Download (Viroids) 1.

Chapter 13
Viruses, Viroids,
and Prions
© 2013 Pearson Education, Inc.
Lectures prepared by Christine L. Case
General Characteristics of Viruses
 Depending on one’s viewpoint, viruses may be
regarded as an exceptionally complex aggregations of
nonliving chemicals, or as exceptionally simple living
microbes.
 Viruses were originally distinguished from other
infectious agents because they are especially small
(filterable) and because they are obligatory intracellular
parasites. They multiply by using the host cell’s
synthesizing machinery to cause the synthesis of
specialized elements that can transfer the viral nucleic
acid to other cells.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 Viruses are entities that
 Contain DNA or RNA (ds or ss)
 Contain a protein coat. Some are enclosed by an
envelope composed of lipids, proteins, and
carbohydrates
 Multiply inside living cells
 Cause the synthesis of specialized structures that can
transfer the viral nucleic acid to other cells
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Host range
 Host range refers to the spectrum of host cells in which
a virus can multiply.
 Most viruses infect only specific types of cells in one
host species.
 Host range is determined by the specific attachment
site on the host cell’s surface and the availability of host
cellular factors.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Viral size
 Viral size is ascertained by electron microscopy (E. M.).
 Viruses range from 20 to 1,000 nm in length.
 A virion (病毒體) is a complete, fully developed viral
particle composed of nucleic acid surrounded by a coat.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
病毒大小
Figure 13.1
Viral Structure
Nucleic acid
 Viruses contain either DNA or RNA, never both, and
the nucleic acid may be single - or double stranded,
linear or circular, or divided into several separate
molecules.
 The proportion of nucleic acid in relation to protein in
viruses ranges from about 1% to about 50%.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Capsid and envelope
 The protein coat surrounding the nucleic acid of a virus
is called the capsid (蛋白鞘，或稱nucleocapsid).
 The capsid is composed of subunits, capsomeres (次蛋
白鞘), which can be a single type of protein or several
types.
 The capsid of some viruses is enclosed by an envelope
(封套) consisting of lipids, proteins, and carbohydrates.
 Viruses whose capsids are not covered by an envelope
are known as nonenveloped viruses
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nonenveloped Viruses – ex: mastadenovirus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.2
Enveloped Viruses – ex: influenzavirus
Figure 13.3
spike: carbohydrate-protein complex that project from the surface
of the envelope
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 Some envelopes are covered with carbohydrate-protein
complexes called spikes (釘狀物).
 Spikes can be used as a means of identification.
 The ability of certain viruses (ex: influenzavirus) to
clump red blood cells is associated with spikes.
 The resulting clumping is called hemagglutination and is
the basis for laboratory tests.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
General morphology
 Viruses may be classified into several different
morphological types on the basis of their capsid
architecture:
 Helical viruses (螺旋)
 Polyhedral viruses (多面)
 Enveloped viruses (套膜)
 Complex viruses (複雜)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Helical Viruses – ex: Ebola virus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.4
Polyhedral Viruses – ex: mastadenovirus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.2
Enveloped Viruses – ex: influenzavirus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.3
Complex Viruses – ex: T-even bacteriophage
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.5a
Complex Viruses – ex: poxvirus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.5b
Taxonomy of Viruses
 Since 1966, the International Committee on Taxonomy
of Viruses (ICTV) has been grouping viruses into
families based on:
 nucleic acid type
 strategy for replication
 morphology
 Genus names end in –virus; family names end in –
viridae; and order names end in –ales.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 Viral species: a group of viruses sharing the same
genetic information and ecological niche (host).
 Common names are used for species. Subspecies are
designated by a number.
 Herpesviridae
 Retroviridae
 Herpesvirus
 Lentivirus
 Human herpes virus,
 Human immunodeficiency
HHV-1, HHV-2, HHV-3
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
virus, HIV-1, HIV-2
Isolation, Cultivation, and Identification of
Viruses
Growing bacteriophages in the laboratory
 The plaque method mixes bacteriophages with host
bacteria and nutrient agar.
 After several viral multiplication cycles, the bacteria in
the area surrounding the original virus are destroyed;
the area of lysis is called a plaque (溶菌斑).
 Each plaque originates with a single viral particle; the
concentration of viruses is given as plaque forming
units (PFU) (溶菌斑形成單位).
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.6
Growing animal viruses in the laboratory
 In living animals:
 Cultivation of some animal viruses requires whole
animals.
 Simian AIDS and feline AIDS provide models for study
of human AIDS.
 In embryonated eggs:
 Some animal viruses can be cultivated in embryonated
eggs.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 Viral grow is signaled
by the death of the
embryo, by embryo
cell damage, or by the
formation of typical
pock or lesions on the
egg membranes.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.7
 In the cell culture:
 Cell cultures are cells growing in culture media in the
laboratory.
 Animal and plants viruses may be grown in cell culture.
 Viral growth can cause cytopathic effects (CPE, 細胞病
變作用) in the cell culture.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.8
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.9
 Primary cell lines, derived from tissue slices, tend to
die out after only a few generation.
