Viruses are obligate intracellular parasites that contain either DNA or RNA, but not both. They cannot replicate without occupying a living host cell and are incapable of metabolism. Viruses have a core of nucleic acid surrounded by a protein coat called a capsid. Some viruses have an additional envelope derived from the host cell. Viruses are classified based on their nucleic acid type and structure. Viral replication requires the virus to enter a host cell and hijack the cell's machinery to produce new viral particles, which are then released to infect new host cells.

1. Virus
General Properties, Classification and
Bacteriophages

2. What are viruses?
All viruses are obligate intracellular parasites; they inhabit a
no-man’s-land between
the living and the non-living worlds, and possess
characteristics of both.
differ radically from the simplest true organisms:
• they cannot be observed using a light microscope
• they have no internal cellular structure
• they contain either DNA or RNA, but not both
• they are incapable of replication unless occupying an
appropriate living host cell
• they are incapable of metabolism
• individuals show no increase in size.

3. Viral structure
Viral particle, or virion, has in
essence just two surrounded by : A
core of nucleic acid, surrounded
and protected by a protein coat or
capsid, the combination of the two
being known as the nucleocapsid.
In certain virus types, the
nucleocapsid is further envelope,
partly derived from host cell
material.

4. The viral genome
• Genetic material of a virus may be either RNA or DNA, and
either of these may be single-stranded or double-stranded.
• Genome may furthermore be circular or linear.
• Size of the genome varies greatly; it may contain as few as
four genes or as many as over 200 .
• These genes may code for both structural and non-structural
proteins; the latter include enzymes such as RNA/DNA
polymerases required for viral replication.

5. Capsid structure
• The characteristic shape of a virus particle is determined by
its protein coat or capsid.
• In the non-enveloped viruses, the capsid represents the
outermost layer, and plays a role in attaching the virus to the
surface of a host cell.
• The capsid is made up of a number of subunits called
capsomers, and may comprise a few different protein types or
just one.

6. • The capsomers have the ability to interact with each other
spontaneously to form the completed capsid by a process of
self-assembly.
• It protect the nucleic acid against harmful environmental
factors such as UV light and desiccation,
• as well as the acid and degradative enzymes encountered
in the gastrointestinal tract.

7. Helical capsids
• A number of plant viruses, including the well-studied tobacco
mosaic virus, have a rodlike structure when viewed under the
electron microscope.
• This is caused by a helical arrangement of capsomers,
resulting in a tube or cylinder, with room in the center for the
nucleic acid element, which fits into a groove on the inside.

8. Icosahedral capsids
• The icosahedron has a low surface-area to volume ratio,
allowing for the maximum amount of nucleic acid to be
packaged.
• An icosahedron is a regular three-dimensional shape with 20
triangular faces, and 12 points or corners The overall effect is
of a roughly spherical structure.

11. The viral envelope
• Envelopes are much more common in animal viruses than in
those of plants.
• The lipid bilayer covering an enveloped virus is derived
from the nuclear or cytoplasmic membrane of a previous
host.
• Embedded in this, however, are proteins (usually
glycoproteins) encoded by the virus’s own genome.

12. • These may project from the surface of the virion as
spikes, which may be instrumental in allowing the
virus to bind to or penetrate its host cell.
• The envelope is more susceptible than the capsid to
environmental pressures, and the virus needs to
remain moist in order to survive.
• Consequently, such viruses are transmitted by
means of body fluids such as blood (e.g. hepatitis B
virus) or respiratory secretions (e.g. influenza virus).

13. Classification of viruses
Baltimore classification (first defined in 1971) -one of seven groups
depending on a combination of their nucleic acid (DNA or RNA),
strandedness (single-stranded or double-stranded), Sense, and
method of replication.
I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
II: ssDNA viruses (+ strand or "sense") DNA (e.g. Parvoviruses)
III: dsRNA viruses (e.g. Reoviruses)
IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g. Picornaviruses,
Togaviruses)
V: (−)ssRNA viruses (− strand or antisense) RNA (e.g.
Orthomyxoviruses, Rhabdoviruses)
VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA intermediate
in life-cycle (e.g. Retroviruses)
VII: dsDNA-RT viruses DNA with RNA intermediate in life-cycle (e.g.
Hepadnaviruses)

