Chapter 13 - Viruses

General Characteristics of VirusesViral Structure

Taxonomy of Viruses

GENERAL CHARACTERISTICS OF VIRUSES

• Depending on one's viewpoint, Viruses may be regarded as exceptionally complex aggregations of nonliving chemicals or as exceptionally simple living microbes.

• Viruses contain a single type of nucleic acid (DNA or RNA) and a protein coat, sometimes enclosed by an envelope composed of lipids, proteins, and carbohydrates.

• Viruses are obligatory intracellular parasites. They multiply by using the host cell's synthesizing machinery to cause the synthesis of specialized elements that can transfer the viral acid to other cells.

Host range• Host range refers to the spectrum of host cells in which a virus

can multiply.• Host range is determined by the specific attachment on the

host cell's surface and the availability of host cellular factors.

Viral Size• Viral size is ascertained by electron microscopy• Viruses range from 20 to 1000 nm in length

Viral Structure• A virion is a complete, fully developed viral particle

composed of nucleic acid surrounded by a coat.

Nucleic acid• Viruses contain either DNA or RNA. Never both, and the

nucleic acid may be single or double stranded, linear or circular, or divided into several separate molecules.

• The proportion of nucleic acid in relation to protein in viruses ranges from about 1% to about 50%

Capsid and Envelope• The protein coat surrounding the nucleic acid of a virus is

called the capsid.• The capsid is composed of subunits, capsomeres, which can

be a single type of protein or several types• The capid of some viruses is enclosed by an envelope

consisting of lipids, proteins and carbohydrates• Some envelopes are covered with carbohydrate-protein

complexes called spikes.

Taxonomy of Viruses• Classification of viruses is based on type of nucleic acid,

strategy for replication, and morphology.• Virus family names end in -viridae; genus names end in -

virus.• A viral species is a group of viruses sharing the same

genetic information and ecological niche.

Isolation, Cultivation, and Identification of Viruses

• Viruses must be grown in living cells.• The easiest viruses to grow are bateriophages.

Growing Bacteriophages in the Laboratory ( Plaque assay)

• The plaque method mixes bacteriophages with the host bacteria nutrient agar.

• After several viral multiplication cycles, the bacteria in the area surrounding the original virus are destroyed and the area of lysis is called a plaque

• Each plaque originates with a single viral particle. • The concentration of viral suspensions is measured by

the number of plaques that are in the terms of plaque forming units (PFU).

Viral Reproduction• Requires host cell• Bacteriophages (prokaryotes)• Animal viruses (eukaryotes)

Bacteriophages Reproduction Cycle• Lytic Cycle

– Mass production of phage– Ends in host cell death (lysis)

• Lysongenic cycle– Virus reproduces through binary fission along with host

cell– Can spontaneously convert to lytic cycle

Bacteriophage Reproduction cycle• Attachment

– Random encounter– Attachment – Receptor connection (lock & key)

• Penetration– Break down of cell wall (phage lysozyme)– Injection of DNA into cytoplasm

Bacteriophage Reproduction cycle (Lytic)

• Biosynthesis – Viral DNA takes over – Host metabolic machinery synthesizes new viral parts– Spontaneous assembly

• Release– Break down of host cell wall– New virion escape to infect more cells

Lysogenic cycle• Viral DNA integrates into host’s genome• Bacteria continues normal metabolic processes (DNA

replication, protein synthesis, binary fission)• Virus remains latent until triggered to enter lytic cycle

Multiplications of Animal Viruses

• Animal viruses may differ from phages while entering the host cell.

• The virus is different due to the prokaryotic and eukaryotic cells.

• Animal viruses may not have all the enzymes found in the phage.

• Multiplication of this virus is shared by the DNA and RNA which contains animal viruses.

Bacteriophages and animal viral multiplication compared

• Tail fibers attach to cell wall proteins

• Viral DNA injected into host cell• Not required• In cytoplasm• Lysogeny• Host cell lysed

•Attachment sites are plasma membrane proteins and glycoproteins.

•Capsid enters by endocytosis or fusion

•Enzymatic removal of capsid proteins

•In nucleus DNA viruses or cytoplasm RNA viruses

•Latency; slow viral infections; cancer

•Enveloped viruses bud out; noneveloped viruses rupture plasma membrane.

There are six phrases in a virus replication

• Attachment• Entry• Uncoating• The Biosynthesis of DNA Viruses• The Biosynthesis of RNA Viruses• Maturation and Release

Chickenpox (Varicellovirus)

• Chickenpox is one of the classic childhood diseases, and one of the most contagious. The affected child or adult may develop hundreds of itchy, fluid-filled blisters that burst and form crusts. Chickenpox is caused by a virus.

Latent Viral Infections• A latent viral infection is one in which the virus remains in

the host cell for long periods without producing an infection.

• Examples are cold sores and shingles.

Persistent Viral Infections

• Persistent viral infections are disease process that occur over a long period and are generally fatal.

• Persistent viral infections are caused by conventional viruses; viruses accumulate over a long period.

Persistent Viral Infections

Prions: Definition

• An infectious agent consisting of a self-replicating protein, with no detectable nucleic acids.

• Involve degeneration of the brain tissue• Cause some infectious diseases

• Prions have a high resistance to all forms of sterilization• Listed as most resistant in the “Decreasing order of

resistance of microorganisms to chemical biocides”

e.g.Mad Cow disease– Great Britain, 1987

Viroids

• Infectious RNA– Short pieces of naked RNA– 300 – 400 nucleotides long– No protein coat

• Some plant diseases are caused by viroids• Conclusively identified as pathogens only of plants