Ch 21: Viruses

true or false. viruses are parasitic entities.

true

true or false. viruses are part of the bacteria family.

false. viruses cannot fit into any domain of life

true or false. viruses cannot infect organisms as diverse as bacteria, plants, and animals.

false. viruses can infect organisms as diverse as bacteria, plants, and animals

true or false. viruses are cellular, have metabolism, grow, and do cellular division.

false. viruses are non-cellular, have no metabolism, no growth, and no cell division

true or false. viruses are copy and replicate themselves.

true.

true or false. viruses are completely dependent on the host for resources to produce progeny viruses

true

what are the 3 main component of a virus?

• capsid

• nucleic acid genome

• envelope

capsid

outer protein coat

nucleic acid genome

single or double-stranded DNA or RNA

envelope (not all viruses have)

membrane covering capsid

where is the envelope derived?

from the plasma membrane of the host

• protein and phospholipids

viruses cannot ________ on their own. they require cellular machinery of ____ cells to survive and replicate.

replicate, host

polymerase chain reaction (PCR)

detects extremely small DNA/RNA of viruses

immunoassays

detect antigens or antibody reaction

Chamberland-Pasteur porcelain filter

removes all bacteria from a water sample

Adolph Meyer 1886

demonstrated that a disease of tobacco plants - tobacco mosaic disease - could be transferred from a diseased plant to a healthy one via liquid plant extracts

Dmitri Ivanowski 1892

demonstrated the disease could be transmitted via liquid plant extracts even after the Chamberland-Pasteur filter had removed all viable bacteria from the extract

• bacteria ruled out as culprit b/c filter took it out → virus

what type of microscopy developed in the 1930s allowed for the first view of viruses?

electron microscopy

what was the first virus to be seen and described?

tobacco mosaic virus (TMV)

scanning electron microscope

scans surface of viruses

transmission electron microscope

scans internal structures of viruses

true or false. there is no known fossil record of the evolution of viruses

true

what are the 3 main hypothesis of virus evolution?

• H1: regressive

• H2: progressive or escapist

• H3: self-replicating

H1: regressive

viruses evolved from free-living cells or from intracellular prokaryotic parasites

H2: progressive or escapist

viruses originated from pieces of RNA and DNA that escaped from a host cell and gained the ability to move between cells

H3: self-replicating

viruses may have originated from self-replicating entities similar to transposons or other mobile genetic elements

viral morphology: size

extremely small, single virion is between 20-250nm in diameter

viral morphology: noncellular

viruses lack almost all cell components except for the nucleic core

viral morphology: make up

made of nucleic core, capsid, and sometimes outer envelope

bacteria size

.1-5 nm

eukaryote size

100nm

helical

long and cylindrical

icosahedral

roughly spherical-shape

enveloped

have membranes surrounding the capsids

complex/head and tail

infect bacteria and have a head that is similar to icosahedral viruses and a tail shaped like helical viruses

the virus core contains nucleic acid

• either DNA or RNA (but not both)

• either single-stranded or double-stranded

• circular or linear

• either in one piece or in multiple segments

viral genome

total genetic content of the virus

viral genomes are very smalle

only contain those genes that encode proteins that the virus cannot get from the host cell

viral DNA directs the ____ cell to make new virus ______

host, copies

true or false. RNA viruses cannot encode their own enzymes

false.

important RNA viral enzyme

RNA-dependent RNA polymerase (RdRp)

RNA viruses use enzymes that make more errors

RdRp makes more errors, lacks error checking ability

true or false. RNA viruses mutate more frequently than DNA viruses

true

true or false. if a virus cannot get a specific protein from the host, they are able to encode the protein

true

true or false. RNA viruses do not have their own enzymes and use the enzymes of the host, specifically DNA polymerase

false. DNA viruses do not have their own enzymes and use the enzymes of the host, specifically DNA polymerase

where do DNA viruses replicate?

in the nucleus

where do RNA viruses duplicate?

in the cytoplasm

true or false. DNA viruses mutate more than RNA viruses

false. DNA viruses (chicken pox) do not mutate as often as RNA viruses (flu virus)

DNA viruses

• often double-stranded, but can be single-stranded

• replication takes place in the nucleus (in most)

• a few have DNA polymerases and can complete replication in the host cell’s cytoplasm

• ex: smallpox virus

where does the smallpox DNA replicate?

in the cytoplasm

RNA viruses

• usually single-stranded, but can be double-stranded

• replication takes place in the cytoplasm (in most)

• mutation happens at a very high rate b/c RNA polymerase does not have proofreading capabilities

• ex. influenza, coronavirus

why do RNA viruses replicate in the cytoplasm?

b/c they encode their own enzymes, do not need DNA polymerase (in nucleus) and its easier for them to do it in the cytoplasm

how were viruses classified in the past?

