Viruses are parasites that can only replicate inside living host cells. They are made up of genetic material surrounded by a protein coat and have no cell structure of their own. Viruses come in many shapes and sizes but are typically 20-400 nanometers. They infect bacteria, plants, and animals. A virus replicates by entering a host cell, releasing its genetic material, and hijacking the cell's machinery to produce new virus particles that eventually cause the cell to burst and release new viruses. Viruses can have DNA or RNA as their genetic material and replicate through either a lytic or lysogenic cycle.

1. VIRUSES

2. We have all gotten viruses…
from bacteria, plants to animals.
Viruses cause colds, flu, warts and diseases such
as measles, AIDS and cancer.
AND not all viruses are harmful to humans.
BUT not all viruses cause diseases,

3. Doesn’t belong to any kingdom
-It’s not a plant or an animal.
-It’s not a fungi, protist, or bacteria.
WHAT IS A VIRUS?
Viruses are parasites—an organism that depends entirely upon another
living organism (a host) for its existence in such a way that it harms that
organism.
Viruses with RNA that transcribe into DNA are called retroviruses
A virus is an infectious agent made up of nucleic acid (DNA or
RNA) wrapped in a protein coat called a capsid.
A virus that infects bacteria is known as a bacteriophage, often
shortened to phage.
The study of viruses is known as virology, and those who study
viruses are known as virologists.
The word virus comes from the Latin, poison (syn. venenum).

4. Definition of a Virus
Sub microscopic entity consisting of a single nucleic acid
surrounded by a protein coat and capable of replication only
within the living cells of bacteria, animals or plants.
The key features of this definition are as follows:
Ø Single type of nucleic acid – either DNA or RNA but not both
Ø Replication of the virus only with a living cell – they are
obligate intracellular parasites.
These characteristics are typical for ALL viruses whether they
infect bacteria, plants or animals.
Viron
• the complete infectious unit of virus particle
• Structurally mature, extracellular virus particles.

5. Viruses have no nucleus, no organelles, no cytoplasm or cell
membrane—Non-cellular
vs
This is why it does NOT belong to any kingdom.

6. Viruses are infectious agents with both living and nonliving
characteristics.
1. Living characteristics of viruses
a. They reproduce, but only in living host cells.
b. They can mutate.
2. Nonliving characteristics of viruses
a. Acellular - no cytoplasm or cellular organelles.
b. They carry out no metabolism on their own and must replicate
using the host cell's metabolic machinery.
In other words, viruses don't grow and divide. Instead, new
viral components are synthesized and assembled within the
infected host cell.
c. The vast majority of viruses possess either DNA or RNA but
not both.

7. Types of Viruses
• DNA Viruses RNA Viruses
• Adenoviruses Coronaviruses
• Iridoviruses Filoviruses
• Herpesviruses Orthomyxoviruses
• Papovaviruses Paramyxoviruses
• Parvoviruses Picornaviruses
• Poxviruses Retroviruses
• Viral Hepatitis Rhabdoviruses

8. How small is a virus?
A small virus has a diameter of about 20nm.
Parvovirus
A large virus have a diameter of up to 400nm.
Poxviruses
Viruses range in size from 20 – 250 nanometers (nm)
If a cell was the size of your classroom, then an average virus
would be the size of a softball.

10. Viral Shape
• helical - rodlike
• isometric - spiral
– Icosahedron
• structure with 20 equilateral triangular
facets
• most efficient symmetrical
arrangement that linear subunits can
form a shell with maximum internal
capacity
• basic design of geodesic dome
• Spherical
• Rod-shaped
• Brick-shaped
• Tadpole-shaped
• Bullet-shaped
• Filament

11. Viral Structure
Viral Structure

12. Bacterial Virus - Structure
(a) Electron Micrograph
(b) diagram of a T4 bacteriophage

13. Virion Structure
Nucleic Acid
Spike
Projections
(Glycoproteins)
Protein
Capsid
Lipid Envelope
Virion
Associated
Polymerase

14. Capsid
1. The protein coat
surrounding the nucleic
acid of a virus is called the
capsid.
2. The capsid is composed of
subunits, capsomeres,
which can be a single type
of protein or several types.

