Bmsc210 m2 p5

genetic element that cannot replicateindependently of a living (host) cell

the study of viruses

• extracellular form of a virus
• – Exists outside host and facilitates transmission from one host cell to another
• – Contains nucleic acid genome surrounded by a protein coat and, in some cases, other layers of material

• – Either DNA or RNA genomes
• – Some circular, but most linear

0.02 to 0.3 μm

smaller

• the protein shell that surrounds the genome of a virus particle (Figure 9.2)
• • Composed of a number of protein molecules arranged in a precise and highly repetitive pattern around the nucleic acid

• -subunit of the capsid
• • Smallest morphological unit visible with an electron microscope

complete complex of nucleic acid and protein packaged in the virion (Figure 9.3)

• – Have membrane surrounding nucleocapsid
• • Lipid bilayer with embedded proteins
• – Envelope makes initial contact with host cell

• rod-shaped viruses (e.g.,tobacco mosaic virus)
• • Length of virus determined by length of nucleic acid
• • Width of virus determined by size and packaging of protein subunits

• – Virions composed of several parts, each with separate shapes and symmetries
• – Bacterial viruses contain complicated structures
• • Icosahedral heads and helical tails

Bacterial viruses are easiest to grow; modelsystems

- Animal viruses (and some plant viruses) can becultivated in tissue or cell cultures

- Plant viruses typically are most difficult because study often requires growth of whole plant

• 20
• 3

number of infectious units per volume of fluid

analogous to the bacterial colony; one way to measure virus infectivity (Figure 9.6)

• -clear zones that develop on lawns of host cells
• • Lawn can be bacterial or tissue culture (Figure 9.7)
• • Each plaque results from infection by a single virus particle

• -degenerative changes in cells associated with the multiplication of certain viruses.
• -lysis is not observed

• -used in quantitative virology
• – The number of plaque-forming units is almost always lower than direct counts by electron microscopy due to
• • Inactive virions
• • Conditions not appropriate for infectivity

• – Some viruses do not show recognizable changes in cell cultures yet cause death or disease in whole animals
• – Virus is diluted
• – Animals are infected with viral dilution
• – End point is calculated (LD50 or ID50)

– Attachment (adsorption) of the virus to asusceptible host cell

– Entry (penetration) of the virion or its nucleic acid

– Synthesis of virus nucleic acid and protein by cellmetabolism as redirected by virus

– Assembly of capsids and packaging of viralgenomes into new virions (maturation)

– Release of mature virions from host cell

• typically characterized by aone-step growth curve (Figure 9.9)
• 􀅖 Latent period: eclipse + maturation

number of virions released

• – Requires complementary receptors on thesurface of a susceptible host and its infectingvirus
• – Receptors on host cell carry out normal functionsfor cell (e.g., uptake proteins, cell to cellinteraction)
• – Receptors include proteins, carbohydrates,glycoproteins, lipids, lipoproteins, or complexes

The attachment of a virus to its host cell results in changes to both virus and cellsurface that facilitate penetration

• -virus of E. coli; one of the mostc omplex penetration mechanisms (Figure 9.10)
• – Virions attach to cells via tail fibers that interact with polysaccharides on E. coli cell envelope
• – Tail fibers retract and tail core makes contact withE. coli cell wall
• – Lysozyme-like enzyme forms small pore inpeptidoglycan
• – Tail sheath contracts and viral DNA passes intocytoplasm

– DNA destruction system; only effective against double-stranded DNA viruses

• -cleave DNA at specific sequences
• – Modification of host’s own DNA at restriction enzyme recognition sites prevents cleavage of own DNA

• – Chemical modification of viral DNA (glycosylation or methylation)
• – Production of proteins that inhibit host cell restriction system

• 5-hydroxymethyl-cytosine
• to avoid digestion by bacteria

• discovered retroviruses andreverse transcriptase
• – Shared 1975 Nobel Prize for Physiology or Medicine
• -Baltimore developed classification scheme for viruses based on relationship of viral genome to its mRNA

