Viruses Living or Not ???????

Characteristics of Viruses Among the smallest biological particles that are capable of causing diseases in living organisms Among the smallest biological particles that are capable of causing diseases in living organisms Constructed of compounds associated with cells Constructed of compounds associated with cells

Not Considered Living A virus is not a bacteria, fungus, protist, plant or animal. A virus is not a bacteria, fungus, protist, plant or animal. They can not carry out cellular functions. They can not carry out cellular functions. A virus can not replicate without infecting cells and then using the organelles and enzymes of the cells of the host. A virus can not replicate without infecting cells and then using the organelles and enzymes of the cells of the host.

A Virus Has Two Essential Features A Nucleic Acid A Nucleic Acid DNA or DNA or RNA RNA But not both But not both A Capsid – a protein coat surrounding the nucleic acid. A Capsid – a protein coat surrounding the nucleic acid.

Some may have a membrane like structure outside the capsid called an envelop Some may have a membrane like structure outside the capsid called an envelop Examples: Examples: Influenza Influenza Chickenpox Chickenpox Herpes-simplex Herpes-simplex HIV HIV

Viral Shape The shape of the virus is determined by either its capsid or its nucleic acid The shape of the virus is determined by either its capsid or its nucleic acid Two examples of shape Two examples of shape Icosahedron has 20 trianglular faces -- herpes simplex, chicken pox and polio Icosahedron has 20 trianglular faces -- herpes simplex, chicken pox and polio Helix is a spiral shape Helix is a spiral shape ( like DNA) -- rabies, measles and tobacco mosaic virus ( like DNA) -- rabies, measles and tobacco mosaic virus

There are Two Types of Viruses DNA DNA Replicated in one of two ways Replicated in one of two ways Directly produce RNA that make new viral proteins Directly produce RNA that make new viral proteins Join with the host cell’s DNA to produce new viral proteins Join with the host cell’s DNA to produce new viral proteins

RNA RNA Viral RNA is released into the host cell’s cytoplasm and uses the ribosomes to produce new viral proteins Viral RNA is released into the host cell’s cytoplasm and uses the ribosomes to produce new viral proteins Some are known as retroviruses containing an enzyme called. These use the RNA as a template to make DNA. This DNA is integrated into the host cell’s DNA. Some are known as retroviruses containing an enzyme called reverse transcriptase. These use the RNA as a template to make DNA. This DNA is integrated into the host cell’s DNA.

Viral Replication

Lytic vs Lysogenic Lytic cycle (virulent phage) Lytic cycle (virulent phage) Release of virus bursts and kills host cell (lysis) Release of virus bursts and kills host cell (lysis) Lysogenic cycle (temperate phage) Lysogenic cycle (temperate phage) Viral DNA integrates into host genome (provirus) Viral DNA integrates into host genome (provirus) Can be transmitted to daughter cells Can be transmitted to daughter cells Can initiate lytic cycle in response to environmental signal (stress) Can initiate lytic cycle in response to environmental signal (stress)

The lytic and lysogenic cycles of phage, a temperate phage Many cell divisions produce a large population of bacteria infected with the prophage. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage (provirus). New phage DNA and proteins are synthesized and assembled into phages. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Certain factors determine whether The phage attaches to a host cell and injects its DNA. Phage DNA circularizes The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered Lysogenic cycleLytic cycle or Prophage/Provirus Bacterial chromosome Phage DNA