Short essay on Viral Reproduction

(A) Lytic cycle:

T₄ bacteriophage is binal virus which attaches to the surface of bacterium E. coli by its tail fibres. Lysozyme creates a hole in the host cell wall. The tail sheath contracts and ejects the viral chromocome/DNA into the bacterium. Viral DNA controls the metabolic machinery of the host cell.

It produces nucleases. Nuclease degrades DNA and m RNAs of the host. Viral DNA is not affected as its cytosine bases are methylated (huydroxy methylated cytosine or HMC). A number of copies of viral DNA are produced followed by synthesis of capsid proteins, polyamines and lysozyme. The components assemble and form phage particles or viruses. The hose cell ruptures or releases.

(B) Lysogenic Cycle:

Lambda phage (X) attaches to the surface of E. coli by means of tail, which produces a hole in host cell wall and injects the phage DNA. Phage DNA produces a repressor (CI), becomes nonvirulent or temperate and attaches to bacterial chromosome at a specific site (e.g. galctose locus in X phage) by means of enzyme intergrase. The viral genome is now called prophage/provirus.

It multiplies along with bacterial genome and is passed on to the progeny. The phenomenon of existence of nonvirulent prophage in the host is called Lysogeny. The host cell is called Lysogenic cell. Occasionally the synthesis of repressor is stopped. The temperate/ nonvirulent phage is now changed to lytic/virulent phage.

The single strand DNA of Ф x 174 or coliphage fd is known as ‘plus strand’. It forms its complementary or ‘negative strand’. The double strand or ‘replicative’ DNA takes over the metabolic machinery of host of synthesise plus strand DNAs and proteins for assembly of new phages.

(C) Pinocytic Reproduction:

The whole virus enters the host cell except the envelope. It is quite common in RNA viruses which are of two types:

(a) RNA-RNA Viruses:

DNA has no role in their multiplication. After entering the host cell, the viruses produce enzyme replicase which helps in producing more genetic RNA genome. The latter also produces m RNAs for synthesis of viral proteins.

(b) RNA-DNA Viruses:

retroviruses (Temin, 1970) e.g. Tumour/Cancer Viruses, HIV. The viruses possess enzyme reverse transcriptase (Temin & Baltimore, 1972, in Rous Sarcoma Virus or RSV of Mouse). The enzyme builds DNA over RNA genome. The phenomenon is called reverse transcription or feminism. The copy DNA (c DNA) builds its complementary strand. The double strand copy DNA attaches to host DNA/chromosome and is now called provirus. It produces chemicals which disturb cellular machinery.

The cell with provirus may dedifferentiate and start dividing leading to cancer (uncontrolled growth forming tumour which sends branches/roots to invade different parts). Some 20 viral oncogenes (genes that trigger cancer) have been recorded. However, no virus has been definitely proved to cause cancer in humans. Humans have some proto-oncogenes (similar to viral oncogenes) which take part in growth but under certain circumstances they get changed to cellular oncogenes.

Jumping genes/transposons can also cause cancer. There are a number of chemicals, physical factors and some viruses (i.e., Hepatitis B, Herpes. Papiloma, Epstein-Barr Virus) which promote Cancer.

HIV (Human Immunodeficiency Virus) destroys immune system of the body and causes AIDS. The provirus ultimately dissociates, transcribes viral RNA genomes, produces reverse transcripture and viral proteins. Different parts assemble to form retroviruses. The latter come out through exocytosis or pass out to other cells through protoplasmic connections.