Virus

Subcellular parasite with genes of DNA or RNA and which replicate inside host cell

2 forms:

1) Virion - inert virus particle that exists outside the host cell. Composed of capsid (protein coat) and nucl. acid. Nucleic acid is DNA or RNA, circular or linear, double- or single-stranded

2) Intracellular form - either lytic or lysogenic

Life Cycle

1) Attachment

2) Entry of viral genome into host cell

3) Replication

4) Synthesis of viral proteins (may or may not require transcription)

5) Assembly

6) Release (may or may not kill the host cell; e.g. M13 doesn't kill)

Entry of virus genome

1) Animal viruses - virion has membrane (taken from last host) that fuses with host membrane; capsid containing the nucl. acid enters the cell; capsid proteins disassemble, freeing the nucl. acid into the cell

2) Bacteriophages - inject nucl. acid into cell. Commandeering of host machinery

Examples of Bacteriophages

1) Qb - linear ssRNA

3 genes - lysis protein, coat protein and replicase

Enters cell, is translated to produce replicase which replicates the RNA by making the complementary (-) strand and then using them as the template for (+) strand synthesis

2) fX174 - circular ssDNA

11 genes, 5 of which overlap

Enters cell, synthesis of (-) strand to create replicative form (RF), then transcription and translation

M13 is another example of circular ssDNA but is released from cell without killing the cell so is useful in bioengineering

3) Lambda, P1, T4, etc - linear dsDNA

May have many genes (T4 has almost 200)

Enters cell, cirularization (usually), replication, transcription, translation

Examples of Plant Viruses

1) Cauliflower mosaic virus (CMV) - circular dsDNA

Infects Brassicaceae (cauliflower, broccoli, cabbage, etc) and causes leaf blotches

(hence called mosaic)

2) Tobacco mosaic virus (TMV) - ssRNA

Most widespread plant virus

Infects tobacco, tomatoes, beets, peppers, etc.

Viroids

Naked piece of circular ssRNA with secondary structure to form a rod-shaped RNA. 10-100x smaller than an RNA virus. Infects plants but only if membrane is injured

Contains no genes, just a signal for replication by the host machinery

May cause disease by binding to a host RNA (7S RNA) that is part of the protein translocation machinery so that abnormal membranes are produced

Examples

1) Potato spindle tuber viroid - first viroid discovered (1967)

2) Coconut cadang-cadang viroid - small (246 bases) but most virulent

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SIUC / College of Science / Microbiology / courses / MICR302

URL: http://www.micro.siu.edu/micro302/virus.html
Last updated: 9-April-98 / laa