Viruses are acellular infectious agents that are too small to be seen with a light microscope. They are composed of nucleic acids surrounded by a protein coat called a capsid. Some viruses have an outer envelope. Viruses must replicate within host cells as they cannot generate their own energy or synthesize proteins. Viruses come in various shapes and sizes determined by the arrangement of subunits in their capsids. Their nucleic acids can be single or double stranded DNA or RNA. Classification is based on nucleic acid type and structure, capsid size/symmetry, and presence of an envelope. Enveloped viruses are more sensitive to inactivation than non-enveloped viruses.

1. General virology:
Introduction, structure,
classification

2. Virus is acellular
• Infectious agents like bacteria, fungi, protozoa, and worms, are either
single cells or composed of many cells.
• Cells are capable of independent replication, can synthesize their own
energy and proteins, and can be seen in the light microscope.
• In contrast, viruses are not cells; they are not capable of independent
replication, can synthesize neither their own energy nor their own
proteins, and are too small to be seen in the light microscope

3. Features of virus
Viruses are characterized by the following features:
• Viruses are particles composed of an internal core containing either DNA
or RNA (but not both) covered by a protective protein coat. Some viruses
have an outer lipoprotein membrane, called an envelope, external to the
coat. Viruses do not have a nucleus, cytoplasm, mitochondria, or
ribosomes.

4. Features of virus

5. Features of virus
• Viruses must reproduce (replicate) within cells, because they cannot
generate energy or synthesize proteins. Because they can reproduce only
within cells, viruses are obligate intracellular parasites.
What the obligate intracellular bacteria?
• Viruses replicate in a manner different from that of cells (i.e., viruses do
not undergo binary fission or mitosis).
• One virus can replicate to produce hundreds of progeny viruses, whereas
one cell divides to produce only two daughter cells.

6. Virus is obligate intracellular parasites Virus producing hundreds of progeny virus

7. Comparison of Viruses and Cells

8. EVOLUTIONARY ORIGIN OF VIRUSES
The origin of viruses is not known.
• Two theories of viral origin can be summarized as follows:
1. Viruses may be derived from DNA or RNA nucleic acid components of
host cells that became able to replicate autonomously and evolve
independently.
2. Viruses may be degenerate forms of intracellular parasites.

9. EVOLUTIONARY ORIGIN OF VIRUSES

10. Structure
Size and shape :
• Viruses range from 20 to 300 nm in diameter.
• The shape of virus particles is determined by the arrangement of the
repeating subunits that form the protein coat (capsid) of the virus.

11. Structure-VIRAL NUCLEIC ACIDS
• The viral nucleic acid (genome) is located internally and can be either single-
or double-stranded DNA or single- or double-stranded RNA. The nucleic acid
can be either linear or circular.
• The DNA is always a single molecule;
• The RNA can exist either as a single molecule or in several pieces. For example,
both influenza virus and rotavirus have a segmented RNA genome. Almost all
viruses contain only a single copy of their genome (i.e., they are haploid). The
exception is the retrovirus family, whose members have two copies of their
RNA genome (i.e., they are diploid)

12. Influenza virus Rota virus HIV virus

13. VIRAL CAPSID & SYMMETRY
• The nucleic acid is surrounded by a protein coat called a capsid made up
of subunits called capsomers. Each capsomer, consisting of one or
several proteins.
• The structure composed of the nucleic acid genome and the capsid
proteins is called the nucleocapsid.
 Viral nucleocapsids have two forms of symmetry:
• (1) Icosahedral,
• (2) Helical,

15. Structure-VIRAL PROTEINS
• The capsid proteins protect the genome DNA or RNA from degradation
by nucleases.
• The proteins on the surface of the virus mediate the attachment of the
virus to specific receptors on the host.
• This interaction of the viral proteins with the cell receptor is the major
determinant of species and organ specificity.
• Outer viral proteins are also important antigens that induce neutralizing
antibody and activate cytotoxic T cells to kill virus-infected cells

16. SARS-CoV-2 binding with ACE HIV binds with CD4 receptor

17. Structure-VIRAL PROTEINS -Specific Viral Receptors

18. Structure-VIRAL PROTEINS-Serotypes
• The term “serotype” is used to describe a subcategory of a virus based
on its surface antigens.
• For example, measles virus has one serotype, polioviruses have three
serotypes, and rhinoviruses have over 100 serotypes.
• This is because all measles viruses have only one antigenic determinant
on their surface protein that induces neutralizing antibody capable of
preventing infection.

19. Structure-VIRAL PROTEINS-Serotypes
Rhinovirus

20. Structure-VIRAL PROTEINS-internal structural protein
• Some of the internal viral proteins are structural (e.g., the capsid
proteins of the enveloped viruses), whereas others are enzymes (e.g.,
the polymerases that synthesize the viral mRNA).
• The internal viral proteins vary depending on the virus.
• Some viruses have a DNA or RNA polymerase attached to the genome;
others do not.

