VirusesVirology - study of viruses

Virologist – scientist that studies Viruses

Introduction to Viruses

• In 1898, Friedrich Loeffler and Paul Frosch: found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria.

• This was the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area between living and non-living states.

• Viruses depend on the host cells that they infect to reproduce.

• When found outside of host cells, viruses exist as: 1. Protein coat or capsid, sometimes enclosed within

a membrane. 2. The capsid encloses either DNA or RNA which

codes for the virus elements. In this form outside the cell, the virus is metabolically inert (dormant)

Influenza Virus

Tobacco Mosaic Virus

Tobacco Mosaic Virus

Discovery of Viruses

• Beijerinck (1897) coined the Latin name "virus" meaning poison for the substance infecting tobacco plants

• Edward Jenner (1796) developed smallpox vaccine using milder cowpox viruses – the first ever vaccine

• 1900 Walter Reed showed that an agent so small it could pass through a filter, which trapped bacteria, caused the human disease yellow fever

• 1918 a pandemic Spanish flu kills 25 million people, more deaths than caused by the first World War

• Wendell Stanley (1935) crystallized sap from tobacco leaves infected with Tobacco Mosaic Virus (TMV) & found virus was made of nucleic acid & protein

• Viruses couldn't be seen until electron microscope invented in the 1930s

• 1950s widespread use of the Salk polio vaccine – killed vaccine

• 1978 last naturally occurring case of smallpox in the world

• 1982 recognition of new virus Human Immunodeficiency Virus (HIV-1)

• 2009 H1N1 Influenza virus – Swine Flu

Characteristics of a Virus

• Not living organisms• Noncellular• Consist of a nucleic acid core (DNA or RNA) and

a protein coat called the capsid• Cannot grow or replicate on their own (inactive

particles)• Can only reproduce inside of a living host cell

using its raw materials & enzymes

• Are extremely small particles ranging from 20 - 400 nanometers on average

• Largest virus is 1000 nanometers in dimension• Some can cause disease (smallpox, measles,

mononucleosis, influenza, colds, AIDS, Ebola Virus

• Some may also cause cancers such as leukemias• Highly host specific (only infect certain cells)• Viruses are classified into 2 main groups by

their nucleic acid --- DNA or RNA Viruses

Virus Structures

Virus Structure

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Bacteriophage – attack bacteria

Bacteriophage

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http://oceanworld.tamu.edu/resources/oceanography-book/Images/Bacteriophage-t4.gif

T4 Bacteriophage – virus that attacks the bacteria E. coli

Lytic and Lysogenic Infections

In the lysogenic cycle, the virus reproduces by first injecting its genetic material, indicated by the red line, into the host cell's genetic instructions.

In the lytic cycle, the virus uses the genetic material of the host cell to make thousands of copies of its own DNA. The cell bursts and releases the virus particles that infect other cells.

The Lytic Cycle

Bacteriophage enzyme lyses thebacterium’s cell wall, releasingnew bacteriophage particles thatcan attack other cells.

Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles

Bacteriophage takes over bacterium’s metabolism, causing synthesis of new bacteriophage proteins and nucleic acids

Bacteriophage injects DNA into bacterium

Bacteriophage attaches to bacterium’s cell wall

Bacteriophage

Bacteriophage DNA

Bacteriophage protein

Bacteriophage protein coat

Bacteriophage DNA

Bacterialchromosome

Lytic Cycle

The Lysogenic Cycle

Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles

Bacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles that can attack other cells

Bacteriophage DNA inserts itself into bacterial chromosome

Bacteriophage DNA (prophage) may replicate with bacterium for many generations

Bacteriophage DNA (prophage) can exit the bacterial chromosome

Bacteriophage DNA forms a circle

Bacteriophage DNA

Bacterialchromosome

Bacteriophage injects DNA into bacterium

Prophage

Lytic Cycle

Lysogenic Cycle

Section 19-3

Retroviruses

• Contains RNA – RNA enters the cells and makes DNA– RNA is copied backward – RNA to DNA

(usually DNA to RNA)• Virus DNA becomes part of host cell’s DNA• Hides in the cell and copies of the virus can be

made at any time.• Causes some cancers and AIDS

http://pathmicro.med.sc.edu/lecture/HIV-color09.jpg

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HIV

Flu Virus (RNA virus)

http://www.chm.bris.ac.uk/webprojects2006/Kelly/influenzafigure1.jpg

attachment to a host cell

Release of new virus particles

• Viroid – infect plants, single stranded RNA molecules that have no capsid – disrupt metabolism and destroy plants