15. Viral core
The viral nucleic acid genome, In the center of the virion, :
Control the viral heredity and variation, responsible for the
infectivity.
Viral Capsid
• The protein shell, or coat, that encloses the nucleic acid
genome.
• Functions:
a. Protect the viral nucleic acid.
b. Participate in the viral infection.
c. Share the antigenicity
Nucleocapsid
• The core of a virus particle consisting of the genome plus a
complex of proteins.
• complex of proteins = Structural proteins +Non- Structural
proteins (Enzymes &Nucleic acid binding proteins)

16. Envelope
• A lipid-containing membrane that surrounds some viral
particles.
• It is acquired during viral maturation by a budding process
through a cellular membrane, Viruses-encoded
glycoproteins are exposed on the surface of the envelope.
• Not all viruses have the envelope, and viruses can be
divided into 2 kinds: enveloped virus and naked virus.
Function
• Antigenicity
some viruses possess neuraminidase
• Infectivity
• Resistance

17. Chemical composition of viruses
• Viral Protein
• Viral Nucleic Acid
• Viral Lipids
• Viral carbohydrate
Viral Nucleic Acid
• DNA-double stranded (ds): linear or circular
Single stranded (ss) : linear or circular
• RNA- ss:segmented or non-segmented
ss:polarity+(sense) or polarity –(non-sense)
ds: linear (only reovirus family)
Viral Protein
• Structural protein (Capsomere)
• Enzyme
• glycoproteins (spike/viral attachment protein, VAP)

18. Viral Genome Structure
• Viral genomes types exhibit great diversity
• Some use DNA others RNA
• Some double stranded, others single stranded
• dsDNA, ssDNA, dsRNA, ssRNA
• In ssRNA, the genome can contain the same base sequences as the
mRNA used to produce viral proteins.
• RNA strand can serve as mRNA and is called a positive-strand virus
• Genome can contain bases complementary to viral mRNA and is
called negative-strand virus
This nucleic acid can be in a variety of physical forms that can be
used as a valuable classification feature.

19. The virions has the following common features:
Simple structure
· The overall structure is not in general complex although they
do perform complex functions
Repeating structure
· They are generally made up a very few proteins (the simple
plant viruses may just have one protein in the virus capsid) OR
proteins which are structurally very similar.
High Level of Redundancy
There is a high degree of redundancy in the virion, which allows
for the partial inactivation of some parts of the virion without
actually destroying the virion completely.
The general exceptions to these comments are the poxviruses.

20.  In order to replicate and make copies of itself, viruses need a host
cell. Any living cell can become a host cell (human, animal, plant,
and even bacterial cells!)
 Without a host cell, viruses cannot function (i.e.-are harmless!)
 Although any cell can theoretically become a host cell, specific
viruses will only infect specific cells
How Do Viruses Work?

21. Virus Replication
1 Virus attachment
and entry
1 2 Uncoating of virion
2
3 Migration of
genome nucleic
acid to nucleus
3
4 Transcription
5 Genome replication
4
5
6 Translation of virus
mRNAs
6
7 Virion assembly
7
8 Release of new
virus particles
8
Steps involved in virus replication process

22. There are many variations on the virus replication and this diagram
illustrates some of the basic features of the cycle.
1. Attachment and entry: viruses recognise specific structures on
the cell surface (referred to as virus receptors), which target the
virus to specific cell types and tissue. The receptor is a normal
component of the cell, which the virus has hijacked for the
infection process.
2.Uncoating: The virion breaks open and releases the virus
genome nucleic acid into the host cell cytoplasm. Further
replication may take place in the cytoplasm or the nucleic acid may
migrate to the cell’s nucleus.
3. Transcription: Virus mRNA is produced using either cellular
enzymes or virus-coded enzymes.
4.Genome replication: This stage can take place in either the
cytoplasm or nucleus of the infected cell.

23. 5. Translation: This stage uses the host cell machinery - ribosomes
and enzymes etc. Various proteins are synthesised
6. Virion Assembly: The newly formed virus proteins and genomic
nucleic acid assemble to produce the new virus particles.
7. Virion release: Various strategies are available for the release of
the progeny virus from the infected cell depending on the
particular virus group.
The virus may bud through the cell membrane at which time it
picks up the envelope surrounding the virus particle OR the virus
may simply cause lysis of the cell resulting in cell death and the
release of progeny virus particles.

