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55 Cards in this Set

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Viruses (four facts)
1) genetic elements that replicate independently of a cell's chromosome but not independently of cells themselves

2) have both extracellular and intracellular form

3) able to exploit metabolic machinery of cells

4) can infect both bacteria and archae
Extracellular form
also known as virion form

allows viruses to exist outside the host for long periods

metabolically inert.. no biosynthetic functions
What are the extracellular forms of viruses made of?
contain a nucleic acid (RNA or DNA) surrounded by a protein coat (capsid) and occassionally other macromolecules
Bad and good things about viruses (2)
1) can introduce beneficial genes or properties encoded in nucleic acids

2) can also cause cell destruction and cell death
Intracellular form
occurs when a virus has invaded host cell

viral replication cocurs

components that make up the virus are synthesized by host cell's machinery
Three characteristics of Viral Genomes
1) may contain RNA or DNA or both and thus may be single or double stranded

2) classified according to their characteristics of their genome

3) also can be classified according to the host they infect
size of viruses usually (in microns) and how many bases are they made up of?
1) average size is .02-.03 microns

2) viral genomes are 1000-5000 kb
why is the small size of the viral genome important?
because it restricts the number of different viral proteins that can be made
What are two words to define viral structure?
1) Capsomere

2) Nucleocapsid
Capsomere
more complex assembly of the protein coat
Nucleocapsid
a combination of the virion nucleic acid and protein coat
Envelope (in virions)
a membrane that surrounds the nucleocapsid
Naked (pertaining to virions)
extracellular virus that does not have an envelope
the two types of virus symmetry
1) Helical symmetry

2) icosahedral symmetry
Helical symmetry
seen in rod-shaped viruses
Icosahedral symmetry
seen in spherical viruses
What is the structure of the Envelope of viruses?
1) consists of a lipid bilayer embedded with proteins and glycoproteins
where is the lipid portion of the envelope of a virus derived from?
from the membrane of the host cell
how is the protein portion encoded in an enveloped virus?
encoded by virus nucleic acid
What types of organisms do enveloped viruses usually infect?
they infect animals
enzymes in virions
1) may play role in infection process

2) bacteriophages produce lysozymes that make small holes in bacterial cell wall

3) some viruses contain their own nucleic acid polymerases
Reverse Transcriptase
enzyme found in RNA viruses called retroviruses
Neuramidases
enzymes that help viruses to release from the host
Growth of viruses in cultured cells
1) many animal and plant viruses can also be grown in cultured cells

2) some cell cultures can be grown indefinitely as permanent cell lines for experimental purposes
Plaque assay
used to quantify the number of virions in a suspension

accurate way to measure virus infectivity
The steps to produce a plaque assay (3)
1) Bacterial culture infected with a virus and produces "lawn" growth in culture

2) virus will cause lysis of the bacteria it infects

3) lysed cells produce "clear areas" called plaques
Five stages of the life cycle of a virus
1) Attachment

2) Penetration

3) Synthesis

4) Assembly

5) Release
Four stages of viral nucleic acid synthesis
1) Eclipse

2) Latent period

3) Maturation

4) Burst size
Eclipse
infectivity of the virus dissappears after adsorption of virus by host cell
Latent Period
replication of viral nucleic acid and protein occurs during this period
Maturation
newly synthesized nucleic acid molecules are packaged inside protein coats
Burst Size
nubmer of new virion particles that are released
Three facts about viral attachment to the host cell
1) attachment of a virion is a highly specific process

2) involves complementary receptors on the surface of a susceptible host cell and the infecting virus

3) if host cell receptor altered by mutation, the virus cannont adsorb and infect
penetration of viruses
not the same in all virus-host cell interactions

may become uncoated prior to entering the host cell

entire virion may enter host cell and must become uncoated after entering the host cell
Restriction endonucleases
An example of prokaryotic DNA destruction systems (because they lack immune systems)

protect their own DNA with protective methyl groups at the restriction sites in their genomes
Genomes of T-even bacteriophages
T2, T4, T6 are closely related viruses

T4 genome is most widely studied
3 parts of T4 genome
Early Proteins

Middle Proteins

Late Proteins
Early and Middle Proteins
primarily enzymes involved in DNA replication and transcription
Late Proteins
head and tail proteins and the enzymes involved in liberating the mature phage particles from the cell
T4 virus infectivity
it is a virulent virus (can kill cells through a lytic life cycle)
How does T4 appropriate the host cells RNA polymerase?
1) may insert anti-sigma factor that causes the host cell RNA polymerase to not recognize host promoters-shuts down host transcription

2) may use its own proteins to covalently modify the sub-units of the host RNA polymerase

3) these modification cause the host RNA polymerase to recognize only phage promoters
Temperate Bacteriophages
Viruses that do not neter a lytic life cycle
lysogeny
a stage entered into by temperate viruses

most virus genes are not expressed

virus genome (prophage) is replicated in synchrony with the host chromosome
Bacteriophage Lamda
a double stranded DNA temperate phage

lytic and lysogenic events in lambda controlled by several promoters and regulatory proteins

often infect E. coli
Genetic Switch
controls whether a lytic or lysogenic pathway will ensue in bacteriophage lambda
For lysogenic pathway to ensue it is necessary that (2)
1) production of all late proteins must be prevented

2) copy of the lambda genome must be integrated into the host chromosome
Three things animal viruses can cause
1) persisten infections

2) Transformation

3) Latent Infections
Persistent infections
produce virus indefinitely

cause lysis of host cell

immediate lytic effect
Trasformation
coversion of a normal cell into a tumor cell
Latent infections
delay between infection by the virus and lytic events

symptoms reappear sporadically
Retrovirus
transfer information from RNA to DNA

contain a genome that is replicated through a DNA intermediate

usea an enzyme caled reverse transcriptase to perform retro-transcription
Viroids
1) smallest known pathogens

2) do not encode any proteins-completely dependent upon host cell enzymes

3) cause some plant diseases (no viroid diseases in animals)
Structure of viroid (3)
1) small, circular single stranded RNA molecules

2) contain no protein coat

3) genomes between 246-399 nucleotides
Prions
1) cause a variety of disease in animals (scrapie in sheep, mad cow in cattle)

2) no known prion diseases in plants
Structure of prion (2)
1) have a distinct extracellular form that is entirely protein

2) do not contain any nucleic acid