8 - General properties of viruses*

Question 1

Obligate intracellular parasites

Answer 1

Require host cells to multiply. Possess no machinery for protein synthesis and energy production

Question 2

Endogenous retroviruses

Answer 2

Viral genomes a part of our own genetic material

Question 3

Generalised structure of Virion

Answer 3

• Non enveloped (Capsid and nucleic acid)
• Enveloped (Capsid, nucleic acid, spike and envelope)

Question 4

Virion

Answer 4

An infectious, complete virus particle composed of nucleic acid (DNA or RNA) surrounded by a protein coat

Question 5

Function of the protein coat

Answer 5

Protection and vehicle for transmission

Question 6

Viral nucleic acid

Answer 6

Can be DNA or RNA, double stranded or single, linear or circular, have several seperate segments (influenza)

Question 7

Different types of viral capsid structure

Answer 7

Icosahedral, Enveloped, complex or helical

Question 8

Capsid

Answer 8

Protein coat surrounding nucleic acid, most of mass of virus and is composed of repeated subunits (capsomeres). Arrgangement of capsomeres characteristic for particular virus.

Question 9

Envelope

Answer 9

• Covers capsid in some viruses
• Combination of lipids, proteins and carbohydrates
• may be derived from hosts own cell membrane
• Protect viral protein from recognition of immune response via adaptive immunity
• Virus without envelope (naked)

Question 10

Peplomers

Answer 10

Protein spikes on surface of some viruses

Question 11

Helical viruses

Answer 11

Rigid or flexible, nucleic acid within hollow cylinder (helical), spiral arrangement of capsomers around nucleic acid

Question 12

What was the Fraenkel, Conrat, Williams experiments

Answer 12

Demonstarted that TMV spontaneously formed when mixtures of purified coat protein and genomic RNA were incubated together

Question 13

What did the Fraenkel, Conrat, Williams exmperiment show

Answer 13

nucleic acid centers of every virus particle were carriers of genetic information that managed viral reproduction

Question 14

Regular complex viruses

Answer 14

• Capsids have components with both helical and icosahedral symmetry (eg, Bacteriophage)
• Additional structures attached capsid (polyhedral) and sheath (helical)

Question 15

Irregular complex viruses

Answer 15

• Non symmetrical capsid structures (eg.Pox viruses)
• Lack clearly identifiable capsids
• Several coats around nucleic acids

Question 16

Steps to replication cycle

Answer 16

1. Attachment of virus to house cell
2. Entry of viral nucleic acid or nucleocapsid
3. Synthesis of viral proteins and nucleic acid
4. Assembly of virions
5. Release of progeny virions

Question 17

how is virus entry into target cells mediated

Answer 17

by receptor molecules

Question 18

Explain entry of enveloped virus by fusing with plasma membrane

Answer 18

1. Viral envelope spikes bind to receptors on surface of host cell
2. Lipid bilayer of viral envelope fuses with host cell membrane
3. Nucleocapsid is released into cytoplasm

Question 19

Explain entry of enveloped virus by endocytosis

Answer 19

1. Viral envelope spikes bind to receptors on surface of host cell
2. Binding to receptor triggers receptor mediated endocytosis
3. Increased acidity allows nucleocapsid to escape from the endosome and enter cytosol

Question 20

Explain entry of nonenveloped virus by endocytosis

Answer 20

1. Capsid proteins bind to receptors on cell surface and trigger receptor mediated endocytosis
2. Nucleic acid is extruded from the endosome into the cytosol

Question 21

Explain the release of virions from cells

Answer 21

1. Viral matrix protein lines the cytoplasmic face of the cell membrane
2. Nucleocapsids are directed to plasma membrane by host cells microtubules
3. Plasma membrane protrudes outward and nucleocapsids are surrounded by matrix lined plasma membrane
4. Neck of protruding membrane is pinched off and mature virion is released

Question 22

Different effects of viral infections on animal cells

Answer 22

1. Acute infections (cell death and virus release)
2. Latent infections (activation leads to acute)
3. Chronic infections (release without cell death)
4. Transformed into malignant cell (inactivation of tumour suppressor protein or insertion of oncogene)

Question 23

what are the two types of persistent infections

Answer 23

Latent and chronic infections

Question 24

Virulent phage

Answer 24

Multiplies immediately upon cell entry, lyses bacterial host cell

Question 25

Temperate phage

Answer 25

Two reproductive options:
- Lytically as virulent phages do
- Remain in host cell without destroying it (Lysogeny)

Question 26

What are the advantages of lysogeny

Answer 26

• Phage remains visible (even within dormant host) but may not rpelicate
• If there are more phages than host cells, lysogeny ensures survival of host cell which will continue to replicate and also replicate integrated phage DNA

Question 27

What can cause a switch from the lysogenic to lytic cycle

Answer 27

Certain environmental stressors such as UV light

Question 28

Phage plaques

Answer 28

• When phages and host bacterial cells are mixed at an appropriate ratio, only a portion of the cells are initially infected.
• When this mixture is plated, the infected cells will be separated from each other.
• The infected cells eventually lyse, releasing progeny phages which infect nearby cells, releasing more phages.
• This continues and ultimately gives rise to a clear area within a lawn of bacteria

Question 29

Diseases causes by viroids

Answer 29

20 different plant diseases including potato spindle tuber and chrysanthemum stunt disease

Question 30

Viroid

Answer 30

• Infectious agents consisting only of RNA.
• Covalently closed, circular ssRNAs that do not encode for proteins
• Alters the specificity of host enzymes to cause them to act on RNA as the substrate instead of RNA

Question 31

what are the two types of bacteriophage reproduction

Answer 31

Lysogenic and lytic cycle