Virus Replication

Genomes of RNA or DNA viruses exist in a considerable variety of sizes and shapes, from small molecules of single-stranded RNA or DNA to large double-stranded molecules that may be linear or circular. Whatever their physical nature, viral RNA or DNA molecules must be replicated efficiently within an infected cell to provide genomes for assembly into progeny virions. Steps in the Replicative Cycle of viruses are still composed of sic steps:

Attachment/Adsorption - Penetration - Uncoating - Biosynthesis - Assembly - Release - Maturation

1) ATTACHMENT / ADSORPTION
Virus attachment consists of specific binding of a VIRAL ATTACHMENT PROTEIN (VAP) to a cellular RECEPTOR. Many examples of virus receptors are now known. Receptor molecules may be proteins (usually glycoproteins - specific molecules), or the sugar residues present on glycoproteins or glycolipids (less specific). Some complex viruses (e.g. Poxviruses, Herpesviruses) may have more than one receptor/receptor-binding protein, therefore, there may be alternative routes of uptake into cells. Specific receptor binding can be side-stepped by antibody-coated virus particles binding to Fc receptor on the surface of monocytes, which results in virus uptake.
The expression (or absence) of receptors on the surface of cells largely determines the TROPISM of most viruses, i.e. the type of cell in which they are able to replicate - important factor in pathogenesis. Attachment is in most cases a reversible process - if penetration does not ensue, the virus can elute from the cell surface. Some viruses have specific mechanisms for "detachment" e.g. Influenza neuraminidase protein. BUT: elution from cell often leads to changes in the virus which decrease or eliminate the possibility of attaching to other cells.

2) PENETRATION
Unlike attachment, viral penetration is an energy-dependent process, i.e. the cell must be metabolically active for this to occur. Three mechanisms may be involved:
TRANSLOCATION of the entire virion across the cell membrane, ENDOCYTOSIS of the virus into intracellular vacuoles; eventually into the cytoplasm. FUSION of the viral envelope with the cell membrane. Requires the presence of a viral fusion protein in the virus envelope, e.g. influenza haemagglutinin; retrovirus envelope glycoprotein. These mechanisms are shown in three different infection cycles for RNA or DNA viruses. Please click on these icons:

3) UNCOATING
A general term for the events which occur after penetration, in which the capsid is removed and the virus genome exposed, usually in the form of a nucleoprotein complex. This might be relatively simple in structure, e.g: Picornaviruses have a small basic protein of ~23 amino acids (VpG) covalently attached to the 5' end of the vRNA genome; or highly complex: Retrovirus cores are highly ordered nucleoprotein complexes which contain, in addition to the diploid RNA genome, the reverse transcriptase enzyme responsible for converting the viral RNA genome into the DNA PROVIRUS. This process occurs inside the core particle. For viruses which replicate in the cytoplasm, e.g. Picornaviruses, the genome is simply released into the cell, but for viruses which replicate in the nucleus, e.g. Herpesviruses, the genome, often with associated nucleoproteins, must be transported through the nuclear membrane. This is achieved by interactions of the nucleoproteins (or capsid) with the cytoskeleton. At the nuclear pores, the capsid is stripped off, and the genome passes into the nucleus.

4) BIOSYNTHESIS: GENOME REPLICATION & GENE EXPRESSION
The replication strategy of the virus depends on the nature of its genome. Viruses can be classified into seven (arbitrary) groups:
I: Double-stranded DNA (Adenoviruses; Herpesviruses; Poxviruses, etc)
II: Single-stranded (+)sense DNA (Parvoviruses)
III: Double-stranded RNA (Reoviruses; Birnaviruses)
IV: Single-stranded (+)sense RNA (Picornaviruses; Togaviruses, etc)
V: Single-stranded (-)sense RNA (Orthomyxoviruses, Rhabdoviruses, etc)
VI: Single-stranded (+)sense RNA with DNA intermediate in life-cycle (Retroviruses)
VII: Double-stranded DNA with RNA intermediate (Hepadnaviruses)

Gene expression Control of viral replication is achieved by tight regulation of gene expression. The methods used depend on nature of the virus genome/replication strategy, e.g: Segmented genomes are usually transcribed to produce monocistronic mRNAs. One advantage of monocistronic mRNAs is that various proteins can be produced in different amounts, rather than in a constant ratio. Non-segmented genomes tend to produce polycistronic mRNA, which is translated to form a polyprotein, processed by proteolytic cleavage to form the mature gene products. To utilize the cellular machinery, viral mRNAs must contain control signals which are recognized by the cell, e.g. ribosome-binding sites, splice signals, polyadenylation signals. Similarly, some DNA viruses, e.g. Papovaviruses, encode a protein which binds to the origin of replication and stimulates cellular DNA polymerase to replicate the virus genome. "Intermediate" viruses, e.g. Adenoviruses, encode their own DNA polymerase, but are still dependent on other cellular factors for replicatition. "Complex" viruses, e.g. Herpesviruses, encode a large number of proteins involved in DNA synthesis, and are largely independent of the cellular machinery. (Animated replication of Herpex Simplex

5) ASSEMBLY
Involves the assembly of all the components necessary for the formation of the mature virion at a particular site in the cell. During this process, the basic structure of the virus is formed.
The site of assembly varies for different viruses, e.g: Picornaviruses, Poxviruses, Reoviruses - In the cytoplasm. Adenoviruses, Papovaviruses, Parvoviruses - In the nucleus. Retroviruses - On the inner surface of the cell membrane.

6) RELEASE
For lytic viruses (most non-enveloped viruses), release is a simple process - the cell breaks open and releases the virus. Enveloped viruses acquire the lipid membrane as the virus buds out through the cell membrane. Virion envelope proteins are picked up during this process as the virus is extruded. Budding may or may not kill the cell, but is controlled by the virus - the physical interaction of the capsid proteins on the inner surface of the cell membrane forces the particle out through the membrane:

7) MATURATION
The stage of the life-cycle at which the virus becomes infectious. Usually involves structural changes in the particle, often resulting from specific cleavage of capsid proteins to form the mature products, which frequently leads to a conformational change in the capsid, or the condensation of nucleoproteins with the genome. For some viruses, assembly and maturation are inseparable, whereas for others, maturation may occur after the virus particle has left the cell.