Assignment On Viruses

Discovery of Viruses

• Beijerinck (1897) coined the Latin name "virus"  meaning poison for  the substance infecting tobacco plants
• Wendell Stanley (1935) crystallized sap from tobacco leaves infected with Tobacco Mosaic Virus (TMV) & found virus was made of nucleic acid & protein

Wendell Stanley	Tobacco Leaf with Virus

• Edward Jenner developed smallpox vaccine using milder cowpox viruses
• Virology - study of viruses
• Deadly viruses are said to be virulent
• Viruses couldn't be seen until electron microscope invented

Viral Characteristics

• Not living organisms
• Noncellular
• Consist of a nucleic acid core (DNA or RNA) and a protein coat called the capsid
• Capsid made of protein subunits called capsomeres
• Cannot grow or replicate on their own (inactive particles)

• Can only reproduce inside of a living host cell using its raw materials & enzymes
• Lack ribosomes & enzymes needed for protein synthesis or metabolism
• 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

Ebola Virus

• Some may also cause cancers such as leukemias
• Virus free cells are rare
• Highly host specific (only infect certain cells)
• Referred to as phages
• Viruses are classified into 2 main groups by their nucleic acid --- DNA or RNA Viruses
•  DNA & RNA viruses are subdivided by capsid shape & whether they do or don't have an envelope

Viral Structure

• DNA or RNA core surrounded by protein sheath called capsid
• Nucleocapsid  includes the viral nucleic acid & its capsid
• Some form lipid rich covering around capsid called the envelope
• Envelope usually formed from host cell membrane
• Envelope may have spikes to help chemically recognize & attach to the host cell
• Shaped determined by the arrangement of proteins making up the capsid
• TMV is rod shaped

• Adenovirus & polio viruses are icosohedral (20 sided)

• Measles & rabies viruses are helical
• T -phages have a head & tail

Bacteriophages or T-Phages

• Among the most complex viruses
• Attack bacterial cells
• Composed of a icosohedral head, tail, base plate, & tail fibers
• Long DNA molecule is inside the head 
• Tail helps inject the viral DNA into host cell
• Tail fibers used to attach to host

Retroviruses

• Contain RNA
• Have an enzyme called reverse transcriptase which helps use the RNA to make DNA
• Use the host cell's ribosomes & raw materials to make viral proteins
• Cause some cancers & AIDS

HIV Virus

Viroids

• Smallest particle able to replicate
• Made of a short, single strand of RNA with no capsid
• Cause disease in plants

Viroid Attack on Potatoes

Prions

• No nucleic acid or capsids
• Made of protein particles that have folded incorrectly
• Attacks the central nervous system
• Cause animal diseases in cows (Mad Cow disease), sheep, & humans

Lytic Cycle

• Viral replication that rapidly kills the host cell causing it to lyse or burst
• Involves 5 steps ----- Adsorption, Injection, Replication, Assembly, & Lysis
• Adsorption --- phage attaches to cell membrane of host
• Injection --- nucleic acid (DNA) of virus injected into host cell
• Replication --- viral DNA inactivates host cell's DNA & uses host's raw materials & ribosomes to make viral DNA, capsids, tails, etc.
• Assembly --- new viral parts are combined to make new phages
• Lysis --- enzymes weaken & destroy the cell membrane causing it to lyse releasing new viruses that infect other cells

Phases of the Lytic Cycle of a Virulent Virus:  Absorption: Virus attaches itself to the cell. Entry: Enzymes weaken the cell wall and nucleic acid is injected into the cell, leaving the empty caspid outside the cell. Many viruses actually enter the host cell intact. Replication: Viral DNA takes control of cell activity. Assembly: All metabolic activity of the cell is directed to assemble new viruses. Release: Enzymes disintegrate the cell in a process called lysis, releasing the new

Source: http://science.howstuffworks.com/virus-human.htm

Lysogenic Cycle

• Replication in which the virus stays inactive inside of the host cell & doesn't immediately kill it
• Viruses are called temperate phages
• Lysogenic steps include adsorption, injection, recombination, cell reproduction, activation, replication, assembly, & lysis
• Recombination ---Viral DNA joins with host cell DNA forming an inactive prophage
• Host cell reproduces  normally until activated by an external stimuli 
• External stimuli unknown, but could be ultraviolet radiation, carcinogens, etc.
• Once activated, prophage forms new viruses & destroys host cell
• HIV is an example of a temperate phage

The Lysogenic Cycle of a Temperate Virus: The virus attaches itself and injects its DNA into the cell. The viral DNA attaches itself to the host DNA, becoming a new set of cell genes called a prophage. When the host cell divides, this new gene is replicated and passed to new cells. This causes no harm to the cell, but may alter its traits. Now there are two possibilities: The prophage survives as a permanent part of the DNA of the host organism. Some external stimuli can cause the prophage to become active, using the cell to produce new viruses.

Source: http://science.howstuffworks.com/virus-human.htm

Viral Control

• Interferon are proteins made by cells to fight viruses
• Two types of viral vaccines exist --- inactivated & attenuated
• Inactivated virus vaccines don't replicate in the host's system
• Attenuated viral vaccines have been genetically altered so they can't cause disease
• Antiviral drugs (AZT, acyclovir, & azidothymidine) interfere with viral DNA synthesis
• Protease Inhibitors interfere with viral capsid production
• New viruses emerge as rain forests are cleared (Ebola virus)
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• PUBLISHED BY     

KABIR HUSSAIN BAIG