Ch.5 Pt. 1 Viruses, Prions, and Viral Diseases

Virion

- extracellular state of a virus

- only capable of infecting a host cell

- consists of >/= 1 molecule of DNA OR RNA enclosed in a coat of protein at min. (nucleocapsid)

Capsid

a protein coat made up of capsomeres

Nucleocapsid

- nucleic acid (DNA or DNA) surrounded by a protein coat (capsid)

- all viruses have this

Capsomere

protein subunit that makes up the capsid

Spike

- viral proteins that are responsible for host cell specificity (host range)

- they attach a virion to the host cell for infection

Envelope

taken from the plasma membrane (sometimes nuclear membrane) of the host cells in which the virus particle was made; has virus proteins inserted into it

Lytic phages (virulent phages)

can ONLY do the lytic cycle

Lysogenic phages (temperate phages)

can do both lytic and lysogenic cycles

Steps in lytic cycle of T4 bacteriohages

attachment > penetration > biosynthesis > maturation > release

Lysogen

a bacterial cell containing a prophage

Lysogeny

Done by many temperate phages by integration of their genome into the host chromosome in a relationship called...

Prophage

Phage DNA integrated within a bacterial chromosome

Generalized transduction

associated with the lytic cycle

Lysogenic conversion

when the presence of a prophage causes a phenotypic change for the lysogen, the infected bacterial cell gains new abilities/characteristics from the new genes on the prophage

multiplication of animal viruses: attachment

viruses attach to cell membrane

multiplication of animal viruses: penetration

- by endocytosis or membrane fusion

- uncoating by viral or host enzymes

Animal virus penetration by membrane fusion

Fusion occurs btwn the viral envelope and the host cell plasma membrane to release nucleocapsid into the cytoplasm, thus penetration and uncoating both occur at the same time.

- Does not need an additional step

Animal virus penetration by endocytosis

- membrane fusion used for uncoating

- sometimes the envelope of virus can fuse w/ the membrane of the endosome after endocytosis, then the nucleocapsid is released into the cytoplasm (can be stimulated by low pH of endosome)

Uncoating

process of getting the viral genetic info into the cytosol since it will often be in an endosome initially

multiplication of animal viruses: biosynthesis

production of nucleic acid and proteins

multiplication of animal viruses: maturation

nucleic acid and capsid proteins assemble

multiplication of animal viruses: release

by budding (enveloped viruses, most common) or rupture

Most common penetration for animal viruses...

endocytosis + membrane fusion

Most common penetration for bacteriophages

direct penetration (phage DNA injected into host cell)

7 classes of the Baltimore system

1. dsDNA viruses

2. ssDNA viruses

3. dsRNA virsues

4. (+) sense ssRNA viruses

5. (-) sense ssRNA viruses

6. RNA reverse transcribing viruses

7. DNA reverse transcribing viruses

dsDNA examples

NE:

- papillomavirus HPV (cervical cancer + warts)

E:

- Hep B

- herpesviridae: HHV, Varricella-Zoster virus (causes chickenpox + shingles)

- herpes viruses

+ssRNA examples

NE:

- Picornaviridae: rhinovirus (most common cause of common cold)

E:

- Coronaviridae: coronavirus (SARS, MERS, COVID-19)

-ssRNA examples

one strand E:

- Rhabdoviridae

- Filoviridae

multiple strands E:

- Orthomyxoviridae: influenzavirus

ssRNA retrovrisues examples

- Lentivirus: HIV

Oncogene

cancer-causing gene

Transformation

activated oncogenes transform normal cells into cancerous cells

Transformed cell

transformed into cancerous cells

Acute viral infections

Virus makes the person acutely ill, and is subsequently cleared from the body by the immune system.

Acute infection examples

- the flu

- colds

Persistent viral infections

Slow production of virions over time; disease processes occurs over a long time period; often is fatal.

Persistant viral infections examples

- Subacute sclerosing panencephalitis (measles virus)

- HIV (AIDS)

Latent viral infections

- virus remains in asymptomatic host cell for long periods

- no virions produced during this phase

- then virus is activated and begins production of new virions

Latent viral infections examples

- cold sores

- shingles

- herpesviruses

DNA viral families

- Papillomaviridae (NE)

- Herpesviridae (E)

Papillomaviridae

Nucleic Acid: dsDNA

Enveloped? NE

Examples: Papillomavirus HPV, cervical cancer + warts

Herpesviridae

Nucleic Acid: dsDNA

Enveloped? E

Examples: Varicellovirus (Varicella-Zoster virus, causes chickenpox + shingles), many herpes viruses cause latent viral infxns

+ ssRNA viral families

- Picornaviridae (NE)

- Coronaviridae (E)

Picornaviridae

Nucleic Acid: + strand ssRNA

Enveloped? NE

Examples: Rhinovirus (most common cause of the common cold)

Coronaviridae

Nucleic Acid: + strand ssRNA

Enveloped? E

Examples: Coronavirus: cause respiratory infections (common cold, SARS, MERS, COVID-19)

Other RNA viral families

- Orthomyxoviridae (E)

- Retroviridae

Orthomyxoviridae

Nucleic Acid: -ssRNA, multiple strands

Enveloped? E

Examples: Influenzavirus

Retroviridae

Nucleic Acid: +ssRNA in virion, dsDNA made in host cell

Enveloped? E

Examples: Lentivirus (HIV)

Host range

determined by viral attachment proteins and corresponding receptors on host cells

Obligate intracellular parasites

cannot reproduce on their own, need to be inside a host cell

Generalized transduction

T4 multiplication cycle (lytic cycle): attachment

phage attaches by tail fibers (spike proteins) to host cell

T4 multiplication cycle (lytic cycle): penetration

direct penetration; phage lysozyme open cell wall, tail sheath

contracts to force phage DNA into cell

T4 multiplication cycle (lytic cycle): biosynthesis

synthesis of phage DNA and proteins (1. early mRNA/proteins, 2. replication, 3. late mRNA/proteins

T4 multiplication cycle (lytic cycle): maturation

assembly of phage particles

T4 multiplication cycle (lytic cycle): release

rupture. phage lysozyme breaks cell wall, lysis of cell

Lytic cycle of bacteriophage

• phage causes lysis and death of host cell

  • generalized transduction may occur

lysogenic cycle of bacteriophage

• prophage DNA incorporated in host DNA

• lysogenic conversion may occur

• specialized transduction may occur