Mircobio - CH 6

viruses

what are the properties of viruses?

• obligate intracellular parasites

• ultramicroscopic in size (nm)

• not cellular in nature, very compact and economical

• do not independentlky fulfill the characteristics of life

• has DNA or RNA, NEVER BOTH

• high specificity for attatchment to host cell

What are the coverings of viruses?

capsids

envelope (not found in all viruses)

What is in the central core of a virus?

DNA or RNA

Matrix proteins, enzymes (not found in all viruses

What is a capsid

a shell that surrounds the nucleic acid (together they are called a nucleocapsid)

what are naked viruses?

only nucleocapsid present

what are enveloped viruses?

additional covering external to capsid present

What protein makes up a capsid?

capsomers

What are helical capsids?

continuous helix of capsomers forming a cylindrical nucleocapsid, not completely closed

what are icosahedral capsids?

3-D symmentrical polygon, with 20 sides, and 12 evenly spaced corners (vortexes), completely closed

Describe the viral envelope.

mostly in animal viruese, accquired when the virus leaves the host cell, exposed proteins on the outside (spikes) are essential for attatchment of virus to the host cell.

CAN WASH AWAY WITH SOAP.

What are the functions of capsids and envelopes?

protects the nucleic acids, binds to a cell surface and assists the penetration, stimulates host immune system

what are poxviruses?

lack a typical capsid and are covered by a dense layer of lipoproteins

what are bacteriophages (phages)?

viruses that infect bacteria, have a helical tail

Describe the pathway of +ssRNA

protein synthesis →translation→viral proteins

genome synthesis→synthesis of complementary strand→-ssRNA→synthesis of complementary strand→Viral genome (+ssRNA)

Describe the pathway of -ssRNA?

synthesis of complementary strand→+ssRNA:

protein synthesis →translation→viral proteins

genome synthesis→synthesis of complementary strand→Viral genome (-ssRNA)

describe the pathway of ±dsRNA?

synthesis of single strand→+ssRNA:

protein synthesis →translation→viral proteins

genome synthesis→synthesis of complementary strand to create dsRNA→Viral genome (±dsRNA)

What is special about DNA viruses?

usually double stranded (ds) but may be single stranded (ss)

circular or linear

very stable, unlikely to mutate

What is special about RNA viruses?

usually ss, maybe ds, may be segemented into seperate RNA pieces

+sense = can make a protein

-sense= has to be make into +sense, then it can form a protein (needs RNA dependent RNA polymerase (rdrp)

What are some pre-formed enzymes required for viral replication?

polymerases- synthetize DNA or RNA

replicases - copy RNA

reverse transcriptionase - synthesis of DNA from RNA (HIV virus)

What is Baltimore classification?

groups of viruses into seven classes based on a single combination of their nucleic acid (DNA or RNA), strandedness (ss or ds), sense, and method of replication

What is the ICTV?

inertnational commitee of taxonomy of viruses,

Order→family→genus→species

What does each order of viruses contain?

multiple families of viruses the share similar characteristics such as a genome type, replicationmechanism, and structure

Class I (dsDNA)

Replication: uses host DNA polymerase for replication and host RNA polymerase for transcription

Ex: adenoviruses, herpesviruses, poxviruses

Class II (ssDNA)

Rep: must be first converted into dsDNA by host DNA polymerases in the nucleus

Ex: paroviruses

Class III (dsRNA)

Rep: uses viral (RdRp) to transcribe mRNA

ex: Reoviruses (rotavirus)

Class IV (+ssRNA)

Rep: genome can act directly as mRNA and be transcribed by host ribosomes

Ex: Coronaviruses, Picornaviruses (polio), Flaviviruses (Dengue Virus)

Class V (-ssRNA)

rep: must carry RdRp to convert genome into +ssRNA before translation

ex: infleunza virus (orthomyxoviruses), Rabies virus (Rhabdovirus), Ebola (filoviruses)

Class VI (+ssRNA) retroviruses

rep: uses reverse transcriptionase to convert RNA genome into DNA, which integrates into the host genome

ex: HIV

Class VII (dsRNA) reverse-transcribing viruses

rep: uses reverse transcriptionase to convert RNA intermediates back into DNA
ex: Hep B

Name ends into -ales

Order

name ends in -viridae

family

name ends in -virus

genus

taxonomy of viruses (species)

members share genetic similarities and host type

common names are used for species

How are strains of viruses determined?

designated by a number and also number of mutations

first step in viral multiplication?

Adsorption- binding of virus to specific molecules on the host cell

second step in viral multiplication

penetration- genome enters the host cell

third step in viral multiplication

uncoating- the viral nucleic acid is released from the capsid

fourth step in viral multiplication

synthesis- viral components are produced (rna molecules, capsomers, and protein spikes)

fifth step in viral multiplication

new viral particles are constructed

sixth step in viral multiplication

release - assembled viruses are released by budding (exocytosis) or cell lysis

what is a host range?

spectrum of cells a virus can infect

what does Hepatitis B infect?

human liver cell

what does poliovirus infect?

primate intestinal and nerve cells

what does rabies infect?

nerve cells of many mammals

what does HIV/AIDs infect?

CD4 protein on WBCs

what are Tropisms?

tissue specificities that viruses have

Describe viral fusion of an enveloped virus.

