Virus multiplication are in Following steps: attached, penetration, biosynthesis, maturation, assembly and release and also discribe the life of Bacteriophage by following two life cycle
Present By Kainat Ramzan
2. General steps in multiplication/ replication of viruses
Multiplication in Animal viruses
Bacteriophage replication - Lytic and Lysogenic cycle
2
Lesson Outcome
3. 3
Virus is a tiny infectious particle consists of nucleic acid and
protein coat.
Obligate intracellular parasites
Acellular - absence of nucleus, organelles, cytoplamic membrane
Viruses use their genome (DNA or RNA) to replicate themselves
in host cell and synthesize viral particle.
Virus in host cell , they will take control of synthetic and genetic
machinery of host cell
Do not divide by binary fission or division
8. 8
Viruses are replicated by a process in which all
components of virus are produced separate and are
assembled into virions
Host specific
- may be a Bacteria, plant and animal
Replication of viruses are studied for first time by
experimenting on bacteriophage of the T series [T2,T4 and
T6]
Two types of the cycle commonly seen in viruses
- Lytic cycle
- Lysogenic cycle
Introduction
9. 9
Viral Multiplication
General phases in life cycle of viruses
• Attachment
• Penetration
• Synthesis
• Maturation
• Release
Viral
Replication
Fig. Virus life cycle
11. 11
Virus are host
specific and
enters into host
or target cell
Virus has host
range and it
may be narrow
or broad
Rabies virus
example for
broad range
virus
HIV is an
example for
narrow range
virus
12. 12
Cell Surface Receptors Used by
Viruses have attachment sites that attach to the receptor sites on the host cell surface
Receptor proteins on the virus particle must fit certain receptor sites on the host cell surface
These receptor proteins in the virus are distributed on surface of the virus
Influenza virus - receptor proteins are the spikes on the surface of the envelope
Adenovirus- receptor proteins are small fibers at the corner of the icosahedron
Receptor sites on the cell vary depending on the nature of the virus:
Rabies virus binds specifically to the acetylcholine receptors found on neural cells
HIV-1 binds specifically to the CD4 on the surface of mature T lymphocytes
Influenza virus binds specifically to sialic acid residue of glycoprotein on the surface of
respiratory epithelium
Susceptibility of the host to virus infection, therefore, depends upon the presence or
absence of receptors on the cell surface
13. In Enveloped viruses- Influenza virus and HIV , glycoprotein spikes bind to
the cell membrane receptors.
13
Fig. Enveloped viruses attachment to host cell membrane
14. 14
Naked Viruses - Adenoviruses, Rhinovirus use molecules on their capsid
that adhere to cell membrane receptors.
Fig. Naked viruses attachment to host cell membrane
15. 2. Penetration
15
Virus enters cell
through one of
the following
processes:
1. Direct fusion
2. Endocytosis
Most Naked
viruses enter
cell by
Endocytosis
Enveloped
viruses- the
envelop fuse
with host’s
plasma
membrane or by
Endocytosis
Flexible cell membrane is penetrated by the whole virus or its nucleic acid
by:
Endocytosis – Entire virus is engulfed and enclosed in a vacuole or vesicle
Fusion – Envelope merges directly with membrane resulting in
nucleocapsid’s entry into cytoplasm
17. 17
Separation of viral nucleic acid from protein capsid.
Lysosomal, cytoplasmic, or viral enzymes may be
involved
Refers to the removal or degradation of capsid, there
by releasing the genome into host cell
The viral genome is transported to the site where
transcription/ replication can begin
3. Uncoating
20. 20
4. Synthetic Phase:
Involves several processes:
• Synthesis of viral proteins in cytoplasm
• Replication of viral genome:
• DNA viruses typically replicate in nucleus
• RNA viruses replicate in cytoplasm
The synthetic stage can be divided in two periods:
• Early period: Synthesis of proteins required for replication of viral
genetic material.
