Introduction to the Viruses (4/10)

Zhou Lecture 3 (Lecture 21)

What are some types of viruses?

SARS-CoV-2 (severe acute respiraory syndrome)

west nile encephalitis virus

ebola

zika

hep A and intestinal infections: souther CA found in sea water near storm drains and sewage outlets (beach goers at risk)

what did louis pasteur do?

postulated that rabies was caused by living things smaller than bacteria

developed first vaccine for rabies

proposed term virus to denote this group of infectious agents

what was important in the year 1980 for virus research?

ivanovski and beijerinck: ID tobacco mosaic virus caused tobacco disease

Loeffler and Frosch: ID foot and mouth virus

agents could pass through porcelain filters designed to trap bacteria: virus filterable

what are viruses? what are the different views?

uniqeu group of biological entities that are specialized for every type of cell

one view: non-living things, cannot exist independent of the host cells

different view: living things, replicate and direct cellular processes

viruses among the biological spectrum

infectious particles (rather than organisms) as either active or inactive

what are unique properties of viruses?

obligate intracellular parasites

ultramicroscopic (smallest 20nm diameter)

not cells, compact, economical

do not independently fulfill characteristics of life

inactive outside of host cells

geometric, form crystals

basic structure: protein shells (capsid) surrounding nucleic acid core

DNA or RNA

viral surface with attachment molecules

hijacking host metabolic machinery for reproduction

lack most enzymes for life

size of viruses

very small: 2000 bacterial viruses fit into one bacteria

animal viruses: 20-450nm

electron microscope need to observe (5Mx magnfiication vs light microscope 2000x)

what are some methods to view viruses?

negative staining

Positive staining

Shadowcasting

negative staining

thin layers of opaque salt to outline shape against dark background and enhance tecture of the viral surface

positive staining

detects internal details of proteins and nucleic acids

shadowcasting

spread metallic vapor from a certain angle, metal coating over the surface approx the viral contours and shadow is cast on unexposed side

what are the parts of viruses necessary to invade and control host cells?

virus particle: covering and central core

• covering: capsid and envelope

  • envelope not in all viruses

• central core: nucleic acid molecule (RNA/DNA) and enzymes

what is the general structure of viruses?

capsid: protein shell that surround the nucleic acid in central core

capside + nucleic acid: nucleocapsid

envelope: some possess modified hose cell membrane that wraps outside of the nucleocapsid

viral capsid

geometric feature: protective outer shell

made of capsomers: spontaneously self-assemble into capsid

depending on the shape of capsomers, the assembled capsids are shaped either as icosahedreal or helical symmetry

Viruses are helical

symmetrical

helical capside: tobacco mosaic virus

rod shaped capsomers

form continuous helix

nucleic acid packed inside, nucleocapsid of naked virus is rigid

nucleocapsid of enveloped helical virus: more flexible - influenza, measles, and rabies

variations of virueses with helical nucleocapsids

naked helical virus: tobacco mosaic virus

enveloped helical virus: influenza virus

icosahedron

viral symmetry

3D, 20 sided object, 12 evenly spaced corners

constructed form single/multiple types of capsomers

variations in the # of capsomers in each virus (polio 32, adenovirus 240)

with or wo envelope

what are the 2 types of icosahedral viruses

papillomaviruses (warts) with no envelope

herpesvirus w enveloped icosahedron

what is a viral envelope and how does it differ from cell membranes?

enveloped viruses use host membrane to make envelopes: bud off the cell membrane, nuclear envelope or ER

differ:

• viral proteins replace cellular proteins

• viral glycoproteins positioned on the envelope for host cell attachment

What are the functions of viral capsid/envelope?

protection of nucleic acids

• capsid of enteric(intestinal) viruses are resistant to acid and protein digesting enzymes of GI tract

• host cell invasion: adsorption and introduction of viral DNA/RNA

• contain antigenic elements for immune response

what are examples of complex viruses?

poxviruses: very large w DNA, lack regualr cpasid but have multiple layers of lipoproteins, coarse surface fibrils

bacteriophage: polyhedral head, helical tail , and fibers for attachment to the host cells

What are morphological features of common viruses?

What is considered the core of a virus?

nucleic acids: DNA(ds) or RNA(ss)

small gneome from 4 genes in hep B virus to hundred of genes in herpesviruses

• bacterium E. coli - 4k genes

• human cell - 25k genes (C. elegans 20k)

some viruses contain ss DNA and ds RNA

what are other substances in the virus particle?

retroviruses: reverse transcriptase

specific polymerases to make DNA and RNA

tRNA: cellular machinery, metabolic enzymes packed in viral particles

Classification of Animal Viruses (DNA, RNA)

DNA viruses: 6 families

RNA virsues: 13 families

Family name: -viridae

Modes of Viral Multiplication

Adsorption: Virus attaches to host

Penetration: Viral DNA enters the bacterial cell.

Two Path Options: Lytic or Lysogenic Cycle

Lytic Cycle

• Viral DNA takes over the bacterial machinery.

• Cell enters eclipse phase: viral parts are made.

• Assembly of complete new virions.

• Maturation of viruses.

• Lysis of the weakened cell → cell bursts.

• Release of viruses to infect new cells.

Lysogenic Cycle

• Viral DNA becomes latent as a prophage inside the bacterial chromosome.

• Bacteria copies the viral DNA every time it divides.

• Can switch into the lytic phase later when triggere

Multiplication cycle in bacteriophages

adsorption: tail fibers bind to specific bacterial surface receptors

injection of phage DNA into the bacteria by pushing inner tube through the bacteria wall into the bacteria

Phage DNA: direct synthesis of phage parts

• capsid heads, parts of tail, new genome, enzyme that weakens cell in preparation of phage release

Spontaneous assembly to generate progeny phages

Lysogeny

silent virus infection

Temperate phages: do not lyse bacteria

viral DNA becomes prophage state > viral replication along bacterial DNA (lysogeny) without killing host

occasionally: viral genome can be activated > cell lysis

multiplication cycles of animal viruses