14. Virus family
Examples (common
names)
Virion
naked/envel
oped
Capsid
symmetry
Nucleic
acid
type
Group
Adenoviridae
Adenovirus,
infectious canine hepatitis
virus
Naked Icosahedral ds I
Papovaviridae
Papillomavirus,
polyomaviridae, simian
vacuolating virus
Naked Icosahedral
ds
circular
I
Parvoviridae
Parvovirus B19, canine
parvovirus
Naked Icosahedral ss II
Herpesviridae
Herpes simplex
virus, varicella-zoster
virus, cytomegalovirus, E
pstein–Barr virus
Enveloped Icosahedral ds I
Poxviridae
Smallpox virus, cow pox
virus, sheep pox virus, orf
virus, monkey pox
virus, vaccinia virus
Complex
coats
Complex ds I
DNA viruses

15. Virus Family
Examples
(common
names)
Capsid
naked/enve
loped
Capsid
Symmetry
Nucleic
acid type
Group
Reoviridae
Reovirus,
rotavirus
Naked Icosahedral ds III
Paramyxoviridae
Measles
virus, mumps
virus,
respiratory
syncytial virus,
Rinderpest
virus, canine
distemper virus
Enveloped Helical ss(-) V
Rhabdoviridae Rabies virus Enveloped Helical ss(-) V
Coronaviridae Corona virus Enveloped Helical ss IV
Hepeviridae
Hepatitis E
virus
Naked Icosahedral ss IV
RNA viruses

16. Viral replication cycles
• One characteristic viruses share in common with true living
organisms is the need to reproduce themselves∗ .
• all viruses are obligate intracellular parasites, and so in
order to replicate, a host cell must be successfully entered.

17. Replication cycles in bacteriophages
• Viruses that infect bacterial cells are called bacteriophages
(phages for short), which means, literally, ‘bacteria eaters’.
• Best understood of all viral replication cycles are those of a
class of bacteriophages which infect E. coli, known as the T-
even phages.
• These are large, complex viruses, with a characteristic head
and tail structure.
• The double-stranded, linear DNA genome contains over 100
genes, and is contained within the icosahedral head.
• The growth cycle is said to be lytic, because it culminates in
the lysis (=bursting) of the host cell.

19. Adsorption (attachment):
• T4 attaches by means of specific tail fiber proteins to
complementary receptors on the host cell’s surface.
• The nature of these receptors is one of the main factors in
determining a virus’s host specificity.
Penetration:
• The enzyme lysozyme, present in the tail of the phage,
weakens the cell wall at the point of attachment, and a
contraction of the tail sheath of the phage
• causes the core to be pushed down into the cell, releasing the
viral DNA into the interior of the bacterium

20. Replication:
• Phage genes cause host protein and nucleic acid synthesis to be
switched off, so that all of the host’s metabolic machinery
becomes dedicated to the synthesis of phage DNA and proteins.
• Host nucleic acids are degraded by phage-encoded enzymes,
thereby providing a supply of nucleotide building blocks.
Assembly:
• Once synthesised in sufficient quantities, capsid and DNA
components assemble spontaneously into viral particles.
• The head and tail regions are synthesised separately, then the
head is filled with the DNA genome, and joined onto the tail.

21. Release
• Phage-encoded lysozyme weakens the cell wall, and leads
to lysis of the cell and release of viral particles.
• During the early phase of infection, the host cell
contains components of phage, but no complete
particles for release (eclipse period).
• The time which elapses between the attachment of a
phage particle to the cell surface and the release of
newly-synthesised phages is the latent period for T4
under optimal conditions, around 22 min.

22. Lysogenic replication cycle
• phage DNA actually becomes incorporated into the host’s
genome as a prophage .
• action of repressor proteins, encoded by the phage,
prevents most of the other phage genes being transcribed.
• genes are, however, replicated along with the bacterial
chromosome, so all the bacterial offspring contain the
incorporated prophage.
• The lysogenic state is ended when the survival of the host
cell is threatened, usually by an environmental factor such
as UV light or a chemical mutagen.

