Viruses

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Chapter 19

• Viruses

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Microbial Model Systems

• Are viruses living organisms?
• Maybe
• The origins of molecular biology lie in early
  studies of viruses that infect bacteria

T4 bacteriophage attacking E. coli
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• Bacteria are prokaryotes with cells much smaller
  and more simply organized than those of
  eukaryotes
• Viruses are smaller and simpler than bacteria

Virus
Bacterium
Animal cell
Animal cell nucleus
0.25 µm
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Viruses

• Viruses were detected indirectly long before they
  were actually seen
• Tobacco mosaic disease stunts growth of tobacco
  plants and gives their leaves a mosaic coloration
• In the late 1800s, researchers hypothesized that
  a particle smaller than bacteria caused the
  disease
• In 1935, this hypothesis was confirmed by
  crystallizing the
  infectious particle, tobacco mosaic virus (TMV)

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Structure of Viruses

• Viruses are not cells
• Viruses are very small infectious particles
  consisting of nucleic acid enclosed in a protein
  coat and, in some cases, a
  membranous envelope
• Viral genomes may consist of
• Double- or single-stranded DNA
• Double- or single-stranded RNA
• Depending on its type of nucleic acid,
• a virus is called a DNA virus or an RNA virus

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Membranousenvelope
Capsomere
RNA
RNA
DNA
Capsid
DNA
Head
Tailsheath
Capsomereof capsid
Tailfiber
Glycoproteins
Glycoprotein
80 ? 225 nm
7090 nm (diameter)
80200 nm (diameter)
18 ? 250 nm
50 nm
50 nm
50 nm
20 nm
(a)
Tobaccomosaic virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4
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RNA
DNA
Capsomere
capsid- is the protein shell that encloses the
viral genome each protein subunit is a capsomere
Capsomere of capsid
Glycoprotein
18 x 250 nm
7090 nm (diameter)
20 nm
50 nm
(a) Tobacco mosaic virus
(b) Adenoviruses
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Membranous envelope

• Some viruses have membranous envelopes
  that help them infect hosts
• surround the capsids of influenza viruses and
  many other viruses found in animals
• are derived from the host cells
  membrane, contain a combination of viral and host
  cell molecules

RNA
Capsid
Glycoproteins
80200 nm (diameter)
50 nm
(c) Influenza viruses
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• Bacteriophages,
• also called phages, are
  viruses that infect bacteria
• They have
• the most complex capsids found among viruses
• an elongated capsid head that encloses their DNA
• a protein tailpiece that attaches the phage to
  the host and injects the phage DNA inside

Head
DNA
Tail sheath
Tail fiber
80 x 225 nm
50 nm
(d) Bacteriophage T4
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Viral Replicative Cycle
VIRUS
Entry into cell and uncoating of DNA
DNA
Capsid
Transcription

• 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
• Once a viral genome has entered a cell, the cell
  begins to manufacture viral proteins
• The virus makes use of host
  enzymes, ribosomes, tRNAs, amino acids, ATP, and
  other molecules

Replication
HOST CELL
host enzymes replicate the viral genome
Viral DNA
mRNA
Viral DNA
Capsid proteins
Self-assembly of new virus particles and their
exit from cell
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Reproductive Cycles of Phages

• Phages are the best understood of all viruses
• Bacteria have defenses against phages, including
  restriction enzymes that recognize and cut up
  certain phage DNA
• Phages have two reproductive mechanisms
• 1. the lytic cycle
• 2. the lysogenic cycle

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The Lytic Cycle
Attachment
Attachment
1
Attachment
1
Attachment
1
Attachment
1
1
Entry of phage DNA and degradation of host DNA
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Entry of phage DNA and degradation of host DNA
2
Entry of phage DNA and degradation of host DNA
2
Entry of phage DNA and degradation of host DNA
2
culminates in the death of the host cell
Release
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digests the hosts cell wall
Phage assembly
Phage assembly
produces new phages
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4
Assembly
Assembly
Synthesis of viral genomes and proteins
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Synthesis of viral genomes and proteins
3
Synthesis of viral genomes and proteins
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• A phage that reproduces only by the lytic cycle
  is called a virulent phage

Head
Tail
Tail fibers
Head
Tail
Tail fibers
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The Lysogenic Cycle

• The lysogenic cycle replicates the phage genome
  without destroying the host
• The viral DNA molecule is incorporated by genetic
  recombination into the
  host cells chromosome this integrated viral DNA
  is known as a prophage

Daughter cell with prophage
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
Bacterial chromosome
Occasionally, a prophage exits the bacterial
chromosome, initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
Certain factors determine whether
The bacterium reproduces normally, copying the
prophage and transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle is induced
Lysogenic cycle is entered
or
Prophage
temperate phage
Phage DNA integrates into the bacterial chromosome
s, becoming a prophage.
New phage DNA and proteins are synthesized and
assembled into phages.
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The phage attaches to ahost cell and injects its
DNA.
PhageDNA
Bacterialchromosome
Prophage
Lysogenic cycle
Lytic cycle
Temperate phage only
Virulent or temperate phage
Destruction of host DNA
Production of new phages
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• Two key variables in classifying viruses that
  infect animals
• DNA or RNA
• single-stranded (ss) double-stranded (ds)

http//en.wikipedia.org/wiki/Baltimore_classificat
ion
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DNA
HPV
RNA
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Retroviruses

• The broadest variety of RNA genomes is found in
  viruses that infect
  animals
• Retroviruses-
• use reverse transcriptase to copy their RNA
  genome into DNA
• Human immunodeficiency virus (HIV)
  is the
  retrovirus that causes
• acquired immunodeficiency syndrome (AIDS)

Class VI. ssRNA template for DNA synthesis
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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 The hosts RNA
polymerase transcribes the proviral DNA into RNA
molecules The RNA molecules function both as
mRNA for synthesis of viral proteins and as
genomes for new virus particles released from the
cell
Membrane of white blood cell
HIV
HOST CELL
reverse transcriptase- catalyzes the
synthesis of a DNA strand complementary to the
viral RNA and a second DNA strand complementary
to the 1st
Viral RNA
RNA-DNA hybrid
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Provirus
Chromosomal DNA
RNA genome for the next viral generation
mRNA
New HIV leaving a cell
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Viral Disease

• Diseases caused by viral infections affect
  humans, agricultural crops, livestock worldwide
• Viruses may damage or kill cells by causing the
  release of hydrolytic enzymes from lysosomes
• Some viruses cause infected cells to produce
  toxins that lead to disease symptoms
• Smaller, less complex entities called
  viroids and prions
  
  also cause disease in plants and animals

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Vaccines

• vaccines-
• are harmless derivatives of pathogenic microbes
  that stimulate the immune system to mount
  defenses against the actual pathogen
• can prevent certain viral illnesses
• Hepatitis B, A
• Polio (IPV)
• Rotavirus
• MMRV- Measles, Mumps, Rubella, Varicella (chicken
  pox)
• HPV
• Other crucial childhood vaccines that protect
  against bacteria
• DTaP (Diptheria, Tetanus, Pertussis whooping
  cough)
• Hib (Haemophilus influenzae type b)
• Pneumococcal

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Viral infections cannot be treated by antibiotics
antiviral drugs- can help to treat, though not
cure,
viral infections by interfering
with viral replication
HIV can be treated with the drug AZT.
AZT has a 100- to 300-fold greater affinity for
the HIV reverse transcriptase, as compared to
the human DNA polymerase
http//en.wikipedia.org/wiki/Zidovudine