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VIRUSES, BACTERIA, and
PRIONS
https://www.msu.edu/course/isb/202/ebertmay/images/HIV%20virus.png
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Concept 19.1
A virus consists of a nucleic acid surrounded by a
protein coat
AP Biology
The Discovery of Viruses: Scientific Inquiry
 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, Wendell Stanley confirmed this hypothesis by crystallizing
the infectious particle, now known as tobacco mosaic virus (TMV)
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Structure of Viruses
 not cells
 small infectious particles consisting of nucleic acid enclosed in a
protein coat and, in some cases, a membranous envelope
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VIRUSES
 Tiny: smaller than ribosomes
http://academic.pgcc.edu/~kroberts/Lecture/Chapter%2013/13-04_SizesOfViruses_0_L.jpg
VIRUSES
 Contain DNA or RNA
 SINGLE or DOUBLE stranded
 NUCLEIC ACID surrounded by
PROTEIN coat = CAPSID
 Some have ENVELOPE outside capsid
http://micro.magnet.fsu.edu/cells/virus.html
Capsids and Envelopes
 A capsid is the protein shell that encloses the viral genome
 Capsids are built from protein subunits called capsomeres
 A capsid can have various structures
AP Biology
Fig. 19-3
RNA
DNA
Capsomere
Membranous
envelope
RNA
Head
DNA
Capsid
Tail
sheath
18
Capsomere
of capsid
Glycoproteins
Glycoprotein
250 nm
70–90 nm (diameter) 80–200 nm (diameter)
20 nm
(a) Tobacco mosaic
virus
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50 nm
(b) Adenoviruses
50 nm
Tail
fiber
80
225 nm
50 nm
(c) Influenza viruses (d) Bacteriophage T4
 Some viruses have membranous envelopes that help them infect
hosts
 These viral envelopes surround the capsids of influenza viruses
and many other viruses found in animals
 Viral envelopes, which are derived from the host cell’s membrane,
contain a combination of viral and host cell molecules
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BACTERIOPHAGE
 viruses that infect bacteria
 no cellular machinery of their own
Can only reproduce in host cells
 have the most complex capsids found among viruses
 have an elongated capsid head that encloses their
DNA
 A protein tail piece attaches the phage to the host
and injects the phage DNA inside
HIV (Human Immunodeficiency Virus)
AIDS virus
 RETROVIRUS (Contains RNA)
 Infects WHITE BLOOD CELLS
 Has REVERSE TRANSCRIPTASE
Enzyme that can use RNA to make DNA
https://www.msu.edu/course/isb/202/ebertmay/images/HIV%20virus.png
Concept 19.2: Viruses reproduce only in host cells
 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
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General Features of Viral Reproductive Cycles
 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
 Viral nucleic acid molecules and capsomeres spontaneously selfassemble into new viruses
Animation: Simplified Viral Reproductive Cycle
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Reproductive Cycles of Phages
 Phages are the best understood of all viruses
 Phages have two reproductive mechanisms:
1. the lytic cycle
2. the lysogenic cycle
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The Lytic Cycle
 a phage reproductive cycle that culminates in the death of the host
cell
 produces new phages and digests the host’s 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
Animation: Phage T4 Lytic Cycle
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The Lysogenic Cycle
 The lysogenic cycle replicates the phage genome without
destroying the host
 The viral DNA molecule is incorporated into the host cell’s
chromosome
 This integrated viral DNA is known as a prophage
 Every time the host divides, it copies the phage DNA and passes
the copies to daughter cells
Animation: Phage Lambda Lysogenic and Lytic Cycles
AP Biology
 An environmental signal can trigger the virus genome to exit
the bacterial chromosome and switch to the lytic mode
 Phages that use both the lytic and lysogenic cycles are called
temperate phages
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Reproductive Cycles of Animal Viruses
• There are two key variables used to classify viruses that infect
animals:


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DNA or RNA?
Single-stranded or double-stranded?
Table 19-1a
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Table 19-1b
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Viral Envelopes
 Many viruses that infect animals have a membranous envelope
 Viral glycoproteins on the envelope bind to specific receptor
molecules on the surface of a host cell
 Some viral envelopes are formed from the host cell’s plasma
membrane as the viral capsids exit
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 Other viral membranes form from the host’s nuclear envelope
and are then replaced by an envelope made from Golgi
apparatus membrane
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Fig. 19-7
Capsid and viral genome
enter the cell
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
Capsid
proteins
ER
Glycoproteins
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Copy of
genome (RNA)
New virus
RNA as Viral Genetic Material
 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 (human immunodeficiency virus) is the retrovirus that causes
AIDS (acquired immunodeficiency syndrome)
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Fig. 19-8
Viral envelope
Glycoprotein
Capsid
Reverse
transcriptase
HIV
RNA (two
identical
strands)
HIV
Membrane of
white blood cell
HOST CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
0.25 µm
DNA
HIV entering a cell
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
New virus
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New HIV leaving a cell
mRNA
Fig. 19-8a
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
RNA (two
identical
strands)
HOST CELL
HIV
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
New virus
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mRNA
 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 host’s 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
Animation: HIV Reproductive Cycle
AP Biology
Concept 19.3: Viruses, viroids, and prions are formidable
pathogens in animals and plants
 Diseases caused by viral infections affect humans, agricultural
crops, and livestock worldwide
 Smaller, less complex entities called viroids and prions also cause
disease in plants and animals, respectively
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Viroids
 circular RNA molecules that infect plants and disrupt their
growth
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PRIONS
 “Misshaped” proteins
 Change the shape of other proteins they contact
 Aggregates of proteins accumulate in brain
PRIONS
 Aggregates of proteins accumulate in brain
 Neurological disorders
 SCRAPIE in sheep
 BOVINE SPONGIFORM ENCEPHALOPATHY
(BSE) = “Mad Cow” disease
 CHRONIC WASTING DISEASE
 KURU
 CREUTZFELD-JAKOB