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