Chapter 18 - Viruses

Structure A virus is a non-living particle A virus is a non-living particle Composed of DNA or RNA and a protein coat Composed of DNA or RNA and a protein coat The outer protein coat is called a capsidThe outer protein coat is called a capsid

Section 19-3 Common Diseases Caused by Viruses Oncogenic viruses Retroviruses Adenoviruses Herpesviruses Poxviruses DNA RNA DNA cancer cancer, AIDS respiratory infections chickenpox smallpox Type of VirusNucleic AcidDisease Go to Section: Grouping Viruses

Viral Infection Scientists learned about viral replication by studying bacteriophages, A bacteriophage is a virus that invades bacteria. It attacks by adhering to a cell wall & injecting its nucleic acids that alter the bacterium’s genetic code. Animal viruses enter their host through endocytosis. Scientists learned about viral replication by studying bacteriophages, A bacteriophage is a virus that invades bacteria. It attacks by adhering to a cell wall & injecting its nucleic acids that alter the bacterium’s genetic code. Animal viruses enter their host through endocytosis.

The reproductive cycle of a virus consists of 2 cycles: the lytic cycle & the lysogenic cycle. The lytic cycle consists of the virus killing the host. It has 5 stages. The reproductive cycle of a virus consists of 2 cycles: the lytic cycle & the lysogenic cycle. The lytic cycle consists of the virus killing the host. It has 5 stages. 1. Adsorption- the phage attaches itself to the cell wall – a chemical bond forms between sites on the tail of the virus & sites on the cell wall – this is like a lock & key 1. Adsorption- the phage attaches itself to the cell wall – a chemical bond forms between sites on the tail of the virus & sites on the cell wall – this is like a lock & key

2. Entry- The phage releases enzymes that break down the wall – the tail is forced through the weakened wall – the nucleic acid of the phage passes through the hollow tail into the host – the empty capsid is left outside 3. Replication- the virus copies itself in the cell – DNA or RNA enters the host’s chromosomes & makes a template from its own – the organism now makes viral proteins & viral RNA 2. Entry- The phage releases enzymes that break down the wall – the tail is forced through the weakened wall – the nucleic acid of the phage passes through the hollow tail into the host – the empty capsid is left outside 3. Replication- the virus copies itself in the cell – DNA or RNA enters the host’s chromosomes & makes a template from its own – the organism now makes viral proteins & viral RNA

4. Assembly - viral nucleic acids & proteins are assembled into new particles called virions 4. Assembly - viral nucleic acids & proteins are assembled into new particles called virions 5. Release - new phages release an enzyme that weakens the cell wall – the host cell lyses & releases newly created viruses- up to 300 new viruses can be produced in 1 cell 5. Release - new phages release an enzyme that weakens the cell wall – the host cell lyses & releases newly created viruses- up to 300 new viruses can be produced in 1 cell

The Lysogenic cycle is considered to be an inactive cycle & may stay this way for several generations. It has 3 stages. 1. Attachment - the nucleic acids of the invading phage attach to the DNA of the host cell – this is now called a prophage The Lysogenic cycle is considered to be an inactive cycle & may stay this way for several generations. It has 3 stages. 1. Attachment - the nucleic acids of the invading phage attach to the DNA of the host cell – this is now called a prophage 2. Replication - the prophage is now replicated along with the DNA of the host 2. Replication - the prophage is now replicated along with the DNA of the host 3. Activation - the prophage enters the lytic cycle & makes new viral parts 3. Activation - the prophage enters the lytic cycle & makes new viral parts

Pathogens are disease-causing organisms. Virulence refers to the ability to cause disease. A virus’ virulence depends on 1) Ability to attach to host, 2) speed of reproduction, & 3) response of the host’s cells. The body’s natural defense against disease is referred to as immunity. Pathogens are disease-causing organisms. Virulence refers to the ability to cause disease. A virus’ virulence depends on 1) Ability to attach to host, 2) speed of reproduction, & 3) response of the host’s cells. The body’s natural defense against disease is referred to as immunity.

Cells called phagocytes engulf & destroy invading organisms. An antigen is a foreign protein or virus while antibodies are produced by the body to fight them. Active immunity is achieved when the body has actually been exposed to antigens through the pathogen itself or through a vaccine. This process stimulates the production of antibodies. Passive immunity is received from the antibodies produced in another person or animal that has developed immunity. This is only temporary. Cells called phagocytes engulf & destroy invading organisms. An antigen is a foreign protein or virus while antibodies are produced by the body to fight them. Active immunity is achieved when the body has actually been exposed to antigens through the pathogen itself or through a vaccine. This process stimulates the production of antibodies. Passive immunity is received from the antibodies produced in another person or animal that has developed immunity. This is only temporary.

Retroviruses & Others A retrovirus is deadly because it transcribes DNA from an RNA template & inserts this into the host. This is made possible by reverse transcriptase. These viruses can have a lengthy latent period. Examples are HIV & Herpes. Prions are “protein infectious particles” that contain no DNA or RNA. These typically infect the brain (mad cow disease). A retrovirus is deadly because it transcribes DNA from an RNA template & inserts this into the host. This is made possible by reverse transcriptase. These viruses can have a lengthy latent period. Examples are HIV & Herpes. Prions are “protein infectious particles” that contain no DNA or RNA. These typically infect the brain (mad cow disease).

Prevention & Treatment Control of viral diseases is accomplished in one of two ways: Control of viral diseases is accomplished in one of two ways: Vaccination to prevent diseaseVaccination to prevent disease Administration of antiviral drugs to infected patientsAdministration of antiviral drugs to infected patients The drugs interefere with nucleic acid synthesisThe drugs interefere with nucleic acid synthesis

Viruses and Cancer Cancer genes may be triggered by certain lysogenic viruses. Cancer genes may be triggered by certain lysogenic viruses.

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