Viruses. What are Viruses? Virus – particles of nucleic acid, protein, and in some cases lipids...
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Transcript of Viruses. What are Viruses? Virus – particles of nucleic acid, protein, and in some cases lipids...
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