Online Counseling Resource
YCMOU ELearning Drive…
School of Architecture, Science and Technology
Yashwantrao Chavan Maharashtra
Open University, Nashik – 422222, India
SBT/SBI/SGS011-CP2-02
Introduction
Programmes and Courses
 SEP –SBT011-CP2-U2
 SEP –SBT011 -CP2–U2
 SEP – SGS011-CP2-U2
School of Science and Technology, Online Counseling Resource…
Credits
 Academic Inputs by
Sonali Alkari
Faculty YCMOU Nagpur Centre,
Faculty LAD college P.G. D of Biotechnology
Research officer Ankur Seeds Pvt Ltd
[email protected]
[email protected]
School of Science and Technology, Online Counseling Resource…
How to Use This Resource

Counselor at each study center should use this presentation to deliver
lecture of 40-60 minutes during Face-To-Face counseling.

Discussion about students difficulties or tutorial with assignments should
follow the lecture for about 40-60 minutes.

Handouts (with 6 slides on each A4 size page) of this presentation should
be provided to each student.

Each student should discuss on the discussion forum all the terms which
could not be understood. This will improve his writing skills and enhance
knowledge level about topics, which shall be immensely useful for end
exam.

Appear several times, for all the Self-Tests, available for this course.

Student can use handouts for last minutes preparation just before end
exam.
© 2007, YCMOU. All Rights Reserved.
4
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Learning Objectives
After studying this module, you should be able to:
 Describe viruses replication
 Discuss different phases in virus replication
 Explain HIV , Transmission, genome and structure,
variability etc.
© 2007, YCMOU. All Rights Reserved.
5
School of Science and Technology, Online Counseling Resource…
Virus Replication
 Viruses do not contain the enzymes and
metabolic precursors necessary for selfreplication.
 They have to get these from the host cells that
they infect.
 Viral replication, therefore, is a process of
separate synthesis of viral components and
assembly of these into new virus particles.
 Replication begins when a virus enters the cell .
 The virus coat is removed by cellular enzymes,
and the virus RNA or DNA comes into contact
with ribosomes (cell organs that synthesize
proteins) inside the cell.
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Virus Replication:1
 There the virus RNA or DNA directs the
synthesis of proteins specified by the viral
nucleic acid.
 The nucleic acid replicates itself, and the
protein subunits constituting the viral coat are
synthesized.
 Thereafter,
the
two
components
are
assembled into a new virus.
 One infecting virus can give rise to thousands
of progeny viruses.
 Some viruses are released by destruction of
the infected cell.
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Virus Replication:2
 Others are released by budding through cell
membranes and do not kill the cell.
 In some instances, infections are “silent”—
that is, viruses may replicate within the cell
but cause no obvious cell damage .
 The RNA-containing viruses are unique among
replicative systems in that the RNA can
replicate itself independently of DNA.
 In some cases, the RNA can function as
messenger RNA, indirectly replicating itself
using the cell's ribosomal and metabolic
precursor systems.
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Virus Replication:3
 In other cases, RNA viruses carry within the
coat an RNA-dependent enzyme that directs
the synthesis of virus RNA.
 Some RNA viruses, which have come to be
known as retroviruses, may produce an
enzyme that can synthesize DNA from the
RNA molecule.
 The DNA thus formed then acts as the viral
genetic material.
 Bacterial viruses and animal viruses differ
somewhat in their interaction with the cell
surface during infection.
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Virus Replication:4
 The “T even” bacteriophage that infects
the bacterium Escherichia coli, for
instance, first attaches to the surface
and injects its DNA directly into the
bacterium.
No
absorption
and
uncoating take place.
 The basic events of virus replication,
however, are the same after the nucleic
acid enters the cell.
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Steps in Viral Replication
The
following steps
replication;-
take
place
during
I. Adsorption
II. Penetration
III.Uncoating
IV.Viral genome replication
V. Maturation
VI.Release
viral
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1. Adsorption
 The first step in infection of a cell is
attachment to the cell surface.
 Attachment is via ionic interactions which are
temperature-independent.
