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Replication
Learning Objectives:
• To be able to describe the process of
replication of the influenza and HIV
virus
• To be able to describe the course of
infection, signs, symptoms and
transmission if Influenza and AIDS
Replication
• Though the details of virus infection and
replication vary greatly with host type,
all viruses share 6 basic steps in their
replication cycles. These are:
– Attachment
– Penetration
– Uncoating
– Replication
– Assembly
– Release
• This is called lytic replication
• The virus must first attach itself to the host cell.
This is usually accomplished through special
glycoprotiens on the exterior of the capsid,
envelope or tail.
• Next, penetration occurs, either of the whole
virus or just the contents of the capsid. If the
entire capsid enters, the genetic material must
be uncoated to make it available to the cell's
replication machinery.
• Replication of genetic material takes place, as
well as the production of capsid and tail
proteins.
• Once all of the necessary parts have been
replicated, individual virus particles are
assembled and released.
• Release often takes place in a destructive
manner, bursting and killing the host cell.
Lysogenic
• Some viruses, when they invade a cell
do not undergo the full infective cycle.
• They integrate themselves into the
hosts DNA.
• When the cell replicates, it also
replicates the virus.
• When the virus reassembles it re-enters
the lytic phase, releasing new copies of
the virus and rupturing the host cell
Viral DNA
injected into
host cell
Lytic cycle
Lysis of cell
Assembly of
new viruses
Lysogenic
cycle
Viral DNA
integrated into
hosts DNA
• In the lytic phase virus particles infect
host cells and are replicated.
• In the lysogenic phase viral genetic
material that has entered the host cell
becomes incorportated in the cell and
lies dormant. It is then passed on to the
progeny of the infected cells.
• Eventually, the lytic phase will start
again, and cells that were never
infected themselves, but carry the viral
genetic material will begin to produce
new virus particles.
• What are the six steps in virus replication
• attachment, penetration, uncoating,
replication, assembly and release.
• What is contained within the capsid of a
virus?
• The capsid contains the genetic material,
either DNA or RNA, and sometimes
enzymes.
• How is the genetic material of viruses
replicated?
• Viruses have no replication capabilities
themselves, so they must use the
replication machinery of their host cells.
Human Influenza Virus (type A,B or C)
• A type of myxovirus (any of a group of RNA
viruses including those that cause influenza
and mumps)
Influenza A viruses infect a wide variety of
mammals, including man, horses, pigs,
ferrets and birds
• Influenza A viruses are a major human
pathogen, associated with epidemics and
pandemics.
• Influenza B viruses infect humans only
and cause disease, but generally not a
severe as A types
• Influenza C viruses infect humans only,
but are not believed to cause disease.
Method of transmission
• Air-bourne droplets infection from the
coughs and sneezes of infected
individuals
• Infected people are infected from a day
before symptoms appear until a week
afterwards
Organ affected
• The virus invades the epithelial cells of
the upper respiratory tract and
reproduces inside them, killing many
cells in the process.
• The dead cells increase the amount and
thickness of the mucus produced
which irritates the throat causing
coughing.
• Secondary bacterial infection is
common
Incubation time
• Infected people are infected from a day
before symptoms appear until a week
afterwards
• Symptoms usually lasts about 7-10 days,
with the fever and aches improving after 2-3
days and the cough and sore throat lasting
longer.
• However, some people have symptoms
which go on for much longer - weakness and
depression sometimes lasts weeks.
Symptoms
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Fever (usually above 38°C)
Chills
Muscle aches and pain
Severe headache
Weakness and tiredness
Loss of appetite
Sore throat
Cough, and
Chest pain
Treatment
Most people got better on their own with
simple relief for their symptoms. This
includes:
• Rest. The most important thing in flu is to
give your body a chance to heal itself by
resting. Lie down and sleep if possible.
• Regular pain relief with aspirin, paracetamol
or ibuprofen
• Keep well-hydrated with plenty of warm
fluids.
• Remedies for sore throat, headache and
blocked nose.
Prevention
• Vaccination is difficult due to genetic
changes in the influenza virus
• Vaccinations based on a variety of
antigens are used to protect vulnerable
groups
• Prevention would require isolation of
flu victims which is not practical.
Human immunodeficiency virus
(HIV)
• HIV is a retrovirus (ie one
that contains a single strand of
RNA and the enzyme reverse
transcriptase)
Method of transmission
• Through infected semen or vaginal
secretions during sexual activity
• Through infected blood in transfusions
or contaminated needles
• Across the placenta or in breast milk
from infected mothers
Organ affected
• HIV in the blood attaches to cells that
carry the ‘CD4’ antigen. These include T
Lymphocytes and macrophage white
blood cells.
Incubation time
• After entering the cell it becomes a provirus
– that is a form of the virus which is capable
of being integrated into the host genome.
• It is replicated over a period of 8 – 10 years
• Eventually the virus particles are
reassembled and emerge into the blood,
rupturing and killing the T cells in the
process. This leaves the immune system
compromised
Symptoms
• During the latent period there are no
symptoms, but the individual is infectious.
• After this period symptoms begin to appear
that include:
– Tiredness
– Fever
– Weight loss
– Diarrhoea
• This is followed by the more serious
symptoms of AIDS
AIDS
• As the immune system is no longer
functioning there is no defence against a
variety of opportunistic infections
• The most common are:
– Kaposi’s sarcoma
– TB
– Pneumonia
• All of these are usually fatal
Treatment
• There is, as yet, no cure or vaccination
for HIV or AIDS
• Drugs such as AZT can delay the onset
of symptoms
Prevention
• Safe sex
• Use only sterile needles
• Screening of blood products
What is a bacteriophage?
b) What do they infect?
c) Why are bacteriophages important industrially?
d) How do they infect a cell?
e) Why is the life cycle of a bacteriophage
described as lytic?
Do you consider viruses to be alive?
Give reasons to justify your answer.
Read the information on RETROVIRUSES.
a) Give 2 features of all retroviruses
b) Name a retrovirus
c) Why are reverse transcriptase and retroviruses
widely used in genetic engineering?