What defines a virus as a "giant virus"?

A giant virus is defined mainly by two features: its unusually large size (often more than 250 nanometers, making it visible under a regular light microscope) and its massive double-stranded DNA genome (usually bigger than 1 megabase), which can carry hundreds to over a thousand genes. These traits set them apart from typical viruses and give them complex structures, often with a thick, fibrous coat around the capsid.

Key Defining Characteristics

• Size: Giant virus particles are much larger than common viruses, usually bigger than 250 nm in diameter, and some can grow beyond 750 nm.
• Genome Size: They have very large double-stranded DNA genomes, ranging from several hundred kilobasepairs up to over 2.5 megabases.
• Gene Content: Their genomes carry hundreds to thousands of protein-coding genes—much more than ordinary viruses and on par with some bacteria or tiny eukaryotes.
• Virion Structure: Their capsids are often pseudo-icosahedral (20-sided) and covered with thick, filament-like fibers.

Unique Features and Capabilities

• Metabolic Genes: Unlike most viruses, some giant viruses have genes for metabolic processes (like glycolysis or the TCA cycle), giving them unusual capabilities.
• Histone-like Proteins: Some also code for histone-like proteins, similar to those in eukaryotic cells, which help in DNA packaging and point to a more complex life cycle.
• Virophage Interaction: Giant viruses themselves can be infected by smaller viruses called virophages, leading to unique multi-layered interactions.

Examples

• Mimivirus: A famous giant virus, about 450 nm wide, with a fuzzy outer layer of fibers.
• Megavirus: Another member of the Mimiviridae family, known for its very large capsid.