Transcript Viruses - Hodder Education

Viruses
What are they? What do they do?
Martin Rowland
Philip Allan Publishers © 2015
Viruses vary in shape and size
•
Bacteriophage T4, 225 nm long
•
Adenovirus, 90 nm
•
Rhinovirus, 30 nm
•
Tobacco mosaic virus — rod-shaped, 250 × 18 nm
•
Vaccinia virus — brick-shaped or ovoid, 300 nm × 200 nm × 100 nm
•
Ebola virus, 970 nm
•
Poliovirus, 30 nm
bacteriophage
Ebola
adenovirus
Compare with Escherichia coli (a bacterium) 3000 × 1000 nm
and a human red blood cell, 10 000 nm diameter, surface membrane 10 nm thick
Philip Allan Publishers © 2015
What do viruses have in
common?
1 Structure:
● Extremely small
● Each consists of a particle (called a virion)
– outer complex of proteins (the capsid)
– inner nucleic acid core (DNA or RNA)
● In addition:
– some have a lipoprotein envelope outside the capsid
– some contain enzymes associated with entering host cell and replication of nucleic acid
Philip Allan Publishers © 2015
A simple example of viral
structure
● Human papillomavirus (HPV) causes warts in humans
● The capsid contains two types of protein:
– L1 (shown in yellow)
– L2 (shown in red)
● The double-stranded DNA (shown in blue) is circular
Philip Allan Publishers © 2015
What do viruses have in
common?
2 Function:
● Viruses have no metabolism of their own, e.g. they do not use ATP and are unable to
produce proteins
● Viruses can only be replicated using the metabolism of another living cell (the host)
– In doing so they cause harm to the host cell
● Outside a suitable host cell, viruses are inert
Philip Allan Publishers © 2015
Five different viruses
● Lambda bacteriophage (λ phage)
● Tobacco mosaic virus (TMV)
● Ebola virus
● Human immunodeficiency virus (HIV)
● Influenza (‘flu’) virus
Philip Allan Publishers © 2015
Lambda bacteriophage
(λ phage)
● Capsid has a head and a tail region
● Nucleic acid is double-stranded DNA,
which is transcribed by the host cell into mRNA
● Infects the bacterium Escherichia coli
Philip Allan Publishers © 2015
Tobacco mosaic virus (TMV)
● Capsid is a spiral of polypeptides
● Nucleic acid is single-stranded RNA, which is transcribed by host cell to form mRNA
● Infects a wide range of plants, especially tobacco and other members of the Solanaceae
family
Philip Allan Publishers © 2015
Ebola virus
● Capsid of protein
● Nucleic acid is single-stranded RNA, which is transcribed by the host cell to form mRNA
● Infects several types of human cell
Philip Allan Publishers © 2015
Human immunodeficiency virus
(HIV)
● Capsid surrounded by phospholipid and glycoprotein envelope
● Nucleic acid is single-stranded RNA, which is transcribed inside the host to form
single-stranded DNA
● Enzymes include reverse transcriptase
● Infects macrophages and T helper cells of humans
Philip Allan Publishers © 2015
Influenza virus
● Capsid surrounded by phospholipid and
glycoprotein envelope
● Eight short, single-stranded,
negative-sense RNA fragments
that are transcribed by the host cell
to form mRNA
● Infects epithelial cells of
the nose, throat and lungs
of mammals
Philip Allan Publishers © 2015
How does a virus infect a cell?
● Proteins on the surface of the virus are complementary to specific proteins (or
glycoproteins) on the surface membrane of a target cell
● For example, HIV attaches to a protein called CD4 found on the surface of T lymphocytes
● If a random collision is perfectly aligned, the viral protein binds to the protein or glycoprotein
on the surface of the target cell
● The viral nucleic acid then enters the target cell
Philip Allan Publishers © 2015
What does the viral
nucleic acid do?
Either:
● Becomes attached to the DNA of the host cell and remains dormant for a period of time
● This period of time is called latency
● Infection by Varicella zoster during childhood causes chickenpox. The virus can show
latency in nerve cells and, in adulthood, reappear as shingles
● Herpes virus (cold sores and genital herpes) can also show latency in nerve cells
Philip Allan Publishers © 2015
What does the viral
nucleic acid do?
or:
● Takes over control of the host cell causing it to
– replicate the virus
– produce new viral capsids
– assemble new virions
– release the new virions
Philip Allan Publishers © 2015
The lytic cycle of the λ phage
Philip Allan Publishers © 2015
‘Life’ cycle of the flu virus
Viral antigens attach to
receptors on lung epithelial
cells and trigger endocytosis
New virus particles leave host
cell, picking up a lipid coat in
the process
Viral RNA and proteins
assembled by host cell to
form new virus particles
Philip Allan Publishers © 2015
Viral RNA enters nucleus of
epithelial cell where it is
transcribed and replicated
Copies of viral RNA leave
nucleus and are translated by
host cell’s ribosomes
How can viral infections
be treated?
● Since viruses have no metabolism, they cannot be ‘killed’
● Vaccines against one or more viral antigens are effective in reducing the risk of infection
● Antivirals prevent one or more stages of the viral life cycle. For example:
– prevent binding of the virus to the target cell
– inhibit virus-specific enzymes, e.g. those that allow the virus to enter the target cell, allow
virus nucleic acid to become incorporated into host DNA, or allow replication of viral
nucleic acid
● Prevention of spread is a key strategy
Philip Allan Publishers © 2015