 The viral attachment protein recognizes
specific receptors, which may be protein,
carbohydrate or lipid, on the outside of the
cell.
 Cells without the appropriate receptors are
not susceptible to the virus.
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2. Penetration
 Penetration rapidly follows adsorption, and
the virus can no longer be recovered from the
intact cell.
 The most common mechanism is receptor
mediated endocytosis, the process by which
many hormones and toxins enter cells.
 The virion is endocytosed
within a cytoplasmic vacuole.
and
contained
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2. Penetration
 Enveloped viruses
 Some enveloped viruses require an acid pH for
fusion to occur and are unable to fuse directly with
the plasma membrane.
 These viruses are taken up by invagination of the
membrane into endosomes.
 As the endosomes become acidified, the latent
fusion activity of the virus proteins becomes
activated by the fall in pH and the virion membrane
fuses with the endosome membrane.
 This results in delivery of the internal components
of the virus to the cytoplasm of the cell
School of Science and Technology, Online Counseling Resource…
Virus Replication
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2. Penetration
 Non-enveloped viruses
Non-enveloped viruses may cross the
plasma membrane directly or may be
taken up into endosomes.
 They then cross (or destroy) the
endosomal membrane.
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3. Uncoating
 Nucleic acid has to be sufficiently uncoated so that virus
replication can begin at this stage.
 A key step in uncoating is the acidification of the content of
the endosome to a pH of about 5, owing to the activity of a
proton pump present in the membrane.
 The low pH causes rearrangement of coat components,
which then expose normally hidden hydrophobic sites.
 They bind to the lipid bilayer of the membrane, causing the
extrusion of the viral core into the cytosol.
 For influenza virus, the acid-sensitive component is the
core HA2 unit of the haemagglutinin, for adenoviruses, it is
the penton base.
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4. Synthesis of Viral Nucleic Acid and Protein
 Many strategies are used.
 Virulent viruses, either DNA and RNA, shut off
cellular protein synthesis and disaggregate cellular
polyribosomes, favouring a shift to viral synthesis.
 The mechanism of protein synthesis shut-off varies
even within the same viral family.
 Poliovirus, using a viral protease, causes cleavage
of a 200 Kd cap-binding protein, which is required
for initiation of translation of capped cellular
messengers.
 In contrast to virulent viruses, moderate viruses
e.g. polyomaviruses may stimulate the synthesis of
host DNA, mRNA, and protein.
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5. Assembly/Maturation
 New virus particles are assembled.
 There may be a maturation step that follows
the initial assembly process.
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6. Release
 Virus may be released due to cell lysis,
or, if enveloped, may bud from the cell.
 Budding viruses (figures 3 and 4) do
not necessarily kill the cell. T
 hus, some budding viruses may be able
to set up persistent infections.
 Not all released viral particles are
infectious.
 The ratio of non-infectious to infectious
particles varies with the virus and the
growth conditions.
School of Science and Technology, Online Counseling Resource…
Virus Replication
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Human Immunodeficiency Virus (HIV)-1
 (HIV) is a retrovirus
Scanning electron micrograph of
HIV-1 (in green) budding from
cultured lymphocyte. Multiple round
bumps on cell surface represent
sites of assembly and budding of
virions
that can lead to
acquired
immunodeficiency
syndrome (AIDS).
 AIDS IS a condition
in humans in which
the immune system
begins
to
fail,
leading
to
lifethreatening
opportunistic
infections.
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HIV:2
 HIV is a member of the genus Lentivirus, part of
the family of Retroviridae.
 Lentiviruses have many common morphologies
and biological properties.
 Many species are infected by lentiviruses, which
are characteristically responsible for longduration illnesses with a long incubation period.
 Lentiviruses are transmitted as single-stranded,
positive-sense, enveloped RNA viruses.
 Upon entry of the target cell, the viral RNA
genome is converted to double-stranded DNA by
a virally encoded reverse transcriptase that is
present in the virus particle.
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HIV:3
