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MICRO 121 > Lecture 2: The World of Viruses - Virus Structure and Function > Flashcards

Lecture 2: The World of Viruses - Virus Structure and Function Flashcards

1
Q

What is a Virus

A
  • Meaning toxin or poison
  • Tiny obligate parasites, unable to sustain themselves and do not replicate on their own
  • Made up of proteins and ONE type of nucleic acid (either DNA or RNA)
  • Main function of a virion (single virus molecule) is to deliver its DNA or RNA into the host cell so that it can be expressed (transcribed and translated) by the host cell
  • Can infect animal cells, bacteria, human cells
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2
Q

Bacteria vs. Viruses

A
  • Incredibly small - need an electron microscope to see them
  • Bacteria self replicate - viruses do not
  • Bacteria have cellular machinery including ribosomes - viruses do not
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3
Q

Virus Structure: Describe the outer structure

A
  • Outermost portion is its envelope made up of glycoproteins - but only some viruses are enveloped
  • Envelope is made of proteins from the host cell
  • Capsid is a protein shell that protects the nucleic acid of the virus
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4
Q

Virus Structure: Describe the capsid

A

Capsid proteins are important for the attachment of viruses to specific host receptors
- Protein shell provides structure and symmetry to the virus
- Consists of assembly of identical protein subunits
- Capsids are either (1) Icosahedral (2) Helical or (3) Spherical

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5
Q

5 Things for Viral Classification

A
  1. Nature of the nucleic acid in the Virion (RNA and DNA)
  2. Symmetry of the capsid (helical, icosahedral)
  3. Presence or absence of an envelope (enveloped or naked)
  4. Structure, size and or morphology of a virus
  5. Tissue or organ tropism (adenovirus, enterovirus, rhinovirus)
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6
Q

Virus Replication Cycle

A
  1. Attachment
  2. Entry and uncoating
  3. Replication and assembly
  4. Egress or release of the virus
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7
Q

Describe Mechanisms of Virion Attachment

A
  1. Direct fusion
  2. Endocytosis (may be receptor mediate or not
  3. Receptor mediate entry (HIV, Hep viruses)
  4. Nucleic acid translocation (rare)
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8
Q

What is Fusion?

A

The virus directly fuses with the host plasma membrane and the nucleic acid is released.

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9
Q

What is Endocytosis?

A

Internalized (endocytosis) into a vacuole, transported to an endosome and then the nucleic acid is released.

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10
Q

Describe Receptor Mediated Entry

A
  • Specific receptors are used by the virus to gain entry into the cell
  • Engagement of the receptors will often lead to changes in the structures of the virus that further help with entry
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11
Q

Describe Nucleic Acid Translocation

A
  • Rare and a feature of nonenveloped viruses
  • Capsid adheres to host cell membrane
  • Partial rearrangement of the virion
  • NA passes directly into host cell
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12
Q

Describe the Lytic Viral Lifecycle

A

Process by which a virus infects a host cell and leads to death of host cell. = KABOOM
- Clinically apparent where you get really sick really fast
Ex: Rhinovirus
1. Bacteriophage attaches to cell wall of host
2. Injects DNA into host
3. Takes over bacterium’s metabolism, causing synthesis of new bacteriophages
4. Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles
5. Bacteriophage enzyme lyses the bacterium’s cell wall releasing new bacteriophage particles that attack new cells

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13
Q

Describe Lysogenic Viral Lifecycle

A

Clinically silent, virus will enter and do nothing
1. Certain factors determine whether lytic cycle is induced, or lysogenic cycle is entered
2. Phage DNA integrates into the bacterial chromosome, becoming a prophage
3. The bacterium reproduces, copying the prophage and transmitting it to daughter cells
4. Cell divisions produce a population of bacteria infected with the prophage
5. Occasionally, a prophage exits the bacterial chromosome, initiating the lytic cycle
Example: Herpes, chickenpox/shingles, HPV