 Certain cell lines, called diploid cell lines, developed
from human embryos can be maintained for about 100
generations and are widely used for culturing viruses
that require a human host.
 Continuous cell lines are transformed (cancerous)
cells and can be maintained in vitro indefinitely.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Viral identification
 Cytopathic effects (CPE)
 Serological tests
 Detect antibodies against viruses in a patient.
 Use antibodies to identify viruses in neutralization tests,
viral hemagglutination, and Western blot.
 Nucleic acids
 Restriction fragment length polymorphisms (RFLPs)
 PCR
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
RFLPs
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Viral Multiplication
 Viruses do not contain enzymes for energy production
or protein synthesis.
 For a virus to multiply, it must invade a host cell and
direct the host’s metabolic machinery to produce viral
enzymes and components.
 Bacteriophage
 Animal virus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 The multiplication of viruses can be demonstrated with
one-step growth curve.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.10
Multiplication of bacteriophages
 Bacteriophages can multiply by two alternative
mechanisms:
 Lytic cycle
 Lysogenic cycle
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
T-even bacteriophages: the lytic cycle
 The T-even bacteriophages that infect E. coli have
been studied extensively.
 Steps:
 Attachment: Phage attaches by tail fibers to host cell.
 Penetration: Phage lysozyme opens cell wall, tail sheath
contracts to force tail core and DNA into cell.
 Biosynthesis: Production of phage DNA and proteins.
 Maturation: Assembly of phage particles.
 Release: Phage lysozyme breaks cell wall.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 During a lytic cycle, a phage causes the lysis and death
of a host cell.
 During the attachment phase of the lytic cycle, sites on
the phage’s tail fibers attach to complementary receptor
sites on the bacterial cell.
 In penetration, phage lysozyme opens a portion of the
bacterial cell wall, the tail sheath contracts to force the
tail core through the cell wall, and phage DNA enters
the bacterial cell. The capsid remains outside.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
1
2
3
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.11, steps 1–2
 In biosynthesis, transcription of phage DNA produces
mRNA coding for proteins necessary for phage
multiplication. Phage DNA is replicated, and capsid
proteins are produced. During the eclipse period,
separate phage DNA and protein can be found.
 During maturation, phage DNA and capsids assemble
into complete viruses.
 During release, phage lysozyme breaks down the
bacterial cell wall, and the multiplied phages are
released.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
4
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.11, steps 3–5
Burst size
Burst time
Biosynthesis &
Maturation
Attachment
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.10
Bacteriophage Lambda (): the lysogenic cycle
 Lysogenic cycle: Prophage DNA incorporated in host
DNA.
 Steps:
 Attachment
 Penetration
 Integration : Viral genome is integrated into host cell
genome. Virus is “latent”.
 Biosynthesis: Viral genome is turned on.
 Assembly
 Release
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 During the lysogenic cycle, prophage genes are
regulated by a repressor coded for by the prophage.
The prophage is replicated each time the cell divides.
 Exposure to certain mutagens can lead to excision of
the prophage and initiation of the lytic cycle.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.12
 There are three important results of lysogeny:
 The lysogenic cells become immune to reinfection with
the same phage.
 Lysogeny may undergo phage conversion. That is, the
host cell may exhibit new properties. Ex: Corynebacterium
diphtheriae, Clostridium botulinum, Vibrio cholerae
 A lysogenic phage can transfer bacterial genes from one
cell to another through transduction. Any genes can be
transferred in generalized transduction, and specific
genes can be transferred in specialized transduction.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Specialized Transduction
2
3
4
5
6
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.13
Multiplication of Animal viruses
 Steps:
 Attachment: Viruses attach to cell membrane.
 Entry by pinocytosis or fusion.
 Uncoating by viral or host enzymes.
 Biosynthesis: Production of nucleic acid and proteins.
 Maturation: Nucleic acid and capsid proteins assemble.
 Release by budding (enveloped viruses) or rupture.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Penetration
 Pinocytosis (胞飲作用)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.14a
 Fusion
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.14b
Biosynthesis
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Table 13.4
Multiplication of DNA Virus – ex: papovavirus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.15
DNA-containing animal viruses
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.16
Multiplication of ssRNA (+) Virus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.17a
Multiplication of
ssRNA (–) Virus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.17b
Multiplication of
dsRNA Virus
Figure 13.17c
RNA-containing animal viruses
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.18
Multiplication
of Retrovirus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.19
Maturation and release
Figure 13.20
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Viruses and Cancer
 The earliest relationship between cancer and viruses
was demonstrated in the early 1900s, when chicken
leukemia and chicken sarcoma were transferred to
healthy animals by cell-free filtrates.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
The transformation of normal cells into
tumor cells
 Activated oncogenes transform normal cells into
cancerous cells.
 American microbiologists J. M Bishop and H. E. Varmus
receive the 1989 Nobel Prize in Medicine for providing
that the cancer-inducing genes carried by viruses are
actually derived from animal cells.
