Lecture: Viral Structure and Classification, Prions
compare and contrast the secondary structures of Cellular PrP and Prion PrP
Cellular PrP are proteins made by mammals and have naturally occuring alpha-helices whereas Prion PrP are disease-causing proteins where the alpha-helices have been altered to form beta-sheets
BSE
bovine spongiform encephalitis
what is the protein subunits in the structure of a viral capsid?
capsomeres
describe the neurological effects of prion disease
disease-causing prions alter the alpha-helices of neurological tissues forming beta-sheets, over time they can lead to spongifrom encephalopathies which is the formation of vacuoles in the brain
prion PrP
disease-causing proteins in which the alpha-helices have been altered to contain beta-sheets
dsDNA
double-stranded DNA
dsRNA
double-stranded RNA
Family Poxviridae
dsDNA, enveloped virus Genus Orthopoxvirus (smallpox) - very complex structure
Family Herpesviridae
dsDNA, enveloped viruses Genera Simplexvirus (oral and genital herpes), Varicellovirus (chickenpox, shingles) - can become dormant but not necessarily encorporated into DNA
Family Papillomavirus
dsDNA, naked virus Genus Papillomavirus (HPV) - potentially cause cancer
Family Adenoviridae
dsDNA, naked viruses Genus Mastadenovirus (the common cold) - possibly alter host phenotype contributing to obesity
describe the assembly stage of bacteriophage replication
during the assembly stage of bacteriophage replication, the host cell begins to put all the parts of the viral capsid together
describe the attachment phase of viral replication
during the attachment phase of viral replication, the virus uses it's glycoprotein spikes or other attachment molecules to initiate chemical attraction between viral proteins and host cell receptors. different viruses have different glycoprotein spikes and/or attachment molecules use different receptors on host cells. these differences are a means of classification among viruses.
describe the entry phase of viral replication
entry into the host cell depends on the type of virus. if the virus is enveloped it could enter one of two ways: endocytosis or fusion. if the virus is naked it could enter one of two ways: endocytosis or directly injecting nucleic acids into the cell. if the virus fuses or undergoes endocytosis, the capsid is uncoated or dismantled.
identify and describe the structure of enveloped viruses
enveloped viruses are just like naked viruses except they have a phospholipid envelope surrounding their nucleocapsid
uncoating
in animal virus replication: the process of taking the capsid off the virus once inside the host cell after the virus has undergone endocytosis
prophage
inactive or dormant phages in a host cell
prion
infectious proteins, proteinaceous infectious agents that do not contain any nucleic acid, change in secondary structure converts native prions into infective prions
describe the nucleic acids that may be used by a virus
may be DNA or RNA but never both. may be single stranded or double stranded. may be circular or linear or in segments. may be coding (+) or non-coding (-)
explain why RNA viruses must supply their own enzymes for RNA replication
messenger RNA cannot be coded from RNA itself and in order to
budding
method by which enveloped viruses who obtain their membranes from the cell membrane of the host cell are released from the host
exocytosis
method by which enveloped viruses who obtain their membranes from the nuclear membrane, ER, or Golgi of the host cell are released from the host
describe and explain the host and cell specificity of virions
most virions are very species specific infecting a single species or genus. virions recognize host cell surface proteins and receptors.
host specificity
most virions infect a single species or genus, most have a specific cell target within the host, very few can infect multiple species ex: bird flu, rabies, etc.
lytic cycle
process used by all bacteriophages which leads to cell death
naked virus
protein coat (capsid) surrounding the nucleocapsid
what are the functions of the viral capsid?
provides protection for viral nucleic acid and means of attachment to host's cells
describe the enzyme reverse transcriptase and its use by retroviruses
reverse transcriptase is an enzyme that allows retroviruses to make dsDNA from an RNA template
cellular PrP
Pr - prion, P - protein, made by all mammals, normal structure with alpa-helices
ssDNA
single-stranded DNA
ssRNA
single-stranded RNA
glycoprotein spikes
special structures that play a role in host cell recognition
Family Filoviridae
ssRNA, enveloped viruses Genera Ebolavirus (Ebola), Marburgvirus - no particular shape associated with these (pleomorphic), can be filamentous
Family Calciviridae
ssRNA, naked virus Genus Norovirus (gastroenteritis) - cause of cruise ship infections
Family Picornaviridae
ssRNA, naked viruses Genera Rhinovirus (common cold), Hepatovirus (hepatitis A), Enterovirus (polio) - 'really small RNA viruses'
Family Orthomyxoviridae
ssRNA, segmented, enveloped virus Genus Influenzvirus (Flu) - genome made up to 8 ssRNA segments which contributes to new strains and able to infect many species
Family Retroviridae
ssRNA, segmented, enveloped, retroviruse Genus Lentivirus (HIV) - RNA is copied to DNA with the reverse transcriptase enzymes carried by the virion
attachment
step 1 of replication: virion recognizes host cell and prepares for entry
entry
step 2 of replication: nucleic acid is injected into host
synthesis
step 3 of replication: the host cell is reprogrammed to make viral componenents
assembly
step 4 of replication: the various components of new viruses are put together inside the host cell
release
step 5 of replication: once the host cell reaches a critical mass of new viruses, the cell lyse and the process continues
describe the synthesis stage of bacteriophage replication
the injected nucleic acids code for the different components of the viral capsid and the host cell starts to build them
explain why different types of animal viruses use different strategies for release
naked viruses are all released by cell lysis. enveloped viruses are released multiple ways depending on where they obtain their envelop from the host cell. if the envelope was taken from the host's cell membrane, the virus is released by budding. if it is taken from the nuclear membrane, ER or Golgi the virus is released by exocytosis.
