Lesson 2: Viruses and prions

• The outside layer of a virus, which is made of proteins, is called the capsid.
• Inside the capsid of a virus is the genetic material, which can be either DNA or RNA (never both).
• Viruses are classified as non-living because they have no organelles to take in nutrients or use energy, cannot make proteins, cannot move, and cannot replicate on their own.
• In order to replicate, a virus must enter a host cell.
• The virus attaches to the host cell using specific receptors on the plasma membrane.
• After the genetic material is inside the host cell, the virus uses the cell to replicate by either the lytic or lysogenic cycle.
• In the lytic cycle, the host cell makes many copies of the viral RNA or DNA.
• in the lytic cycle the viral genes instruct the host cells to make more viral protein capsids and enzymes.
• in the lytic cycle Viruses leave the cell by exocytosis or by causing the cell to burst.
• Viruses that replicate using the lytic cycle are usually active infections.
• An active infection is the presence and replication of a harmful organism within a living being, causing illness or disease, which start to appear one to four days after exposure.
• In the lysogenic cycle, viral DNA integrates into a chromosome in a host cell.
• In the lysogenic cycle Viral genes may remain dormant for some time.
• in the lysogenic cycle Activated viral genes result in the lytic cycle.
• Many disease-causing viruses have lysogenic cycles.
• Retroviruses have RNA instead of DNA for their genetic material.
• Retroviruses have a lipid envelope surrounding the capsid obtained from the plasma membrane of a host cell.
• in retroviruses In the host cytoplasm, reverse transcriptase synthesizes DNA from an RNA template; DNA integrates into the host chromosome until activated.
• The best-known example is Human immunodeficiency virus (HIV), which causes AIDS.
• A protein that can cause infection or disease is called a prion
• Prions are normally spiral shaped.
• Prions does not contain DNA or RNA
• Mutations cause incorrect folding; mutated prions cause diseases.
• Mutated prions can cause normal proteins to mutate.
• In CJD, prions infect nerve cells in the brain, causing them to burst.
• Some scientists think that prions can be transmitted across species.