VirusesBiology 342

Protein Coats and Nucleic AcidAn infectious agent consisting of only a

nucleic acid molecule within a protein coat.They are all very small requiring electron

microscopy to see most*.They all require a host cell to replicate.

*some pox viruses can be seen using a light microscope

Virus Factoids

Viruses are the smallest living things known to science.

Viruses are everywhere – in the air, soil, rivers, and oceans, even in the deepest caves miles underground and under mile thick layers of ice in Antarctica.

Vast numbers of viruses live on and in us – particularly in our lungs.

We are even part virus…the human genome contains more DNA from viruses than our own genes.

Viruses affect global climate and all other living things on the earth.

Viruses…Move DNA between speciesProvide new genetic material for evolutionRegulate vast populations of organismsHave very simple organizationAre obligatory intracellular parasitesAre most frequently host specificAre considered “infectious agents” because…

Viruses produced in one cell can invade others (spread)

Viruses cause functional alterations of host (death)

Viruses: Alive or Not?Characters of living things Virus – yes or no

Cells with nucleic acid (DNA) Use host cells; have either DNA or RNA

Reproduce (asexual or sexual) YES - by using host cells

Growth and development NO – do not grow or divide

Use energy (metabolism) YES – use the host cell’s

Respond to stimuli (environment) ? - inert outside cell – respond inside

Homeostasis (internal balance) NO - inert outside cell

Heredity (pass genetic traits) YES

Evolve (adapt through time) YES

* Are viruses exceptionally simple microbes or exceptionally complex chemicals?

Major Viral TypesBacteriophages – viruses that use bacteria Plant viruses – viruses that use plantsAnimal viruses – viruses that use animals

BacteriophagesInfects and replicates within a bacteriaConsists of proteins with either DNA or RNA

genomeReplicate after injecting their genome into

the cytoplasm of the bacterial cellMost common and most diverse group of

virusesWidely distributed, especially in oceans

infecting most marine bacteria.Phage Therapy

Plant VirusesMost have RNA rather than DNAMost are rod-shaped with spirally arranged

proteins on the outsideOften infect plants through injuriesSeed and pollen bornePlant transmission occurs through vectors

Insect borne – e.g., leaf hoppersNematode borne – e.g., root feedersProtozoan borne – e.g., parasites of roots

Animal VirusesRNA and DNA genomesOften have an Outer Envelope (membrane)

with projecting spikes of proteins which help virus enter and leave the cell

RNA enveloped virus get their envelope from the host cell membrane and reproduce in the cell’s cytoplasm

DNA enveloped virus get their envelope from the host nuclear membrane and reproduce in the cell’s nucleus

Great Viral Plagues of HistoryInfluenza – Killed ~ 80 million people in 100 years

(1890 to 1990). Responsible for ~ 1/3rd of death among soldiers in WWI.

Smallpox – death toll unknown – killed millions in Europe over centuries and devastated the Native Americans after European contact in the 1500’s, killing an estimated 90% of the indigenous population.

Yellow fever – death toll unknown – likely killed thousands in Central and North America. Along with Malaria it nearly prevented the construction of the Panama Canal.

History of Viral Disease

smallpox

Yellow fever

Influenza

How Small are Viruses?A single grain of salt has a diameter of ~ 0.3

mm - A line of 10 skin cells is equal to one grain of

salt -A line of 100 bacteria cells is equal to one

skin cell –A line of 1,000 viruses equals one bacterial

cell.

Timeline of Virus Awareness1892 – found the pathogen for mosaic

tobacco disease was so small it would pass through a ceramic filter.

1901 – Yellow fever is found to be caused by a virus carried by mosquitoes.

1915 – Bacteriophage viruses that infect bacteria are recognized.

1930 – First virus seen by human eye with electron microscope.

1949 – Poliovirus grown in human cell culture.

1966 – Epstein-Bar first virus shown to cause cancer in humans.

Viral ShapesHelical capsid – e.g., Tobacco Mosaic virus

Icosahedra capsid – most common typee.g., rhinovirus

Bullet-shaped capsid – e.g., rabies virus

Lunar Lander shape – e.g., bacteriophages

Enveloped Viruses

Some viruses create an envelope around their capsid and nucleic acid. The envelope is derived from portions of the host’s cell membrane.

The envelope is made of membrane phospholipids, and importantly glycoproteins.

The glycoproteins identify and bind to receptors on the host membrane.

The viral envelope then fuses with the host membrane and the viral capsid and genome enter the cell.

Examples: herpes viruses, poxviruses, hepatitis, and retroviruses.

Naked VirusesNaked viruses do not have an enclosing envelope

and consist only of the capsid and nucleic acid.Have a protein coat with subunits called capsomeres.The capsomeres have adhesins to attaches the virus to the host cell surface.Naked viruses are known to last a long time in the environment.Examples: poliovirus and papillomavirus

Endocytosis and the Trojan HorseEndocytosis – host cells absorb important

proteins through a process of engulfing them because big molecules can’t pass through the cell membrane.

Viruses exploit endocytosis masquerading as important molecules (Trojan Horse) to get absorbed into host cell.

Once inside the virus exploits the cell’s machinery for their own advantage, that is, to replicate.

Attachment and Absorption of Host Cell

Naked Virus

Enveloped Virus

Viral Escape after ReplicationEnveloped virus buds from host cell membrane

Viral Infection Cycles

Enveloped viruses are easily inactivated outside of host

Naked Viruses remain infective for long periods outside the hostPapillomavirus – skin warts, genital warts,

cervical cancer – passed via host skin and genital contact

Rhinoviruses – common colds – passed via surface contact

Polioviruses – poliomyelitis – passed via contact with feces contaminated food or surfaces and rarely via oral contact persons to person.

Families of VirusesDNA or RNA, single or double stranded, with or without an

envelope

Virus Taxonomy

Mutation rates of DNA viruses and RNA virusesDNA viruses mutate more slowly than RNA

viruses:

Enzymes used to copy DNA (DNA Polymerases) can proofread/edit, correct mistakes, make fewer mistakes, thus fewer mutations

Enzymes which copy RNA (RNA Polymerases) do not proofread/edit, therefore make many mistakes that equal mutations

Mutations ratesDNA viruses (e.g., Herpes, pox virus, HPV): 1 mutation/100,000,000-10,000,000,000

nucleotides

RNA viruses (e.g., HIV, influenza virus): potential for

1 mutation/10,000-100,000 nucleotides (at least 1,000 times more mutations than DNA viruses)

* Viruses evolve faster than anything else on the planet

Emerging Infectious VirusesHow do new viruses arise?What causes some to emerge as major

threats?