Question

22. Discuss the criteria used for classifying viruses into families and genera.

23. Compare and contrast the steps of viral multiplication cycles in bacteriophages and animal viruses.

Answer 1

22. Viruses are classified based on morphology, chemical composition, and mode of replication. Human viruses are grouped into 21 families. The criteria used for classifying viruses into families and genera are 1) type and size of nucleic acid 2) capsid shape and size and 3) presence of lipid envelope that is derived from host cell and surround the nucleocapsid.

a) Type and size of nucleic acid: Based on chemical composition, there are DNA and RNA viruses. DNA viruses have DNA as the nucleic acid while RNA viruses have RNA as nucleic acid. RNA viruses, although comprising of 70% of all viruses, have high mutation rate due to error rate of enzymes involved in their replication. RNA may be single or double stranded. Single strand of RNA may be sense (plus) or antisense (minus). The minus strand is a complementary strand to the sense strand and cannot be used for translation. Reverse transcriptase enzyme catalyses formation of plus strand synthesis in these viruses. Single stranded RNA is present in most plant, animal, and bacterial viruses.

Double stranded RNA viruses have segmented genomes that code for the 3 enzymes for RNA replication, 3 major capsid proteins, and numerous small structural proteins. They have both sense and antisense strands. DNA viruses can have single and double stranded DNA.

b) Type of nucleocapsid: viruses can have helical or icosahedral symmetry in a 3D arrangement. In helical symmetry, identical protein subunits are self assembled in a helical symmetry around the nucleic acid. In icosahedral symmetry, there are 60 identical subunits, mostly composed of a single protein. Triangulation number determines the number of subunits that can be incorporated. If T=2, the capsid has 120 subunits. There is minimal surface area: volume ratio but difficult to self assemble.

c) Type of envelope: A glycoprotein envelope may surround the nucleocapsid in many viruses, which is of host cell origin. This envelope has a lipoprotein bilayer with materials of both host cell and viral origin. Lipids are obtained from host cell membrane during viral budding. However, the proteins in cell membrane are replaced by viral proteins. Spikes from such glycoproteins on envelope help to attach to cell surfaces.

23)

Bacteriophage multiplication cycle	Animal virus multiplication cycle
There are three steps in multiplication cycle: adsorption, attachment, and replication.	Steps involved are absorption, penetration, un-coating, replication, assembly and release from host cell
Adsorption of virus on bacterium is reversible and mediated by tail fibers	Adsorption is through specific receptors on the animal cell membrane
There is no fusion with the host cell membrane.	Either the virus envelope fuses with the host cell membrane or there is penetration of virus by endocytosis.
Nucleic acid is injected into host cell through the cell wall into the cytoplasm	Nucleic acid is not injected into host cell cytoplasm
There is no un-coating of the viral nucleic acid	Un-coating involving removal of capsid proteins by enzymes is required for the nuclei acid
Replication occurs with the help of host cell enzymes	Replication is complex, depending on the type of nucleic acid and is highly regulated.
Biosynthesis occurs in cytoplasm	Biosynthesis occurs in nucleus for DNA viruses and cytoplasm for DNA viruses.
Assembly of the phage particle occurs followed by release from the host cell.	Assembly of the different viral component occurs followed by encapsulation. Encapsulation: Capsid is produced first as an empty shell. This shell is then used to encapsulate the nucleic acid.
Host cell is lysed. The viral nucleic acid may also remain integrated in host cell DNA in lysogenic cycle.	These viruses show latency.

Both bacteriophages and animal viruses are specific for the species they invade. Both viruses show adsorption, attachment, and replication. Both viruses can have either DNA or RNA as nucleic acid.