V ruses Are they Alive?

Virus Characteristics: A virus is a submicroscopic infectious particle composed of a protein coat (capsid) and a nucleic acid core. This core may contain DNA or RNA.

Virus Characteristics: Viruses are not cells! Viruses cannot carry out many of the characteristics of life including having a metabolism, and thus are not considered alive.

Virus Characteristics: Viruses are classified by the type of nucleic acid they contain, and the shape of their protein capsid.

Various Viral Shapes and Sizes: Spherical—Ex. Influenza virus Cylindrical—tobacco mosaic virus Bacteriophage—polyhedral head with tail fibers These infect bacteria Ex. T4

Virus Characteristics: Some Typical Viruses and their Shapes T4 Bacteriophage Tobacco Mosaic virus Influenza Virus

Viral Shapes

More Viral Shapes

Virus Characteristics: Reproduction is the only life function they can perform. But they can’t do this by themselves, they depend on a host cell in order to reproduce. Therefore, viruses are called obligate intracellular parasites.

Virus Characteristics: The word virus is Latin for Poison, because viruses were known to cause diseases but could not be isolated, before modern science and medicine. In 1935 the first virus was isolated from diseased tobacco plants.

Viral diseases include: Viruses and Diseases Viruses cause a variety of diseases among all groups of living organisms. Viral diseases include: the flu common cold herpes measles chicken pox small pox encephalitis Antibiotics are not effective against viruses. Vaccination offers protection for uninfected individuals.

Viruses and Disease Polio—vaccine in 1955 by Jonas Salk Measles and mumps—MMR vaccination for infants AIDS due to HIV virus Influenza is “ the flu” Rabies affects mammals Common cold due to over 200 types of Rhinovirus

EXAMPLES OF VIRUSES The Adenovirus is a DNA virus that causes colds and "pink eye". The Papillomavirus is a DNA virus that causes warts. These infectious particles are small, about 15 nm in diameter.

Viral Infections There are two cycles in which viruses are able to reproduce within a host cell: 1. Lytic cycle 2. Lysogenic cycle

Virus Life Cycles: The Lytic Cycle

Virus Life Cycles: The Lytic Cycle Adsorption or Attachment: at a Specific Receptor Site (SRS) on the “host” cell’s membrane.

Virus Life Cycles: The Lytic Cycle Entry or Penetration: the virus either injects its nucleic acid (DNA) into the cell or the entire virus enters the cell and then releases its DNA

Viral DNA is replicated into more viral DNA Virus Life Cycles: The Lytic Cycle Replication: the viral DNA takes over the “host” cell, using its resources and organelles. Viral DNA is transcribed into viral mRNA which is translated into protein capsids and enzymes Viral DNA is replicated into more viral DNA

Virus Life Cycles: The Lytic Cycle Formation (also called assembly or maturation): Some of the enzymes formed during replication are used to assemble new viruses from the viral DNA copies and the new protein capsids.

Virus Life Cycles: The Lytic Cycle Lysis and Release: other enzymes formed during replication are used to lyse (break open) the cell, so the new viruses are released to go infect other cells.

Virus Life Cycles: The Lytic Cycle

Virus Life Cycles: The Lysogenic Cycle

Virus Life Cycles: The Lysogenic Cycle Adsorption or Attachment: (Same as the Lytic Cycle)

Virus Life Cycles: The Lysogenic Cycle Entry or Penetration: (Same as the Lytic Cycle)

Virus Life Cycles: The Lysogenic Cycle Prophage or Provirus Formation: the viral DNA becomes part of the “host” cell’s chromosome. The “host” cell is now called a prophage or a provirus.

Virus Life Cycles: The Lysogenic Cycle Cell Division: (mitosis and cytokinesis) daughter cells are produced that have the viral DNA incorporated into the chromosome (a way of making more viruses and cells for them to infect-Reproduction).

Virus Life Cycles: The Lysogenic Cycle Spontaneous Induction or Trigger Event: something causes the viral DNA in the “host” cell to take over the cell.

Virus Life Cycles: The Lysogenic Cycle Replication: (Same as the Lytic Cycle)

Virus Life Cycles: The Lysogenic Cycle Formation (also called assembly or maturation): (Same as the Lytic Cycle)

Virus Life Cycles: The Lysogenic Cycle Lysis and Release: (Same as the Lytic Cycle)

Virus Life Cycles: The Lysogenic Cycle

RNA Viruses: In the Lytic Cycle: Transcription does not take place since they don’t have DNA. Instead the viral RNA replicates and will serve as the mRNA, which is translated to form protein coats and enzymes.

In the Lysogenic Cycle: RNA Viruses: In the Lysogenic Cycle: The viral RNA uses an enzyme called reverse transcriptase to transcribe into DNA. The DNA then becomes incorporated into the “host cell’s chromosome to form the prophage or provirus.

RNA Viruses: In the Lysogenic Cycle: Because these viruses go through “backwards” or reverse transcription they are called Retroviruses. HIV (the virus that causes AIDS) is a retrovirus.

Emergent Viruses Viruses are usually quite specific as to their hosts and even to the types of cells they infect in a multicellular host. Recently some viruses appear to have shifted their host: HIV, hantavirus, and ebola appear to be either viruses shifting to a new (human) host or else viruses whose existence and effects are just now being realized by scientists and the general public.

Origin of Viruses Because of their relative simplicity, many people once assumed that viruses represented and ancestral form of life. However, some scientists are now suggesting that viruses originated from their host cells by being nucleic acids that broke free and maintained their ability to replicate parasitically within the host. If this is true, viruses would be more closely related to their host cell than to other viruses.

*Prions are believed to be the cause of Mad Cow Disease. Viroids and Prions Viruses would appear to be the simplest form of infectious particle. The discovery of viroids, nucleic acid without a protein capsid and prions, infectious proteins, subtracts another level of complexity. Both viroids and prions can cause diseases. *Prions are believed to be the cause of Mad Cow Disease.

Bovine Spongiform Encephalitis

Your assignment before the test is to research if there are any “good” viruses.. Use books, parents, doctors, or your best bet might be the internet. Write a one paragraph summary and include a copy of any information you find. Good Luck !!!!