Chapter 13: Viruses, Viroids, and Prions

why are viruses not considered to be alive?

they are not made of cells, they can only replicate using a host cell’s machinery

in 1886…

tobacco mosaic disease found to be transferred from plant to plant

in 1892…

the causative agent of tobacco mosaic disease could pass through pores of a filter

in 1935…

tobacco mosaic virus was purified, enabling the study of its structure using electron microscopy

how are viruses different from other life forms

contain a single type of nucleic acid, contains a protein coat, uses host cell for replication, and responsible for synthesis of structures that transfer viral nucleic acid to other cells

host range tropism

spectrum of host cells a virus can infect

how is host range determined?

by the ability of the virus to attach to the host cell and reproduce (involves viral proteins and receptor on the host cell)

why do viruses exhibit host tropism

their surface proteins can only fit with specific receptors on certain host cells

viruses are substantially smaller than

bacteria

giant viruses

susceptible to smaller viruses (virophages) due to their size

virion

complete fully developed infectious viral particle found outside of a host cell

what are virions composed of

nucleic acid surrounded by a protein coat

capsid

proteins coat that surrounds a virus’s genome (IN ALL VIRUSES)

envelope

lipid layer that covers the capsid

when is the envelope formed

from the plasma membrane when a virus exits a host cell

what are spike proteins projected off of

the envelope

helical virus

nucleic acid within a hollow, cylindrical capsid

polyhedral

many sided often icosahedral (20 faces)

complex virus shape

lacks symmetry (bacteriophage for example)

viruses within a DNA genome need

an enzyme that reads DNA and synthesizes DNA; uses host cell DNA polymerase

viruses with an RNA genome need

an enzyme that reads DNA and synthesizes RNA

how do viruses in RNA genome get RNA polymerase

RNA viruses encode their own RNA dependent RNA polymerase

what other mechanism does structure of the viral genome impact

transcription (synthesizing mRNA)

what differs in Baltimore classification

DNA vs RNA genome, double stranded or single stranded, positive sense or negative sense

retroviruses

have an RNA genome but convert their genome to DNA version so it can integrate into the host cell genome

retroviruses encode

their own RNA dependent DNA polymerase called reverse transcriptase

what is the envelope made of

phospholipid bilayer acquired from the infected host

are envelopes sensitive to chemical and physical treatments

yes

how do viruses bind to the receptor molecules

using fusion proteins

nonenveloped viruses have

fusion proteins on their capsids

enveloped viruses have

fusion proteins on their envelopes

bacteriophage

virus that infects bacteria

plaques

formed by bacteriophages; zones of bacterial cell lysis when the phage infects a bacterial cell and replicates to high numbers and lyses the cell or infects neighboring cells

some viruses can be grown in

embryonated eggs to grow vaccines

cell lines from viruses grown in cell cultures

primary: derives from tissue and only survive a few generations, or continuous: derived from cancerous cells (immortal)

lytic life cycle function

converts a bacterial cell into a phage producing factor resulting in the lysis of a bacterial cell

lytic cell step 1: attachment

phage attaches to host cell by the tail fibers to a receptor of the bacterial cell

lytic life cycle step 2: penetration

DNA is injected into the host cell

lytic cycle step 3: biosynthesis

production of phage DNA and proteins

lytic cycle step 4: maturation

assembly of phage particles

lytic cycle step 5: release

phage lyse the bacterial cell and release it into the environment

lysogenic cycle

phage genome integrates into bacterial genome: prophage and then passed down

temperate phage

choose between lytic and lysogenic cycles

animal virus life cycle step 1

attachment: virus binds to receptor cell on host cell

animal virus life cycle step 2

entry: virus enters host cell through injection, receptor-mediated endocytosis, or fusion

animal virus life cycle step 3

uncoating: loss of capsid, releases nucleic acid into the host cell

animal virus life cycle step 4

biosynthesis: production of nucleic acid and proteins depends on Baltimore classification

animal virus life cycle step 5

maturation: nucleic acid and capsid proteins assemble

animal virus life cycle step 6

release: leaves the host cell by rupture or budding (in enveloped viruses)

latent infection

lay dormant for period of time; virus is present but does not cause symptoms

acute infection

sudden and rapid, short term infection that is usually curable

chronic/persistent infection

slow and gradual long term infection that is usually managed long term and not cured

prion

misfolded proteins that induce misfolding of host proteins; do not contain DNA or RNA

when was the first prion disease discovered

in 1982 in sheep

prion diseases affect

the brain or neurological function; only diagnosed on postmortem brain tissue analysis

prevention of prion diseases

ban on animal product feeds, removal of brain tissue by slaughterhouses, import control on animal products from countries detected

mechanism of prions

amplification of prions occurs through conversion rather than replication by converting an existing protein into the prion form

what does the prion form of a host protein look like

formation of beta and pleated sheets

when scrapie protein comes into contact with normal host protein

it is converted into scrapie and many scrapie proteins are produced aggregating to form amyloids which accumulate and cause cell death

sporadic prion diseases

spontaneous conversion to scrapie protein

acquired prion disease

through ingestion of prions from an infected animal