Chapter 4 VIRUSES
第四章 病毒
A virus is a submicroscopic infectious particle composed of a protein coat and a nucleic acid core. Viruses, like cells, carry genetic information encoded in their nucleic acid, and can undergo mutations and reproduce; however, they cannot carry out metabolism, and thus are not considered alive. Viruses are classified by the type of nucleic acid they contain, and the shape of their protein capsule.

General Characteristics of Viruses

1.      Depending on one's viewpoint, viruses may be regarded as   exceptionally   complex   aggregations of nonliving chemicals or as exceptionally simple living microbes.

2.      Viruses contain a single type of nucleic acid (DNA or RNA) and a protein coat, sometimes enclosed by an envelope composed of lipids, proteins, and carbohydrates.

3.      Viruses are obligatory intracellular parasites. They multiply by using the host cell's synthesizing machinery tocause the synthesis of specialized elements that can transfer the viral nucleic acid to other cells.

4.      A virion is a complete, fully developed viral particle composed of nucleic acid surrounded by a coat.

HOST RANGE                  

1.       Host range refers to the spectrum of host cells in which a virus can multiply.

2.       Depending on its host range, a virus is generally classified as an animal virus, bacterial virus (bacteriophage), or plant virus. A virus can infect only certain species within each class.

3.       Host range is determined by the specific attachment site on the host cell's surface and the availability of host cellular factors.

SIZE                            

1.       Viral size is determined by filtration through membrane filters, ultracentrifugation, and electron microscopy.

2.       Viruses range from 20 to 300nm in diameter.

Viral Structure

NUCLEIC ACID                    

1.      The proportion of nucleic acid in relation to protein in viruses ranges from about 1% to about 50%.

2.      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.

CAPSID AND ENVELOPE         

1.      The protein coat surrounding the nucleic acid of a virus is called the capsid（衣殼）.

2.      The capsid is composed of subunits, the capsomeres（衣殼粒）,which can be a single type of protein or several types.

3.      The capsid of some viruses is enclosed by an envelope consisting of lipids, proteins, and carbohydrates.

4.      Some envelopes are covered with carbohydrate-protein complexes called spikes（刺突）.

5.      Viruses without envelopes are called naked viruses.

GENERAL MORPHOLOGY          

1.  Helical （螺旋對(duì)稱(chēng)）viruses (for example, tobacco mosaic virus) resemble long rods, and their capsids are hollow cylinders surrounding the nucleic acid.

2.   Polyhedral（多面體） viruses (for example, adenovirus，腺病毒) are many sided. Usually the capsid is an icosahedron（二十面體）. Enveloped viruses are covered by an envelope and are roughly spherical but highly pleomorphic（多型性的）. There are also enveloped helical viruses (for example, influenza virus) and enveloped polyhedral viruses (for example, herpes simplex virus).

3.  Complex viruses have complex structures. For example, many bacteriophages have a polyhedral capsid with a helical tail attached.

Taxonomy of Viruses

1.      Classification of viruses is based on type of nucleic acid, morphological class, size of capsid, and number of capsomeres.

2.      Other classification schemes take into account the virus's susceptibility to microbial control agents, immunological properties, site of multiplication, and method of transmission.

Isolation, Cultivation, and Identification of Viruses  

1.              Viruses must be grown in living cells.

2.               The easiest viruses to grow are bacteriophages.

GROWTH OF BACTERIOPHAGES IN THE LABORATORY

The plaque method：

1.              Mixes bacteriophages with host bacteria and nutrient agar.

2.              After several viral multiplication cycles, the bacteria in the area surrounding the original virus are destroyed; the area of lysis is called a plaque.

3.              Each plaque can originate with a single viral particle or with more than one; the concentration of viruses is given as plaque-forming units.

GROWTH OF ANIMAL VIRUSES IN THE LABORATORY 

1.      Cultivation of some animal viruses requires whole animals.

2.      Some animal viruses can be cultivated in embryonated eggs.

3.      Cell cultures are cells growing in culture media in the laboratory.

4.      Viral growth can cause cytopathic effects in the cell culture.

VIRAL IDENTIFICATION        

1.  Serological tests are used most often to identify viruses. Viruses may be identified by restriction enzyme fragments and nucleic acid base sequencing.

2.  Viral Multiplication 

1.     Viruses do not contain enzymes for energy production or protein synthesis.

2.     For a virus to multiply, it must invade a host cell and direct the host's metabolic machinery to produce viral enzymes and components.

