Virus structure!

Virus
A virus is a sub cellular organism with a parasitic intracellular life cycle. It contains either RNA or DNA but not both. A virus is a non-living thing since it does not meet the requirements of a living thing. But once it has a host, it starts to infect the host. A virus is made up of a virion which contains nucleic acid genome surrounded by protein. The basic objective of a virion is to transfer the viral genome to a cell where it starts to replicate. This requires 2 things which are structures to contain and protect nucleic acid genome and specific receptors on the virion surface which allows virus to enter the target cell.

Classification of virus

Virus Structure - Capsid
The capsid is the protein structure surrounding the viral genome which is assembled around the nucleic acid. The whole part is called nucleocapsid. The use of small protein subunits reduces amount of genetic code capacity that has to be dedicated to produce capsid proteins which are crucial as big viral genomes are very small by cellular standards. Protein components naturally align in the energetically favourable state thus certain structure is preferred. These structures have mostly icosahedral or helical symmetry.

Icosahedral Helical

Envelope
The envelope is derived from membranes of the host cell. Envelopment is roughly a passive process of picking up membrane from the cell. Before envelopment takes places, specific changes occur to the membrane. Changes to membrane fluidity is resulting from preferential incorporation of specific lipids. The most apparent change is presence of protein spikes or fringe when virion is viewed by electron microscope. The viral proteins have to be present in order to platform specific virus functions like binding to host cell.

Viral Genome Size
A large viral genome is still small by cellular standards means that virus can produce many proteins allowing a complex structure whereas viruses with small genome are restricted. Therefore bigger viral genome means that the complexity of the structure is more than those with small viral genome.

Viral genome types
• Double stranded(ds) DNA – These are often among largest of viral genomes
• Single stranded (ss) DNA – These are usually small genomes
• Ds RNA- Compared to all RNA genomes these are smaller than most DNA genome
• Ss RNA - Can be subdivided into (+) & (-) sense. Positive sense can function as mRNA and those that are complementary to mRNA are negative sense.

Viruses with RNA genomes use DNA intermediate stage to produce the RNA genome.
Viruses with DNA genomes use RNA intermediate stage to produce DNA genome

Virus cycle
Attachment
Entry
Uncoating
replication and expression
Assembly
Exit

Replication of Virus
A virion must protect the viral genetic material and it has to be capable of delivering it into host cell. The surface of any virus must have receptor/effectors to bind to the host cell. Binding is mediated on one side by specific viral proteins and the other side by cellular structures. The progeny virus genomes assemble into new viruses and are used to make more viruses.