Lecture Notes: Virus Structure and Composition

General Overview

• Viruses, unlike eukaryotic and prokaryotic cells, are simpler structures.
• Contain either DNA or RNA, but not both.

Components of a Virus

Nucleic Acid

• Viruses have nucleic acid (either DNA or RNA).

Capsid

• Protein coat surrounding the nucleic acid.
• Shapes include icosahedral, helical, and complex.
• Made of proteins called capsomeres.

Naked Viruses

• Viruses without an envelope.
• Example: Papillomavirus.
• Have spike proteins on the capsid to recognize host cells.

Enveloped Viruses

• Many have an additional lipid layer called an envelope.
• Derived from host cell membranes (plasma, nuclear, ER, Golgi).
• Spike proteins necessary for host cell recognition are on the envelope.
• Example: Influenza virus.

Capsid Shapes

• Helical Capsid: Examples include Tobacco Mosaic Virus and Coronaviruses.
• Icosahedral Capsid: Example includes Adenoviruses.
• Complex Capsid: Example includes Bacteriophages, which inject DNA into bacterial cells.

Viral Infection and Release

• Enveloped viruses leave host cells via budding, taking part of the host cell membrane with them.
• Viral spikes replace host proteins in the membrane for recognition of new host cells.

Viral Genomes

• Can be either DNA or RNA.
• Range in gene count from a few (Hepatitis B) to thousands (Mimiviruses and Pandoraviruses).
• Genome size does not correlate with organism complexity.

DNA vs. RNA Viruses

• DNA can be single or double-stranded in viruses.
• RNA can also be single or double-stranded.
• RNA can be positive or negative stranded, affecting viral classification.

Retroviruses

• Class of RNA viruses that convert RNA to DNA in host cells.
• Example: HIV.

Classification of Viruses

• Grouped by genetic material type, strand type, and presence of an envelope.
• Common viruses: Rhinoviruses (common cold), Rotavirus (diarrhea).

Next Lecture

• Focus on the viral life cycle.