23. • Inactivation of the repressor protein allows the phage
DNA to be excised, and adopt a circular form in the
cytoplasm.
• In this form, it initiates a lytic cycle, resulting in
destruction of the host cell. Example of a temperate
phage is bacteriophage λ (Lambda), which infects certain
strains of E. coli.
• Bacterial strains that can incorporate phage DNA in this
way are termed lysogens.

25. Temperate phage
• Some bacteria and bacteriophages (bacterial viruses) whose
relationships are partially symbiotic.
• Bacterium may carry such a phage as a part of its
chromosome, while in turn, the phage does not lyse the
bacterial cell.
• Such bacterial strains are called lysogenic, and the
phenomenon is termed as lysogeny.

26. • When integrated within the bacterial chromosome, the
phage ʎ is called a prophage or provirus and replicates
only with the host chromosome.
• The bacteriophage that can enter into a lysogenic
relationship with its host is called temperate phage.
• Examples of temperate bacteriophages are lambda (ʎ), ɸ
80, P1, P2 and Mu etc.

27. Induction of a prophage may occur spontaneously or artificially,
e.g., an exposure to U.V. light, X-rays or chemical mutagens,
such as, nitrogen mustards and organic peroxide.
Induction converts the prophage into a free “virulent” phage
through excision of the phage genome from that of its host.
The free phage multiplies vegetatively and causes the lysis of
the bacterial cell. High doses of UV may cause the loss of a
prophage from the bacterial cell; this process is known as
curing.
The curing of lysogenic bacteria may also occur spontaneously
with a frequency of 1 per 105 cells per generation.

30. • Flagella are absent but gliding movements are known in a
number of cyanobacteria.
• The name Oscillatoria has been given to a common blue
green alga on the basis of pendulum like oscillating
movements of its anterior region

31. Cell Structure of Cyanobacteria:
• Cyanobacterial cells are larger and more elaborate than
bacteria.
• Cell structure is typically prokaryotic— one envelope
organisation with peptidoglycan wall, naked DNA, 70S
ribosomes and absence of membrane bound structures like
endoplasmic reticulum, mitochondria, Golgi bodies, plastids,
lysosomes, sap vacuoles
• The cell wall is four layered with peptidoglycan present in the
second layer.
• The outer part of the protoplast contains a number of
photosynthetic thylakoids called chromoplasm and thylakoids
lie freely in the cytoplasm.
• Their membranes contain chlorophyll a, carotenes and
xanthophyll’s, but Chlorophyll b is absent.

33. • Attached to the thylakoid membranes are
small granules known as phycobilisomes.
• The latter possess accessory photosynthetic
pigments known as phycobilins.
• The phycobilins are of three types—
phycocyanin (blue), allophycocyanin (blue) and
phycoerythrin (red).
• Differential formation of phycobilins produces
specific colouration which is adapted to
absorbing maximum amount of solar radiation.
• Therefore, cyanobacteria are not always
blue green as they may appear purplish, violet,
brownish, etc.

34. • Instead of typical vacuoles or sap
vacuoles, gas vacuoles or pseudo-
vacuoles are found.
• Each gas vacuole consists of a number
of submicroscopic units called gas
vesicles.
• Gas vacuoles function as light screen;
provide buoyancy regulating mechanism
and pneumatic strength.
• A naked, circular, double stranded DNA
lies coiled generally in the central part of
the cytoplasm known as centroplasm.
• The coiled up DNA is equivalent to a
single chromosome of higher organisms.

35. • It is often called nucleoid.
• Like bacteria, small circular DNA segments
may also occur in addition to nucleoid.
• They are known as plasmids or
transposons.
• 70S ribosomes occur here and there.
Semicircular group of coiled membranes
often attaches the plasma membrane with
the nucleoid.
• Four types of inclusions occur in the cells.
They are α-granules (cyanophycean
starch), β-granules (lipid droplets), volutin
granules and polyhedral bodies (ribulose
biphosphate carboxylase).

