Viruses-size,shape & types

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T4 phage
 M13 (General properties and structure,classification,reproduction)
 Retrovirus
 Herpes simplex virus (Classification, reproduction )
Cytocidal infection and cell damage
 TMV (Morphology, taxonomy, and reproduction)


WHAT ARE
VIRUSES?
A virus is a non-cellular
particle made up of
genetic material and
protein that can invade
living cells.
3

VIRAL HISTORY
copyright cmassengale
4

Beijerinck (1897)
coined the Latin name
“virus” meaning poison
He studied filtered plant
juices & found they
caused healthy plants to
become sick
copyright cmassengale 5

Wendell Stanley (1935)
crystallized sap from
sick tobacco plants
He discovered viruses
were made of nucleic
acid and protein

Edward Jenner (1796)
developed a smallpox
vaccine using milder
cowpox viruses
Deadly viruses are said to
be virulent
Smallpox has been
eradicated in the world
today

VIEWING VIRUSES
Viruses are smaller than
the smallest cell
Measured in nanometers
Viruses couldn’t be seen
until the electron
microscope was invented
in the 20th century

VIRAL
STRUCTURE
copyright cmassengale
10

Non living structures
Noncellular
Contain a protein coat called the capsid
Have a nucleic acid core containing DNA or
RNA
Capable of reproducing only when inside a
HOST cell

Some viruses are enclosed
in an protective envelope
Some viruses may have
spikes to help attach to the
host cell
Most viruses infect only
SPECIFIC host cells
CAPSID
ENVELOPE
DNA
SPIKES

Viral capsids (coats)
are made of
individual protein
subunits
Individual subunits
are called
capsomeres
CAPSOMERES

Outside of host cells, viruses
are inactive
Lack ribosomes and enzymes
needed for metabolism
Use the raw materials and
enzymes of the host cell to be
able to reproduce
EBOLA VIRUS
HIV VIRUS

capsid nucleic acid
lipid
envelope
surface
proteins
capsid
nucleic acid
lipid envelope
Surface proteins capsidsurface
proteins
nucleic acid
helical
(rabies)
polyhedral
(foot-and-mouth
disease)
enveloped
(influenza)

CHARACTERISTICS
•Some viruses cause disease
•Smallpox, measles,
mononucleosis, influenza,
colds, warts, AIDS, Ebola
•Some viruses may cause some
cancers like leukemia
•Virus-free cells are rare
MEASLES

•Viruses come in a variety of shapes
•Some may be helical shape like the
Ebola virus
•Some may be polyhedral shapes
like the influenza virus
•Others have more complex shapes
like bacteriophages

19
20-sided with 12 corners
Vary in the number of
capsomers
Each capsomer may be
made of 1 or several
proteins
Some are enveloped

21
 Viral entry into host cells occurs through one of the following
methods:
Endocytosis
Direct fusion
Nucleic acid translocation

22
 All of the virus is engulfed
and enclosed in a vacuole

23
 Host cell membrane fuses with the virus

24
Non enveloped virus injecting its
nucleic acid to the host cell

Viruses are obligate intracellular parasites, which
means they can replicate only within a host cell
Each virus has a host range, a limited number of
host cells that it can infect
© 2011 Pearson Education, Inc.

Once a viral genome has entered a cell, the cell
begins to manufacture viral proteins
The virus makes use of host enzymes, ribosomes,
tRNAs, amino acids, ATP, and other molecules
Viral nucleic acid molecules and capsomeres
spontaneously self-assemble into new viruses

VIRUS
2
1
3
4
Entry and
uncoating
Replication
Transcription
and manufacture of
capsid proteins
Self-assembly of
new virus particles
and their exit from
the cell
DNA
Capsid
HOST
CELL
Viral DNA
Viral
DNA
mRNA
Capsid
proteins

Phages are the best understood of all viruses
Phages have two reproductive mechanisms: the
lytic cycle and the lysogenic cycle

The lytic cycle is a phage replicative cycle that
culminates in the death of the host cell
The lytic cycle produces new phages and lyses
(breaks open) the host’s cell wall, releasing the
progeny viruses
A phage that reproduces only by the lytic cycle is
called a virulent phage
Bacteria have defenses against phages, including
restriction enzymes that recognize and cut up
certain phage DNA
Animation: Phage T4 Lytic Cycle