MULTIPLICATION OF BACTERIOPHAGES

T-Even Bacteriophages       

1.     The T-even bacteriophages that infect E coli have been studied extensively.

2.     In attachment, sites on the phage's tail fibers attach to complementary receptor sites on the bacterial cell.

3.     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.

4.     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.

5.     During maturation, phage DNA and capsids are assembled into complete viruses.

6.     During release, phage lysozyme breaks down the bacterial cell wall, and the multiplied phages are released.

7.     The time from phage adsorption to release is called burst time (20 to 40 minutes). Burst size, the number of newly synthesized phages produced from a single infected cell, ranges from 50 to 200.

Lysogeny                    

1.    During a lytic cycle, a phage causes the lysis and death of a host cell.

2.    Some viruses can either cause lysis or have their DNA incorporated as a prophage into the DNA of the host cell. The latter situation is called lysogeny.

3.    Prophage genes are regulated by a represser coded for by the prophage. The prophage is replicated each time the cell divides.

4.    Exposure to certain mutagens can lead to excision of the prophage and initiation of the lytic cycle.

5.    Because of lysogeny, lysogenic cells become immune to reinfection with the same phage, and the host cell can exhibit new properties.

6.    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.

MULTIPLICATION OF ANIMAL VIRUSES  

Animal viruses attach to the plasma membrane of the host cell.

1.    Penetration of enveloped viruses occurs by endocytosis.

2.    Animal viruses are uncoated by viral or host cell enzymes. The DNA of most DNA viruses is released into the nucleus of the host cell. Transcription of viral DNA and translation produce viral DNA and, later, capsid protein. Capsid protein is synthesized in the cytoplasm of the host cell.

3.    Retroviruses carry reverse transcriptase (RNA-dependent DNA polymerase), which transcribes DNA from RNA.

4.     After maturation, viruses are released. One method of release (and envelope formation) is budding. Naked viruses are released through ruptures in the host cell membrane.

Effects of Animal Viral Infection on Host Cells  

1.      Cytopathic effects (CPE) are abnormalities that lead to damage or death of a host cell.

2.      Cytopathic effects include inclusion bodies, polykaryocytes, and altered function.

Interferon

Interferon is produced by virus-infected cells and protects neighboring cells from viral infection.

Viruses and Diseases
Viruses cause a variety of diseases among all groups of living organisms. Viral diseases include the flu, common cold, herpes, measles, chicken pox, small pox, and encephalitis. Antibiotics are not effective against viruses. Vaccination offers protection for uninfected individuals.

Emergent Viruses

Viruses are usually quite specific as to their hosts and even to the types of cells they infect in a multicellular host. Recently some viruses appear to have shifted their host: HIV, hantavirus, and ebola appear to be either viruses shifting to a new (human) host or else viruses whose existence and effects are just now being realized by scientists and the general public.

Viruses and Cancer

1.      An excess of tissue due to unusually rapid cell multiplication is called a tumor. Tumors are malignant (cancerous) or benign (noncancerous). Metastasis refers to the spread of cancer to other parts of the body.

2.      Tumors are usually named by attachment of the suffixoma to the name of the tissue from which the tumor arises.

3.      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.

TRANSFORMATION OF NORMAL CELLS INTO TUMOR CELLS  

1.      Eucaryotic cells have proto-oncogenes that code for proteins necessary for the cells' normal growth. When activated to oncogenes, these genes transform normal cells into cancerous cells.

2.      Viruses capable of producing tumors are called oncogenic viruses.

3.      Several DNA viruses and retroviruses are oncogenic.

4.  The genetic material of oncogenic viruses becomes integrated into the host cell's DNA.Transformed cells lose contact inhibition, contain virusspecific antigens (TSTA and T antigen), exhibit chromosomal abnormalities, and can produce tumors when injected into susceptible animals.

ACTIVATION OF ONCOGENES

A single mutation can result in the production of a protein required for transformation.

1.      Transduction of oncogenes could result in oncogene products being made in abnormal amounts or at the wrong time.

2.      Translocation of oncogenes could remove normal controls.

3.      Gene amplification causes unusually large amounts of oncogene products.

DNA-CONTAINING ONCOGENIC VIRUSES 

1.      Oncogenic viruses are found among adenoviruses, hepesviruses, poxviruses, and papovaviruses.

2.      The EB virus, a herpesvirus, causes infectious mononu cleosis and has been implicated in Burkitt's lymphoma
and nasopharyngeal carcinoma. One type of herpes simplex virus (HHV 2), associated with over 90% of genital
herpes infections, might be implicated in cervical cancer.

