BI-112 Virus-Viroid Wi25

VIROIDS

Origins of Life

  • First Cellular Life: ~3.5-3.8 billion years ago (bya)

  • Pre-cellular life molecules: ~4 billion years ago

    • Key players: RNA, lipids, proteins

    • Jack Szostak's contributions to proto-cell formation

RNA World Hypothesis

  • Video resources available on RNA world

  • Microscopy evidence of vesicle growth and division

  • Model protocell membranes consisting of fatty acid vesicles from various acids:

    • Oleic acid

    • Myristoleic acid

    • Capric acid

Characteristics of Viroids

  • Defined as "rogue" RNAs, first identified in 1971

  • Structure: naked circular RNA, 200-400 nucleotides long

  • Size: Approximately 10 times smaller than the smallest virus

  • Unique feature: Lack of genes (non-coding RNA)

  • Self-replication capabilities are partial

Pathogenic Nature of Viroids

  • Rogue RNAs are known to cause diseases in plants

    • 99% of known viroids affect plant life

  • Transmission methods:

    • Insects that bite plants

    • Spread through cell-to-cell communication and vascular tissues

Examples of Plant Diseases Caused by Viroids

  • Notable diseases:

    • Citrus exocortis

    • Potato spindle tuber

    • Cadang-cadang in coconuts

    • Avocado sunblotch

    • Tomato planto macho

  • Hundreds of viroid varieties exist, with characterized images available.

Theories of Viroid Origin

  • Explanations for viroids' origin:

    • Considered relict or "re-invention" from the RNA world

    • Proposed endogenous origin: escaped RNA from plants or viruses

Genetic Complexity

  • Introns discussed as non-coding regions in host genes:

    • Sequence flow from gene DNA to mRNA, excluding introns before translation

Viroids in Humans

  • Unique instance of viroid-like RNA in humans:

    • Delta agent within Hepatitis B virus

    • Causes Hepatitis D through interaction with the Hepatitis B virus

VIRUSES

What is a Virus?

Historical Context

  • Early mentions of variolation and vaccine development:

    • Edward Jenner's work (utilized cowpox with subjects like Blossum and Sarah Nelms)

Size Comparison of Cells and Viruses

  • Eukaryotic Cell: 0.01 mm (10 μm)

  • Bacterium: 0.001 mm (1 μm)

  • Virus: 0.0001 mm (100 nm)

Virus Structure

  1. Composed of nucleic acid (single or double-stranded DNA or RNA)

  2. Surrounded by a protein coat known as a capsid (an essential feature of all viruses)

  3. Possible additional structures:

    • Sometimes includes "stuffing" with other proteins packed around the nucleic acid core

    • Special enzymes may be present, important for infection and replication

  4. Outer envelope that may be present:

    • Lipid bilayer derived from the host cell's membrane

    • Non-enveloped viruses have surface proteins, some from the virus itself.

Are Viruses Alive?

  • Arguments against viruses being classified as living:

    • Not composed of cells

    • Metabolically inert (no energy-producing reactions)

    • Cannot reproduce independently without a host

    • Can be crystallized like pure compounds

    • Synthetic creation of viruses is achievable (example: Polio, 2002)

  • Possible argument in favor of the viral existence:

    • May form independent genetic lineages with hereditary traits and exhibit adaptation/evolution.

Origin of Viruses

  • Lack of fossil evidence leads to three main theories:

    1. Viruses as relics of the proto-cell world.

    2. Evolution from parasitic bacteria that streamlined over time.

    3. Origin from escaped genetic material (instead of introns).

Virus Infection & Replication

  • Hosts for viruses are ubiquitous; found in every known organism.

  • Transmission methods include:

    • Water, food, and body fluids

    • Airborne vectors like mosquitoes

  • Phases of viral infection:

    • Attachment to host cells

    • Entry through membrane fusion or endocytosis (Trojan horse method)

Attachment Mechanism

  • Lock & Key mechanism described:

    • Virus protein identifies and docks with a specific host receptor protein.

    • Some viruses may require two different receptors for successful binding.

Penetration Processes

  • Mechanisms for viral entry into cells:

    1. Direct fusion with lipid membrane if present.

    2. Endocytosis via binding to specific membrane receptors.

Plant Virus Entry

  • Plant cells possess a protective wall; entry facilitated via:

    • Injection from insects

    • Breakage in the wall, allowing flow through plasmodesmata.

Composition of Viruses

  • All viruses consist of:

    • DNA or RNA

    • Protein coat

    • An outer lipid envelope (if present)

    • Hence, potential answers to questions on virus composition include all available structures.

Vaccine Etymology

  • "Vacca" meaning cow refers to the origins of the word vaccine.

Virus and Bacterial Interactions

  • Statement about bacteria's capabilities concerning viruses:

    • Bacteria cannot "catch" a virus (False claim).

Cellular Machinery hijacking by Viruses

  • Viruses hijack host machinery to replicate via:

    • Strategies outlined in the Baltimore Virus Classification System:

      1. Double-stranded DNA viruses (e.g., Herpes, Hep B) integrate into host genome.

      2. Single-stranded RNA viruses (e.g., influenza, HIV) either function directly as mRNA or utilize reverse transcription for integration.

Assembly and Release of Viruses

  • The lifecycle of viral replication includes:

    • Assembly of viral components at sites like ER and Golgi within the host cell.

    • Release methods vary: Lytic infections lead to quick cell lysis while persistent and lysogenic forms incorporate an extended silent phase.

Example Viruses and Characteristics

  • Virus profiles include:

    • Influenza (ssRNA with fragmented genome)

    • Coronavirus (structure and replication details)

    • Certain cancer-causing viruses (associated viral connections)

    • HIV specifics regarding infection and replication pathways.