INFLUENZA VIRUS a model for learning about disease

6.2 The Position of Viruses in the Biological Spectrum  Can infect every type of cell  Cannot exist independently from the host cell, so aren’t considered living things  However, sinc

6.1 The Search for the Elusive

6.2 The Position of Viruses in the

Biological Spectrum

 Can infect every type of cell

 Cannot exist independently from the host cell, so aren’t considered living things

 However, since they can direct life processes they are often considered more than lifeless molecules

 Referred to as infectious particles, either active or inactive

 Obligate intracellular parasites

 Cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic

machinery to make and release new viruses

6.3 The General Structure of

Viruses

Figure 6.1

Size Range

 Smallest infectious agents

 Most are so small, they can only be seen with an electron microscope

 Animal viruses

 Proviruses- around 20 nm in diameter

 Mimiviruses- up to 450 nm in length

 Viewing viruses

 Special stains and an electron microscope

 Negative staining outlines the shape

 Positive staining shows internal details

 Shadowcasting technique

Figure 6.2

Viral Components: Capsids,

Nucleic Acids, and Envelopes

 Molecular structure- composed of regular,

repeating subunits that give rise to their crystalline appearance

 Contain only those parts needed to invade and

control a host cell

 External coating

• Capsid

• Envelope- in 13 of the 20 families of animal viruses

• If no envelope, called naked virus

 Fully formed virus that is able to establish an

infection in a host cell- virion

Figure 6.4

The Viral Capsid: The Protective

Outer Shell

 Constructed from identical subunits called

capsomers

 Made up of protein molecules

 Two different types

 Helical

• Rod-shaped capsomers

• Assemble in to helical nucleocapsid

Figure 6.5

 Icosahderal

evenly spaced corners

are wide variations

Figure 6.7

Figure 6.10

The Viral Envelope

cell membrane in the form of an envelope

 In the envelope, some or all of the regular membrane proteins are replaced with viral proteins

 Some proteins form a binding layer

between the envelope and the capsid

 Glycoproteins remain exposed as spikes

(peplomers)- essential for attachment

Functions of the Viral

Capsid/Envelope

 Protects nucleic acids

 Help introduce the viral DNA or RNA into a suitable host cell

 Stimulate the immune system to produce antibodies that can protect the host cells against future infections

Nucleic Acids: At the Core of a

Virus

information carried by an organism

quite small

invade host cells and redirect their activity

DNA and RNA

RNA Viruses

 Mostly single-stranded

for immediate translation into proteins

converted into the proper form to be made

into proteins

 Segmented- individual genes exist on

separate pieces of RNA

Other Substances in the Virus

Particle

 Other Substances in the Virus Particle

within the host cell

6.4 How Viruses are Classified and

Named

Viruses, 2000

system, but it is not an official designation

6.5 Modes of Viral Multiplication

 The host cell is absolutely necessary for viral multiplication

Figure 6.11

Multiplication Cycles in Animal

 Virus encounters susceptible host cells

 Adsorbs specifically to receptor sites on the cell membrane

• Because of the exact fit required, viruses

have a limited host range

Figure 6.12

 Flexible cell membrane of the host is

penetrated by the whole virus or its nucleic acid

 Endocytosis: entire virus engulfed by the cell and enclosed in a vacuole or vesicle

 The viral envelope can also directly fuse

with the host cell membrane

Figure 6.13

host’s synthetic and metabolic machinery

they are replicated and assembled

RNA

viral proteins (translation)

the cytoplasm

 Mature virus particles are constructed from the growing pool of parts

released when the cell lyses or ruptures

 Enveloped viruses are liberated by

Figure 6.15

Damage to the Host Cell and

Persistent Infections

damage to the cell that alters its

microscopic appearance

 Inclusion bodies- compacted masses of viruses or damaged cell organelles

Figure 6.16

 Important for the diagnosis of viral infections

 Some viral infections maintain a carrier

relationship

 The cell harbors the virus and is not immediately lysed

 Persistent infections- from a few weeks to the

remainder of the host’s life

 Some viruses remain in a chronic latent state, periodically becoming activated

 Some viruses enter their host cell and

permanently alter its genetic material, leading to cancer

 Oncogenic viruses

 Their effect is called transformation

 Oncoviruses- mammalian viruses capable of

initiating tumors

Viruses that Infect Bacteria

 Often make the bacteria they infect more pathogenic for humans

T-even Phages

 Adsorb to host bacteria

 The nucleic acid penetrates the host after being injected through a rigid tube inserted through the bacterial membrane and wall

 Entry of the nucleic acid causes the cessation of host cell DNA replication and protein synthesis

 The host cell machinery is then used for viral replication and

synthesis of viral proteins

 As the host cell produces new parts, they spontaneously assemble

Figure 6.17

Lysogeny: The Silent Virus

Infection

that undergo adsorption and penetration but are not replicated or released

immediately

 Instead the viral DNA enters an inactive

have the temperate phage DNA

bacterium acquires a new trait from its

temperate phage

Figure 6.20

6.6 Techniques in Cultivating and

Identifying Animal Viruses

cycles, genetics, and effects on host cells

pigs, and rabbits

used

propagation

Figure 6.21

Using Cell (Tissue) Culture

 Animal cell cultures are grown in sterile

chambers with special media

 Cultured cells grow in the form of a

monolayer

 Primary or continuous

Figure 6.22

6.7 Medical Importance of

Viruses

 Most common cause of acute infections

that do not result in hospitalization

 Most do not cause death but those that do can have very high mortality rates

 Others can lead to long-term debility

6.8 Other Noncellular Infectious

Agents

 Spongiform encephalopathies

 Chronic, persistent diseases

 Long period of latency

 Deposition of protein fibrils in the brain tissue- prions

 Satellite viruses

 Defective forms of viruses

 Dependent on other viruses for replication

6.9 Treatment of Animal Viral

 Interferon shows potential for treating and preventing viral infections

 Vaccines stimulate immunity