Chapter 13 Viruses, Viroids and Prions

Question 1

Viruses

Answer 1

• 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

Question 2

Features of Viruses

Answer 2

• 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

Question 3

Host Range

Answer 3

• 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

Question 4

Viral Size

Answer 4

• Electron microscope is required to view viruses

- Range from 20-1000nm in length

Question 5

Viral Architecture

Answer 5

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

Question 6

Nucleic Acid

Answer 6

• 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

Question 7

Capsid

Answer 7

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

Question 8

Envelope

Answer 8

• 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

Question 9

Polyhedral (Shape)

Answer 9

Usually icosahedral - Shape with 20 triangular faces

Question 10

Helical (Shape)

Answer 10

Long rods - can be rigid or flexible

Question 11

Enveloped (Shape)

Answer 11

Roughly spherical - dictated by lipid bilayer

Question 12

Complex (Shape)

Answer 12

• Polyhedral head with a helical tail

- Only found in bacteriophages

Question 13

Classification of Viruses

Answer 13

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

Question 14

Naming of Viruses

Answer 14

-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

Question 15

Adsorption - attachment to host cell

Answer 15

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

Question 16

Penetration - entry into host cell

Answer 16

• Naked virus enters the cell via endocytosis

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

Question 17

Uncoating

Answer 17

Viral nucleic acid is freed from the capsid

Question 18

Biosynthesis

Answer 18

• 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

Question 19

Maturation and Assembly

Answer 19

New virions are assembled

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

Question 20

Release

Answer 20

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

Question 21

Interaction Between Viruses and Animal Hosts

Answer 21

• 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

Question 22

Acute Infections

Answer 22

• 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)

Question 23

Acute infections with Late Complications

Answer 23

• 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)

Question 24

Persistent Viral Infections

Answer 24

• 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

Question 25

Latent Viral Infections

Answer 25

- 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

Question 26

Chronic Viral Infection

Answer 26

- 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

Question 27

Tumors

Answer 27

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

Question 28

2 Types of Genes that cause Cell Growth

Answer 28

- 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

Question 29

Cancer Causing Viruses (Oncogenic Viruses)

Answer 29

- 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

Question 30

Hepatitis B and Hepatitis C

Answer 30

- Viruses associated with cancer | - Believed to cause almost all cases of liver cancer

Question 31

Epstein-Barr Virus

Answer 31

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

Question 32

Human Papillomavirus (HPV)

Answer 32

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

Question 33

Viroids (Virus-like infectious particles)

Answer 33

- Naked RNA - No protein coat - Results in some diseases in plants - not yet found in animals

Question 34

Prions (Virus-like infectious particles)

Answer 34

- 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

Question 35

Mode of infection

Answer 35

-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