infectious particles

what are viruses

obligate intracellular parasites

where do viruses replicate

only inside host cells

what are host cells for viruses

archaea, bacteria, eukarya

how do viruses exist outside of host cells

as inert infectious particles, virions

what is the structure of a virus

nucleic acid coated with a capsid (protein coat)

How many types of nucleic acid are found in a single virion?

Only one type — either DNA or RNA, never both.

Can viral genomes be single- or double-stranded?

Yes — both DNA and RNA viruses can have single-stranded or double-stranded genomes.

how many proteins does a simple virus have

3 proteins

how many proteins does a complex virus have

>100 proteins

what does the host cell provide for viruses

nucleic acids, amino acids, ribosomes, and energy

capsid

protein coat around nucleic acid

capsomere

protein subunit that makes up the capsid

nucleocapsid

nucleic acid and capsid together

envelope

lipid containing layer with embedded proteins

where does the envelope come from in animal viruses

cytoplasmic membrane of host cells

Where do envelope proteins in viruses come from?

They are encoded by the viral genome.

Are envelope proteins the same in all viruses

no, they are specific to each virus

What is the main function of viral envelope proteins

They help the virus attach to the next host cell

what does the shape of the virus determine

the nature of the capsomere

what are spike proteins

adhesin proteins

what is the function of spike proteins

allow virus to attach to cells and infect them

what is an example of a helical virus

tobacco mosaic virus

how many identical capsomeres do tobacco mosaic viruses have

2130

where is tobacco mosaic virus typically found

in plants

what is the most common shape of a polyhedral virus

icosahedron (20 triangular faces)

how many capsomeres does a icosahedron polyhedral virus have

3 capsomeres

what limits the number of capsomeres possible

the geometry

how many faces/clusters does human HPV have

72 faces 5 clusters of capsomeres

what is the most complex virus

bacteriophages

what are viroids

closed circle of single stranded RNA

what are viroids a pathogen of

plants, and they dont affect mammals

why arent viroids a virus

because they lack the capsid

when do viroids start to replicate

when they infect plants

how do viroids move around plant

through their plasmodesma

what does replication depend on

the host machinery

how do viroids cause diseases

by overtaking the host machinery

what transmits viroids

insects or rabbits eg

what are 2 examples of the disease they cause

1. cadang cadang coconut disease

2. potato spindle tuber

what are prions

misfolded versions of normal protein that can cause disease

how many proteins do prions have

only one protein

what do prions cause

neurological degenerative disorders

what are examples of the neurological degenerative disorders

1. scrapie

2. bovine spongiform encephalopathy

3. Creutzfeldt-Jakob disease

4. vCJD

5. kuru

explain the human kuru disease

in new guinea, women eat brain of deceased, prion is transmitted

what happens when prions misfold

1. misfolding of PrPc makes PrPsc

2. PrPsc induces misfolding of PrPc

3. PrPsc accumulates in the cell

4. it induces more misfolding

5. constant cycle kills neurons

6. affects neighboring neurons too

what is the family of viruses

-viridae

what is the genus of viruses

-virus

what is classification of viruses based on

characteristics

what characteristics are considered for classifying viruses

1. nature of host

2. type of diseased caused

3. life cycle

4. naked of enveloped

5. type of nucleic acid

6. type of strandedness

what are the 2 possible RNA genomes

+ve strand and -ve strand

explain the +ve strand of RNA genomes

same strand as RNA, can be translated directly to amino acids

explain the -ve strand of RNA genomes

complementary to RNA, first transcribed to +ve strand, then translated into aminoc acids

what transcribes the -ve strand of RNA genomes

RNA dependent RNA polymerase

who supplies the RNA dependent RNA polymerase

the virus already has it

1st step of the life cycle of the virus

attachment

2nd step of the life cycle of the virus

penetration

3rd step of the life cycle of the virus

biosyntehsis

4th step of the life cycle of the virus

biosynthesis

5th step of the life cycle of the virus

maturation

6th step of the life cycle of the virus

lysis

7th step of the life cycle of the virus

release

are bacteriophage naked or enveloped viruses

can be both, mostly naked

how do naked bacteriophages infect cells

by direct penetration, capsid stays on surface of cell

what is injected in direct penetration

the nucleic acid (RNA/DNA)

how do enveloped viruses infect cells

1. nucleocapsid enters the cell

2. viral genome is released

3. capsid disintegrates inside cell

what is viropexis

enveloped viruses enter cells through phagocytosis

what does an enveloped cell create when it enters the cell

a phagosome

what happens to the enveloped cell

lysosomes degrade the capsid

explain attachment

binding to specific receptors on cell surfeace, virus is introduced

explain penetration

virus genome enters the cell

explain biosynthesis

transcription/translation (protein synthesis)

explain maturation

assembly of virus components, nucleic acid, nucleocapsid, accessory proteins form new virions

explain release

mature virions leave cell by lysis/budding, plant viruses exit by/transmit by vectors

1st step of virus replication

innoculation

2nd step of virus replication

eclipse/latent period

3rd step of virus replication

burst/release

4th step of virus replication

burst size

explain the latent (eclipse) period

time necessary for host cells to replicate viral genome/synthesize viral components

explain release (burst)

virions detected outside of cell, lysis where peptidolgycan layer is destroyed, budding

explain burst size

number of virions released

release by budding steps

1. viral capsid inside host cell

2. leaves cell by budding

3. creates enveloped around nucleocapsid

4. viral glycoproteins present on envelope

What bacteria do the best-studied bacteriophages infect?

E. coli, a Gram-negative bacterium.

what type of genome do most phages contain

linear dsDNA genomes

are phage genomes naked or enveloped

most naked, some enveloped by lipids

what are the 2 types of phages

1. virulent

2. temperate

what does the virulent phage do

infect host cell = always replicate --> host cell lysis

takes the lytic pathway

what do temperate phages do

infect a cell through either lytic or lysogenic pathway

genome is incorporated into bacterial host genome

where/how does T4 bacteriophage attach

attaches to core region of LPS by tail fibers

what happens after attachment of T4 bacteriophage

1. tail sheath contracts

2. forces central core through membrane

3. tail lysozymes digest peptidoglycan layer

4. forms small pore

what/where is injected into cell after T4 bacteriophage attaches

phage DNA injected into cytoplasm

what is a function of virulent phages

replication and release of new virions

what is a function of temperate phages

phage genome integrates with chromosome, results in prolonged and latent state of infection

What kind of structure do most dsDNA phage genomes have

linear with cohesive ends, regions of single-stranded complementary DNA

What happens to cohesive ends of phage DNA after entering the host cell

cohesive ends join, forming a circular double-stranded DNA molecule

explain the lytic cycle

1. phage attaches to host and injects DNA

2. phage DNA circularizes, enters lytic cycle

3. new phage DNA and proteins synthesized (assembled into virions)

4. cell lyses, releases virions

explain the lysogenic cycle

1. phage attaches to host and injects DNA

2. phage DNA circularizes, enters lysogenic cycle

3. DNA integrates within bacterial chromosome

4. lysegonic bacterium reproduces (cell division)

5. prophage is excised (sometimes)

6. prophage enters lytic cycle

what happens when DNA circularizes

integrates into attB site on bacterial chromosome

what allows phage genome to bind to the attB site

phage genome has attP, it is homologous to attB

what allows integration of genomes

lambda integrase

prophage

bacteriophage DNA that is embedded in the bacterial host's DNA

lysogen

a bacterium containing a prophage