lysogenic or lytic are called
  temperate phages.

15
Fig. 19-6
Daughter cell with prophage
Phage DNA
The phage injects its DNA.
Cell divisions produce population of bacteria
infected with the prophage.
Phage DNA circularizes.
Phage
Bacterial chromosome
Occasionally, a prophage exits the
bacterial chromosome, initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
The bacterium reproduces, copying the prophage
and transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle is induced
Lysogenic cycle is entered
or
Prophage
Phage DNA integrates into the bacterial
chromosome, becoming a prophage.
New phage DNA and proteins are synthesized
and assembled into phages.
16
Animal Viruses

• Classified as DNA or RNA viruses, single or
  double-stranded
• Many have envelopes with glycoproteins that are
  specific for receptors.
• The glycoproteins are made by the ER and added to
  the host cells membrane which envelopes the
  emerging viruses.

17
Fig. 19-7
Capsid and viral genome enter the cell
Capsid
RNA
HOST CELL
Envelope (with glycoproteins)
Viral genome (RNA)
Template
mRNA
Notice the viral mRNA codes for Glycoproteins
that are added to the cell membrane. RNA viruses
often have Codes for their own enzymes unlike
DNA Viruses.
Capsid proteins
ER
Copy of genome (RNA)
Glyco- proteins
New virus
18
Table 19-1a
19
Table 19-1b
20
RNA Viruses

• The broadest variety of RNA genomes is found in
  viruses that infect animals
• Retroviruses use reverse transcriptase to copy
  their RNA genome into DNA (HIV is ex.)
• The viral DNA that is integrated into the host
  genome is called a provirus
• Unlike a prophage, a provirus remains a permanent
  resident of the host cell

http//highered.mcgraw-hill.com/olcweb/cgi/pluginp
op.cgi?itswf535535/sites/dl/free/0072437316
/120088/micro41.swfHIV Replication
21
Fig. 19-8a
Viral envelope
Glycoprotein
Capsid
RNA DNA
RNA (two identical strands)
HOST CELL
Reverse transcriptase
HIV
Reverse transcriptase
Viral RNA
RNA-DNA hybrid
DNA
NUCLEUS
Provirus
http//highered.mcgraw-hill.com/olcweb/cgi/pluginp
op.cgi?itswf535535/sites/dl/free/0072437316
/120088/micro41.swfHIV Replication
Chromosomal DNA
RNA genome for the next viral generation
mRNA
New virus
22
Fig. 19-8b
Membrane of white blood cell
HIV
0.25 µm
HIV entering a cell
New HIV leaving a cell
23
Evolution of Viruses

• Since viruses can reproduce only within cells,
  they probably evolved as bits of cellular nucleic
  acid
• Candidates for the source of viral genomes are
  plasmids and transposons (small mobile DNA
  segments)
• Mimivirus, a double-stranded DNA virus, is the
  largest virus yet discovered. not any more.
  Mega Virus

24
Mimivirus and megavirus
Mimivirus was first isolated in 1992 from amoeba
growing in a water tower. Megavirus was isolated
from infecting amoeba with mimiviruses.
Which came first, the cell or the mimivirus?
25
(No Transcript)
26
How fast can viruses evolve?

• When viruses face an obstacle to infecting the
  cells they normally infect, how long does it take
  for them to evolve to successfully invade them
  again? A new study has a frightening answer just
  a little more than two weeks.
• how fast viruses evolve lambda virus

27
Viral diseases in animals

• Symptoms caused by
• - toxins
• - bodys defense mechanisms
• Vaccines weakened or derivatives of viral
  particles capable of causing an immune response
• Antibiotics not effective
• Some antiviral medications interfere with viral
  nucleic acid synthesis

28
Why are antibiotics ineffective against viruses?

• They target 70s ribosomes, cell walls, or
  bacteria-specific enzymes
• High rates of mutation in viral protein coats and
  enzymes make it difficult to develop vaccines and
  drugs against viruses

29
Where do new viruses come from?

• Mutations of existing viruses
• The dissemination of an existing virus to a more
  widespread population
• Or spread between species
• Epidemic general outbreak of a disease
• Pandemic global epidemic

30
Fig. 19-9
(a) The 1918 flu pandemic
0.5 µm
(b) Influenza A H5N1 virus
(c) Vaccinating ducks
31
Plant viruses

• More than 2,000 types of viral diseases of plants
  are known and cause spots on leaves and fruits,
  stunted growth, and damaged flowers or roots
• Most plant viruses have an RNA genome
• Plant viral disease can spread by vertical
  transmission from parent plant or by horizontal
  transmission from an external source.

32
Fig. 19-10
33
Viroids and Prions The Simplest Infectious Agents

• Viroids are circular RNA molecules that infect
  plants and disrupt their growth
• Prions are slow-acting, virtually indestructible
  infectious misfolded proteins that cause brain
  diseases in mammals
• Prions propagate by converting normal proteins
  into the prion version
• Scrapie in sheep, mad cow disease, and
  Creutzfeldt-Jakob disease in humans are all
  caused by prions

34
Viroids in Plants
35
Misfolding of proteins to form prions
Fig. 19-11
Remember Prion - Protein
Original prion
Prion
Aggregates of prions
New prion
Normal protein
36
Scrapie in sheep
37
How Prions Arise
38
Why is it hard to treat viroid and prion
infections?

• Due to their simple structure, it is difficult to
  attack them without attaching native cell
  proteins or RNA

39
Hybrid Viruses
hybrid viruses
avian-human flu
viral replication