VIRUS

VIRUS

DEFINITION:

A virus (meaning a toxin or poision) is a small infectious agent that replicates in the cell of  an organism.

The causative agent of an infectious disease any of a large group of submicroscopic infective agents that are regarded either as extremely simple microorganisms or as extremely complex molecules, that are capable of growth and multiplication only in living cells, and that cause various important diseases in humans, animals, or plants.

VIRAL SIZES:

·         Smallest 0.02µm, 20 nanometers (polio virus)

·         Largest 0.3µm, 300 nanometers (smallpox virus)

HOST RANGE:

• Bunyaviruses: animals and plants
• Partitiviruses: plants and fungi
• Reoviruses: animals and plants
• Rhabdoviruses: animals and plants
• Phycodnaviruses: protozoa and plants
• Picornavirus-like viruses: plants and animals
• Totiviruses: protozoa / fungi and insects – tentative

CHARACTERISTICS OF VIRUS:

Living characteristics of viruses

a.       They reproduce at a fantastic rate, but only in living host cells.

Nonliving characteristics of viruses

a.       They are acellular, that is, they contain no cytoplasm or cellular organelles.

b.      They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell.

c.       The vast majority of viruses possess either DNA or RNA but not both.

GENERAL CHARACTERISTICS:

a.       Viruses are a cellular, non-cytoplasmic infectious agents.

b.      They are smaller than bacteria, and this can pass through bacteriological filter.

c.       Viruses are transmissible from disease to healthy organisms.

d.      All viruses are obligate parasites and can multiply only within the living host cells.

e.       Viruses contain only a single type of nucleic acid either DNA or RNA.

f.       Viruses are host specific that they infect only a single species and definite cells of the host organisms.

g.      Viruses are effective in very small doses. They are highly resistant to germicides and extremes of physical conditions.

HELICAL VIRUS

CLASSIFICATION OF VIRUS:

Virus can be classified on following chracteristics:

Ø  NUCLEIC ACID:

·         ss DNA (single stranded Deoxyribonucleic acid)

·        

ENVELOPED

ds DNA (double stranded Deoxyribonucleic acid)

·         ss RNA (single stranded ribonucleic acid)

·         ds RNA( double stranded ribonucleic acid)

Ø  MORPHOLOGY:

·         Helical

·         Polyhyderal

·         Enveloped

·         Complex

Ø  REPLICATION:

·         Lytic cycle

·         Lysogenic cycle

STRUCTURE AND MORPHOLOGY:

HEAD: is protein membrane stuffed with molecule of either DNA or RNA, consists of two parts CAPSID and ENVELOP.

COLLAR: base of the head, conecting head and tail.

HELICAL SHEATH: protein covering surrounding the hollow core.

COMPLEX VIRUS

TAIL: plays a vital role in the attachment to the host cell and the transfer of the viral DNA during the infectious process.

In the lysogenic cycle, the virus reproduces by first injecting its genetic material, indicated by the red line, into the host cell's genetic instructions

HEXAGONAL PLATE: anchors the virus on the cell surface, and help to extrude the viral DNA through the hollow core into the cell.

REPRODUCTION IN VIRUS:

Viruses uses their host cell to produce their copies.

Virus replicates by two different methods:

Ø  LYSOGENIC CYCLE

Ø  LYTIC CYCLE

LYSOGENIC CYCLE

The lysogenic cycle is complementary to the lytic cycle for viral entry and reproduction within cells. While the lytic cycle is common to both animal viruses and bacterial phages, the lysogenic cycle is more commonly found in animal viruses.

The following are the steps of the lysogenic cycle:

1) Viral genome enters cell

2) Viral genome integrates into host cell genome

3) Host cell DNA polymerase copies viral chromosomes

4) Cell divides, and virus chromosomes are transmitted to cell's daughter cells

5) At any moment when the virus is "triggered", the viral genome detaches from the host cell's DNA and enters stage 2 of the lytic cycle. While it is unclear as of yet what exactly constitutes a "trigger" that activates the viral DNA from the latent stage entered in Step 4, common symptoms that appear to "trigger" the viral DNA are hormones, high stress levels (adrenaline), and free energy within the infected cell.

An example of a virus that enter the lysogenic cycle is herpes, which first enters the lytic cycle after infecting a human, then the lysogenic cycle before travelling to the nervous system where it resides in the nerve fibers as an episomal element. After a long period of time (months to years) in a latent stage, the herpes virus is often reactivated to the lytic stage during which it causes severe nervous system damage.

LYTIC CYCLE

lytic cycle is a viral replication cycle in which a virus takes over a host cell's genetic material and uses the host cell's structures and energy to replicate until the host cell bursts, killing it. 
-A phage reproductive cycle that results in death of host cell.

-A virulent phage: a phage that reproduces only by a lytic cycle.

Step 1: A phage binds its tail receptors to receptor cells on the outside of a cell.

Step 2: Part of its tail contracts and allows the phage to enter the cell. The cell's DNA becomes hydrolyzed.

Step 3: By using the cell's resources, the phage produces proteins and copies of itself.

Step 4: Three separate sets of proteins become individual phages.

Step 5: The phage produces an enzyme that destroys the cell's bacterial wall and allows fluid to

enter. This causes the cell to burst and release 100+ phage particles.

Figure

A generalized representation of the replication of two viruses. Replication of a DNA virus is shown in (1); replication of an RNA virus is displayed in (2).