Lesson #6 - Viruses, Viroids, and Prions

Question 1

Viruses

Answer 1

• Viruses are intracellular parasites that can reproduce
  ONLY within a host cell.
• They do not have:
• Enzymes for metabolism
• Ribosomes
• Other equipment to make proteins
• They get these from their host cells instead, preventing the host cell from replicating its own DNA!
• Because they are unable to carry out the basic functions of life
• without a host, viruses are considered to be NON-LIVING

Question 2

Components of a virus

Answer 2

• All viruses have one type of nucleic acid: DNA or RNA 🡪 the genetic information (instructions) of the virus
• Viral nucleic acid is covered by a protein coat (called a CAPSID)
• An outer membrane layer, called an ENVELOPE, surrounds the capsid in some viruses
• Viruses are very small 🡪 the largest virus is still smaller than the smallest bacteria

Question 3

Viral targets

Answer 3

• Some viruses target only one or a few types of organism🡪
• HOST SPECIFIC
• Example: Swine flu (only humans or swine/pigs)
• Viruses that only infect bacteria cells are called BACTERIOPHAGES
• Others are able to go from one species to another 🡪 BREAK THE SPECIES BARRIER
• Example: Rabies
• Viruses use a“lock and key” fit to identify hosts.

Question 4

How viruses enter the cell

Answer 4

1. Endocytosis (common in animal cells 🡪 the whole virus is engulfed by the cell)
   or
2. Virus comes in contact with the cell and injects it’s genetic information to enter the cell (common in bacteriophages)

Question 5

Dangers of a viral infection

Answer 5

In some cases, viral damage is easily repaired (respiratory epithelium after a cold), but in others, infection causes permanent damage (nerve cells after polio)

Question 6

The lytic cycle

Answer 6

• The viral genes immediately turn the host cell into a virus-producing factory
• The viral DNA uses the cells machinery to create new
  DNA and capsids, ready for assembly
• The cell soon lyses (bursts) and releases its viral
  products.
• Culminates in the death of the host cell
• Example: Influenza Virus

Question 7

the lysogenic cycle

Answer 7

• Replication of the viral genome without destroying the host cell.
• The viral genome gets integrated into the DNA, and becomes replicated with the rest of the cell’s DNA
• Each new daughter cell will then have the integrated DNA incorporated as part of its genome
• Eventually, an environmental factor will cause the viral DNA to separate from the rest of the DNA, and will enter the lytic cycle
• EXAMPLE: HIV Virus

Question 8

How humans deal with viruses

Answer 8

• Best action is preventative measures with VACCINES
• Vaccines are harmless versions of the virus that stimulate the immune system to build defenses against the pathogen.
• Vaccines can help prevent viral infections, but they can do little to cure most viral infection once they occur 🡪 they are PROACTIVE treatments
• Some recently-developed drugs do combat some viruses by interfering with certain parts of viral replication

Question 9

Common misconception

Answer 9

• Should you take antibiotics to cure a viral infection?
• Antibiotics kill bacteria by inhibiting enzyme or processes specific to bacteria
• However, they are powerless again viruses, which have few or no enzymes of their own.

Question 10

Viroids and prions

Answer 10

• Viroids 🡪 even smaller and simpler than viruses!
• Consist of tiny molecules of naked circular RNA that infect PLANTS.
• These RNA molecules can disrupt plant metabolism and stunt plant growth, and can wipe out an entire crop

Question 11

Prions

Answer 11

• Prions are abnormally shaped proteins that cause the
  degeneration of mammalian brain and nervous tissues of infected animals
• Prions interact with normal proteins, causing them to change shape and also become infected
• Common example – bovine spongiform encephalopathy, or “mad cow disease”
• If humans eat beef with infected
  tissues, they can develop the disease known as Creutzfeldt-Jakob disease