Viruses

What are Viruses?

• Virus – particles of nucleic acid, protein, and in some cases lipids– Not alive – Do NOT exhibit characteristics of life– Can only reproduce when inside a host cell– Cannot maintain homeostasis

• Very simply a virus is genetic information enclosed in a protein coat

Viral Structure

• Viruses contain DNA or RNA but never both

• DNA or RNA is surrounded by a protein coat call a capsid– Proteins in the capsid give the virus

different shapes– Shape of the virus causes the virus to be

cell specific (can only infect a certain type of cell)

Viral Structure

• Some viruses are enclosed by a protective protein envelope

• Some viruses have spikes to help attach to the host cell

CAPSID

ENVELOPE

DNA

SPIKES

Virus vs CellCharacteristic Virus Cell

Structure DNA or RNA core, capsid

Cell membrane, cytoplasm; eukaryotes also contain nucleus & organelles

Reproduction Only within host Independent cell division either asexually or sexually

Genetic Code DNA or RNA DNA

Growth & Development

No Yes, in multicellular organisms, cells increase in number & differenetiate

Obtain & Use Energy

No Yes

Response to Environment

No Yes

Change Over Time Yes Yes

Viruses & Disease

**Remember ALL viruses are parasitic which means they require a living host and they cause harm to that

host**

Bacteriophages• A virus that infects

bacteria is known as a bacteriophage or phage

• Bacteriophages are used for research tools in genetics– Ex: to “program”

bacteria to produce insulin for the treatment of diabetes

Retroviruses• Group of viruses that contain RNA as

their genetic material• Retroviruses have a complicated

reproductive cycle because they must carry out reverse transcription

• Contain a special enzyme call reverse transcriptase– This enzyme helps a cell transcribe DNA

from RNA

• Examples: HIV and feline leukemia

ENZYME

Fighting Viral Infections• Antiviral drugs work

by interfering with the genetic material or the formation of the capsid– This will prevent the

virus from replicating in the host cell

• Examples: tamiflu, acyclovir

Benefits of Viruses

• Not all viruses are bad!• Example:

– Vaccines for viral diseases are made from weaker strains of the virus

– A weaker virus is capable of stimulating an immune response and creating immunity, but not causing illness

Viral Hosts• Remember that a host is necessary for

viral replication• Broad host range = infect several

species– Ex: Swine flu and rabies

• Narrow host range = infect only one species

• Very narrow host range = infect specific tissue types of one species– Ex: Human cold virus attack cells of the

respiratory system

Viral Cycles

There are three initial steps that are common to all viral infections:

1. Virus attach to the cell membrane of the host cell

2. Trick cell into allowing it inside3. Virus releases its genetic information

(DNA or RNA) into the host cell- Replication – Viral DNA is copied- Transcription – Viral DNA is converted into

RNA- Translated – Viral proteins are made from the

RNA

Viral Cycles

Once inside the host cell, there are two ways that a virus can take over and reproduce:

1. Lytic Infection2. Lysogenic Infection

Lytic Infection

1. Attachment to the cell2. Penetration (injection) of viral DNA or

RNA3. Virus reprograms host to copy viral

DNA and make viral proteins4. New viruses assemble and mature5. Cell lyses (bursts) and releases the

new viruses to attack other cells6. Results in death of the host cell

Lysogenic Infection

– Virus attaches to host and injects DNA– Viral DNA joins host DNA forming a

prophage– Viral DNA replicates along with host

DNA– Remain inactive for a period

(generations)– Eventually viral DNA exits host cell and

enters lytic cycle