Virus spikes attatch to cell membrane via recpetors →envelope fuses with the membrane(forms receptor-spike complex) →capsid is released into cytoplasm where nucleic acid is free

describe viral endocytosis of an enveloped virus.

specific attatchment→engulfment→virus in vescicle→vesicle,envelope,and capsid break down→free DNA

describe viral endocytosis of a naked virus.

adhesion of virus to host receptors→engulfment into vesicle→viral RNA is released from vescicle

where are DNA viruses replicated and assembled?

the nucleus

where are RNA viruses replicated and assembled?

the cytoplasm

what is the first step in assembly of viruses?

the capsid is first laid down as an empty shell that will serve as a recpetacle for the nucleic acid strand.

what is the first step in maturation and release of enveloped viruses?

viral protiens that will become envelope spikes insert into host cytoplasmic membrane

what is the second step in maturation and release of enveloped viruses?

viral matric protein coast inside of cytoplsmic membrane

what is the third step in maturation and release of enveloped viruses?

nucleocapsid extrudes from the host cell, becoming coated with matrix proteins and envelope with protein spikes.

what is the fourth step in maturation and release of enveloped viruses?

new virus is released

what is a virion?

a fully formed, extracellular virus particle that is virulent and able to establish infection in a host

What are the two ways mature viruses are released?

budding and lysis.

describe budding/exocytosis.

happens only in enveloped viruses, nucleocapsid binds to membrane which pinches off and shed the viruses gradually, cell is not immediately destroyed.

describe lysis/rupturing.

happens in noneveloped and complex viruses, viruses released when cell dies and ruptures

What are some cytopathic effects?

effects that damage the cell and alter micrscopic appearance:

• disoreinetation of individual cells

• gross changes in shape or size

• intracellular changes (inclusion bodies/syncytium)

what are the cytopathic effects of smallpox virus?

cells round up; inclusions appear in cytoplasm

what are the cytopathic effects of herpes virus?

cells fuse to form multinucleated syncytia; nuclear inclusions

what are the cytopathic effects of adenovirus?

clumping of cells; nuclear inclusions

what are the cytopathic effects of poliovirus?

cell lysis; no inclusions

what are the cytopathic effects of Reovirus?

cell elargement; vacuoles and inclusons in cytoplasm

what are the cytopathic effects of influenza virus?

cells round up; no inclusions

what are the cytopathic effects of rabies virus?

no cells change in shape; cytoplasmic inclusions (negri bodies)

what are the cytopathic effects of measles virus?

syncytica form (multinucleated giant cells)

what are persistent infections?

cell harbors the virus and is not immediately lysed. can last weeks or host’s lifetime; several can periodically reactivate (chronic latent state)

Sign of herpes simplex virus reinfection?

cold sores and genital herpes

sign of herpes zoster virus reinfection?

chickenpox and shingles?

what is oncogenic?

when some animal viruses enter the host cell and permanently alter it’s genetic material resulting in cancer.

describe a transformed cell.

have an increased rate of growth, alterations in chromosomes, and the capacity to divide for indefinite time periods resulting in tumors

what are two times of mammalian oncoviruses?

papillomavirus - cervical cancer

epstein-barr virus - burkitt’s lymphoma

what are bacteriophages (phages)?

bacterial viruses

describe the lysogenic state of bacteria?

the viral DNA is inserted at specific sites on the bacterial chromosome. the viral dna is duplicated along with the regular genome and can provide adaptive genes for the host bacterium.

first step in the lytic cycle of a bacteriophage?

adsorption

second step in the lytic cycle of a bacteriophage?

penetration

third step in the lytic cycle of a bacteriophage?

duplication of phage components; replication of virus genetic material

foruth step in the lytic cycle of a bacteriophage?

assembly of new virions (capsid formed first→strand of nucleic acid inserted→sheath (tail) and tall fibers are added)

fifth step in the lytic cycle of a bacteriophage?

matruation

sixth step in the lytic cycle of a bacteriophage?

lysis of weakend cell

seventh step in the lytic cycle of a bacteriophage?

release of viruses

what happens if the virus enters a reversible state of lysogeny?

DNA is incorporated into the host’s gentic material

what happens when a phage doesn’t complete the lytic cycle?

the viral genome inserts into bacterial genome and becomes inactive prophage (cell not lysed). prophage is retained and copied during normal cell division resulting in the transfer of temperate page genome to all host cell progeny (lysogeny)

what is a temperate genome?

some DNA phages undergo adsorption and penetration but don’t replicate

when happens when induction occurs?

activation of lysogenic prophage followed by viral replication and cell lysis

what does lysogeny result in?

spread of the virus but without killing the host cell

What can phage genes in the bacterial chromosome cause?

production of toxins or enzymes that cause pathology (isogenic conversion)

ex: Corynebacterium diphtheriae, Vibrio cholerae, Clostridium botulinum

what are prions?

misfolded proteins, contain no DNA or RNA, extremely resistant to sterilization techniques

What can prions cause?

transmissible spongiform encephalopathy - fatal nuerodegenerative diseases

What are prions most common in?

Animals:

• Scrapie in sheep and goats

• bovine spongiform encephalopathies (BSE), also known as mad cow disease

• Wasting disease in elk

• Humans - Cruetzfeldt-Jakob Syndrome (CJS)

First step in prion infection?

prion protein enters the cell

second step in prion infection?

upon contact with normal proteins, the prions are able to shifty the configuration of normal proteins, converting them into prions

Third step in prion infection?

when this process creates large numbers of prions, they bind tightly together and form elongate chains

Fourth step in prion infection?

as the chains build up, they form fibers within the cell, which interfere within the cell’s function and destroy it

what are satellite viruses?

dependent on other viruses for replication

Examples of satellite viruses?

adeno-associated virus- replicates only in cell’s infect with adenovirus

delta agent- naked strong strand of RNA expressed only in the presence of Hep B virus

what are viroids?

short pieces of RNA, no protein coat; only been identified in plants