Virus – Morphology, Classification and Mode of Replication

  • Virus depends upon the living host to reproduce and multiply. They cannot replicate when they are outside the living host.
  • They are also known to be acellular because they do not contain cellular components such as cell organelles and ribosomes but it also contains certain enzyme.
  • A complete virus is known as a virion which consists of a nucleic acid, an outer protein coated capsid and occasionally an envelope.
  • Viruses are 100 times smaller than bacteria and are known to be a microscopic parasite whose size ranges from 5 to 300 nanometers.
  • In terms of size, large sized virus known till date is a Giant Mimivirus which was discovered in 1992 as Acanthamoeba polyphaga mimivirus (APMV) which causes pneumonia and has a diameter size of approximately 750nm with 500nm diameter of extended capsid in its outer layer.
  • Viruses can either be DNA or RNA as their unique genetic material which can further be either single stranded (ss) or double stranded (ds) surrounded by viral protein.
  • The main purpose of virus is to attach and penetrate inside the host cell and express its genome by various processes like transcription and translation.

We will learn about classification, morphology and virus replication in this article.

Morphology of Virus

Viruses are generally categorized into four groups:

  1. Filamentous – Plant virus like TMV (tobacco mosaic virus).
  2. Isometric/icosahedral – They are rough in shape. Like polio or herpes virus.
  3. Envelope – Enveloped virus include animal virus like HIV which contains an envelope protein outside their cell.
  4. Head and tail – Head and tail having virus infects bacteria. For example – Bacteriophage have head similar to icosahedral and filamentous like tail.

The morphology and shape of the virion and its outer envelope provides us an information about how and what sort of infection the virus may cause which is useful in classifying the viruses.

Structure and function of viral parts:

Structure of virus
Structure of virus

By Ben Taylor ,

Virus consists of major three components:

  • Nucleic acid
  • Capsid, a protein coat
  • And sometimes an envelope

  Nucleic acid:

  • Nucleic acid are either DNA or RNA where viruses use it as a genetic material.
  • Depending upon taxonomy, genomes can be either double stranded or single stranded and circular or linear.
  • DNA containing  viruses like chicken pox, Hepatitis B, herpes virus directs the replication of the hosts cell protein to synthesize new similar copies of DNA with further transcription and translation process to obtain viral proteins.
  • RNA containing viruses like Hepatitis C, measels and rabies virus encodes RNA polymerase enzyme that converts RNA into DNA which is carried out by the virus cell. There often occurs a misinterpretation during transcription which is the reason why RNA virus have more mutations than DNA virus which causes them to rapidly adapt according to the several changes.


  • Capsid functions as a proteinous shell to protect the viral genetic material i.e, nucleic acid from the digestive enzymes and also helps in attachment to the specific receptor of the host cell during replication and injection of infectious nuclei.
  • Capsomere is composition of  protein capsid subunits.
  • Capsid must be assembled together to build a three dimensional structure.
  • Two basic patterns that the virus capsid follows are:
  • Helix symmetry
  • Icosahedral symmetry
  • Larger viruses have complex structure consisting of both these symmetries.
  • The T4 bacteriophage has a complex structure that infects E. coli bacterium

Lipid Envelope:

  • Virus envelope consists of phospholipids and protein structure which is acquired from the part of the membrane of host cells.
  • Some of the viruses contains an envelope as the additional protective covering consisting of a glyocosylate transmembrane protein on the surface. These protein has a main role in identification, attachment and causing viral infection to the host cell.
  • It also determines the range of host and composition of antigenicity.
  •  Adenovirus causes respiratory illness in humans which is a non-enveloped animal virus.
  • Polio virus, Papilloma virus and Hepatitis A are also non-enveloped virus.
  • Enveloped viruses like HIV, chicken pox, influenza virus, mump virus are more fragile towards temperature, pH, disinfectants and some antiviral drugs. Therefore, non-enveloped viruses are more resistant to these changes.

Baltimore classification of virus

On the basis of combination of genome type and method of replication, viruses are divided into seven groups:

Baltimore classification of Virus
Baltimore classification of Virus
  1. Class I: dsDNA viruses (Adenovirus, Herpesvirus, Poxvirus)
  2. Class II: ssDNA viruses (+) sense DNA (Parvovirus)
  3. Class III: dsRNA viruses (Reovirus)
  4. Class IV: ssRNA viruses (+) sense RNA (Picornavirus, Togavirus)
  5. Class V: ssRNA viruses (-) sense RNA (Orthomyxovirus, Rhabdovirus)
  6. Class VI: ssRNA RT viruses (+) sense RNA with DNA intermediate in life cycle (Retrovirus)
  7. Class VII: dsDNA RT viruses (Hepadnavirus)

Mode of replication:

In influenza virus infection, viral glycoproteins attach the virus to a host epithelial cell. As a result, the virus is engulfed. Viral RNA and viral proteins are made and assembled into new virions that are released by budding

Most of the viruses are destroyed and eliminated by hosts immune response or by the lysis or apoptosis. Animal virus such as Human Immunodeficiency virus (HIV) processes by budding and the body immune system are not capable of eliminating them. Therefore, the cells for a long time remains in the hosts body by replicating and damaging the hosts normal cell hence making the hosts immune system weak.

For a virus to reproduce , it must attach to the host cell surface and must be able to penetrate and infect its genetic material inside the host cell where they can multiply and spread rapidly.

Virus mode of replication involves 6 major steps:

Steps of Replication of Virus

  1. Attachment: Protein produced on the surface of the virus capsid or envelope binds to the specific receptor of the host cell surface. This binding process acts as a lock and key method where each key fits only with a specific lock.
  2. Penetration: HIV and influenza virus with an envelope enters the cell with a fusion of host cell membrane wheras non-enveloped virus enters by translocation or through receptor mediated endocytosis. While penetrating, viral capsid protein goes into several conformationa changes.
  3. Uncoating/disassembly: The viral nucleic acid are released by the viral enzyme or host enzyme that degrades the viral capsid and uncoats the genetic material and makes it available for transcription and translation.
  4. Genome replication: Transcription and translation are initiated after the uncoating of viral genetic material where DNA and RNA virus replicates in terms of different taxonomic grouping.

DNA virus use protein cells and enzymes of the host to transcribe into mRNA and translate to protein synthesis.

While RNA virus use the RNA template and enzymes to get transcribed into mRNA .

These replication processes lead up the de novo synthesis.

  • Assembly: After replication, the newly replicated genomic material are assembled together and are packed inside the new nucleocapsid of the virus. This process is also the maturation stage before release into the host cell.
  • Release: New viruses gets ready to be released in the host organism by lysis or by budding.

Enveloped virus are released by budding and usually do not kill the infected cell. So, they are termed as cytopathic virus.

Wheras, lysis is termed as cytolytic which results in the death of the host cell.

After viruses are released by various processes, some remains in the host cell which proceeds in making the circulating antibodies and remaining viral protein are processed and presented to the MHC class I molecules which are recognized by memory T cells.



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