21. VIRAL ENVELOPE
• In addition to the capsid and internal proteins, there are two other types of
proteins, both of which are associated with the envelope.
• The envelope is a lipoprotein membrane composed of lipid derived from the
host cell membrane and protein that is virus-specific
• There are frequently glycoproteins in the form of spikelike projections on the
surface, which attach to host cell receptors during the entry of the virus into
the cell.
• Another protein, the matrix protein, mediates the interaction between the
capsid proteins and the envelope

22. Release of influenza virus by plasma membrane budding

23. VIRAL ENVELOPE
• Presence of an envelope confers instability on the virus. Enveloped
viruses are more sensitive to heat, drying, detergents, and lipid solvents
such as alcohol and ether than are nonenveloped (nucleocapsid) viruses,
which are composed only of nucleic acid and capsid proteins.
• Virtually all viruses that are transmitted by the fecal–oral route (those
that have to survive in the environment) do not have an envelope; that
is, they are naked nucleocapsid viruses.

24. ATYPICAL VIRUS-LIKE AGENTS
• Defective viruses : Composed of viral nucleic acid and proteins but
cannot replicate without a “helper” virus. Defective viruses usually have
a mutation or a deletion of part of their genetic material. Because these
defective particles can interfere with the growth of the infectious
particles, it has been hypothesized that the defective viruses may aid in
recovery from an infection by limiting the ability of the infectious
particles to grow.

25. ATYPICAL VIRUS-LIKE AGENTS
• Pseudovirions : Contain host cell DNA instead of viral DNA within the
capsid. They are formed during infection with certain viruses when the
host cell DNA is fragmented and pieces of it are incorporated within the
capsid protein. Pseudovirions can infect cells, but they do not replicate.

26. ATYPICAL VIRUS-LIKE AGENTS
• Viroids : Consist solely of a single molecule of circular RNA without a
protein coat or envelope. Viroids replicate, but the mechanism is unclear.
They cause several plant diseases but are not implicated in any human
disease.

27. ATYPICAL VIRUS-LIKE AGENTS
• Prions: Infectious particles that are composed solely of protein (i.e., they
contain no detectable nucleic acid). They are implicated as the cause of certain
“slow” diseases called transmissible spongiform encephalopathies, which
include such diseases as Creutzfeldt-Jakob disease in humans and kuru in
humans and mad cow disease and scrapie in animals.
• The term spongiform refers to the spongelike appearance of the brain seen in
these diseases. The holes of the sponge are vacuoles resulting from dead
neurons.

28. ATYPICAL VIRUS-LIKE AGENTS
• Prion proteins are encoded by a cellular gene
• Prions are highly resistant to inactivation by ultraviolet light, heat, and
other inactivating agents.
• Because prions are normal human proteins, they do not elicit an
inflammatory response or an antibody response in humans.

29. Comparison of Prions and Conventional Viruses

30. PRINCIPLES OF CLASSIFICATION
The two major components of the virus used in classification are
• (1) the nucleic acid (its molecular weight and structure) and
• (2) the capsid (its size and symmetry and whether it is enveloped).

31. Viral genomes

32. Classification of DNA Viruses

33. DNA Virus

34. Important Features of DNA Viruses

35. DNA virus

36. Classification of RNA Viruses

37. Important Features of RNA Viruses

38. RNA virus

39. Classification of RNA virus

40. Polarity
• Positive polarity is defined as an RNA with the same base sequence as
the mRNA. These viruses use their RNA genome directly as mRNA
• RNA with negative polarity has a base sequence that is complementary
to the mRNA. An mRNA must be transcribed by using the negative strand
as a template.
• For example, if the mRNA sequence is an A-C-U-G, an RNA with negative
polarity would be U-G-A-C and an RNA with positive polarity would be A-
C-U-G.

41. Polarity

42. Polarity

43. Question
• If a virus has an envelope, it is more easily inactivated by lipid solvents and
detergents than viruses that do not have an envelope. Which one of the
following viruses is the most sensitive to inactivation by lipid solvents and
detergents?
(A) Coxsackie virus
(B) Hepatitis A virus
(C) Herpes simplex virus
(D) Poliovirus
(E) Rotavirus

44. Question
• The proteins on the external surface of viruses serve several important
functions. Regarding these proteins, which one of the following statements is
most accurate?
(A) They are the antigens against which neutralizing antibodies are formed.
(B) They are the polymerases that synthesize viral messenger RNA.
(C) They are the proteases that degrade cellular proteins leading to cell death.
(D) They are the proteins that regulate viral transcription.
(E) Change in conformation of these proteins can result in prionmediated
diseases such as Creutzfeldt-Jakob disease.