• nucleic acid type

• capsid structure

• enveloped/non-enveloped (influenza, HIV/ common cold)

• genome structure

what is the current way to classify viruses?

how viruses send out their messenger RNA

central dogma

1. replication: DNA polymerase (DNA → DNA)

2. transcription: RNA polymerase (DNA → RNA)

3. translation: ribosome (RNA → protein)

reverse transcription

uses reverse transcriptase to convert RNA → DNA

• used by retroviruses (HIV)

what is the most commonly and currently used system of virus classification?

baltimore classficaiton

who is the baltimore classification developed by?

nobel-prize winning biologist David Baltimore in the early 1970s

how does the baltimore classification group viruses?

according to how their mRNA is produced during the replicative cycle of the virus

Group I: characteristics

ds-DNA

Group I: mode of mRNA production

mRNA is transcribed directly from the DNA template

• uses host enzymes to make mRNA, uses DNA polymerase

Group I: example

Herpes simplex (herpesvirus)

Group II: characteristics

ss-DNA

Group II: mode of mRNA production

DNA is converted to a double-stranded form before RNA is transcribed

• ssDNA → dsDNA → mRNA

Group II: example

canine parvovirus (parvovirus)

Group III: characteristics

ds-RNA

Group III: mode of mRNA production

mRNA is transcribed from the RNA genome by using its own enzyme

• dsRNA → separates into ssRNA → mRNA

Group III: example

childhood gastroenteritis (rotavirus)

Group IV: characteristics

ssRNA (+) polar

Group IV: mode of mRNA production

genome functions as mRNA (+)

Group IV: example

common cold (picornavirus) and COVID-19

Group V: characteristics

ssRNA (-) polar

Group V: mode of mRNA production

mRNA is transcribe from the RNA genome

• genome is complementary to mRNA

• ssRNA→ mRNA RdRp

Group V: example

rabies (rhabdovirus)

Group VI: characteristics

ss RNA viruses w/ reverse transcriptase

Group VI: mode of mRNA production

reverse transcriptase makes DNA from the RNA genome: DNA is then incorporated in the host genome: mRNA is transcribed from the incorporated DNA

• converts RNA → DNA, DNA is then converted in the host genome

• unless someone has HIV, they won’t have reverse transcriptase

Group VI: example

human immunodeficiency virus (HIV)

Group VII: characteristics

dsDNA w/ reverse transcriptase

Group VII: mode of mRNA production

the viral genome is dsDNA, but viral DNA is replicated through an RNA intermediate: the RNA may serve directly as mRNA or as a template to make mRNA

• dsDNA → ssRNA intermediate, functions as mRNA necessary for genome replication Rbds DNA

Group VII: example

hepatitis B virus (hepadnavirus)

steps of viral infection

1. attachment

2. entry

3. replication and assembly

4. egress (release)

Attachment

• receptors on the surface of the host cell bind to virus capsid proteins or virus envelope glycoproteins

  • have caro groups attached to them

• viruses can attach only to cells that have the right receptor molecules

• therefore, viruses can be very specific about what species or cell type they can infect

what must be compatible for attachment of the virus to occur?

protein of the virus and receptor of the host

what is the protein on HIV?

GP120: attachment must happen before HIV enter the immune cell

HIV, an __________, ____________ virus, attaches to the CD4 receptor of an immune cell and fuses w/ the cell _________.

enveloped, icosahedral, membrane

entry

viruses may enter eukaryotic cells by (a) endocytosis, or if enveloped, by (b) fusion w/ the cell’s membrane

endocytosis

virus engulfment → becomes a vesicle, envelope and capsid break down → free DNA

fusion w/ the cell’s membrane

envelope of the virus fuses w/ the plasma membrane of the host

what does replication and assembly depend on?

the viral genome

replication and assembly: DNA viruses

usually use host-cell proteins and enzymes to replicate the viral DNA and to transcribe viral mRNA → then used to direct viral protein synthesis

replication and assembly: RNA viruses

• RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA

• the viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins and assemble new virions

• uses RdRp, RNA genome contains the code for RdRp, host actually makes RdRp

replication and assembly: RNA retroviruses

• have an RNA genome that must have reverse transcribed into DNA, which then is incorporated into the host cell genome

• DNA directs synthesis and assembly of new viruses

• RNA→ DNA→host

what type of cells does reverse transcriptase not occur in?

uninfected host cells

what is reverse transcriptase derived from?

the expression of viral genes within the infected host cells

true or false. only infected host cell have reverse transcriptase.

true

what do DNA viruses use to replicate the viral DNA?

host-cell proteins and enzymes

what do RNA viruses use as a template for synthesis of viral genomic RNA and mRNA?

RNA core

what does the viral mRNA direct the host cell to synthesize and assemble?

to synthesize viral enzymes and capsid proteins and to assemble new virions

what is the DNA viruses viral mRNA used for?

to direct viral protein synthesis