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VIRUSES Chapter 13, Slides of Virology

Texas A&M University (A&M)Virology

DNA viruses are produced directly; the host cell builds viral components from the genetic instructions provided by the viral DNA b. Retro RNA viruses induce ...

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VIRUSES Chapter 13
I. Properties of Viruses
Virology is the study of viruses and virus-like proteins
A. Nucleic acid enclosed by a protein coat
1. morphology: polygonal, helical or complex
a. Viruses range in size from 20 to 250 nanometers (nm)
b. Ordinarily too small to be seen by light microscopy, viruses usually can be detected with
electron microscopy
2. Contain DNA or RNA, never both
Viruses have a unique composition
a. Some viruses are composed of only one type of nucleic acid, which makes up their genetic
code; others, such as Prions, are composed entirely of protein and contain no nucleic acid
b. Deoxyribonucleic acid (DNA) viruses are composed of DNA
c. Ribonucleic acid (RNA) viruses are composed of RNA
3. Lack properties of cells such as membranes, ribosomes, enzymes, and ATP synthesizing
mechanisms
B. Obligate intracellular parasites
1. Act obligate intracellular parasites, viruses lack the capacity for independent metabolism and
reproduce only in living hosts
a. The hosts that they infect are bacteria, plant cells, animal cells, human cells, and any of the
preceding cells in tissue cultures
b. Viral infection of human cells can cause such diseases as acquired immunodeficiency
syndrome (AIDS), chicken pox, hepatitis, and polio
C. Classification
1. Genome structure – DNA or RNA, single-stranded or double-stranded, segmented or a
single molecule.
2. Virus particle structure - isometric (icosahedral), helical (rod-shaped), or pleomorphic
(irregular in shape)
3. Presence or absence of a viral envelope.
4. The host the virus attacks.
General characteristic
1. Viruses attach to specific receptor sites on host cells
a. The ability of a virus to infect an organism is determined by the types and presence of
receptor sites on host cells
b. Plant viruses tend to infect only plants and animal viruses tend to infect only animals
because of the nature of the receptor sites
c. Receptor sites are proteins on the surface of cells
2. Once a virus enters a host cell through the receptor site, it takes over the host cell's metabolism
and uses the host cell's components to produce more virus particles
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VIRUSES Chapter 13

I. Properties of Viruses

Virology is the study of viruses and virus-like proteins

A. Nucleic acid enclosed by a protein coat

  1. morphology: polygonal, helical or complex a. Viruses range in size from 20 to 250 nanometers (nm) b. Ordinarily too small to be seen by light microscopy, viruses usually can be detected with electron microscopy
  2. Contain DNA or RNA, never both Viruses have a unique composition a. Some viruses are composed of only one type of nucleic acid, which makes up their genetic code; others, such as Prions, are composed entirely of protein and contain no nucleic acid b. Deoxyribonucleic acid (DNA) viruses are composed of DNA c. Ribonucleic acid (RNA) viruses are composed of RNA
  3. Lack properties of cells such as membranes, ribosomes, enzymes, and ATP synthesizing mechanisms

B. Obligate intracellular parasites

  1. Act obligate intracellular parasites, viruses lack the capacity for independent metabolism and reproduce only in living hosts a. The hosts that they infect are bacteria, plant cells, animal cells, human cells, and any of the preceding cells in tissue cultures b. Viral infection of human cells can cause such diseases as acquired immunodeficiency syndrome (AIDS), chicken pox, hepatitis, and polio

C. Classification

  1. Genome structure – DNA or RNA, single-stranded or double-stranded, segmented or a single molecule.
  2. Virus particle structure - isometric (icosahedral), helical (rod-shaped), or pleomorphic (irregular in shape)
  3. Presence or absence of a viral envelope.
  4. The host the virus attacks.

General characteristic

  1. Viruses attach to specific receptor sites on host cells a. The ability of a virus to infect an organism is determined by the types and presence of receptor sites on host cells b. Plant viruses tend to infect only plants and animal viruses tend to infect only animals because of the nature of the receptor sites c. Receptor sites are proteins on the surface of cells
  2. Once a virus enters a host cell through the receptor site, it takes over the host cell's metabolism and uses the host cell's components to produce more virus particles

a. DNA viruses are produced directly; the host cell builds viral components from the genetic instructions provided by the viral DNA b. Retro RNA viruses induce production of messenger RNA or transcriptase enzymes, which copy viral RNA back into DNA for use by the host cell

  1. Morphology and size: a. The morphology of viruses is simpler than that of bacteria (1) The virus particle has a rod-shaped central core composed of nucleic acid (2) Covering the core is a protein coat called a capsid (3) An envelope derived from the host cell membrane may protect the viral capsids by camouflaging them with host antigens (a) Viruses with envelopes tend to be resistant to the host's immune system (b) Viruses without envelopes are more common in plants
  2. Individual virus particles are called virions a. Virions can be shaped as cubes, spheres, icosahedra (20-sided polygons), or helices b. They can also occur as complex forms composed of multiple copies of the shapes mentioned above
  3. Accessory structures (tails, envelopes, spikes, etc.)

