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Microbiology > Chapter 13 Viruses, Viroids and Prions > Flashcards

Chapter 13 Viruses, Viroids and Prions Flashcards

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Biology 101 flashcards
1
Q

Viruses

A
  • Acellular particles capable of infecting host cells and causing disease
  • Not free-living - required a host cell in which to multiply
    1) Obligate intracellular parasites
    2) Use host metabolic systems and usually disrupt normal hots cell function
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2
Q

Features of Viruses

A
  • Acellular - do not have plasma membrane
  • Contain a single type of nucleic acid - either DNA or RNA
    Surrounded by protein coat
    May or may not have additional envelope of lipids
  • Have very few of their own enzymes
    Take over enzymes of their host
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3
Q

Host Range

A
  • Viruses can infect animals, plants, fungi, protozoa and bacteria
  • Most viruses are specific for a single host species
    1) To infect a cell the viruses must recognize features on the host cell surfaces
    2) Example: some viruses recognize the fimbriae of a certain bacterial species
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4
Q

Viral Size

A
  • Electron microscope is required to view viruses

- Range from 20-1000nm in length

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5
Q

Viral Architecture

A

1) Nucleic Acid
2) Capsid
3) Envelope (not present in all viruses)

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6
Q

Nucleic Acid

A
  • Can have either DNA or RNA as the genetic material - not both
  • Can be single stranded or double stranded
  • Can be linear or circular
  • Can be in several pieces - segmented
  • Total amount of nucleic acid = a few thousand to 250,000 base pairs
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7
Q

Capsid

A
  • Protein coat surrounding the nucleic acid
  • Made up of individual proteins called capsomeres
  • Nucleic acid and Capsid-Nucleocaspid
  • Minimum requires structure for a virus
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8
Q

Envelope

A
  • Not present in all viruses
  • Lipid bilayer acquired from the host cell
  • External coating around the nucleocapsid
  • Additional viral proteins inserted into envelope - called spikes
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9
Q

Polyhedral (Shape)

A

Usually icosahedral - Shape with 20 triangular faces

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10
Q

Helical (Shape)

A

Long rods - can be rigid or flexible

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11
Q

Enveloped (Shape)

A

Roughly spherical - dictated by lipid bilayer

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12
Q

Complex (Shape)

A
  • Polyhedral head with a helical tail

- Only found in bacteriophages

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13
Q

Classification of Viruses

A

Based on:

1) Nucleic Acid Type
- DNA or RNA
- Single stranded or double stranded
- Segmented or single molecule
2) Capsid Structure
- Polyhedral
- Helical
3) Presence of envelope

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14
Q

Naming of Viruses

A

-Family - ends with suffix -viridae
- Genus - ends with suffix - virus
- Species - specific epithets are not used ( given a descriptor name)
Example: Family Herpesviridae, Genus: Simplexvirus, Species: Human herpesvirus 2

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15
Q

Adsorption - attachment to host cell

A

Viruses have attachment sites - recognize protein of glycoprotein of host membrane

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16
Q

Penetration - entry into host cell

A
  • Naked virus enters the cell via endocytosis

- Most enveloped viruses enter by fusion - lipids of envelope fuse with host cytoplasmic membrane

17
Q

Uncoating

A

Viral nucleic acid is freed from the capsid

18
Q

Biosynthesis

A
  • Viral nucleic acids are replicated
  • DNA replication occurs in the nucleus
  • RNA replication occurs in the cytoplasm
  • Viral proteins (capsomeres) are synthesized in the cytoplasm
  • Biosynthesis relies on the host metabolic machinery (Ex. Replication and transcription enzymes, ribosomes
19
Q

Maturation and Assembly

A

New virions are assembled

  • Capsomeres form the capsid
  • Nucleic acid enters capsid - forms the nucleocapsid
20
Q

Release

A

Naked Viruses - burst out, rupture host cell - host cell dies
Enveloped viruses - bud out, virus pushes through cytoplasmic membrane
- Steady release of mature viruses
- Host cell stays alive for a long time

21
Q

Interaction Between Viruses and Animal Hosts

A
  • Host defence plays major role in outcome of viral infection (protects against otherwise lethal infection
  • Most healthy humans carry a number of viruses, and antibodies to viruses
  • If virus is transferred from the immune host to another individual - can result in infection
22
Q

Acute Infections

A
  • Usually short duration
  • Disease symptoms result from tissue damage
  • Lysis of host cells - release and spread of virus particles
  • Host defence systems gradually eliminate virus (may take days or months)
  • Host may develop long lasting immunity (Ex. Mumps, Polio, Influenza)
23
Q

Acute infections with Late Complications

A
  • After acute period, some non-infectious particles remain - can cause serious disease later
  • Ex. Measles - Subacute sclerosis panencephalitis (Fetal brain disorder - occurs up to ten years after recovery from measles)
24
Q

Persistent Viral Infections

A
  • Virus is continuously present in body, but may or may not cause disease (ie. may be no symptoms)
  • Infected host can still serve as a reservoir- can transmit virus to others
25
Q

Latent Viral Infections

A
  • Acute infection followed by symptomless period
  • The virus integrates a copy of its DNA into a host cell chromosome and remains dormant
    1) Provirus
    2) Disease can be reactivated years later
    3) Symptoms may be different
    Example: Chicken Pox can later become shingles
26
Q

Chronic Viral Infection

A
  • After the acute period, infectious virus remains present at all times - may or may not cause noticeable symptoms
    Example: Hepatitis (Serum hepatitis virus) Has acute period, and then patient has it for the rest of their life
27
Q

Tumors

A

Tumor - abnormal growth of tissue
Benign Tumor - Does not spread
Malignant Tumor - metastasizes and invades nearby tissues (Cancer)

28
Q

2 Types of Genes that cause Cell Growth

A
  • Proto-oncogenes - Genes that stimulate cell growth
  • Tumor suppressor genes - Genes that inhibit cell growth
  • Mutations in these genes can lead to uncontrolled cell growth, formation and cancer
29
Q

Cancer Causing Viruses (Oncogenic Viruses)

A
  • Carry oncogenes: genes that interfere with the cell’s control mechanisms
  • Most are DNA viruses
    1) Integrate viral DNA into the host chromosome as a provirus
    2) Oncogenes continue to be expresses
30
Q

Hepatitis B and Hepatitis C

A
  • Viruses associated with cancer

- Believed to cause almost all cases of liver cancer

31
Q

Epstein-Barr Virus

A
  • Virus associated with cancer
  • Causes infectious mononucleosis
  • May cause lymphoma (cancer of WBC) and some cancers of the nose or throat
32
Q

Human Papillomavirus (HPV)

A
  • Virus associated with cancer
  • Sexually transmitted - genital warts
  • Believed to cause almost all cases of cervical cancer
33
Q

Viroids (Virus-like infectious particles)

A
  • Naked RNA
  • No protein coat
  • Results in some diseases in plants - not yet found in animals
34
Q

Prions (Virus-like infectious particles)

A
  • Infectious protein particles
  • No genetic material (RNA or DNA)
  • Linked to several human and animal diseases
    1) Transmissible several spongiform encephalopathies
    2) Sponge - like holes in the brain
35
Q

Mode of infection

A

-Seem to be transmitted through food
- Not usually destroyed by high temperatures - can be destroyed by autoclaving in a solution o sodium hydroxide (strong base)
- Onset of disease in humans occurs several years after infection
Not clear why or how it accumulates in the brain
Always fatal - no treatment or cure

Microbiology (17 decks)

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