Virus| Structure , Characteristics, Features, Reproduction| Medical World

INTRODUCTION

The term virus is derived from a latin word, meaning poison and was first given by beijerinck. The branch of science which deals with the virus is called virology and the specialised of this branch are designated as virologist.

DEFINITION OF VIRUS

Virus are biological entities as they exhibit characters of both inanimate and animate organisms. Virus are ultra-microscopic disease causing nucleoprotein entities, capable of being introduced into the living cells of specific organisms and capable of multiplying or being multiplied within the living cells.

FEATURES OF VIRUS

1. Virus are only visible under electron microscope; they are a kind of organism lying in between living and non-living objects.
2. Virus show their existence in water, land and air.
3. Cytoplasm is absent within the body of virus, hence non-cellular.
4. Virus are very infectious obligatory parasites.
5. The disease producing virus particles can only multiply within living organisms
6. Living characters of virus are reproduction and mutability.

WHY VIRUS IS CALLED ACELLULAR?

Virus is called acellular due to the following reason:

1. Virus have no power to grow and work independently.
2. For different function of the cell a minimum space of 5000nm diameter is necessary . But virus have no such size. Viruses have diameter between 100nm-2000nm.
3. Virus can be crystallised. No living cell shows such phenomenon
4. Enzymes, cytoplasm. Other cell organelles are totally absent within the virus.

STRUCTURE OF VIRUS

1. External structure: Externally virus may be spherical, rod shaped, cuboidal or spermatozoa shaped.
2. Internal structure : virus is called viron and each virus particle is called virion electronic microscopy shows that a virus particle is made up of two parts, an inner core of nucleic acid and an outer core of protein. The nucleic acid component is either RNA or DNA, which lies centrally , and is surrounded by a membrane made up of protein.

The two main parts of the virus body are

1. Capsid or outer membrane
2. Nucleoid - the parts lying within the capsid

Capsid

the protein coat of a virus particle is called the capsid. The membrane is composed of a number of identical sub-units known as the capsomeres. The nucleic acid core and capsid together form the nucleocapsid. The number of capsomeres, their shape and constitution differ considerably in various kind of virus. The capsid of a virus is physiologically inactive. However, it protects the nucleic acid i.e. viral genome, from unfavourable environmental conditions and helps the virus to penetrate within the body of host cell.

This part is left outside the host cell all the time of reproduction. Each capsomere is composed of a few monomers where both of which are attached together by bonds. The virus particles in some cases on the outer side of capsid is surrounded by means of an envelope. The envelope is composed of viral protein and lipid of the host cell. In some virus, like vaccinia virus, the capsid is composed of carbohydrate, fat, biotin, riboflavin, and copper inanition to 89% protein. The capsid of influenza virus posses starch and fatty substances. Besides this they posses numerous projected structure of spikes. The spikes are 10 nm. Long and situated 7-8 nm.each virus capsid posses specific number of enzymes so that the virus can enter the host cell penetrating the cell wall.

In some animal virus like Herpes virus, Pox virus, the envelope situated outside the capsid is composed together of viral protein and lipid derived from the hot cell. The existence of lipids keeps the envelope loose and flexible. This type of lipid envelope virus are called lipoviruses.

Nucleoid

The central core of virus composed of nucleic acid is called nucleoid. This part is covered by means of an outer coat of protein termed as capsid. Nucleic acid is composed RNA or DNA. Most of the plant viruses e.g. Tobacco mosaic virus .

CHARACTERSTICS OF VIRUS

Influenza virus

Definition

the virus which produces the influenza disease is called influenza virus.

Structure of virus

1. It is a helical virion, contains a single RNA strands and is surrounded by an envelope, provided with a series of spikes.
2. The surrounding envelope is called capsid.
3. In the spike of influenza virus haemoglutanin is present. Haemoglutanin is protein in nature and is capable of clotting the red blood cells or erythrocytes of the host cell.
4. During their, replication, enzymes like neuraminidase dissolves their cell membrane. Spike containing haemoglutanin is made up of polypeptide like HA1 and HA2. Four NA polypeptides form the neuraminidase spike. The segmented nucleocapsid looks like a double helix.

Bacteriophage

Definition

Those virus which kills harmful bacteria and are thus beneficial is called bacteriophage or phage virus.

Structure

The phage virus is tadpole shaped consisting of a hexagonal head, neck, long tail and base plate

Head

The head is 6 faced with two protein layers. Inside the hollow cavity of the head lies the double standard DNA. The capsid of the head is formed of several capsomeres. Capsid generally posses a cuboidal symmetry.

Neck

The neck is tube like connecting the head and the tail portion, and is often surrounded by a circular disc. The cooler at the mid part. The posterior end of the neck is protected into the front hole of the tail core.

Tail

The tube like portion consists of two parts, the inner hollow core and the surrounding outer protein sheath, is called tail sheath.

Base plate

The tail ends in a base plate bearing long slender thread like tail fibres which help in anchoring the virion to the host cell. The base plate has a pin or spike attached to its lower surface at each corner.

REPRODUCTION OF VIRUS

Virus in its extracellular phase are incapable of reproduction, metabolism etc. when they attach themselves with the living host cell then they undergo a series of processes to cause their own replication.

Viral reproduction or replication involves five stages generally-

Adsorption

it means attachment of viral particle to the surface of the susceptible host cell. Adsorption of bacteriophage though involve complex process, but adsorption of plant and animal virus appears to be less complex where the virus enters the host cell either by phagocytosis or through vectors.

Penetration

In bacteriophage , a hole is made on the host cell surface by the enzyme lysozyme secreted by tail fibres with a simultaneous contraction of tail protein sheath forcing the tail core into the cell just like an injection syringe, through which the DNA pass into the host cell. The infective virion before penetration is called extracellular virion and the nucleic acid portion after penetration is termed as intracellular virion.

Eclips phase

In this period synthesis of components of progeny phages takes place. In plant and animal virus the protein coat is separated inside the host cell leaving aside the free nucleic acid to start operations. Nucleic acid takes hold of the host cell metabolic process and synthesises its own components , the protein and nucleic acid.

Maturation

After eclipse stage , the component of progeny virus combine to form several complete virions. In the sequence of viral synthesis, viral nucleic acid is synthesised first then protein followed by capsid. The page DNA condenses within the bacterial cytoplasm and is enveloped by head protein which in turn attaches itself with the phage tail.

Release of virus

after maturation the complete progeny virions are released to the surrounding medium by several processes, such as

1. Lysis- sudden rupture of the cell wall.
2. Extrusion of cytoplasm
3. Release of individual or groups of particles.

The life cycle of the phage can be divided into the following three phases:

1. Infective phase
2. Vegetative phase
3. Progeny formation phase

The whole process is called Lytic cycle, the replication cycle of a phase virus.