Structure of Viruses. Prepared By : Abdulrahman M. El-Sha\'er Supervised By : DR. Abdelraouf Elmanama. SIZE & SHAPE. Viruses range from 20 to 300 nm in diameter; this corresponds roughly to a range of sizes from that of the
Prepared By :
Abdulrahman M. El-Sha\'er
Supervised By :
DR. Abdelraouf Elmanama
Viruses range from 20 to 300 nm in diameter; this
corresponds roughly to a range of sizes from that of the
largest protein to that of the smallest cell.
Their shapes are frequently referred to in
colloquial terms, eg, spheres, rods, bullets, or bricks, but in reality they are complex structures of precise geometric symmetry (see below). The shape of virus particles is determined by the arrangement of the repeating subunits that form the protein coat (capsid) of the virus.
The viral nucleic acid (genome) is located internally and can be either single or double-stranded DNA or single- or double-stranded RNA.
Only viruses have genetic material composed of single-stranded DNA or of single-stranded or double-stranded RNA. The nucleic acid can be either linear or circular.
RNA can exist either as a single molecule or in several
pieces. For example, both influenza virus and rotavirus
have a segmented RNA genome. Almost all viruses
contain only a single copy of their genome; ie, they are
haploid. The exception is the retrovirus family, whose
members have two copies of their RNA genome; ie,
they are diploid.
The nucleic acid is surrounded by a protein coat called
a capsid, made up of subunits called capsomers. Each
capsomer, consisting of one or several proteins, can be
seen in the electron microscope as a spherical particle,
sometimes with a central hole.
The structure composed of the nucleic acid genome
and the capsid proteins is called the nucleocapsid. The
arrangement of capsomers gives the virus structure its
The helix can be either rigid or flexible. All human viruses that have a helical nucleocapsid are enclosed by an outer membrane called an envelope, ie, there are no naked helical viruses. Viruses that have an icosahedral nudeocapsid can be either enveloped or naked .
identical protein subunits is 2-fold: (1) it reduces the
need for genetic information, and (2) it promotes self-
assembly; ie, no enzyme or energy is required.
In fact, functional virus particles have been assembled in the test tube by combining the purified nucleic acid with the purified proteins in the absence of cells, energy source, and enzymes.
Viral proteins serve several important functions.
1- outer capsid proteins protect the genetic material and
mediate the attachment of the virus to specific recep-
tors on the host cell surface.
2- This interaction of the viral proteins with the cell receptor is the major determinant of species and organ specificity. Outer viral proteins are also important antigens that induce neutralizing antibody and activate cytotoxic T cells to kill virus-infected cells.
Some of the internal viral proteins are structural (eg,
the capsi d proteins of the enveloped viruses), whereas
others are enzymes (eg, the polymerases that synthesize the viral mRNA).
Some viruses produce proteins that act as "superantigens" similar in their action to the superantigens produced by bacteria, such as the toxic shock syndrome toxin of Staphylococcus aureus .
In ad d ition to the capsid and internal proteins, there are
two other types of proteins, both of which are associated with the envelope. The envelope is a lipoprotein membrane composed of lipid d erive d from the host cell membrane and protein that is virus-specific.
Furthermore, there are frequently glycoproteins in the form of spike-like projections on the surface, which attach to host cell receptors during the entry of the virus into the cell. other protein, the matrix protein, mediates the interaction between the capsid proteins and the envelope.
In general, the presence of an envelope confers instability on the virus. Enveloped viruses are more sensitive to heat, drying, detergents, and lipid solvents such as alcohol and ether than are nonenveloped (nucleocapsid) viruses, which are composed only of nucleic acid and capsid proteins.
that virtually all viruses that are transmitted by the fecal-oral route (those that have to survive in the environment) do not have an envelope, that is, they are nake d nucleocapsid viruses.
These include viruses suchas hepatitis A virus, poliovirus, coxsackievirus, echovirus, Norwalk virus, and rotavirus.
In contrast, envelope d viruses are most often transmitted by d irect contact, such as by bloo d or by sexual transmission.
Other enveloped viruses are transmitted directly by insect bite, eg, yellow fever virus and West Nile virus, or by animal bite, eg, rabies virus.
Many other enveloped viruses are transmitted from
person to person in respiratory aerosol droplets, such as influenza virus, measles virus, rubella virus, respiratory syncytial virus, and varicella-zoster virus.
They therefore can also be transmitted by hands that
make contact with the virus on contaminated surfaces.
The surface proteins of the virus, whether they are
the capsid proteins or the envelope glycoproteins, are
the principal antigens against which the host mounts its immune response to viruses.
Viruses that have multiple serorypes, ie, have antigenic
variants, have an enhanced ability to evade our host defenses because antibody against one serotype will not protect against another serotype.
There are four exceptions to the typical virus as de-
scribed above :
(1) Defective viruses are composed of viral nucleic
acid and proteins but cannot replicate witho,ut a "helper"
virus, which provides the missing function. Defective
viruses usually have a mutation or a deletion of part of
their genetic material. During the growth of most
human viruses, many more defective than infectious
virus particles are produced.
Because these defective particles can interfere with the growth of the infectious particles, it has been hypothesized that the defective viruses may aid in recovery from an infection by limiting the ability of the infectious particles to grow.
viroids replicate but the mechanism is unclear. They cause several plant diseases but are not implicated in any human disease.
Because neither DNA nor RNA has been detected in prions, they are clearly different from viruses . Furthermore, electron microscopy reveals filament rather than virus particles. Prions are much more resistant to inactivation by ultraviolet light and heat than are viruses. They are remarkably resistant to formaldehyde and nucleases. However, they are inacti-vated by hypochlorite, NaOH, and autoclaving.
and other medical supplies that cannot be autoclaved.
Prions are composed of a single glycoprotein with a
molecular weight of 27,000-30,000. With scrapie pri-
ons as the model, it was found that this protein is en-
coded by a single cellular gene. This gene is found in
equal numbers in the cells of both infected and unin-
fected animals. Furthermore, the amount of prion pro-
tein mRNA is the same in uninfected as in infected
cells. In view of these findings, posttranslational modi-
fications of the prion protein are hypothesized to be the
important distinction between the protein found in in-
fected and uninfected cells.
from the normal alpha-helical form (known as PrP c, or prion protein cellular) to the abnormal beta-pleated
sheet form (known as PrP sc, or prion protein scrapie) is the important modification.
The abnormal form then recruits additional normal forms to change their configuration, and the number of abnormal pathogenic particles increases. Although prions are composed only of proteins, specific cellular RNAs enhance the conversion of the normal alpha-helical form to the pathologic beta-pleated sheet form.
There is some evidence that it is one of the signal transduction proteins in neurons and that it is a copper-binding protein.
Prion proteins in infected brain tissue form rod-shaped
particles that are morphologically and histochemically
indistinguishable from amyloid, a substance found in
the brain tissue of individuals with various central nervous system diseases (as well as diseases of other organs).