Reproduction in Viruses, Viral Diseases & Sub-Viral Agents
1. Reproduction in Viruses
Viral reproduction occurs mainly through two primary mechanisms:
(a) Phagic Reproduction
Only the nucleic acid of the virus enters the host cell. It is further categorized into two types:
- Lytic Cycle: Occurs in virulent phages (e.g., $T_4$ bacteriophages). Causes the destruction/lysis of the host cell.
- Lysogenic Cycle: Occurs in temperate viruses (e.g., $\lambda$ phage). Viral DNA integrates into the host genome without immediate cell destruction.
(b) Pinocytic Reproduction
Found in viruses like TMV, HIV, Hepatitis B, etc., in which the whole virus particle enters the host cell (except the outer envelope, if present).
2. Viral Diseases
A. Viral Diseases of Humans
| # | Name of the Disease | Causal Agent |
|---|---|---|
| 1 | Influenza | Influenza virus |
| 2 | Small pox | Variola virus |
| 3 | Mumps | Paramyxovirus |
| 4 | AIDS | Retroviruses (HIV) |
| 5 | Poliomyelitis | Polio virus |
| 6 | German measles | Rubella virus |
| 7 | Measles | Measles virus |
B. Viral Diseases of Plants
| # | Name of the Disease | Causal Agent |
|---|---|---|
| 1 | Tobacco mosaic | TMV |
| 2 | Cucumber mosaic | Cucumber mosaic virus |
| 3 | Potato leaf roll | Potato leaf roll virus |
| 4 | Bunchy top of banana | Banana bunchy top virus |
C. Common Symptoms in Plants
- Mosaic formation
- Leaf rolling and curling
- Yellowing and vein clearing
- Dwarfing and stunted growth
3. Sub-Viral Agents
Sub-viral agents are infectious agents that lack one of the essential components of standard viruses (e.g., lacking either the protein coat or nucleic acid). Examples include Viroids, Prions, and Virusoids.
| Viroids | Prions | Virusoids |
|---|---|---|
| Discovered by T.O. Diener. | Discovered by Alper et al. / Prusiner. | Discovered by Randles et al. |
| Smallest self-replicating particles. | Are abnormally folded proteins. | Are RNA viruses contained inside the capsid of other larger viruses. |
| Infectious RNA particles devoid of protein coat. | Agent is similar in size to viruses. | They replicate within the host cell but do not cause infection alone. |
| Obligate parasites with low molecular weight. | Cause certain infectious neurological diseases like:
|
Require a helper virus for multiplication/replication. |
| Cause diseases in plants only (e.g., Potato Spindle Tuber Disease - PSTVd). | Cause diseases in animals and humans. | Associated with satellite RNAs. |
