Home | | Botany | | Botany | Viruses : Size, Shape And Structure of a Virus

Chapter: 11th 12th standard bio zoology Human Body higher secondary school

| Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail |

Viruses : Size, Shape And Structure of a Virus

Viruses : Size, Shape And Structure of a Virus
Viruses are very minute particles that they can be seen only under electron microscope. They are measured in millimicrons ( 1 millimicron = 1/1000micron). (1micron - 1/1000 millimeter). Generally they vary from 2.0 mm to 300 mm in size.

Viruses : Size and Shape

 

Viruses are very minute particles that they can be seen only under electron microscope. They are measured in millimicrons ( 1 millimicron = 1/1000micron). (1micron - 1/1000 millimeter). Generally they vary from 2.0 mm to 300 mm in size.

 
Very small size and ability to pass through bacterial filters are classic attributes of viruses. The following methods are used to determine the size of the viruses.

Direct observation by using electron microscope:
 
Filtration through membranes of graded porosity: In this method viruses are made to pass through a series of membranes of known pore size, the approximate size of any virus can be measured by determining which membrane allows the virus to pass through and which membrane holds it back.

Sedimentation by ultra centrifugation : The relationship between the size and shape of a particle and its rate of sedimentation permits determination of particle size.

 

Comparative measurements:


The following data is used for reference.


a. Staphylococcus has a diameter of 1000 mm.

b. Bacteriophage varies in size from 10-100 nm.

 

Broadly speaking viruses occur in three main shapes:

1. Cubic symmetry: polyhedral or spherical - eg. Adeno virus, HIV

2. Helical symmetry: e  g        Tobacco Mosaic virus (TMV), Influenza virus.

3. Complex or atypical eg. Bacteriophage, Pox virus.

 

Structure of a virus

 

A virus is composed of two major parts    

1. Capsid  (the protein coat) 

2. Nucleic acid.

The capsid is the outer protein coat. It is protective in function.

It is often composed of many identical subunits   called capsomeres. Some  of  the viruses have an outer covering called envelope eg. HIV.  They are called enveloped viruses.

Others are called naked viruses or non- enveloped viruses. The capsid is in close contact with the nucleic acid and hence known as nucleocapsid. The nucleic acid forms the central core. Unlike any living cell a virus contains either DNA or RNA, but never both. The infective nature of the virus is attributed to the nucleic acid while host specificity is attributed to the protein coat.


Virion

 An intact, infective virus particle which is non-replicating outside a host cell is called virion.


Viroids

 A viroid is a circular molecule of ss RNA without a capsid. Viroids cause several economically important plant diseases, including Citrus exocortis.

 

Prions(pronounced 'preeons' )

 They are proteinaceous infectious particles. They are the causative agents for about a dozen fatal degenerative disorders of the central nervous systems of humans and other animals. eg. Creutzfeldt-Jacob Disease(CJD), Bovine Spongiform Encephalopathy (BSE)-Commonly known as mad cow disease, etc .They are very unique among infectious agents because they contain no genetic material i.e DNA/RNA. Stanley Prusiner did most of the work on prions and was awarded Nobel Prize in 1998.


Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail


Copyright © 2018-2020 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.