Chapter 13-Viruses, Viroids, and Prions

Virus

Nonliving tracellular parasite that depends on host for replication, at a minimum consists of nucleic acid surrounded by a protein coast

Smallest virus

~10nm

Largest virus

~800nm

Virion

Fully developed viral particle

Capsid

Protein coat of virus

Nucleocapsid

Nucleic acid (DNA or RNA) together with capsid

Envelope

A lipid bilayer outside the capsid of some viruses

Spikes

Carbohydrate-protein compmlexes that project from the surface of certain viruses

Icosahedral

Flat surfaces, forming equilateral triangles

Helical

Filamentous or rodlike appearance

Complex

Has complicated structure, such as a bacteriophage

How many known viruses are there?

More than 6,000

What criteria are typically used to classify viruses?

Genome (DNA or RNA)

Number of nucleic acid strands (double or single [+/-])

Outer covering (enveloped or non-enveloped)

How many known virus families are there?

More than 100

Suffix of virus families

-viridae

How many known Genera are there?

More than 450

Suffix of Genera

virus

Number of Virus species

>2,800, name often name of disease

Lytic Bacteriophage

Exit host and cell is lysed

Productive infection

New particles formed

T4 phage

dsDNA takes ~30 minutes

Five step process of lytic bacteriophage

Attachment

Genome entry

Synthesis

Assembly

Release

Lysogenic bacteriophage

Incorporates into host genome and cell is not damaged

Prophage

Replicates along with host cell chromosome

Model of lysogenic bacteriophage

Lambda phage

Three step process of lysogenic bacteriophage

Attachment

Genome entry

Integration

Temperate bacteriophage

Option of lytic or lysogenic cell

Seven step process of a temperate bacteriophage

Attachment

Entry

Integratoin

Excision

Synthesis

Assembly

Release

Prevent phage attachment

Alter or cover specific receptors on cell surface (ie capusles, slime layers, and biofilms)

Restriction enzymes

Recognize and cut short nucleotide sequences (that is the incoming phage DNA)

CRISPR system

recently discovered clusters of regularly interspersed short palindromic repeats

Allows bacteria to recognize and block repeat phage infections

Plaque assay

Used to quantitate phage particles in samples (sewage, seawater, soil, etc)

Process of plaque assay

Soft agar inoculated with bacterial host and phage, poured over surface of agar in petri dish

Bacterial lawn forms

Counting plaque forming units PFU yields titer or the concentration of phage

Plaques

Zones of clearing from bacterial lysis

Attachment

Viruses bind to receptors

Usually glycoproteins on cytoplasmic membrane of host

Penetration and uncoating

Enveloped iruses via fusion or endocytosis

Non-eveloped viruses only via endocytosis

Synthesis and replication

Viral proteins must be synthesized by replicating and expressing viral genes

Three gneral replication strategies depending on type of genome of virus

Three general replication processes of viruses

DNA virueses: nucleus

RNA virueses: cyptoplasm

Reverse transcribing viruses (encode reverse transciptase that makes DNA from RNA)

Assembly

Protein cpasid forms; genome and enzymes packaged

Takes place in nucleus, cytoplasm, or both

Release

Most enveloped viruses released via budding

Non-enveloped viruses relased when host cell dies, often by apoptosis initiated by virus or host

Acute infection

Rapid onset and short duration (influenza virus, mumps virus, and rhinovirus)

Persistent infection

continues for years or lifetime and may or may not have symptoms

Chronic infection

Continuous production of low levels of virus particles (hepatitis B virus)

Latent infections

Viral genome (provirus) remains silent in host cell, but can reactivate (varicella zoster virus and herpes simplex virus)

Proto-oncogenes

Stimulate cell growth (over activation can lead to cancer)

Tumor suppressor genes

Inhibit cell growth (inactivation can lead to cancer)

Oncogenic viruses

Can cause tumors/cancer (viral oncogenes can interfere with host control mechanisms)

Classification of viruses

Obligate intracellular parasites (require host)

Inoculate live animals or fertilized chicken eggs

Cell or tissue cultures

Cell or tissue cultures

Animal cells used as host, disassociated and grown as single cells or monolayer

Drawback is cells only divide limited number of times (50-100)

Tumor cells often used because multiply indefinitely

Quantitating animals viruses

Direct counts ia electron microscopy (if concentration is high enough)

Plaque assays using monolayer of tissue culture cells

Dilution yielding

Results from Quantal assayID50 infective dose

LD 50 lethal dose

Hemagglutination

Causes clumping of RBCs, yields relative concentration

Phages cannot infect

animals

How do plants get infected?

Do not attach to cell receptors, enter via wounds in cell wall, spread through cell openings (plasmodesmata)

Plants rarely recover, lack specific immunity

What transmits plant viruses?

Insects, soil, humans, grafting, and contaminated seeds, tubers, and pollen

Viroids

Small single-stranded infectious RNA molecules that are only known to infect plants

Genome about 1/10th of the smallest RNA virus

Prions

Proteinaceous infectious agents, composed of proteins, no nucleic acids

Diseases caused by prions

human: Creutzfeldt Jakob diease and kuru

animal: scrapie, mad cow, and chronic wasting disease

Accumulation of prion protein in neural tissue

Neurons die, tissue develop holes, brain function dteriorates (transmissible spongiform encephalopathy)

Hypothesized to be caused by misfolding of normal proteins