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14
Q

How to classify viruses

A
  • Nucleic Acid
  • DNA/RNA, single or double stranded
  • How the virus replicates
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15
Q

dsDNA Virus Replication

A
  • Viral DNA is transcribed to viral mRNA by the VIRAL polymerase
  • The mRNA is then translated to make proteins and enzymes that allow for new virus particle production
  • Uses the host RNA polymerase to make RNA (capsid proteins, DNA polymerase)
  • The newly created DNA polymerase can then replicate the virus DNA
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16
Q

Examples of dsDNA Viruses

A
  • Herpes Simplex Virus 1 and 2
  • Cytomegalovirus
  • Varicella zoster virus
  • Epstein Barr virus
    (All member of the herpes family)
17
Q

RNA Viral Replication

A
  • Much simpler
  • Some can act directly as mRNA and be read on the ribosome
  • Some more complex i.e: must bring extra replicative enzymes with them
18
Q

RNA Virus Examples

A
  • Poliovirus
  • Rotavirus
  • West Nile Virus
  • Influenza Virus
  • Hep A Virus
  • HIV
  • Hep C Virus
    SARS-CoV-1 and SARS-CoV-2 (Covid-19)
19
Q

What is a latent viral infection?

A

Persistence of viral genomes, but not infectious visions, in the host cells without the destruction of the infected cell.
Examples:
1. Herpes simplex virus and varicella-zoster virus established latency in sensory neurons
2. HIV-1 can avoid host immune responses and antiretroviral drugs through the latent infection of resting memory CD4(+) T cells

20
Q

Describe the integration of viral DNA

A
  • For certain viruses, viral DNA can also become integrated within host cell DNA
  • Can lead to latent infection
  • Virus may start to replicate at later times
    • Triggers for reactivation include: Stress, UV light, Hormones, Immune suppression and many unknown triggers
21
Q

Negative Sense RNA Viruses

A
  • Has to be converted to a positive stranded genome prior to mRNA and protein production
  • Virion associated RNA polymerase results in a positive sense RNA that can them be transcribed to proteins
22
Q

(+) Positive Sense RNA Virus

A
  • RNA directly to mRNA and can be used to make proteins
  • Examples: Coronaviruses
  • No intermediary steps
23
Q

Retroviruses

A
  • Reverse Transcriptase enzyme creates a singly strand of viral DNA complementary to the retroviral RNA
  • ssDNA copied to form complementary DNA
  • dsDNA now enters the host cell nucleus a later infection results
24
Q

Ways to detect viruses

A

Bit more difficult
- Direct detection with Electron Microscopy
- Serology
- Amplification of Nucleic Acid (PCR)
- Antigen Detection

25
Q

Electron Microscopy

A
  • Used more in the past when viruses difficult to grow
  • Must use an Electron Microscope - impractical and not very sensitive
    **Can only tell you what family of viruses the virus is from not the actual virus
26
Q

What are virus cultures?

A
  • Growth in tissue culture
  • Not all viruses can be cultured
  • Slow (days to weeks)
  • Requires specific cell lines and many different tissues types must be inoculated
  • Cytopathic Effect (CPE)
27
Q

What is CPE for Virus Cultures

A

Cytopathic Effect
- Effects the virus infection has on the appearance of cells in culture viewed by light microscopy after a patient specimen is inoculated onto the cell culture and the virus has has a chance to replication
- CPE tends to be fairly nonspecific

28
Q

What is the Serologic Response

A
  • Detection of the immune response by the host against the infectious agent
  • Detecting specific immunoglobulin - Hosts antibody response
  • Common ways of detecting viruses that have very narrow window of viremia (West Nile, Zika virus) and those that are not culturable (Hep B)
  • Often can be used when the virus is at low levels soon after infection
29
Q

What is the amplification of nucleic acids?

A
  • Polymerase Chain Reaction (this is how SARS-CoV-2 is being detected)
  • Find a specific segment of RNA and DNA
30
Q

What is antigen detection?

A
  • Direct test on a patient specimen
  • Detecting specific antigens (antigen = antibody generator) of the particular organism you are looking for
  • Similar to pregnancy test

MICRO 121 (19 decks)

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