Every virion, regardless of the virus it belongs to, contains two essential components: a nucleic acid genome and a protein shell called a capsid. Together, these form the nucleocapsid, the minimal structure that defines a complete virus particle. Some virions carry additional features like an outer membrane envelope or specialized enzymes, but those are extras found only in certain virus families. The genome and the capsid are universal.
The Genome: DNA or RNA, but Never Both
At the core of every virion sits its genetic material. Unlike living cells, which always use double-stranded DNA, viruses show enormous variety in how they store their genetic instructions. A viral genome can be DNA or RNA, single-stranded or double-stranded, linear or circular. It can be as small as 2 kilobases (roughly 2,000 genetic “letters”) or stretch to 2,500 kilobases, a thousand-fold difference. Some viruses pack their entire genome into one continuous molecule, while others split it across multiple separate segments that all need to be present for the virus to replicate.
RNA viruses add even more variety. Some carry positive-sense RNA, meaning their genome can be read directly by the host cell’s machinery like a ready-made instruction manual. Others carry negative-sense RNA, which has to be copied into a complementary strand before the cell can use it. A third category, called ambisense viruses, carry RNA that is partly positive-sense and partly negative-sense on the same segment. Retroviruses, like HIV, package two copies of their RNA genome along with a cellular molecule that helps kickstart replication once inside the host.
Despite all this variation, the principle holds: every virion contains nucleic acid. Without it, the particle has no instructions to hijack a host cell, and no way to produce new copies of itself.
The Capsid: A Protein Shell Built From Repeating Units
Surrounding the genome is the capsid, a protective coat assembled from many copies of one or a few protein types. These individual protein building blocks are called capsomeres. Because viruses encode very few genes compared to cells, they can’t afford to build their outer shell from dozens of unique proteins. Instead, they use the same small set of proteins over and over, arranging them into a regular, repeating pattern. This is an elegant solution: a small genome can code for just one or two shell proteins, and those proteins self-assemble into a structure large enough to enclose the entire genome.
The capsid does three critical jobs. First, it physically shields the genome from enzymes in the environment that would otherwise shred exposed nucleic acid. Second, it serves as the vehicle that carries the genome from one host cell to the next. Third, in many viruses, proteins on the capsid surface are what latch onto specific receptors on a target cell, initiating infection. The capsid essentially decides which cell types, and sometimes which species, a virus can infect.
Capsid Shapes: Icosahedral, Helical, and Complex
Capsids aren’t randomly shaped. They follow one of a few geometric blueprints dictated by the physics of protein assembly. The two most common symmetries are icosahedral and helical.
An icosahedral capsid looks roughly spherical and is built like a geodesic dome, with 20 triangular faces, 12 vertices, and a minimum of 60 protein subunits. Many viruses use far more than 60 subunits, scaling up according to a mathematical rule called the triangulation number. Herpes simplex virus, for example, builds its capsid from 162 capsomeres arranged in a triangulation number of 16. Icosahedral symmetry is extremely common and shows up in viruses as different as the common cold (rhinoviruses) and poliovirus.
Helical capsids, by contrast, form a rod or tube shape. The protein subunits spiral around the nucleic acid like steps on a spiral staircase, with each subunit sitting in a nearly identical position relative to its neighbors. Tobacco mosaic virus is the classic example, and many RNA viruses that infect animals, including measles and rabies, use helical nucleocapsids inside an outer envelope.
A third category, sometimes called complex symmetry, applies to viruses that don’t fit neatly into either camp. Poxviruses, for instance, are brick-shaped with an intricate internal structure. Some bacteriophages (viruses that infect bacteria) combine an icosahedral head with a helical tail, creating a shape that looks almost mechanical.
The Nucleocapsid: Genome Plus Capsid as One Unit
Virologists often refer to the genome-plus-capsid package as the nucleocapsid. This term highlights the fact that these two components function as a single integrated unit. The nucleocapsid protein associates with both itself and with the genome, forming a closed cavity that stores and protects the genetic material. In coronaviruses, for instance, the nucleocapsid protein oligomerizes (links together in multiple copies) to create a sealed shell around the RNA, providing the genome’s first line of defense against the host environment.
For the simplest viruses, the nucleocapsid is the entire virion. Nothing else is needed. These “naked” viruses tend to be tougher in the outside environment because their protein shell is exposed and relatively resistant to drying, detergents, and temperature changes.
What Some Virions Have That Others Don’t
Beyond the two universal components, many virions carry additional structures. The most notable is the viral envelope, a lipid membrane stolen from the host cell during the virus’s exit. Envelopes are common among animal viruses (influenza, HIV, Ebola, SARS-CoV-2 all have them) but are not universal. Many viruses, including norovirus and adenovirus, lack an envelope entirely.
Embedded in the envelope are glycoproteins, spikes that protrude from the surface and handle the job of recognizing and fusing with host cells. In enveloped viruses, these glycoproteins take over the cell-attachment role that the capsid performs in naked viruses. Some enveloped viruses also contain a layer called the tegument, a protein-filled zone between the capsid and the envelope. Herpes simplex virus is a well-known example: its tegument carries proteins that immediately go to work inside the host cell upon infection, before the viral genome is even unpacked.
Certain viruses also package enzymes inside the virion. Retroviruses carry reverse transcriptase, the enzyme that converts their RNA genome into DNA. Negative-sense RNA viruses carry their own RNA-dependent RNA polymerase because the host cell has no machinery to read negative-sense RNA on its own. These enzymes are essential for those particular virus families, but they are not universal to all virions.
Why Only Two Components Are Truly Universal
The reason the genome and capsid are the only features shared by every virion comes down to what a virus fundamentally is: a set of genetic instructions wrapped in a protective delivery package. Envelopes, teguments, and packaged enzymes are evolutionary extras that certain lineages have acquired to suit their specific infection strategies. A virus that infects a plant cell by being injected through an insect’s mouthparts faces different challenges than one that fuses with a human lung cell, and their structures reflect that.
But no matter how a virus infects, replicates, or spreads, it always needs genetic material to encode its proteins and a capsid to protect and deliver that material. Strip away every optional feature, and what remains is the nucleocapsid: the irreducible core of every virion in existence.