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Viruses

Viruses are 20-300 nm, a cellular living organisms as they are generally made up of a core of genetic material (nucleoid) of only one type of nucleic acid, either DNA (mostly double stranded but in some cases single stranded) or RNA (mostly single stranded but in some viruses double stranded), that is surrounded by a protective protein coat (capsid) and lack the cellular structures like cytoplasm, various organelles, etc. The capsid is made up of capsomeres the number of which varies from 12 (bacteriophage фXl 74) to 2 130 (plant virus TMV). They may contain a few enzymes, e.g., lysozyme that helps in the penetration of host cell wall, reverse transcriptase in some RNA viruses (e.g., retroviruses with single stranded RNA) to produce temporary viral DNA that integrates into the host DNA and is required for the formation of genomic RNA as well as the mRNA.

In some viruses additional envelope may be present around the protein coat.

 

They are obligate (strictly) parasites and require a host cell for their multiplication where they neither divide nor grow in size and their various components are formed independently that assemble together to produce the daughter viruses. The viruses have very little water and they neither respire nor have any independent metabolism (like synthesis of proteins, nucleic acids, etc.) and the antibiotics are not effective against them. Outside the host cell (in cell free environment) they are found in the inactive state and become active only after entering their host cell, where they utilize the synthetic machinery of the host cell. Therefore, they cannot be cultured in cell-free medium (can be cultured only in the presence of their host cells).

 

The viruses have been classified in many ways depending upon their specificity to hosts, their structure, nature of nucleic acid, etc. On the basis of the specificity to their hosts the viruses have been classified into different groups:

 

Bacteriophages

 

They infect bacteria, like E. coli, and contain generally double stranded DNA (e.g., 1’2, T4 bacteriophages) or rarely single stranded DNA (bacteriophage ф X 1 74 and M l 3 with circular DNA) or single stranded RNA (bacteriophages R-17, MS-2) as their genome.

 

Their body generally has a polyhedral (commonly icosahedral with 20 faces, but may be

Tetrahedral with 4 faces or dodecahedral with 1 2 faces) head (bacteriophages ф XI74, A., T1, T5) that may be associated with a helical tail (e.g., bacteriophages 1’2. T4) or it may be filamentous

(bacteriophage M l3) in shape.

 

The T2 bacteriophage infects E. coli and bas an icosahedral (20-sided) head (with 95 nm length and 65 nm breadth) and a helical tail (115 nm long and 17 nm wide). Around 2000 capsomeres of head enclose a double stranded DNA molecule as the nucleoid. The tail has a contractile sheath consisting of 144 capsids that are arranged helically and enclose a core tube. This tail contains the enzyme lysozyme that helps in the penetration of the host cell wall through which the phage DNA is passed into the host cell. A proteinaceous neck with circular collar is found between the head and the tail. The basal part of the tail is occupied with a base plate that is made up of protein that bears 6 spikes (tail pins) and 6 long tail fibres at the comers. The tail fibres remain folded in the free form but spread out at the time of infection to help the attachment of the bacteriophage at the cell wall of the bacterium (Fig. 1 .44).

For multiplication, the T2 bacteriophage lands on the host cel1 wall with the help of their tail fibres and then the tail pins make contact with the cell wall. Ultimately, when the tail comes in contact with the bacterial cell, the tail sheath contracts and the tail core tube passes through a central hole in the base plate into the cell wall and the DNA is injected into the host cell (Fig. 1 .45). The bacterial DNA is then broken down by the phage coded nucleases and the phage utilizes the host’s protein synthesis machinery (ribosomes, RNA polymerase, etc.) to synthesise its own proteins. The injected phage DNA replicates to produce many copies of the phage genome that gets incorporated into the newly synthesized protein capsids to produce progeny phages. The progeny phages are released by sudden lysis (bursting) of the host cell that occurs within 20-60 min of infection. The individual progeny phages may again infect a fresh host cell and this multiplication cycle is known as lytic cycle (Fig. 1 .46).

Fig. 1.45 Attachment of T2 phage and injection of its DNA into the host cell.

Fig. 1.46 Life cycle ofT2 bacteriophage.

 

Some bacteriophages, like A.-phage, mainly have the lysogenic cycle. In the lysogenic cycle, the phage DNA after being injected into the host cell gets integrated into the bacterial chromosomal DNA (and in this form the phage is known as the prophage that is noninfectious) that replicates along with the host chromosome. In this form the phage is called temperate phage and the host bacterium is known as lysogenic bacterium. Sometimes, the prophage enters the lytic cycle when the prophage accidentally gets detached from the host chromosome to produce and release progeny phages by bursting the host cell (Fig. 1 .47).

Fig. 1.47 Structure (top) and life cycle of A.-phage.

 

Cyanophages

 

They infect cyanobacteria (blue-green algae) and most of them have the stmcture similar to the bacteriophages with icsahedral head, with or without a tail. Examples of some of the cyanophages are LPP-1 (infecting Ly nbya, Plectonema and Phormidium), SM-1 (hosts Sy nechococcus and Microcystis), N-1 (infects Nostoc muscorum), etc.

 

Plant Viruses

 

Plant viruses usually have single stranded RNA as their nucleoid (e.g., TMV) some have double stranded DNA as the genetic material, e.g., CaMV (Cauliflower Mosaic Vi