Introduction to Viruses: Structure, Genomes, and Symmetry

Viruses: Definition and Basic Properties

What is a Virus?

Viruses are unique genetic elements that require a living host cell to multiply. They exist in an acellular form known as the virion, which facilitates transmission between host cells.

• Virus: A genetic element capable of replication only inside living cells.

• Virion: The extracellular, acellular form of a virus. It contains the viral genome surrounded by a protein coat (capsid), and sometimes additional layers such as an envelope.

• Capsid: The protein shell that encases the viral genome, composed of subunits called capsomeres.

• Envelope: In some viruses, a lipid membrane surrounds the capsid, derived from the host cell membrane and containing viral proteins.

Example: The diagram shows a naked virus (with only nucleocapsid) and an enveloped virus (with an additional envelope layer).

Viral Genomes

Types and Organization

Viral genomes are highly diverse and can be composed of either DNA or RNA, which may be single-stranded or double-stranded, and linear or circular in shape.

• Genome Types:

  • DNA viruses: ssDNA (single-stranded), dsDNA (double-stranded)

  • RNA viruses: ssRNA (single-stranded), dsRNA (double-stranded)

  • Retroviruses: RNA genome that is reverse-transcribed into DNA

• Size: Most viral genomes are much smaller than those of cells.

• Structure: Genomes may be linear or circular.

Example: The diagram compares genome types among viruses, showing the diversity in nucleic acid composition.

Structure of the Virion

Shapes and Sizes

Viruses exhibit a wide variety of shapes and sizes, but most are significantly smaller than prokaryotic cells, typically ranging from 0.02 to 0.3 μm in diameter.

• Shape Diversity: Viruses can be rod-shaped, spherical, or complex.

• Size Comparison: Viruses are much smaller than bacteria such as Escherichia coli.

Example: The slide shows various bacteriophages and human RNA viruses, with a size comparison to a bacterial cell.

Capsid and Capsomeres

The capsid is a highly organized protein shell that protects the viral genome. It is made up of repeating protein subunits called capsomeres, arranged in a precise and repetitive pattern.

• Capsid: Provides structural integrity and protection for the viral genome.

• Capsomere: The individual protein subunit of the capsid.

Example: Electron micrographs show the arrangement of capsomeres in helical viruses.

Symmetry of Nucleocapsids

Viral nucleocapsids are arranged with specific symmetry, which determines the overall shape of the virion.

• Helical Symmetry: Rod-shaped viruses, such as Tobacco mosaic virus, have capsomeres arranged in a spiral around the nucleic acid.

• Icosahedral Symmetry: Spherical viruses, such as Human papillomavirus, have capsomeres arranged in a 20-sided (icosahedral) structure.

Example: Diagrams and micrographs illustrate the differences between helical and icosahedral symmetry.

Table: Comparison of Helical and Icosahedral Symmetry

Symmetry Type	Shape	Example Virus
Helical	Rod-shaped	Tobacco mosaic virus
Icosahedral	Spherical (20-sided)	Human papillomavirus

Additional info: The arrangement of capsomeres and the type of symmetry are critical for the assembly and stability of the virion.