A2.3 Viruses (HL)
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What are some characteristics common to all viruses?
Small, fixed size Contain nucleic acid (RNA or DNA) Enclosed by a protein capsid Lack cytoplasm inside the capsid Possess few, if any, enzymes
Why are viruses not considered alive?
Viruses don’t perform all life functions, relying on infected cells for reproduction.
How do viruses infect host cells?
They attach to specific host cells using specialized sites on their capsid.
What determines a virus’s ability to infect a host cell?
The unique composition and structure of its capsid.
What is the role of an envelope in some viruses?
it aids in host cell recognition and attachment.
What occupies the space inside a virus capsid?
Nucleic acid and sometimes enzymes.
How do viruses replicate?
They can’t replicate on their own and rely on host cells for reproduction.
What’s the range of sizes for most viruses?
10-400 nm, comparable to large protein macromolecules.
What tools are needed to study virus structure due to their size?
Electron microscope.
uniqueness and variation in shapes of viruses
polyhedral spherical Helical Complex
example of uniqueness and variation in shapes of viruses
polyhedral (adenovirus) spherical (influenza) Helical (tobacco mosaic virus) Complex (bacteriophage)
Three specific viruses and their shapes (that i need to know)
bacteriophage lambda coronaviruses human immunodeficiency virus (HIV)
Important features of bacteriophage lambda are:
• a capsid head that protects the double-stranded DNA core • tail fibres that attach the virus to the host cell • a tail sheath that consists of proteins that contract to drive the tail tube through the host cell’s outer membrane • DNA that is injected through the tail into the host cell.
Important features of coronaviruses are:
• a spherical shape • single-stranded RNA as its genetic material • an envelope outside the capsid • numerous projections of spike proteins on the envelope, creating a “corona”.
Important features of HIV are:
• it has an envelope outside the capsid • two identical single strands of RNA, protected by the capsid • within the viral RNA, reverse transcriptase is encoded, which allows the production of DNA using the viral RNA as a model • it is known as a retrovirus because it makes a DNA copy of its RNA code • the envelope spikes of HIV are made of protein and carbohydrate.
What defines viruses as obligatory intracellular parasites?
They rely on living host cells for multiplication and metabolic functions.
How do viruses determine the cells they infect?
Specific attachment points on the virus correspond with host cells.
Why do viruses show specificity towards certain host cells?
Certain properties in host cells are necessary for the virus to carry out its functions.
What dictates a virus’s choice of host cell?
Attachment points and properties required in the host cell for virus functions.
What are some host cells targeted by specific viruses?
-White blood cells (CD4 cells) by HIV -Spinal nerve cells by Poliovirus -Liver cells by Hepatitis virus -Cells of the intestinal system by Norovirus
What is the medical challenge in treating viral infections?
Controlling the virus without disrupting normal host cell function.
What steps are involved in the reproduction of all viruses?
• Attach to a specific host cell site. • Incorporate genetic material into the host cell’s cytoplasm. • Utilize host cell processes to produce virus components. • Assemble viral components into new virus entities. • Release the new viruses into the host cell’s environment.
What distinguishes bacteriophages?
They infect bacteria and exhibit unique reproductive cycles.
What is the lytic cycle?
A viral replication cycle involving five stages: attachment, penetration, biosynthesis, maturation, and release.