D. Incubation periods

  1. Incubation periods for viral diseases range from days to years a. Many infections, such as mumps and chicken pox, incubate for a few days, whereas the human immunodeficiency virus may not produce symptoms for 5 to 10 years b. Latent, or long-term, infections are possible (1) These infections are characterized by asymptomatic periods, followed by the reappearance of symptoms (2) Reactivation of a latent virus results in new viral reproduction

E. Viral types

  • I: Double-stranded DNA (e.g. Adenoviruses, Herpesviruses, Poxviruses)
  • II: Single-stranded (+)sense DNA (e.g. Parvoviruses)
  • III: Double-stranded RNA (e.g. Reoviruses)
  • IV: Single-stranded (+)sense RNA (e.g. Picornaviruses, Togaviruses)
  • V: Single-stranded (-)sense RNA (e.g. Orthomyxoviruses, Rhabdoviruses)
  • VI: Single-stranded (+)sense RNA with DNA intermediate in life-cycle (e.g. Retroviruses)
  • VII: Double-stranded DNA with RNA intermediate (e.g. Hepadnaviruses)

As an example of viral classification, the chicken pox virus, Varicella zoster (VZV), belongs to family Herpesviridae, subfamily Alphaherpesvirinae and genus Varicellovirus. It remains unranked in terms of order. VZV is in Group I of the Baltimore Classification because it is a dsDNA virus that does not use reverse transcriptase.

II. DNA Viruses

A. General information

  1. DNA viruses, which have only deoxyribonucleic acid in their core, typically occur as icosahedral forms
  2. After inserting their DNA into the host DNA, the viruses are duplicated during replication, translation, and transcription (see Chapter 13 for more information on DNA replication, translation, and transcription)
  3. Cell cultures allow microbiologists to grow these viruses for study a. Research with DNA viruses has led to advances in genetics, gene therapy, vaccination and prevention of disease, and cancer treatment b. Vaccines to combat hepatitis B, for instance, are produced by recombinant-gene technology
  4. DNA viruses often form cellular masses known as inclusion bodies, which are helpful in identification

B. Means of identification

  1. DNA viruses can be identified by the signs and symptoms of the diseases that they cause
  2. If the virus forms inclusion bodies in host cells, these inclusions may be seen when the organism is examined under a microscope
  3. Another means of identification are the specific antibodies produced in response to viral antigens; these antibodies can be measured in the blood (a) Serological: Western blot test
  1. Growing of Virus a. Egg or cell line culture
  2. Nucleic Acid reproduction (molecular methods) to identify by a. RFLP (restriction fragment length polymorphism) b. PCR ( polymerase chain reaction)
  3. Electron microscope

C. Taxonomic classification: Adenovirus

  1. This group of viruses infects mammals and birds through airborne and fecaloral transmission
  2. Adenoviruses affect the upper respiratory tract, intestinal tract, and conjunctivae
  3. This virus affects liver cells, causing fever, malaise, jaundice, and (occasionally) death h
  4. It is unrelated to any other known human virus

III. Bacteriophages

A. Fine structure

  1. Capsid (head), {sheath, tail fiber, baseplate, pin} = Tail

B. Detection and quantification (see lab 15)

C. Life cycle of a lytic phage

1. Molecular events during 5 distinct stages :

a. 1- attachment; 2 - penetration; 3 - biosynthesis 4 - maturation 5 - release

2. Eclipse period

a. The period of time is when viral multiplication is complete, yet, infective virions are not present.

3. Burst time

a. averages 20 - 40 minutes b. the number of phage particles released from a single cell is referred to burst size Ranging from 50 to 200 particles

D. Life cycle of a lysogenic phage

Lysogeny is a state of cell chromosome where a bacteriophage genome has been inserted into the bacterial chromosome by nonreciprocal recombination occurring between the phage chromosome and the bacterial chromosome. This insertion occurs at specific locations in each of chromosomes where there in homology of sequences in the two chromosomes.

Lysogenic conversion is the state of a cell that shows new properties like ability to form cytotoxins. The tox gene, coding for a toxic protein affecting eukaryotic cells, is on the phage genome that is expressed in the bacterium without causing lysis of the bacterial cell and production of more phage. The tox gene that is located in a corynephage chromosome codes for diphtheria toxin that kills susceptible human cells. When this phage chromosome becomes inserted into chromosome of the bacterium Corynebacterium diphtheriae, human infection with this microbe leads to formation and release of diphtheria toxin in the human host producing symptoms of diphtheria.