RNA-CONTAINING ONCOGENIC VIRUSES 

1.      Among the RNA viruses, only retroviruses seem to be oncogenic.

2.      HTLV 1 and HTLV 2 have been associated with human leukemia and lymphoma.

3.      The virus's ability to produce tumors is related to the production of reverse transcriptase. The DNA synthe sized from the viral RNA becomes incorporated as a provirus into the host cell's DNA.

Plant Viruses              

Plant viruses must enter plant hosts through wounds or with invasive parasites, such as insects.

Viroids and Prions

Viroids are common plant pathogens which are a serious economic problem.

The RNA genomes of viroids are 246-375 nucleotides in length and share many similarities:

They are all single stranded covalent circles
There is extensive intramolecular base pairing
A DNA-directed RNA polymerase makes both plus and minus strands
Replication does not depend on the presence of a helper virus
No proteins are encoded
Virusoids and Satellites
Virusoids or satellite RNAs are also several hundred nucleotides long circular and single stranded. They depend on a helper virus for replication.

 

Chapter 4 virus     review questions

1.      Viruses were first detected because they are filterable. What do we mean by the term filterable, and how could this property have helped their detection before invention of the electron microscope?

2.      Why do we classify viruses as obligatory intracellular parasites?

3.      List the four properties that define a virus. What is a virion?

4.      Describe the three morphological classes of viruses, then diagram and give an example of each.

5.  Describe how bacteriophages are detected and enumerated by the plaque method.

6.  Describe the multiplication of a T-even bacteriophage. Be sure to include the essential features of attachment, penetration, biosynthesis, maturation, and release.

7.  Recall from Chapter 1 that Koch's postulates are used todetermine the etiology of a disease. Why is it difficult to determine the etiology of a viral infection such as influenza?

8.  Assume that this strand of RNA is the nucleic acid for an RNA-containing animal virus: UAGUCAAGGU.

(a)      Describe the steps of RNA replication for a virus that contains a + strand of RNA.

(b)     Describe the steps of RNA replication for a virus that contains a - strand of RNA.

(c)      Describe the steps of RNA replication for a virus that contains double-stranded RNA.

(d)     Describe the steps of RNA replication for a virus that contains reverse transcriptase.

9.            In some viruses, capsomeres function as enzymes as well as structural supports. Of what advantage is this to the virus.

10.     Prophages and proviruses have been described as being similar to bacterial plasmids. What similar properties do they exhibit? How are they different?

 

第四章 病毒
A virus is a submicroscopic infectious particle composed of a protein coat and a nucleic acid core. Viruses, like cells, carry genetic information encoded in their nucleic acid, and can undergo mutations and reproduce; however, they cannot carry out metabolism, and thus are not considered alive. Viruses are classified by the type of nucleic acid they contain, and the shape of their protein capsule.

General Characteristics of Viruses

1.      Depending on one's viewpoint, viruses may be regarded as   exceptionally   complex   aggregations of nonliving chemicals or as exceptionally simple living microbes.

2.      Viruses contain a single type of nucleic acid (DNA or RNA) and a protein coat, sometimes enclosed by an envelope composed of lipids, proteins, and carbohydrates.

3.      Viruses are obligatory intracellular parasites. They multiply by using the host cell's synthesizing machinery tocause the synthesis of specialized elements that can transfer the viral nucleic acid to other cells.

4.      A virion is a complete, fully developed viral particle composed of nucleic acid surrounded by a coat.

HOST RANGE                  

1.       Host range refers to the spectrum of host cells in which a virus can multiply.

2.       Depending on its host range, a virus is generally classified as an animal virus, bacterial virus (bacteriophage), or plant virus. A virus can infect only certain species within each class.

3.       Host range is determined by the specific attachment site on the host cell's surface and the availability of host cellular factors.

SIZE                            

1.       Viral size is determined by filtration through membrane filters, ultracentrifugation, and electron microscopy.

2.       Viruses range from 20 to 300nm in diameter.

Viral Structure

NUCLEIC ACID                    

1.      The proportion of nucleic acid in relation to protein in viruses ranges from about 1% to about 50%.

2.      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.

CAPSID AND ENVELOPE         

1.      The protein coat surrounding the nucleic acid of a virus is called the capsid（衣殼）.

2.      The capsid is composed of subunits, the capsomeres（衣殼粒）,which can be a single type of protein or several types.

3.      The capsid of some viruses is enclosed by an envelope consisting of lipids, proteins, and carbohydrates.

4.      Some envelopes are covered with carbohydrate-protein complexes called spikes（刺突）.

5.      Viruses without envelopes are called naked viruses.

GENERAL MORPHOLOGY          

1.  Helical （螺旋對(duì)稱(chēng)）viruses (for example, tobacco mosaic virus) resemble long rods, and their capsids are hollow cylinders surrounding the nucleic acid.

