Viruses, Viroids, and Prions

What are Viruses?

• Definition: An infective agent consisting of nucleic acid surrounded by a protein coat, capable of replication ONLY within the living cells of bacteria, animals, or plants.

• Characteristics:

  • Replicate inside hosts using host metabolic machinery

  • Genomes consist of nucleic acid, DNA, or RNA

  • Cannot carry out metabolic rates or reproduce independently

  • No membrane-bound organelles or cellular membrane

  • Do not grow in size, respond to direct environment, or follow the Central Dogma

Viral Genome Types

• Viral genomes are much smaller compared to bacteria

• Either DNA or RNA (NEVER both)

• DNA Viruses:

  • Non-segmented

  • Linear or circular

  • ds or ss

• RNA Viruses:

  • Non-segmented or segmented

  • Linear or circular

  • ds or ss

  • (-) or (+) sense

• Sense (+):

  • Like mRNA, direct production of protein

• Sense (-):

  • Must be converted to (+) strand to produce protein

Viral Morphology

• Viruses are MUCH smaller than bacteria

• Three main morphologies:

  • Helical/Tube -

    • Long rods

    • Rigid or flexible

  • Polyhedral -

    • Most are “3-D,” have 20 faces and 12 corners

  • Complex -

    • Complicated structures

    • Capsids w/ other structures attached

• Capsid: Repetitive units of one/few proteins, protects, attachment/penetration of host cell

• Some viruses are enveloped

  • Host acquired

  • Usually spherical

  • Include viral glycoproteins

    • avoids immune system + host binding

Host Range

• All cellular organisms can be infected by a virus

• Narrow host range: Species-specific. Ex - the smallpox virus can only infect humans

• Broad range: Can infect different species of mammals. Ex - rabies virus

  • Viruses must recognize specific receptor sites on host cells

Life Cycle of Bacteriophages

• Bacteriophages: Viruses that infect bacteria

• Has two life cycles:

  • Lytic life cycle*

  • Lysogenic life cycle*

Lytic Life Cycle

• 5 Steps

  • 1.) Attachment (Locate and Stick)

    • Attraction between phage “tail fibers” and proteins on the bacterium

  • 2.) Entry (Getting in)

    • Tail releases enzyme, phage lysozyme breaks down cell wall. The tail contracts and the core is driven through the cell wall and membrane into the host like a needle. Viral enzymes + DNA pass through the tail core into host. Capsid remains outside the bacterial cell.

  • 3.) Synthesis (Making pieces)

    • Phage genetic info takes over machinery of bacteria cell by turning off synthesis of bacterial components and redirecting the bacterial synthetic system to make phage components

  • 4.) Assembly (Putting it together)

    • Maturation - Cell is full of viral components, spontaneous assembly of capsids and nucleic acid to make viral particles

  • 5.) Lysis (Getting out)

    • Destruction of cell wall and membrane by lysozyme. Virus released. Bacterium destroyed, lysed or eaten.

Lysogenic Life Cycle

• Only 2 steps:

  • Attachment

  • Entry

• After entry, there is NO synthesis

  • Phage DNA integrates into the bacterial chromosome called a prophage*

    • Dormant/inactive

  • The bacterium carrying the prophage is a lysogen. The prophage is replicated along with the bacteria’s DNA during cell division. Doesn’t kill the bacteria.

  • Induction - Occurs occasionally when environmental factors trigger the excision of DNA, starting the lytic cycle

What are the consequences of Lysogeny?

• Host becomes immune to reinfection by the same virus, but can be infected by other viruses

• Phage conversion

  • Host can exhibit new properties like picking up toxin genes.

• Transduction*

  • The transfer of bacterial genes from one bacteria to another

What’s the difference between the Lytic and Lysogenic life cycles?

• Lytic - 5 steps

• Lysogenic - 2 steps, doesn’t kill host bacteria immediately

Provirus vs. Prophage

• Provirus:

  • Genetic material of a retrovirus

  • Host: Eukaryotic cells

  • Do not come out

  • Can inhabit a host for years without any symptoms

  • Changes in the immune system can cause activation and acute infection

    • Shingles

• Prophage:

  • Genetic material of a bacteriophage

  • Host: Bacteria

Latency vs. Oncogenisis

• Latency:

  • Same as lysogeny of phage

  • Viral genome integrates into the host chromosome

  • Dormancy stage

• Oncogenesis:

  • Process in which normal cells transform into cancer cells.

  • Abnormal cell division

  • Viral latency may disrupt genetic control, leading to abnormal growth

Prions*

• An infectious protein

• Normal cellular protein → becomes infectious

• Genetic predisposition

Viroids*

• Infectious particles

• Non-capsid

• Circular RNA

• Extremely small

• Found in plants

• Smaller RNA pieces

Life Cycle of Animal Viruses

• Assemble in the nucleus or cytoplasm

• Release by budding with or without an envelope or by lysis (lytic cycle)

Replication of Viral Genomes