  1. Penetration 3 methods of viral penetration into eukaryotic host cell a. endocytosis : folding of the cell membrane inward engulfing enveloped virus 1) Enveloped virus uncoats within cytoplasm of host release nucleic acid b. Fusion : Envelope of Enveloped viruses fuse with host plasma membrane (1) releasing capsid into cell’s cytoplasm HIV works in this manner c. Direct penetration of nucleic acid into cytoplasm through membrane of host without capsid entering the cytoplasm
  2. Uncoating a. Viral nucleic acid separates from its protein coat.

b. Enzymes degrade the proteins of the viral capsid releasing the virus nucleic acid Poliovirus works in this manner

C. Replication & Maturation

Biosynthesis of Nucleic Acid Virus uses the host system to make new nucleic acid

Proteins and capsid are synthesized in cytoplasm.

Proteins migrate into nucleus and are assembled into active virus.

Released from host cell

DNA viruses

Transcription and translation using host enzymes

Exception is Poxviruses - All component synthesized in cytoplasm. Poxviruses use their own transcriptase enzyme

D. Importance: Interactions with host cells

  1. Active infections caused by DNA viruses include hepatitis, infectious monomucleosis, Burkitt’s lymphoma, chicken pox, and small pox.
  2. Latent infections caused by DNA viruses include genital herpes and pharyngitis cause by adenovruses
  3. Oncogenic (cancer-causing) and potentially oncogenic DNA viruses are common; such viruses cause cancer of the liver and genitourinary tract, as well as lymphoma and papilloma.
  4. Prions : infectious protein particles: a. the gene of PrP is located on chromosome 20 in the humans. PrP produced by cells isc^ c secreted to the cell surface. This type of disease runs in the family line.

b. PrP sc^ is reacts with PrP on the cell surface converting the PrP to PrPc^ c^ sc

c. PrP scis taken in by endocytosis and accumulates in the lysome.

V. RNA Viruses A. General information

  1. RNA viruses, which have only ribonucleic acid in their core, occur as cubic, icosahedral, helical, and spherical forms
  2. They contain transcriptase enzymes that copy viral RNA back into DNA; the DNA can then be used for viral replication
  3. These viruses often form cellular inclusions that are useful in identification
  4. Non-retro RNA viruses replicate within the cytoplasm of the host a. Positive (+) strand RNA synthesize negative (-) strand RNA 1) -RNA templates for further synthesis of +RNA 1) +RNA acts as (^) mRNA templating the protein synthesis

B. Means of identification

  1. RNA viruses can be identified by the signs and symptoms of the diseases that they cause
  2. Host cell inclusion bodies may be formed and can be seen during microscopic examination
  3. Electron microscopy also is useful for identifying RNA viruse
  4. Specific antibodies produced to viral antigens can be measured in the blood
  5. Cell cultures permit growth of these viruses for study

C. Importance

  1. Active infection caused by RNA viruses includes various diseases of plants, animals, and humans; human diseases include colds, encephalitis, mumps, and measles
  2. Latent infections are also possible
  3. Oncogenic or potentially oncogenic viruses are common; RNA viruses produce many types of leukemia

D. Taxonomic classification: Arenavirus

  1. This class of viruses infects rodents, primates, and humans through airborne transmission and vector bites
  2. Arenaviruses affect the central nervous system, blood, lungs, and kidneys
  3. Examples include lymphocytic choriomeningitis virus, Lassa fever virus, and South American hemorrhagic fever virus

E. Taxonomic classification: Bunyavirus

  1. These viruses infect mammals and humans through bites from mosquitoes and flies
  2. Bunya viruses affect the central nervous system and cause febrile illnesses
  3. California virus (a cause of encephalitis) is a bunyavirus

F. Taxonomic classification: Coronavirus

  1. These viruses are so named because, under an electron microscope, they resemble a corona or crown
  2. Coronaviruses affect the upper respiratory tract in adults and the lower respiratory tract in children
  3. Viral strains that cause the common cold belong to this group

G. Taxonomic classification: Influenzavirus

  1. The influenzaviruses infect birds, mammals, and humans through airborne transmission
  2. This virus causes an acute infection of the respiratory tract
  3. Three major strains of influenzavirus have been identified-types A, B, and C

or mosquitoes

  1. These viruses affect the skin, mucous membranes, nervous system, and internal organs, causing inflammation, fever, rash, or encephalitis
  2. Togaviruses comprise the German measles, or rubella, virus and numerous viruses carried from animals to humans by mosquitoes and ticks

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