Cellular Life
Introduction
The life form which exists without a cellular structure is known as acellular or non-cellular life. The primary candidates for non-cellular life are viruses. The majority of biologists consider viruses to be nonliving because they are not capable of autopoiesis (the ability of reproduction) without a host.
The other examples of acellular life are viroids which are the smallest infectious agents consisting solely of short strands of circular single-stranded RNA without protein coat. The prions are infectious agents composed entirely of protein, capable of multiplying itself and transferable from one host to another.
5.1 Viruses Discovery and Structure
A virus is a biological agent that reproduces only inside the cells of a living host. Viruses can infect all types of life forms i.e., from animals and plants to microorganisms including bacteria.
In 1884 the French microbiologist Charles Chamberland made a filter paper for filtration of bacteria. In 1892, Russian biologist Ivanovsky used this filter to determine the cause of tobacco mosaic disease.
In his experiment, he proved that tobacco mosaic disease was not caused by bacteria but caused by another infectious agent that can pass through filter paper. He called these filterable viruses. His view was confirmed by American virologist W.M. Stanley in 1935 when he observed the tobacco mosaic virus under an Electron Microscope.
In the early 20 century (1915, 1917) Twort and Herelle discovered bacteriophages (viruses that infect bacteria). Since then thousands of species of viruses have been discovered and microbiologists speculate that there are millions of species of
viruses still to be discovered.
5.1.1 Viruses Living or Non-Living
Viruses show the characteristics of both living and non-living characteristics of viruses include: They have their own genetic material. They undergo mutation. Can reproduce inside the host cell by using host metabolic machinery. Get destroyed by ultraviolet radiations and Occur in different varieties or strains.
The non-living characteristics of viruses are non-cellular particles. Generally lack enzymes and the living mes and depend upon host enzymes and coenzymes for their metabolic activities. Can be crystallized and stored in a laboratory.
Do not respire and use the energy of the host for their activities. Therefore, depending upon the ambivalent(fluctuating) nature of characteristics possessed by the Viruses; they are considered on the boundary line between living and nonliving things.
5.1.2 Classification of Viruses
Viruses may be classified based on morphology, type of host they infect, presence or absence of outer covering, and types of nucleic acid. Classification of viruses based on structure (morphology)
1. based on capsid:
Some viruses have helical capsids such as tobacco mosaic virus (TMV).
Many have polyhedral capsids, that contain a glycoprotein spike at each vertex, such as adenovirus. Viruses possess an outer envelope studded with glycoprotein spikes, such as Influenza viruses. Viruses like bacteriophages possess complex capsids consisting of a polyhedral head and tail apparatus.
Based on genome (DNA and RNA):
Double-stranded DNA viruses (dsDNA Viruses). e.g., Adenoviruses, Herpes viruses, Pox viruses. Single-stranded DNA viruses (ssDNA Viruses), e.g., Parvoviruses (small viruses of vertebrates and invertebrates) cause a rash.
Double-stranded RNA viruses (dsRNA viruses), e.g., Reoviruses, cause diarrhea.
Single-stranded RNA reverses transcribing viruses template for DNA (SSRNA-RT viruses), e.g., HIV (retrovirus).
Classification of viruses based on the host they infect:
Bacteriophages attack bacteria and plant viruses which cause more than 2,000 types of plant including Potato yellow dwarf viruses. Animal viruses cause many diseases to animals and foot disease in livestock, papovavirus causes mumps and viruses.
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