 Viruses capable of producing tumors are called
oncogenic viruses, or oncoviruses.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 Transformed cells:
 contain a virus-specific antigen on the surface (tumor
specific transplantation antigen, TSTA), or an antigen in
their nucleus (T antigen)
 exhibit chromosomal abnormalities
 have increased growth, loss of contact inhibition
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Oncogenic viruses
 DNA oncogenic viruses
 Adenoviridae
 RNA oncogenic viruses
 Retroviridae
 Papovaviridae
 Viral RNA is
transcribed to DNA
which can integrate
into host DNA
 Hepadnaviridae
 ex: HTLV 1, HTLV 2
 Herpesviridae
 Poxviridae
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Latent Viral Infection
 A latent viral infection (潛伏性感染) is one in which the
virus remains in equilibrium with in the host cell for
long periods without producing an infection. (same as
lysogenic state)
 All human herpesvirus remain all life time. They are
reactivated by immunosupression (e.g. AIDS).
 ex: cold sores (唇疱疹)
 ex: shingles (帶狀疱疹)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Persistent Viral Infection
 Persistent (chronic) viral infections (持續性感染) are
disease processes that occur over a long period and
are generally fatal.
 Persistent viral infections are caused by conventional
viruses; viruses accumulate over a long period.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Latent and persistent viral infection
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.21
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Table 13.5
Prions
 Prions are infectious proteins first discovered in the
1980’s by American neurobiologist Stanley Prusiner in
sheep scrapie (羊搔癢症).
 The name prion is coined for proteinaceous infectious
particle.
 Spongiform encephalopathies: Sheep scrapie,
Creutzfeldt-Jakob disease (CJD), GerstmannSträussler-Scheinker syndrome, fatal familial insomnia,
mad cow disease
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 Prion diseases are due to an altered protein; the cause
can be a mutation in the normal gene (on chromosome
20) for PrPC or contact with an altered protein (PrPSc).
 PrPC: Normal cellular prion protein, on cell surface
 PrPSc: Scrapie protein; accumulates in brain cells
forming plaques
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
How a protein can be infectious
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.22
Plant Viruses and Viroids
 Plant viruses must enter plant hosts through wounds
or with invasive parasites, such as insects, nematodes,
fungi. e.g. bean mosiac virus, wound tumor virus (corn,
sugarcane), potato yellow dwarf virus.
 Some plant viruses also multiply in insect (vector) cells.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Some Plant Viruses
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
 Viroids (類病毒) are infectious pieces of RNA that
cause some plant diseases, such as potato spindle
tuber viroid (PSTV) disease.
PSTV
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
類病毒 (Viroids)
1. 單股環狀的 RNA 分子，無蛋白質外殼，核酸基因也不
製造任何蛋白質。
2. 1961年首先自馬鈴薯紡錘狀疾病塊莖中發現。
3. 1971年由 T. O. Diener 命名為類病毒 (Viroids)。
4. 分子大小約為 300~400 個核苷酸, 為一般病毒的數千分
之一。
5. 只發現於植物，於植物細胞核內複製，造成植物疾病。
6. 致病原因不明，可能是干擾宿主 mRNA 的形成。
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
類病毒
T7 噬菌體DNA
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
被感染的蕃茄
正常馬鈴薯
染病的馬鈴薯
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Clinical Focus
禽流感、猪流感與人流感
2009/04/27華視新聞：
類似SARS、禽流感，猪流感人傳人
目前世衛組織警告，猪流感疫情可
能擴大到全球，事實上猪流感和SARS、
禽流感一樣都會人傳人，猪流感會發燒、
嘔吐腹瀉，載上口罩可以避免被傳染，
潛伏期有一到七天，而且青壯年最容易
被感染。
猪流感病毒來勢洶洶，科學家發現這是一種A型流感病毒
H1N1，結合人類、鳥類和猪的三種流感病毒，同時感染在猪隻
身上，所產生的混合變種病毒不但會在猪隻之間相互傳染，還會
由猪傳染到人身上，再藉由口沫和空氣在人類之間交互傳染。尤
其是25到45歲之間，免疫力最強的青壯年最容易受到感染。
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
什麼是「H1、N1」？
 目前發現一共有16種H和9種N。
 人類的流感病毒僅有三型：H1N1、H1N2和H3N2。
 鳥類的流感卻所有都有。
 禽流感理論上不傳染給人，但卻會藉由中間媒介物 (如：
猪) 形成新的突變種，傳染給人類。
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
人類歷史上因為流感造成的死亡記事
1918-19
H1N1，造成全球5千萬人死亡。
1957-58
H2N2，1957年2月底首先於中國發現，病毒
是人流感和禽流感混合的突變種，在美國造成
7萬人死亡。
1968-69
H3N2，病毒是人流感和禽流感混合的突變
種，在美國造成3.4萬人死亡。
2009-10
H1N1，在世界造成1.4萬人死亡。在第一例
出現後三個月即有疫苗上市。
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Table B
Table A