Viral Structure and Genome

Viral Genomes Capsid Structure Envelope Structure References

Viral Genomes

Viral genomes are the genetic material that a virus uses to replicate and produce new virions. These genomes can be composed of either DNA or RNA, and they vary greatly in size, structure, and complexity. Viral genomes can be:

• DNA or RNA:
  • DNA viruses have genomes composed of DNA, which can be single-stranded (ssDNA) or double-stranded (dsDNA).
  • RNA viruses have genomes composed of RNA, which can be single-stranded (ssRNA) or double-stranded (dsRNA).
• Linear or Circular:
  • Some viral genomes are linear, while others are circular.
• Segmented or Non-segmented:
  • A segmented genome is divided into multiple pieces, each coding for different proteins, while a non-segmented genome is a single continuous piece of nucleic acid.
• Positive-sense or Negative-sense RNA:
  • In RNA viruses, positive-sense RNA acts as mRNA and can be directly translated into proteins by the host cell's machinery, whereas negative-sense RNA must be transcribed into positive-sense RNA before translation.
• Size:
  • Viral genomes range from just a few thousand nucleotides, as seen in some RNA viruses, to over a million base pairs in large DNA viruses like poxviruses.

Capsid Structure

The capsid is the protein shell that encases and protects the viral genome. It plays a critical role in the virus's ability to infect host cells. The capsid is composed of protein subunits called capsomeres, which assemble into a highly organized structure. The three primary capsid structures are:

• Icosahedral:
  • This is a symmetrical structure composed of 20 triangular faces. It is one of the most efficient ways to enclose a space and is found in many viruses, including adenoviruses and herpesviruses.
• Helical:
  • In helical viruses, the capsid proteins wind around the nucleic acid, forming a rod-like structure. Examples include tobacco mosaic virus and many RNA viruses like the influenza virus.
• Complex:
  • Some viruses, like bacteriophages, have complex capsid structures that do not fit neatly into the icosahedral or helical categories. They often have additional components, such as tails or fibers, that aid in host cell recognition and attachment.

Envelope Structure

Some viruses possess an envelope, a lipid bilayer derived from the host cell membrane, surrounding the capsid. This envelope is embedded with viral glycoproteins that play crucial roles in the infection process, including host cell recognition, attachment, and entry. The envelope structure can be described as follows:

• Lipid Bilayer:
  • The envelope is composed of a lipid bilayer, which is acquired from the host cell during viral budding. This bilayer is similar to the host cell membrane but includes viral proteins.
• Glycoproteins:
  • Embedded in the envelope are viral glycoproteins, which are critical for binding to host cell receptors. These glycoproteins can vary greatly between different viruses. Examples include the hemagglutinin and neuraminidase of the influenza virus.
• Matrix Proteins:
  • Beneath the lipid bilayer, some viruses have matrix proteins that stabilize the interaction between the envelope and the capsid. These proteins also play roles in the assembly and release of new virions.
• Susceptibility to Environmental Factors:
  • Enveloped viruses are generally more sensitive to environmental factors such as detergents, drying, and temperature changes, compared to non-enveloped viruses. This sensitivity is due to the delicate nature of the lipid bilayer.

References

1. Flint, S. J., Enquist, L. W., Racaniello, V. R., & Skalka, A. M. (2015). Principles of Virology, Volume I: Molecular Biology. American Society for Microbiology Press.
2. Cann, A. J. (2015). Principles of Molecular Virology. Academic Press.
3. Fields, B. N., Knipe, D. M., & Howley, P. M. (2007). Fields Virology. Lippincott Williams & Wilkins.