MicroorganismsMicroorganisms

Chapter 25Chapter 25

VirusesViruses

Viruses DefinedViruses Defined

A virus is a nonliving A virus is a nonliving particle composed of particle composed of a nucleic acid and a a nucleic acid and a protein coatprotein coat

Viruses cause many Viruses cause many diseases in living diseases in living organisms.organisms.

VirologyVirology

The science of studying viruses is called The science of studying viruses is called Virology.Virology.

Viruses were once thought to perhaps be Viruses were once thought to perhaps be the ancestors of bacteria.the ancestors of bacteria.

Viruses are use to provide clues to the Viruses are use to provide clues to the biochemistry of living organisms.biochemistry of living organisms.

Characteristics of VirusesCharacteristics of Viruses

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

Viral StructureViral Structure

Viruses have 2 Viruses have 2 essential featuresessential features A nucleic acid-DNA or A nucleic acid-DNA or

RNARNA A protein coat called a A protein coat called a

Capsid that surrounds Capsid that surrounds the nucleic acidthe nucleic acid

Virus ShapeVirus Shape

Shape determined by Shape determined by its capsid or its its capsid or its nucleic acidnucleic acid Icosahedrons-Icosahedrons-

Geometric shape with Geometric shape with 20 sides20 sides

Examples: Chicken Examples: Chicken Pox and PolioPox and Polio

Helix-resembles a Helix-resembles a coiled springcoiled spring

Examples: Measles Examples: Measles and Rabiesand Rabies

ClassificationClassification

DNA virusesDNA viruses 1. Direct production of more viruses through 1. Direct production of more viruses through

transcription.transcription. 2. Become joined with host’s DNA2. Become joined with host’s DNA

RNA virusesRNA viruses Uses host cell’s ribosomes to produce new Uses host cell’s ribosomes to produce new

viruses.viruses.

Virus Replication-Lytic CycleVirus Replication-Lytic Cycle

The Lytic CycleThe Lytic Cycle 1. Virus attaches to host 1. Virus attaches to host

cell receptor sitescell receptor sites 2. Virus produces an 2. Virus produces an

enzyme to weaken cell wall enzyme to weaken cell wall presses its sheath against presses its sheath against the cell leaving its Capsid the cell leaving its Capsid outside of the host cell and outside of the host cell and then injects its DNA then injects its DNA

3. The virus then 3. The virus then manufactures new manufactures new

DNA and CapsidsDNA and Capsids4. The new viruses cause 4. The new viruses cause the cell membrane to the cell membrane to disentegrate called Lysis disentegrate called Lysis and are released .and are released .

Virus Replication-Lysogenic CycleVirus Replication-Lysogenic Cycle

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

Viruses and Human DiseaseViruses and Human Disease

3 common viral diseases are3 common viral diseases are The common cold-Influenza Virus which is The common cold-Influenza Virus which is

transmitted by coming in contact with virus transmitted by coming in contact with virus particles shed by contact and mucus droplets.particles shed by contact and mucus droplets.

The rabies virus-transmitted by the bite of an The rabies virus-transmitted by the bite of an infected animal.infected animal.

Chicken Pox-transmitted by direct contact with Chicken Pox-transmitted by direct contact with the skin rash.the skin rash.

Viruses and Disease-ControlViruses and Disease-Control

Vaccination-to prevent the diseaseVaccination-to prevent the diseaseAdministration of Antiviral Drugs-drugs Administration of Antiviral Drugs-drugs that interfere with the viruses ability to that interfere with the viruses ability to make its nucleic acids.make its nucleic acids. Acyclovir-used to treat Herpes SimplexAcyclovir-used to treat Herpes Simplex Azidothymidine (AZT)-used to treat HIVAzidothymidine (AZT)-used to treat HIV Protease Inhibitors- used to treat HIVProtease Inhibitors- used to treat HIV

Antibiotics are not effective in the Antibiotics are not effective in the treatment of viral diseases.treatment of viral diseases.