Attachment
2
1
Entry of phage
DNA and
degradation
of host DNA

Attachment
2
1
3
Entry of phage
DNA and
degradation
of host DNA
Synthesis of
viral genomes
and proteins

Attachment
2
1
4
3
Entry of phage
DNA and
degradation
of host DNA
Synthesis of
viral genomes
and proteins
Assembly
Phage assembly
Head Tail Tail
fibers

Attachment
2
1
5
4
3
Entry of phage
DNA and
degradation
of host DNA
Release
Synthesis of
viral genomes
and proteins
Assembly
Phage assembly
Head Tail Tail
fibers

The lysogenic cycle replicates the phage genome
without destroying the host
The viral DNA molecule is incorporated into the
host cell’s chromosome
This integrated viral DNA is known as a prophage
Every time the host divides, it copies the phage
DNA and passes the copies to daughter cells
Animation: Phage Lambda Lysogenic and Lytic Cycles

An environmental signal can trigger the virus
genome to exit the bacterial chromosome and
switch to the lytic mode
Phages that use both the lytic and lysogenic cycles
are called temperate phages

lytic cycle
is induced
or
Phage DNA
circularizes.
Certain factors
determine whether
lysogenic cycle
is entered
Lysogenic cycle
Prophage
Daughter cell
with prophage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Cell divisions
produce a
population of
bacteria infected
with the prophage.
The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.

New phage DNA and proteins
are synthesized and assembled
into phages.
The cell lyses, releasing phages.
Phage
Phage
DNA
The phage
injects its DNA.
Bacterial
chromosome
Lytic cycle
lytic cycle
is induced
or
Phage DNA
circularizes.
Certain factors
determine whether
lysogenic cycle
is entered
Lysogenic cycle
Prophage
Daughter cell
with prophage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Cell divisions
produce a
population of
bacteria infected
with the prophage.
The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.

The lytic cycle of the bacteriophage: a lunar analogy
Lytic Cycle Lysogenic Cycle
The lytic cycle causes the host bacterium to undergo
cell lysis, that is, cell destruction.
The lysogenic cycle does not cause cell lysis or cell
destruction.
The lytic cycle can lead to production of 100 to 200
progeny phages.
The DNA of the phage gets integrated into the
bacterial chromosome and no progeny are produced
mostly.
Lytic cycle cannot be converted into the lysogenic
cycle.
Lysogenic cycle can be converted into lysogenic cycle
when the host cell is exposed to chemical or physical
agents.

Family names end in -viridae
Genus names end in -virus
Viral species: A group of viruses sharing the
same genetic information and ecological niche
(host).
 Common names are used for species
Subspecies are designated by a number

Herpesviridae
Herpesvirus
Human herpes virus 1, HHV 2, HHV 3
Retroviridae
Lentivirus
Human Immunodeficiency Virus 1, HIV 2

COMMON COLD

 Smaller than viruses
 Strands or circles of RNA
 No protein-coding genes
 No protein coat
 Cause many plant diseases
Viroid
Small, single-stranded, covalently closed circular RNA molecules
existing as highly base-paired
rod-like structures; they do not possess capsids
They range in size from 246 to 375 nucleotides in length. The
extracellular form of the viroid is naked RNA—there is no capsid
of any kind
The RNA molecule contains no protein-encoding genes, and the
viroid is therefore totally dependent on host functions for its
replication
The RNAs of viroids have been shown to contain
inverted repeated base sequences at their 3' and 5' ends, a
characteristic of transposable elements and retroviruses. Thus, it
is likely that they have evolved
from transposable elements or retroviruses by the deletion of
internal sequences
44

Prion
A kind of infectious protein that can resist the digestion of
proteinase
The cellular form of the prion protein (PrPc) is encoded by the
host’s chromosomal DNA
An abnormal isoform of this protein (PrPres) is the only known
component of the prion and is associated with transmissibility.
Kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann-
Sträussler-Scheinker disease, fatal familial insomnia
, and Bovine spongiform encephalopathy (BSE)
Dr.T.V.Rao MD 45

•RNA or DNA Virus
•Do or do NOT have an envelope
•Capsid shape
•HOST they infect

Viruses-size,shape & types

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