• Late period: Nucleic acid replication and synthesis of capsid and
envelope proteins
22. Biosynthesis of DNA viruses
22
DNA viruses replicate in the
nucleus of the host cell by
using viral enzymes
They synthesize their capsid
and other proteins in the
cytoplasm by using host cell
enzymes
Proteins enter into the nucleus
and assemble with the newly
synthesized DNA to form
virions
These virions are transported
along with endoplasmic
reticulum to the membrane of
the host cell for release.
Herpesvirus, papovavirus,
adenovirus, and hepadnavirus
synthesize their nucleic acid in
the host cell nucleus
Poxvirus is an exception,
because all of its components
are synthesized in the
cytoplasm.
23. The RNA viruses
multiply in the
cytoplasm of
the host cell
In Reoviruses, the
double-stranded
RNA is transcribed
to mRNA by viral
polymerase.
Single-stranded RNA viruses are classified into
two categories, depending on the method of
mRNA transcription
Positive strand RNA virus,
the viral RNA itself acts as the
mRNA. The viral RNA is
translated directly into viral
proteins in host cell cytoplasm.
Negative strand RNA
viruses, the RNA is anti-
sense, with polarity
opposite to that of mRNA.
They possess their own
RNA polymerase for
transcription of mRNA
23
Biosynthesis of RNA viruses
25. 25
Assembly/ Maturation
Involves the collection of all components necessary for
formation of virion. The assembly of the protein capsid is the
first step in viral maturation
During maturation, the envelope protein is encoded by the
viral genes and is incorporated into the plasma membrane of
the infected host cell
The assembly of various viral components into virions may
take place in the nucleus (e.g., herpesvirus and
adenoviruses) or cytoplasm (e.g., picornaviruses and
poxviruses).
26. 26
There are two main mechanisms of release:
Lysis of cells: Naked viruses and pox
viruses leave cell by rupturing the cell
membrane.
• Usually results in death of the host cell.
• Example: Poliovirus
Budding: Enveloped viruses incorporate viral
proteins in specific areas of a membrane and bud
through the membrane.
• Envelope contains host lipids and carbohydrates.
• Host cell does not necessarily die.
• Example: Human Immunodeficiency Virus
27. 27
A poliovirus infected cell can release over 100,000 virions
About 3,000-4,000 virions are released from a single cell infected with
Poxvirus.
Number of virions released by infected cells is variable, controlled by factors
such as size of the virus and health of the host cell.
A single infected cell may release a large number of daughter virions. The time
taken for a single cycle of replication is about 15–30 hours.
32. 32
Bacteriophage, also called phage or bacterial virus, any of a group
of viruses that infect bacteria.
Discovered independently by Frederick W. Twort in Great Britain
(1915) and Felix d’Herelle in France (1917).
Bacteriophage, meaning “bacteria eater”
Bacteriophage
33. 33
Lytic: Cell bursts at end of cycle
• Attachment or adsorption: Virus tail binds to specific receptors on the cell surface.
• Penetration: Virus injects genetic material (DNA) into cell. Tail releases lysozyme, capsid remains
outside.
• Biosynthesis: Viral proteins and nucleic acids are made.
• Maturation: Bacteriophage capsids and DNA are assembled into complete virions.
• Release: Bacteriophage virions are released from the cell. Plasma membrane breaks open and cell
lyses.
Burst time: Time from attachment to release of new virions (20-40 minutes).
Burst size: Number of new phage particles that emerge from a single cell (50-200)
37. Bacteriophage: Lysogenic Cycle:
37
1. Attachment and Penetration: Virus tail
binds to specific receptors on the cell
surface and injects genetic material (DNA)
into cell.
2. Circularization: Phage DNA circularizes and
enters either lytic or lysogenic cycle.
3. Integration: Phage DNA integrates with
bacterial chromosome and becomes a
prophage.
4. Excision: Prophage DNA is removed due to
a stimulus (e.g.: chemicals, UV radiation)
and initiates a lytic cycle.