2.   Polyhedral（多面體） viruses (for example, adenovirus，腺病毒) are many sided. Usually the capsid is an icosahedron（二十面體）. Enveloped viruses are covered by an envelope and are roughly spherical but highly pleomorphic（多型性的）. There are also enveloped helical viruses (for example, influenza virus) and enveloped polyhedral viruses (for example, herpes simplex virus).

3.  Complex viruses have complex structures. For example, many bacteriophages have a polyhedral capsid with a helical tail attached.

Taxonomy of Viruses

1.      Classification of viruses is based on type of nucleic acid, morphological class, size of capsid, and number of capsomeres.

2.      Other classification schemes take into account the virus's susceptibility to microbial control agents, immunological properties, site of multiplication, and method of transmission.

Isolation, Cultivation, and Identification of Viruses  

1.              Viruses must be grown in living cells.

2.               The easiest viruses to grow are bacteriophages.

GROWTH OF BACTERIOPHAGES IN THE LABORATORY

The plaque method：

1.              Mixes bacteriophages with host bacteria and nutrient agar.

2.              After several viral multiplication cycles, the bacteria in the area surrounding the original virus are destroyed; the area of lysis is called a plaque.

3.              Each plaque can originate with a single viral particle or with more than one; the concentration of viruses is given as plaque-forming units.

GROWTH OF ANIMAL VIRUSES IN THE LABORATORY 

1.      Cultivation of some animal viruses requires whole animals.

2.      Some animal viruses can be cultivated in embryonated eggs.

3.      Cell cultures are cells growing in culture media in the laboratory.

4.      Viral growth can cause cytopathic effects in the cell culture.

VIRAL IDENTIFICATION        

1.  Serological tests are used most often to identify viruses. Viruses may be identified by restriction enzyme fragments and nucleic acid base sequencing.

2.  Viral Multiplication 

1.     Viruses do not contain enzymes for energy production or protein synthesis.

2.     For a virus to multiply, it must invade a host cell and direct the host's metabolic machinery to produce viral enzymes and components.

MULTIPLICATION OF BACTERIOPHAGES

T-Even Bacteriophages       

1.     The T-even bacteriophages that infect E coli have been studied extensively.

2.     In attachment, sites on the phage's tail fibers attach to complementary receptor sites on the bacterial cell.

3.     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.

4.     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.

5.     During maturation, phage DNA and capsids are assembled into complete viruses.

6.     During release, phage lysozyme breaks down the bacterial cell wall, and the multiplied phages are released.

7.     The time from phage adsorption to release is called burst time (20 to 40 minutes). Burst size, the number of newly synthesized phages produced from a single infected cell, ranges from 50 to 200.

Lysogeny                    

1.    During a lytic cycle, a phage causes the lysis and death of a host cell.

2.    Some viruses can either cause lysis or have their DNA incorporated as a prophage into the DNA of the host cell. The latter situation is called lysogeny.

3.    Prophage genes are regulated by a represser coded for by the prophage. The prophage is replicated each time the cell divides.

4.    Exposure to certain mutagens can lead to excision of the prophage and initiation of the lytic cycle.

5.    Because of lysogeny, lysogenic cells become immune to reinfection with the same phage, and the host cell can exhibit new properties.

6.    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.

MULTIPLICATION OF ANIMAL VIRUSES  

Animal viruses attach to the plasma membrane of the host cell.

1.    Penetration of enveloped viruses occurs by endocytosis.

2.    Animal viruses are uncoated by viral or host cell enzymes. The DNA of most DNA viruses is released into the nucleus of the host cell. Transcription of viral DNA and translation produce viral DNA and, later, capsid protein. Capsid protein is synthesized in the cytoplasm of the host cell.

3.    Retroviruses carry reverse transcriptase (RNA-dependent DNA polymerase), which transcribes DNA from RNA.

4.     After maturation, viruses are released. One method of release (and envelope formation) is budding. Naked viruses are released through ruptures in the host cell membrane.

Effects of Animal Viral Infection on Host Cells  

1.      Cytopathic effects (CPE) are abnormalities that lead to damage or death of a host cell.

2.      Cytopathic effects include inclusion bodies, polykaryocytes, and altered function.

Interferon

Interferon is produced by virus-infected cells and protects neighboring cells from viral infection.

Viruses and Diseases
Viruses cause a variety of diseases among all groups of living organisms. Viral diseases include the flu, common cold, herpes, measles, chicken pox, small pox, and encephalitis. Antibiotics are not effective against viruses. Vaccination offers protection for uninfected individuals.