24. Virus Replication
DNA/RNA is
copied.
DNA/RNA injected
into cell.
Virus attaches
to cell.
Virus copies
itself.
Cell bursts (lyses) and
releases new viruses.
Step
1
Step 2 Step
3
Step
4
Step
5

25. RNA or DNA core (center),
protein coat (capsid)
Copies itself only inside
host cell--REPLICATION
DNA or RNA
NO
Cell membrane, cytoplasm,
genetic material, organelles
Asexual or Sexual
DNA and RNA
YES—Multicellular Organisms
YES
Structure
Reproduction
Genetic Material
Growth and
Development
Response to
Environment
Change over time
Obtain and Use
Energy
NO
NO
NO YES
YES

26. Bacteriophages
• Bacteriophages - viruses that infect bacteria
– Bacteriophages are among the most common biological
entities on Earth.
– The term is commonly used in its shortened form, phage.
– some named as members of a “T” series
• One of the densest natural sources for phages and other viruses is
sea water, where up to 9×108 virions per milliliter have been found
in microbial mats at the surface, and up to 70% of marine bacteria
may be infected by phages.
• They have been used for over 60 years as an alternative to
antibiotics in the former Soviet Union and Eastern Europe.
• They are seen as a possible therapy against multi drug resistant
strains of many bacteria.

27. Bacteriophages
• Structure

28. Lytic Cycle
• tail fiber contacts lipoproteins of host bacterial cell wall
– tail contracts and tail tube passes through opening in base plate,
piercing bacterial cell wall
• contents injected into host cytoplasm
– will kill infected cell by lysis
– virulent viruses
Picture of phages on outside of cell


29. Lysogenic cycle
• Does not immediately kill the cell
• integrate their nucleic acid into the genome of the infected host cell
(prophage).
• prophage - phage genome inserted as part of the linear
structure of the DNA chromosome of a bacterium
– The integration of a virus into a cellular genome is termed
lysogeny.

31. Viral multiplication
Bacteriophages – Detailed notes
During a lytic cycle, a phage causes the lysis and death of a
host cell.
Lysogeny. DNA incorporated as a prophage into the DNA of
the host cell

32. Lytic Cycle
• The multiplication cycle of these phages
can be divided into five distinct stages:
– Attachment
– Penetration
– Biosynthesis
– Maturation
– Release

33. Lytic Cycle: Attachment
• During the
attachment phase of
the lytic cycle,
– Chance collision
– Sites on the phage’s
tail fibers attach to
complementary
receptor sites on the
– bacterial cell.

34. Lytic Cycle: Penetration
• Phage lysozymes opens a portion of the bacterial cell wall,
• tail sheath contracts to force the tail core through the cell wall,
• DNA enters the bacterial cell and the capsid remains outside.

35. Lytic Cycle: Biosynthesis,
• Phage DNA is replicated
• Phage DNA produces mRNA coding for proteins necessary for
phage multiplication
• capsids and proteins are produced

36. Lytic Cycle: Maturation
• Phage DNA and capsids
are assembling into
complete viruses

37. Lytic Cycle: Release (lysis)
• phage lysozyme
breaks down the
bacterial cell wall,
and the multiplied
phages are
released

38. Lysogeny
• Some viruses (lysogenic phages) do not
always cause lysis and death of the host cell
when they multiply.
• These viruses may incorporate their DNA
into the host cell’s DNA to begin a lyogenic
cycle.
• In lysogeny, the phage remains latent or
inactive

39. Characteristics of lysogeny
• Lysogenic cells are immune to reinfection
by the same phage.
– Repressor proteins stop transcription of all
other phage genes.
• Host cell may exhibit new properties (phage
conversion)
– Bacteria may acquire new genes from
previously infected cells
• Special transduction

40. A Bacteriophage one-step growth curve

41. Vocabulary
• Burst time: The time from phage attachment to
release (AVG 20 to 40 min).
• Burst size: The number of newly synthesized
phages from a single infected cell (50-200).
• Eclipse period The time period when whole
virons can not be found. It is the time from the
end of penetration to the beginning of release.

42. Viroids
• tiny, naked molecules of RNA that are an important infectious
disease agent in plants
– recent outbreak killed 10 million coconut palms in Phillipines
• Not clear how they cause disease
• Viroid nucleotide sequences resemble sequences of introns within
ribosomal RNA genes
– capable of catalyzing destruction of chromosome integrity??
300-400 nucleotides long