36. • Heterocyst of
Cyanobacteria:
• It is a large-sized pale
coloured thick-walled cell
which occurs in terminal,
intercalary or lateral position
in filamentous
cyanobacteria, e.g., Nostoc.
• The thick wall is
impermeable to oxygen but
permeable to nitrogen.
Mucilage sheath is absent.
Photosystem II is absent.
• Heterocyst is dependent for
its nourishment on adjacent
vegetative cells. It has
enzyme nitrogenase.
Heterocyst is specialised to
perform nitrogen fixation.

37. Algal
divisions

38. General characteristics of Phaeophyceae
•Pheophyceae are called commonly known as brown algae
•Photosynthetic pigments: They possesses brown colored
photosynthetic pigments fucoxanthin and β-carotenoids in addition
to chlorophyll a and c.
•Habitat: They are almost marine, very few are fresh water eg.
•Thallus: they are multicellular brown algae. No unicellular and
colonial (motile or non-motile) brown algae till known.
•Storage form of food: laminarin starch, manitol (alcohol) and
some store iodine also.
•Reproduction: vegetative, asexual and sexual methods
•Vegetative: fragmentation.
•Asexual: asexual spores (motile zoospores).
•Sexual: isogamous or oogamous type gametic fusion.
CLASSIFICATION OF ALGAE
On the basis of photosynthetic pigments algae classified
into three classes.

39. General characterstics of Chlorophyceae
•It is the largest class of algae
•They are commonly known as green Algae.
•Photosynthetic pigments: They possesses chlorophyll a,
chlorophyll b and small amount of β-carotenoids.
•The chloroplasts shows various shape ie. Spiral shape in Spirogyra,
cup shaped in Chlamydomonas, star shaped in Zygnema, girdle
shaped in Ulothrix
•Habitat: Mostly freshwater (Spirogyra, Oedogonium,
Chlamydomonas, Volvox, etc), some are marine
(Sargassum, Laminaria, etc) and some are parasitic (Polysiphonia,
Harvevella, Cephaleuros)
•Distribution: they are cosmopolitan in distribution
•They are unicellular as well as multicellular.
•Each cell is eukaryotic
•Thalllus: their body structure, shape and size varies.
•Examples: Chlamydomonas: unicellular free living
•Volvox: colonial form
•Spirogyra: multicellular, unbranched filamentous form
•Ulva: multicellular, parenchymatous form

40. 3. Rhodophyceae (Red algae)
General characteristics of Rhodophyceae
•Rhodophyceae are commonly known as Red Algae
•Photosynthetic pigments: They possesses Red colored
photosynthetic pigments r-phycocyanin and r-phycoerythrin along
with chlorophyll a, d, xanthophyll and β-carotenoid
•Habitat: They are aquatic, mostly marine. Some are freshwater
e.g. Batrachospermum.
•Thallus: Red algae show a variety of life forms-
•Examples: Unicellular- Porphyridium,
•multicellular- Goniotrichum,
•Parenchymatous- Porphyra,
•unicellular colonies-Chroothece,

41. Asexual Reproduction
• Asexual reproduction is the production of progeny without
the union of cells or nuclear material.
• Many small algae reproduce asexually by ordinary cell
division or by fragmentation, whereas larger algae reproduce
by spores.
• Some red algae produce monospores (walled, nonflagellate,
spherical cells) that are carried by water currents and
upon germination produce a new organism.
• Some green algae produce nonmotile spores
called aplanospores, while others produce zoospores, which
lack true cell walls and bear one or more flagella.
• These flagella allow zoospores to swim to a
favourable environment, whereas monospores and
aplanospores have to rely on passive transport by water
currents.

42. Sexual Reproduction
• Sexual reproduction is characterized by the process of
meiosis, in which progeny cells receive half of their genetic
information from each parent cell.
• A sexually reproducing organism typically has two phases in
its life cycle.
• In the first stage, each cell has a single set of chromosomes
and is called haploid, whereas in the second stage each cell
has two sets of chromosomes and is called diploid.
• When one haploid gamete fuses with another haploid gamete
during fertilization, the resulting combination, with two sets of
chromosomes, is called a zygote.
• Either immediately or at some later time, a diploid cell directly
or indirectly undergoes a special reductive cell-division
process (meiosis).
• During meiosis the chromosome number of a diploid
sporophyte is halved, and the resulting daughter cells are
haploid.