A virion is the complete, infectious form of a virus that contains the viral genetic material and protein capsule. It has a nucleic acid core of either DNA or RNA surrounded by a protein capsid, and sometimes an outer envelope. A virion is able to infect host cells and initiate viral replication. In contrast, a viroid is an infectious agent composed only of circular RNA that does not encode any proteins but can replicate using the host cell's machinery, causing disease in plants. Prions are protein-based infectious particles that cause transmissible spongiform encephalopathies by adopting an abnormal folded shape. They are much smaller than viruses and do not contain any nucleic acids.

1. Virion Vs. Viroid
&
Prions
Vs.

2. Viron
• A virion is a complete functional virus that has the
capacity to infect living tissue. This means that it includes
the genetic material, the capsid, the envelope and the
membrane proteins that allow the virus to bind to its host
and enter it.
• Structural unit of the virus
• It has two essential structures: DNA or RNA and capsid.
• Sometimes, glycoprotein spicules can be added to these
basic structures
• VIRUSES can be found either inside a cell (intracellular)
or outside of a cell (extracellular). If it is found
extracellular, the virus is called a virion. A virion
contains a protein coating called a capsid, which
surrounds the core of the virus containing the nucleic
acid (either DNA or RNA).

3. Bio-molecules found in virions
• Genetic material, either DNA or RNA, single or double stranded,
nucleoprotein capsid, maybe an envelope usually receptor
proteins or enzymes that permit binding or entry into the host.
• It is an extracellular virus consisting of a protein coat (capsid)
surrounding a nucleic acid core, either DNA or RNA, together
known as a nucleocapsid called an envelope.
• A virus will not have an envelope within a cell. If the cell was
burst artificially, then these virus particles cannot be called virion
because they will lack certain proteins that will make them
infectious even though the genetic material is present.
• Although viruses lack cell membrane, some viruses have a viral
membrane surrounding its capsid. These are called enveloped
virion. One without an envelop is called a naked or non-enveloped
virion.

4. • The virion shell or capsid protects the interior
core that includes the genome and other
proteins. After the virion binds to the surface
of a specific host cell, its DNA or RNA is
injected into the host cell and viral replication
occurs, resulting in the spread of the infection
to other host cells.
• A virion is the infectious particle that is
designed for transmitting the nucleic acid
genome among hosts or host cells. Virions are
produced in the cytoplasm of complex viral
‘factories,' the virus.

6. Viroids
• Viroids are infectious, non-protein‐coding, highly
structured small circular ribonucleic acids (RNAs) able
to replicate autonomously and induce diseases in higher
plants.
• Infectious agents composed only of closed, circular
ssRNAs
• Do not encode gene products
• Requires host cell DNA-dependent RNA polymerase to
replicate
• Cause plant diseases
• Some found in infected host cell nucleolus, others
found in chloroplast
• May cause disease by triggering RNA silencing

7. Discovery of viroids

8. • Viroids and viruses differ in structure, function and
evolutionary origin
• Viroids move intracellularly, cell‐to‐cell through
plasmodesmata, and long distance through the phloem.
• Viroids may infect their host plants latently or induce
different pathogenic alterations including death.
• Viroids, the smallest known pathogens, are naked, circular,
singlestranded RNA molecules that do not encode protein yet
replicate autonomously when introduced into host plants.
• Symptoms of viroid infection in plants include stunting of
growth, deformation of leaves and fruit, stem necrosis, and
death.
• E.g., Potato spindle Tuber Viroid, Coconut Cadang Cadang
Viroid

11. Prions
• Pronounced “pree-on”
• Shortened term for:
• Proteinaceous Infections Particle
• Causes TSE (Transmissible Spongiform Disease) which
attacks the central nervous system (the brain).
• Discovered by Prusiner in 1982 in
Scrapie(neurological disease of sheep)
• Won Nobel prize in Physiology and medicine in 1997

12. Basic Structure
• Normal prions contain
about 200-250 amino
acids twisted into three
telephone chord-like coils
known as helices, with
tails of more amino acids.
• The mutated, and
infectious, form is built
from the same amino
acids but take a different
shape.
• 100 times smaller than
the smallest known virus.

14. Differences From Bacteria &
Viruses
• Prions do not contain nucleic acid; they don’t
have DNA or RNA.
• They are extremely resistant to heat and
chemicals.
• Prions are very difficult to decompose
biologically; they survive in soil for many
years.

15. Controversy
• DNA and RNA are the only substances now
known to replicate in body tissues, so how do
prions make copies of themselves without any
nucleic acids?
• Some believe TSEs are caused by an
unidentified slow-acting virus.
• Others believe a small virus accompanies a
prion and they work together to cause disease.