Emergent Viruses

Viruses are usually quite specific as to their hosts and even to the types of cells they infect in a multicellular host. Recently some viruses appear to have shifted their host: HIV, hantavirus, and ebola appear to be either viruses shifting to a new (human) host or else viruses whose existence and effects are just now being realized by scientists and the general public.

Viruses and Cancer

1.      An excess of tissue due to unusually rapid cell multiplication is called a tumor. Tumors are malignant (cancerous) or benign (noncancerous). Metastasis refers to the spread of cancer to other parts of the body.

2.      Tumors are usually named by attachment of the suffixoma to the name of the tissue from which the tumor arises.

3.      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.

TRANSFORMATION OF NORMAL CELLS INTO TUMOR CELLS  

1.      Eucaryotic cells have proto-oncogenes that code for proteins necessary for the cells' normal growth. When activated to oncogenes, these genes transform normal cells into cancerous cells.

2.      Viruses capable of producing tumors are called oncogenic viruses.

3.      Several DNA viruses and retroviruses are oncogenic.

4.  The genetic material of oncogenic viruses becomes integrated into the host cell's DNA.Transformed cells lose contact inhibition, contain virusspecific antigens (TSTA and T antigen), exhibit chromosomal abnormalities, and can produce tumors when injected into susceptible animals.

ACTIVATION OF ONCOGENES

A single mutation can result in the production of a protein required for transformation.

1.      Transduction of oncogenes could result in oncogene products being made in abnormal amounts or at the wrong time.

2.      Translocation of oncogenes could remove normal controls.

3.      Gene amplification causes unusually large amounts of oncogene products.

DNA-CONTAINING ONCOGENIC VIRUSES 

1.      Oncogenic viruses are found among adenoviruses, hepesviruses, poxviruses, and papovaviruses.

2.      The EB virus, a herpesvirus, causes infectious mononu cleosis and has been implicated in Burkitt's lymphoma
and nasopharyngeal carcinoma. One type of herpes simplex virus (HHV 2), associated with over 90% of genital
herpes infections, might be implicated in cervical cancer.

RNA-CONTAINING ONCOGENIC VIRUSES 

1.      Among the RNA viruses, only retroviruses seem to be oncogenic.

2.      HTLV 1 and HTLV 2 have been associated with human leukemia and lymphoma.

3.      The virus's ability to produce tumors is related to the production of reverse transcriptase. The DNA synthe sized from the viral RNA becomes incorporated as a provirus into the host cell's DNA.

Plant Viruses              

Plant viruses must enter plant hosts through wounds or with invasive parasites, such as insects.

Viroids and Prions

Viroids are common plant pathogens which are a serious economic problem.

The RNA genomes of viroids are 246-375 nucleotides in length and share many similarities:

They are all single stranded covalent circles
There is extensive intramolecular base pairing
A DNA-directed RNA polymerase makes both plus and minus strands
Replication does not depend on the presence of a helper virus
No proteins are encoded
Virusoids and Satellites
Virusoids or satellite RNAs are also several hundred nucleotides long circular and single stranded. They depend on a helper virus for replication.

 

Chapter 4 virus     review questions

1.      Viruses were first detected because they are filterable. What do we mean by the term filterable, and how could this property have helped their detection before invention of the electron microscope?

2.      Why do we classify viruses as obligatory intracellular parasites?

3.      List the four properties that define a virus. What is a virion?

4.      Describe the three morphological classes of viruses, then diagram and give an example of each.

5.  Describe how bacteriophages are detected and enumerated by the plaque method.

6.  Describe the multiplication of a T-even bacteriophage. Be sure to include the essential features of attachment, penetration, biosynthesis, maturation, and release.

7.  Recall from Chapter 1 that Koch's postulates are used todetermine the etiology of a disease. Why is it difficult to determine the etiology of a viral infection such as influenza?

8.  Assume that this strand of RNA is the nucleic acid for an RNA-containing animal virus: UAGUCAAGGU.

(a)      Describe the steps of RNA replication for a virus that contains a + strand of RNA.

(b)     Describe the steps of RNA replication for a virus that contains a - strand of RNA.

(c)      Describe the steps of RNA replication for a virus that contains double-stranded RNA.

(d)     Describe the steps of RNA replication for a virus that contains reverse transcriptase.

9.            In some viruses, capsomeres function as enzymes as well as structural supports. Of what advantage is this to the virus.

10.     Prophages and proviruses have been described as being similar to bacterial plasmids. What similar properties do they exhibit? How are they different?