8 - General properties of viruses* Flashcards
Obligate intracellular parasites
Require host cells to multiply. Possess no machinery for protein synthesis and energy production
Endogenous retroviruses
Viral genomes a part of our own genetic material
Generalised structure of Virion
- Non enveloped (Capsid and nucleic acid)
- Enveloped (Capsid, nucleic acid, spike and envelope)
Virion
An infectious, complete virus particle composed of nucleic acid (DNA or RNA) surrounded by a protein coat
Function of the protein coat
Protection and vehicle for transmission
Viral nucleic acid
Can be DNA or RNA, double stranded or single, linear or circular, have several seperate segments (influenza)
Different types of viral capsid structure
Icosahedral, Enveloped, complex or helical
Capsid
Protein coat surrounding nucleic acid, most of mass of virus and is composed of repeated subunits (capsomeres). Arrgangement of capsomeres characteristic for particular virus.
Envelope
- 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)
Peplomers
Protein spikes on surface of some viruses
Helical viruses
Rigid or flexible, nucleic acid within hollow cylinder (helical), spiral arrangement of capsomers around nucleic acid
What was the Fraenkel, Conrat, Williams experiments
Demonstarted that TMV spontaneously formed when mixtures of purified coat protein and genomic RNA were incubated together
What did the Fraenkel, Conrat, Williams exmperiment show
nucleic acid centers of every virus particle were carriers of genetic information that managed viral reproduction
Regular complex viruses
- Capsids have components with both helical and icosahedral symmetry (eg, Bacteriophage)
- Additional structures attached capsid (polyhedral) and sheath (helical)
Irregular complex viruses
- Non symmetrical capsid structures (eg.Pox viruses)
- Lack clearly identifiable capsids
- Several coats around nucleic acids
Steps to replication cycle
- Attachment of virus to house cell
- Entry of viral nucleic acid or nucleocapsid
- Synthesis of viral proteins and nucleic acid
- Assembly of virions
- Release of progeny virions
how is virus entry into target cells mediated
by receptor molecules
Explain entry of enveloped virus by fusing with plasma membrane
- Viral envelope spikes bind to receptors on surface of host cell
- Lipid bilayer of viral envelope fuses with host cell membrane
- Nucleocapsid is released into cytoplasm
Explain entry of enveloped virus by endocytosis
- Viral envelope spikes bind to receptors on surface of host cell
- Binding to receptor triggers receptor mediated endocytosis
- Increased acidity allows nucleocapsid to escape from the endosome and enter cytosol
Explain entry of nonenveloped virus by endocytosis
- Capsid proteins bind to receptors on cell surface and trigger receptor mediated endocytosis
- Nucleic acid is extruded from the endosome into the cytosol
Explain the release of virions from cells
- Viral matrix protein lines the cytoplasmic face of the cell membrane
- Nucleocapsids are directed to plasma membrane by host cells microtubules
- Plasma membrane protrudes outward and nucleocapsids are surrounded by matrix lined plasma membrane
- Neck of protruding membrane is pinched off and mature virion is released
Different effects of viral infections on animal cells
- Acute infections (cell death and virus release)
- Latent infections (activation leads to acute)
- Chronic infections (release without cell death)
- Transformed into malignant cell (inactivation of tumour suppressor protein or insertion of oncogene)
what are the two types of persistent infections
Latent and chronic infections
Virulent phage
Multiplies immediately upon cell entry, lyses bacterial host cell
Temperate phage
Two reproductive options:
- Lytically as virulent phages do
- Remain in host cell without destroying it (Lysogeny)
What are the advantages of lysogeny
- 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
What can cause a switch from the lysogenic to lytic cycle
Certain environmental stressors such as UV light
Phage plaques
- 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
Diseases causes by viroids
20 different plant diseases including potato spindle tuber and chrysanthemum stunt disease
Viroid
- 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
what are the two types of bacteriophage reproduction
Lysogenic and lytic cycle
