General morphology of Viruses
Virus particle has one nucleic acid molecule integrated into a protein shell.
Envelope a lipoprotein membrane is present in some plant viruses.
Virion is a complete virus particle with capsid,nucleic acid and envelope.
Protein coat is called capsid.It encloses nucleic acid.
Nucleocapsid is nucleic acid surrounded by capsid.Single protein molecule of capsid is known as structural unit.Structural unit is also referred to as building block.
Capsomere is assembly of protein molecules.Depending on protein subunits present they are called dimers,trimers,pentamers,hexamers.This is used for capsid structure of spherical viruses.
Thus virus particles are aggregates of many protein molecules enclosing nucleic acid.
Structural units in helical viruses are joined together as individuals. They are also the morphological units.
The "virus" name(virus=poison) was given by Louis Pasteur to many causal events of infectious diseases.Later virus was used for microorganisms that pass through candles, gradocol membranes.
Viruses may be classified into several groups according to their shape:
Influenza virus,measles,arbovirus,Japanese encephalitis come in this group.They are 18 to 150 nm in size.
Tobacco mosaic,potato blight viruses come in this group. They are 300 nm in length and 15 nm in width.
It is characteristic for lower plant viruses(phages).They are 47-104 to 10-225 nm in size.
Cowpox virus,smallpox virus,adenoviruses,papilloma viruses,enterovirus fall in this group. They are 30 and 350 nm in size.
Virus sizes are of different sizes.Most viruses are between 10 and 200 nm.Poliomyelitis, yellow fever and foot and mouth disease viruses are small in diameter about 25 nm whereas viruses like Vaccinia(small pox virus) are large in diameter about 250 nm.Most of the viruses are invisible by optical microscope except large viruses. The size of viruses can be known by electron microscopy, ultrafiltration, ultra centrifugal I on.
A virus is made of nucleic acid(DNA or RNA),a wall of protein that has enzymes with which it penetrates into the cells of tissue.The cell wall has been called capsid as it isade of capsomers.
The structural capsomere number in virus's capsid is constant (252 in adenovirus,32 in poliomyelitis, 2000 in tobacco mosaic virus).Nuclepcapsid is nucleic acid enclosed by capsid.
Symmetry is important in structure of virions.Symmetric packings of viruses have only axes of symmetry.Symmetry axis is a straight line on which if a figure rotates to a definite angle it coincides with itself.According to protein subunits packing character, viruses are categorized into 2 types of symmetry: cubic symmetry and spiral symmetry. In spiral,the packing becomes a closed cylindrical pattern. In cubic, the curve lines of the hexagonal subunit layer will be lines at an angle of 120 degrees in relation to one another.Surface elements are equilateral triangles.
Tobacco mosaic virus is thought to typically represent spiral symmetry viruses.It is hollow rod shaped 17 nm in diameter and 310 nm in length. The virus is built of protein subunits. Between protein subunits is single stranded RNA.Viruses of spiral symmetry are male E.Colo cell phages ,influenza and parainfluenza viruses.
Cubic symmetry viruses are polyhedron in shape. They cause herpes,poliomyelitis, papilloma,adenovirus.
There are some viruses which have combined symmetry. They have an intermediate layer(inner capsid) between outer capsid and nucleoid.Virus vaccinia comes in this group.
Viral body volumes decrease in hypertonic solutions such as saccharose and increase in hypotonic solutions.
Intercellular inclusions are formed in many infections of viruses.
Crick and Watson and Kendrew and Crick on their study on TMV put forward theory of virus capsid.Virus capsid is formed by single type of protein molecules which are packed in exact,regular and symmetrical manner.All molecules of protein are identical in structure.
Protein subunits of different viruses are different.
Building block assembly follows geometric principles so that it has a stable state.This type of state comes into existence when building blocks make identical bonds with neighbors. This can be done when protein subunits are arranged helically or in icosahedron. Therefore spheres and rods 2 basic symmetrical arrangements of subunits occur.
Nucleic Acid of Viruses
Genome of viruses is either single stranded or double stranded RNA or single stranded or double stranded DNA.
The genome of RNA may be positive sense when it can function as mRNA directly.
The genome of RNA which is of positive polarity may be monopartite when a single strand of RNA contains all genetic information or multipartite when genetic information is spread over 2 or more than 2 RNA segments.
A.Single stranded positive sense RNA
Most of the plant viruses have single stranded positive sense RNA as genome.
B.Single stranded negative sense RNA
Such genome is present in plant rhabviruses.
C.Double stranded RNA
Such genome is present only in phytoreoviruses.
D. DNA genome
Such genome occurs in caulimoviruses and geminiviruses.
Shape of viruses
Viruses can be grouped into 3 classes based on their shape:
These viruses are also known as elongated viruses. They are helical in symmetry. Their length ranges from 180 nm to 1250 nm.Their diameter is between 10 to 22 nm.These viruses are further divided into rod shaped and flexuous viruses.
These are also known as bullet shaped or bacilliform viruses because their both ends are rounded.
These are known as spherical, polyhedral or icosahedral viruses. Each face of icosahedron is an equilateral triangle with isometrical symmetry.Diameter of viruses is about 25 to 30 nm while some may have 70 to 100 nm diameter.