• – Class I are double-stranded (ds) DNA viruses
• – Class II are single-stranded (ss) DNA viruses
• – Class III are dsRNA
• – Class IV and V are ssRNA (+ or )
• – Class VI are retroviruses
• – Class VII are dsDNA viruses that replicate through an RNA intermediate

converted to dsDna and then uses Rna polymerase to synthesize mRna

use of RNA dependant RNA polymerase to replicate and create mRNA

• genome
• proteins

messenger RNA (mRNA)

mRNA

viral RNA

virion

configuration of the genome of a single-stranded DNA or RNA virus (mRNA is said to be in plus (+) configuration; its complement is in minus () configuration)

single-stranded RNAgenome with same orientation as its mRNA

single-stranded RNA genome with orientation complementary to its mRNA

mRNA

• – synthesized soon after infection
• – necessary for replication of virus nucleic acid– typically act catalytically
• – synthesized in smaller amounts

• • Synthesized later
• • Include proteins of virus coat
• • Typically structural components
• • Synthesized in larger amounts

• entericbacteria
• – Examples of hosts: E. coli, Salmonella enterica

dsDNA

viruses lyse host cells after infection

• viruses replicate their genomes in tandem with host genome and without killing host
• • Virus can also be lytic

• – First viruses studied in detail contained linear, dsDNA genomes that infect enteric bacteria
• – Examples include T1, T2, …, T7
• • T2, T4, and T6 are closely related viruses; T4 most extensively studied

• has a dsDNA genome that is circularly permuted and terminally redundant (Figure 9.13)
• – Both factors affect genome packagingDNA contains the modified base5-hydroxymethylcytosine (Figure 9.14)
• – DNA is resistant to virtually all knownrestriction enzymes

early, middle, and late

enzymes needed for DNA replication and transcription

 head and tail proteins and enzymes required to liberate mature phage particles

• can undergo a stable genetic relationship within the host (Figure 9.16)
• – But can also kill cells through lytic cycle

state where most virus genes are not expressed and virus genome (prophage) isreplicated in synchrony with host chromosome

• - a bacterium containing a prophage 
• - Under certain conditions lysogenic viruses may revert to the lytic pathway and begin to produce virions

• – Linear, dsDNA genome
• – Complementary, single-stranded regions 12 nucleotides long at the 5' terminus of each strand(Figure 9.18)
• – Upon penetration, DNA ends base-pair, forming the cos site, and the DNA ligates and forms double-stranded circle
• – When lambda is lysogenic, its DNA integrates into E. coli chromosome at the lambda attachment site (att)
• – When it enters lytic pathway, lambdasynthesizes long, linear concatamers of DNAby rolling circle replication (Figure 9.19)

• • cI protein (the lambda repressor): causes repression of lambda lytic events
• • Cro repressor: controls activation of lytic events

enters

nucleus

modes

• animal
• bacterial
• – As animal viruses leave host cell, they canremove part of host cell’s lipid bilayer for theirenvelope

• release of virions from host cell does not result in cell lysis
• • Infected cell remains alive and continues to produce virus

delay between infection by the virus and lytic events

conversion of normal cell into tumor cell

two or more cells become one cell with many nuclei

• RNA viruses that replicatethrough a DNA intermediate
• – Enveloped viruses (Figure 9.23a)
• – Contain a reverse transcriptase (copiesinformation from its RNA genome into DNA),integrase, and protease
• – Virion contains specific tRNA molecules

– Two identical ssRNA molecules of the plus (+)orientation

• • gag: encode structural proteins
• • pol: encode reverse transcriptase andintegrase
• • env: encode envelope proteins

• – Entrance into the cell
• – Removal of virion envelope at the membrane
• – Reverse transcription of one of the two RNAgenomes
• – Integration of retroviral DNA into host genome
• – Transcription of retroviral DNA
• – Assembly and packaging of genomic RNA
• – Budding of enveloped virions; release from cell