Question

write an essay describing the structure, genome organisation and replication strategies of viruses

Answer 1

STRUCTURE

Viruses are non-cellular organisms that are characterized by having an inert crystalline structure outside the living cell. They are obligate parasite and also contain a genetic material, RNA or DNA but never both. A virus is a nucleoprotein and the genetic material is infectious. A protein coat called as capsid (made up of small capsomeres arranged either in helical or polyhedral geometric forms) is present which protects the nucleic acid.

GENOME ORGANIZATION

A virus has either DNA or RNA, but never both. DNA and RNA molecules can be double or single stranded, linear or circular. A genome is a unique piece of nucleic acid among multiple pieces comprising one whole viral genome. The single stranded genomes consist of an unpaired nucleic acid, analogous to one-half of the ladder. The double stranded genomes consist of complimentary pair of nucleic acids, analogous to a ladder.

REPLICATION STRATEGIES

Since, viruses are acellular they do not replicate via cell division. Instead, these viruses uses the host machinery to grow and replicate to make multiple copies of themselves (lytic cycle).The other way is lysogenic cycle(viral genome is incorporated by genetic recombination into a specific place in the host’s chromosome).

Most productive viral infections follow similar steps in the virus replication cycle: attachment, penetration, uncoating, replication, assembly, and release.

• Attachment
  A virus attaches to a specific receptor site on the host cell membrane through attachment proteins in the capsid or via glycoproteins embedded in the viral envelope.
• Penetration
  The nucleic acid of bacteriophages enters the host cell naked, leaving the capsid outside the cell. At this stage, the bacteriophage is termed as a

• Replication followed by assembly

  DNA viruses usually use host cell proteins and enzymes to make additional DNA that is transcribed to mRNA, which is then used to direct protein synthesis. RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA. The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and to assemble new virions.

• Release
  The last stage of viral replication is the release of the new virions produced in the host organism.

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