VIRUSES

                               VIRUSES

Definition of Viruses

Viruses are microscopic non-cellular parasitic particle made up of genetic material and protein that can only reproduce or replicate in a living cells.

The study of viruses is called VIROLOGY. 

A  complete virus particle as it exist outside the cell is called VIRION or VIRAL particle.

Viruses can infect all types of cells including  Bacteria,Protozoa,Plants ,Animal,Human.

       DIFFRENCES BETWEEN EUKARYOTES,PROKARYOTE&VIRUSES

Eukaryotes

Prokaryotes

Viruses

Nucleus with membrane

No membrane bound nucleus

No nucleus

Only plants have cell wall

Has cell wall

No membranes

Contains many organelles

Only a few oraganelles

No organelles

Has a lipid bi-layer membrane surrounding it

Has a capsule surrounding it

Surrounded by a protein coat called ‘Capsid’

   

                                 GENERAL CHARACTERISTICS OF VIRUSES

Small Size: Cannot be viewed with a light microscope size range is 30-400nm, can be viewed using an electron microscope which uses beams of electron to magnify size of objects.

Characteristic Shape: Spherical,helical,rod  or polyhedral,sometimes with tails or envelopes.Most  common polyhedral is the icosahedron which has 20 triangular faces.

Obligate Intracellular parasite: Viruses do not contain within their coat the machinery for replication.For this they depend upon a host cell and this accounts for their existence as obligate intracellular parasites.Each virus can only infect certain species of cells .This refers to the Virus host range.

No built-in metabolic machinery: Viruses cannot have no metabolic enzymes and cannot generate their own energy.

No Ribosomes: Viruses cannot synthesize their own protein.For this they utilize host cell ribosomes during replication.

Only one type of nucleic acid: Viruses contain either DNA or RNA(never both) as their genetic material.The nucleic acid can be single stranded or double stranded

Do not grow in size: Unlike cells viruses do not grow in size and mass leading to a division process rather viruses grow by separate synthesis and assembly of their component resulting in the production of a mature virus.

                             STRUCTURE OF A VIRUSES

Inner Core: Contains genetic information in the form of RNA or DNA (never both)

Capsid(protein coat): Serves to protect genetic information inside viral particle  and permit attachment to appropriate host.

Outer envelope:Complex viruses also have a quasi-memebrane or envelope (protein and phospholipids) around the capsid to enhance adhesion ta an appropriate host.

Capsid Construction:Made of identical protein subunit termed capsomeres.

The protein spikes on viruses surface that allows it to bind and fuse with host cells.

                  CLASSIFICATION OF VIRUSES

Viruses are classified based on genetic material, their strategy of replication and their structure.

Type of genetic material: Individual viruses or viral particles are called virions contain genetic in the form of either DNA or RNA,DNA is double stranded either circular or linear arrangement,RNA Single stranded usually linear and may be segmented or non segmented.

Structure: The viral protective shell or capsid may be helical i.e spiral shaped or icosahedral i.e having 20 triangular sides.The most elaborate virus is the Bacteriophage.

                           HOW  VIRUSES ARE NAMED

Named after diseases e.g Measeles virus,Small pox virus. 

Named after the places where the disease first reported e.g Newcastle disease disease virus,Ebola virus,Norwalk virus,Bunyaviridae.  

 Host and signs of disease e.g Tobacco mosaic virus,Cauliflower mosaic virus,Brome mosaic virus.  

Latin and Greek words e.g Coronaviridae- ‘crown’,Parvoviridae-‘small’. 

Virus Discoverers e.g Epstein-Barr virus. 

How they where originally thought to be contracted e.g Dengue virus (‘evil spirit’), Influenza virus (the ‘influence of bad air’).

Combination of the above e.g Rous Sarcoma virus

                       VIRAL REPLICATION

Viruses require living cells for reproduction.The cell that may be infected is named ‘Host cell’ or ‘Target cell’.Viruses are obligate intracellular parasites that are dependent on cellular energy for reproduction and cellular machinery for synthesis of their components.There are six steps of viral replication

Attachment (Adsorption): This is the first step in viral replication.Specific molecules on the surface of the virions attach to specific receptor on the plasma membrane of cell.Specific binding protiens of the virus interact with specific recdeptors on the target cell surface.Adsorption occurs to specific cellular receptors.

Penetration: The virus must enter the cell.They can penetrate cells in a variety of ways.Electron microscopy show that virions can enter cells by different mechanisms.Many envelope viruses penetrate into cell with the help of ‘Fusion’ with the cell membrane,and the nucleocapsid to be released into the cytoplasm.’Endocytosis’ is a specific process of uptake of essential macromolecules.Virus enters into intracellular vacuoles and then into the cytoplasm.

Uncoating:Is the step in which viral enzymes degrade the viral capsid ;that then exposes the viral genome to permit multiplication of the virus.

Viral Replication:This involves the synthesis of the viral mRNA(transcription) and genome replication.The virus uses the host cellular machinery to replicate make functional and structural protiens.Strategies for genomic expression for different genomic expression for different taxonomic group of viruses are different.All virulent viruses shut off cellular protein synthesis and disaggregate cellular ribosomes favouring a shift to viral synthesis.

Assembly:This is the process of assembly of the new components into virions.This involves bringing together newly formed viral nucleic acid and the structural protiens to form the nucleocapsid of the virus.

Release: Viruses can be released from the host cell by lysis,a process that kills the cell by bursting its membrane and cell wall.Some viruses e.g HIV typically are released from host cell by budding.During this process,the virus acquires its envelope,which is a modified piece of host’s plasma or other internal membrane.This newly formed virus particles are released ready to infect other cells.The virus replication circle can be as short as a couple of hours for certain small viruses or as long as several days for some large viruses.

               

       ROLE OF VIRUSES IN DISEASE

Viruses can cause many human diseases.For example COVID-19,small pox,the commoncold,measles,mumps,rubella,chickenpox,hepatitis,polio,rabies,acquired immune deficiency syndrome etc.

Covid-19 is caused by the SARS-CoV-2 virus.It causes mild to severe respiratory illness,including death.The virus that cause COVID-19,enters the body through the mouth,nose or eyes.It then travels to the back of the nasal passage and mucuos membrane in the back of the throat.It attaches to cells there and begin to multiply and moves into the lung tissue.From there ,the virus can spread to other body tissues.The best preventive measures includes getting vaccinated,wearing a mask during the times of high transmission,Staying 6 feet apart,washing hand often and avoiding sick people.

Tobacco mosaic virus is a single strand RNA virus that infects plants especially tobacco and other members of the family Solanacae.TMV was the first virus to be discovered.The infection causes characteristic pattern ‘mosaic’-like mottling and discoloration on the leaves hence the name.TMV can multiply only inside a living cell but it can survive in a dormant state in dead tissue ,retaining its ability to infect growing plants for years after the infected plant part died.TMV spread from plant to plant through workers hands,clothing,or on tools.The infected plant stock is discarded immediately.