 This viral DNA is then integrated into the
cellular DNA by a virally encoded integrase,
along with host cellular co-factors, so that the
genome can be transcribed.
 Once the virus has infected the cell, two
pathways are possible: either the virus
becomes latent and the infected cell continues
to function, or the virus becomes active and
replicates, and a large number of virus
particles are liberated that can then infect
other cells.
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Routes of HIV Transmission
 The four major routes of transmission are
unprotected sexual intercourse , contaminated
needles, breast milk, and transmission from
an infected mother to her baby at birth.
 Screening of blood products for HIV has
largely eliminated transmission through blood
transfusions or infected blood products in the
developed world.
 HIV primarily infects vital cells in the human
immune system such as helper T cells
(specifically CD4+ T cells), macrophages and
dendritic cells.
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HIV Infection
 HIV infection leads to low levels of CD4+ T cells
through three main mechanisms:
 Firstly, direct viral killing of infected cells;
 Secondly, increased rates of apoptosis in
infected cells; and
 Thirdly, killing of infected CD4+ T cells by
CD8
 cytotoxic lymphocytes that recognize infected
cells. When CD4+ T cell numbers decline below a
critical level, cell-mediated immunity is lost, and
the
body
becomes
progressively
more
susceptible to opportunistic infections.
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Structure and Genome:1
 HIV is different in structure from other retroviruses.
 It is about 120 nm in diameter (120 billionths of a
meter; around 60 times smaller than a red blood cell,
yet large for being a virus) and roughly spherical.
 It is composed of two copies of positive single-stranded
RNA that codes for the virus's nine genes enclosed by a
conical capsid composed of 2,000 copies of the viral
protein p24.
 The single-stranded RNA is tightly bound to
nucleocapsid proteins, p7 and enzymes needed for the
development
of
the
virion
such
as
reverse
transcriptase, proteases, ribonuclease and integrase.
 A matrix composed of the viral protein p17 surrounds
the capsid ensuring the integrity of the virion particle.
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Structure and Genome:2
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HIV Replication Cycle
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Genetic Variability
 HIV differs from many viruses in that it has very high
genetic variability.
 This diversity is a result of its fast replication cycle,
with the generation of 109 to 1010 virions every day,
coupled with a high mutation rate of approximately 3
x 10-5 per nucleotide base per cycle of replication and
recombinogenic properties of reverse transcriptase.
School of Science and Technology, Online Counseling Resource…
What You Learn…
You have learnt :
 Viral replication, therefore, is a process of
separate synthesis of viral components
and assembly of these into new virus
particles
 The various steps of viral replication are
Adsorption, Penetration,Uncoating Viral
genome
replication,
Maturation
and
Release .
 HIV is a retrovirus that can lead to
acquired
immunodeficiency
syndrome
(AIDS)
 HIV differs from many viruses in that it
has very high genetic variability.
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Critical Thinking Questions
1. Describe is in detail the process of
virus replication?
2. What is
virus?
Human
immunodeficiency
3. Write a short note on HIV transmission
and infection.
© 2007, YCMOU. All Rights Reserved.
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School of Science and Technology, Online Counseling Resource…
Hints For Critical Thinking Question
1. process of separate synthesis of viral
components, various steps in virus
replication.
2. Details of HIV, infection, transmission
and ultimate effect.
3. four major routes of transmission and
three main mechanisms of HIV
© 2007, YCMOU. All Rights Reserved.
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School of Science and Technology, Online Counseling Resource…
Study Tips
 Book1
 Title:The Living World
 Author: George Johnson
 Book2
 Title: ABC Of Biology
 Publisher: Holy Faith
 Book3
 Title: Biological Science
 Author: Taylor, Green & Stout
School of Science and Technology, Online Counseling Resource…
Study Tips
www.en.wikipedia.org
Microsoft Encarta Encyclopedia
http://en.wikipedia.org/wiki/
Wikipedia the free encyclopedia
School of Science and Technology, Online Counseling Resource…
End of the Presentation
Thank You !