Acellular Organisms

 CH. 2 SYSTEMATICS OF LIVING ORGANISMS

2.14 Acellular Organisms:

    Viruses, Viroids and Lichens

a. Viruses

•The name virus was first used by Louis Pasteur.

•Virus means venom or poison.

•Viruses are obligate parasites.

•They were given the name 'virus' by M. W. Beijerinck, after observation that they were able to migrate in an agar gel.

•Thus, being and infectious soluble agent, he called the filtrate as ‘contagium vivum fluidum’.

•W. M. Stanley demonstrated that

–Viruses are inert outside the host cell and

–They can be crystallized.

–They are made up of proteins.

•Viruses lack their own cell machinery.

•They have protein coat (capsid) around nucleic acid strand.

–Thus considered to be acellular organisms.

•Viruses are inactive outside a host cell; but once they enter their specific host cell, they take charge of cellular machinery of host cell and duplicate themselves.

•Viruses thus can be called infectious nucleoprotein particles.

Types of viruses :

•As per genetic material, viruses are grouped as DNA or RNA viruses.

–Do you know ?

•Viruses have either DNA or RNA as their genetic material but never DNA as well as RNA.

•Protein coat called capsid.

•Capsid is made up of smaller units, the capsomeres.

•Capsomeres are arranged in polyhedral or helical forms.

•Capsid protects genetic material.

•The genetic material in viruses is either

–single-stranded RNA or

–single or double stranded RNA or

–double-stranded DNA.

•Viruses that infect bacterial cells are called bacteriophages.

–They normally have double stranded DNA.

•Viruses cause disorders in plants and animals as well as in humans

–Disorders in plants like-

•leaf curling,

•yellowing,

•mosaic formation etc.

–Disorders in animals like –

•foot disease and

•mouth disease

•swine flu etc.

–Disorders in Humans –

•Viruses are the causative agents of many diseases in humans.

–Small pox,

–mumps,

–Herpes,

–common cold,  etc.

•The list includes AIDS too!

b. Viroids:-

•Potato spindle tuber disease was found to be caused by single stranded RNA which lacks protein coat.

•T. O. Diener in 1971 reported that this is

–low molecular weight RNA and

–smaller in size than viruses.

•These infectious RNA strands are called viroids.

c. Lichens:

•Lichen is co-existence of algae and fungi for mutual benefit.

•Algal member is called the phycobiont.

–mostly belongs to

•cyanobacteria (blue-green algae) or

•green algae.

•Fungal member is called mycobiont.

•They are excellent example of symbiosis.

•The algal component of lichens provides food to fungal part while fungus provides shelter to alga and also absorbed water and minerals to alga.

•The association is intense and it is difficult to identify them as separate living beings.

•Though found in extreme environments like snow clad poles, lichens are sensitive to pollution.

•They are not found in polluted regions.

–Hence are considered as pollution indicators.

•Lichens also play important role in soil formation by using specific acid productions.

•Prions :

•In modern medicine, certain infectious neurological diseases were found to be transmitted by abnormally folded proteins.

•These proteins are called prions.

•The word prion comes from ‘proteinaceous infectious particle’.

  e.g.   Mad cow disease in cattle,

    Jacob's disease in human.

Find more information about prions.

-Created By-

Shri. Deshmukh A. B.