identify and describe the structure of naked viruses
naked viruses are nucleic acids and some accessory proteins surrounded by capsid. the nucleic acids and the capsid together are called a nucleocapsid.
describe the release phase of viral replication
naked viruses are released by lysis - just like bacteriophages. if enveloped viruses get their envelopes from the cell membrane, they are released by budding, if they get their envelopes from the nucleus, ER or Golgi they are released by exocytosis.
describe the entry stage of bacteriophage replication
once attached, the bacteriophage injects it's nucleic acid into the host
describe the attachment stage of bacteriophage replication
once the bacteriophage recognizes it's host cell's membrane proteins/receptors it attaches
describe the release stage of bacteriophage replication
once the host cell reaches a critical mass of new phages, the cells lyse and new viruses spill into the environment. the process repeats.
differentiate between viral structure inside and outside a host cell
outside a host cell, a virus is called a virion and is composed of a nucleocapsid (nucleic acids + capsid) and possibly an envelope. inside the host cell, a virus is called a virus and is only it's nucleic acids.
temperate phages
phages that induce the lysogenic cycle in a virus by becoming part of the host cell's genome through incorporating it's DNA into the host cell and going dormant (becoming a prophage)
describe the assembly phase of viral replication
DNA viruses are typically assembled in the nucleus. RNA viruses are typically assembled in the cytoplasm.
describe the synthesis phase of viral replication
DNA viruses typically enter the nucleus and use host enzymes. RNA viruses typically replicated in the cytoplasm and must carry their own enzymes in order to turn their RNA into DNA.
explain why different types of animal viruses use different strategies for entry
animal viruses are either naked or enveloped which determines their mode of entry and release into and out of host cells. there are two modes of entry employed by naked viruses: direct injection of nucleic acids into the host cell or endocytosis. enveloped viruses also employ two methods of entry: endocytosis or fusion. enveloped viruses are able to fuse directly with the host's cell membrane because the composition of their envelope is similar to that of the host. naked viruses do not have an envelope and are therefore unable to fuse with the host cell.
Kuru
a human spongiform encephalitis found in Papua New Guinea
lysogeny
a modified lytic replication cycle where a temperate phage enters the host cell and becomes part of the bacterial chromosome
capsid
a protective protein structure surrounding nucleic acids of a virion
virion
a virus outside a host cell consisting of nucleic acids enclosed by a capsid
virus
acellular infectious agents: no cytoplasmic membrane, no cytoplasm, no organelles, do not metabolize, and are very small
endocytosis
an active transport process in which a substance is engulfed by and moves into a cell
differentiate between lytic and lysogenic cycles
the lytic cycle ultimately results in lysis and death of the host cell, whereas the lysogenic cycle is a modified lytic cycle. during the lysogenic cycle a temperate phage enters the host cell and becomes part of the bacterial chromosome. once incorporated into the chromosome the virus in now called a prophage and can be dormant until it is induced to become active. once induced the cell will go into the assembly stage of replication and will ultimately be lysed as it releases the newly manufactured virions
nucleocapsid
the nucleic acids and it's protective capsid
lysis
the obliteration of a cell in which the cell membrane is destroyed and the components leak out
capsomeres
the protein subunits of a capsid
genome
the sum of all the genetic material in a cell or virus
name and describe the functions of the viral envelope
the viral envelope is composed of a phospholipid bilayer and other proteins that play a role in host recognition. the envelope was acquired from the host cell during viral release. depending on where the virus was assembled within the host the envelope could be taken from the nuclear envelope, ER, Golgi, or the cell membrane.
where do DNA viruses typically synthesize their viral components?
they typically enter the nucleus and use host enzymes
where do RNA viruses typically synthesize their viral components?
they typically replicate in the cytoplasm and MUST CARRY THEIR OWN ENZYMES
enveloped virus
virions that have a phospholipid membrane surrounding their nucleocapsid
describe the general characteristics of viruses
viruses are acellular agents with no cytoplasmic membrane, no cytoplasm, no organelles, they do not metabolize, and they are very small. they completely depend on host cells for all of these needs.
describe how viruses are classified
viruses are classified primarily on their extracellular state (virions), on whether they are naked or enveloped, and whether they have double or single stranded DNA, or single stranded RNA. they can also be classified as to if they are segmented enveloped retroviruses or segmented enveloped viruses.
explain why most biologists consider viruses to be non-living biological entities
viruses have no cytoplasmic membranes, no cytoplasm, no organelles, and they do not metabolize. despite this, they do have some sort of genetic material either in the form of RNA or DNA but both the RNA and DNA can be double or single-stranded.
bacteriophage
viruses that infect bacteria
describe how viruses rely upon the host cell for replication
viruses use the 'machinery' of a cell to produce parts to build more viruses.
induction
when a dormant phage becomes active
explain how lysogenic conversion alters bacterial phenotype and give examples
when a temperate phage enters a host cell and becomes part of the bacteria's chromosome it is possible to alter the phenotype of that bacteria. an example of this would be V. cholerae which is a prophage that once it incorporates its genetic information into the host cell the cell will then start producing cholera toxin. whereas before the lysogenic conversion, the cell was completely incapable of producing the toxin.
lysogenic conversion
when the presence of a prophage alters the phenotype of a bacterium ex: prophages code for cholera and diphtheria toxins
acellular
without cells