• Prion- contain only protein (no DNA or RNA), forms protein clumps in nervous tissue

Mad Cow Disease

Virus or Prion

Comparing Viruses and Living Cells

Prokaryotes Unicellular organisms that lack a nucleus

smallest of all the cellsCommon term - Bacteria

Biology4kids

Section 19-1

Concept Map

are classified into the kingdoms of

Eubacteria

live in harsh environments such as

include a variety of lifestyles such as

Bacteria

Archaebacteria

Infecting large organisms

Thick mudLiving in soilAnimal

digestive tracts

Salty lakes Hot springs

A. Classifying Prokaryotes1. Eubacteria – “true bacteria”

common, all around us2. Archaebacteria – Ancient Bacteria

Extreme environments (hot sulfur springs)

Common Diseases Caused by Bacteria

Tooth decay

Lyme disease

Tetanus

Tuberculosis

Salmonella food poisoning

Pneumonia

Cholera

Streptococcus mutans

Borrelia burgdorferi

Clostridium tetani

Mycobacterium tuberculosis

Salmonella enteritidis

Streptococcus pneumoniae

Vibrio cholerae

Regular dental hygiene

Protection from tick bites

Current tetanus vaccination

Vaccination

Proper food-handling practices

Maintaining good health

Clean water supplies

Disease Pathogen Prevention

B. Structure of a Eubacterium

PeptidoglycanCell wall Cell membrane

Ribosome

Flagellum DNA Pili

C. Identifying Prokaryotes1. Shapes• Bacilli – rod shaped• Cocci – spherical• Spirilla – spiral shaped

harmonscience7

2. Cell Walls –made of peptidoglycan

Eubacteria – two types of cell walls

1. Gram-positive – stain purple, stains the thick peptidoglycan cell walls

Ex . Streptococcus

2. Gram –negative – stain pink or light red, Ex. E. coli

3. Movement

Do they move or how they move?

1.Propelled by flagella2.Spiral forward3.Glide along a layer of excreted slime

C. Obtaining Energy

1.Photoautotroph – photosynthetic prokaryotes (bacteria)

2.Chemoautotroph – energy from chemical reactions

3. Heterotroph – obtains energy from food it consumes (organic compounds)

4. Photoheterotroph – photosynthetic and needs organic compounds

Volcanic Ocean Vent

What type of Prokaryote survives in this environment?

Could a photoheterotroph survive in this environment?

E. Releasing Energy -Bacteria use cellular respiration, fermentation or both

1. Obligate Aerobes – need Oxygen (02) to live

2. Obligate Anaerobes - do not need Oxygen (02) and are poisoned by it

botulism

3. Facultative Anaerobes – do not require Oxygen (02) to live, but are not harmed by it either (live in our gut)

F. Growth and Reproduction1.Binary Fission = Asexual – double in size

and DNA, split in half. 2. Conjugation = Sexual, two bacteria

exchange genetic material – can create a new strain.

F. Growth and Reproduction (cont.)3. Spore Formation = Endospore -

Formed during harsh conditions – hibernates until conditions are good.

Ex. Drought, extreme heat, no food, frozen food

• Bacteria in NatureA. Decomposers – break down organic

matter

B. Nitrogen Fixers – converts nitrogen into a form plants can us N2 converted to NH3 (ammonia)

• Bacteria in Nature (cont.)C. Bacteria and Disease – 2 ways

1. Damage the tissues to make food2. Release toxins (poisons)

D. Human Uses of Bacteria(yogurt, cheese, mine minerals, oil spill clean up,

medicine etc.)

The Nitrogen Cycle

N2 in Atmosphere

NH3

NO3-

and NO2-

•Required to make proteins

•most of the nitrogen is in the air as N2

H. Controlling Bacteria • Sterilization by heat • Disinfectants • Food Storage and Processing • Antibiotics • Vaccines

Germ Theory – infectious diseases are caused by microorganisms, or germs

Koch’s Postulate1. Pathogen should be found in sick organism

2. Pathogen should be isolated & grown in pure culture

3. Purified pathogens are placed in a new, healthy organism and cause disease in new organism

4. Injected pathogen should be reisolated from second host & be identical to original pathogen