Microorganisms Chapter 25 Viruses

Viruses Defined A virus is a nonliving particle composed of a nucleic acid and a protein coat Viruses cause many diseases in living organisms.

Virology The science of studying viruses is called Virology. Viruses were once thought to perhaps be the ancestors of bacteria. Viruses are use to provide clues to the biochemistry of living organisms.

Characteristics of Viruses Smallest biological particle capable of causing disease. Viruses do not have a nucleus, cytoplasm, organelles or cell membrane. Viruses are only able to replicate by using the organelles of the cells they infect.

Viral Structure Viruses have 2 essential features A nucleic acid-DNA or RNA A nucleic acid-DNA or RNA A protein coat called a Capsid that surrounds the nucleic acid A protein coat called a Capsid that surrounds the nucleic acid

Virus Shape Shape determined by its capsid or its nucleic acid Icosahedrons- Geometric shape with 20 sides Icosahedrons- Geometric shape with 20 sides Examples: Chicken Pox and Polio Examples: Chicken Pox and Polio Helix-resembles a coiled spring Helix-resembles a coiled spring Examples: Measles and Rabies Examples: Measles and Rabies

Classification DNA viruses 1. Direct production of more viruses through transcription. 1. Direct production of more viruses through transcription. 2. Become joined with host’s DNA 2. Become joined with host’s DNA RNA viruses Uses host cell’s ribosomes to produce new viruses. Uses host cell’s ribosomes to produce new viruses.

Virus Replication-Lytic Cycle The Lytic Cycle 1. Virus attaches to host cell receptor sites 1. Virus attaches to host cell receptor sites 2. Virus produces an enzyme to weaken cell wall presses its sheath against the cell leaving its Capsid outside of the host cell and then injects its DNA 2. Virus produces an enzyme to weaken cell wall presses its sheath against the cell leaving its Capsid outside of the host cell and then injects its DNA 3. The virus then manufactures new 3. The virus then manufactures new DNA and Capsids DNA and Capsids 4. The new viruses cause the cell membrane to disentegrate called Lysis and are released.

Virus Replication-Lysogenic Cycle Viruses that stay within the cell for extended periods of time and use the Lysogenic Cycle to replicate are called temperate viruses. DNA of invading virus integrated with that of host DNA-genome of host referred to as a Prophage. Certain stimuli can cause a prophage to enter the Lytic Cycle, destroying the host.

Viruses and Human Disease 3 common viral diseases are The common cold-Influenza Virus which is transmitted by coming in contact with virus particles shed by contact and mucus droplets. The common cold-Influenza Virus which is transmitted by coming in contact with virus particles shed by contact and mucus droplets. The rabies virus-transmitted by the bite of an infected animal. The rabies virus-transmitted by the bite of an infected animal. Chicken Pox-transmitted by direct contact with the skin rash. Chicken Pox-transmitted by direct contact with the skin rash.

Viruses and Disease-Control Vaccination-to prevent the disease Administration of Antiviral Drugs-drugs that interfere with the viruses ability to make its nucleic acids. Acyclovir-used to treat Herpes Simplex Acyclovir-used to treat Herpes Simplex Azidothymidine (AZT)-used to treat HIV Azidothymidine (AZT)-used to treat HIV Protease Inhibitors- used to treat HIV Protease Inhibitors- used to treat HIV Antibiotics are not effective in the treatment of viral diseases.